Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability
For supramolecular compounds with donor and acceptor fragments, coupled with each other by the π-conjugated methyn chain, it was ascertained an influence of Cl and Br atoms on the π-conjugation and charge transfer in them in molecular and aggregate states. Compound, which contains Br atom, crysta...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
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Piryatinski, Yu.P. Sevryukova, M.M. 2017-06-03T04:42:43Z 2017-06-03T04:42:43Z 2008 Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability / Yu.P. Piryatinski, М.М. Sevryukova // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 3. — С. 292-302. — Бібліогр.: 21 назв. — англ. 1560-8034 PACS 71.35.-y, 78.55.-m https://nasplib.isofts.kiev.ua/handle/123456789/119048 For supramolecular compounds with donor and acceptor fragments, coupled with each other by the π-conjugated methyn chain, it was ascertained an influence of Cl and Br atoms on the π-conjugation and charge transfer in them in molecular and aggregate states. Compound, which contains Br atom, crystallizes into a noncentrosymmetric polar structure and under infrared (1064 nm) laser excitation it is observed the second harmonic generation and intense up-conversion photoluminescence in the visible spectral region. For compounds containing Cl atom, when passing into a crystalline state, only up-conversion photoluminescence is observed, which can be considered as twophoton excited. Possible mechanisms of intramolecular charge transfer and collective interactions in investigated molecular structures under one- and two-photon excitation are considered. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability Article published earlier |
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Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
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Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability Piryatinski, Yu.P. Sevryukova, M.M. |
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Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
| title_full |
Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
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Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
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Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
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optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability |
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Piryatinski, Yu.P. Sevryukova, M.M. |
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Piryatinski, Yu.P. Sevryukova, M.M. |
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2008 |
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English |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Article |
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For supramolecular compounds with donor and acceptor fragments, coupled
with each other by the π-conjugated methyn chain, it was ascertained an influence of
Cl and Br atoms on the π-conjugation and charge transfer in them in molecular and
aggregate states. Compound, which contains Br atom, crystallizes into a noncentrosymmetric
polar structure and under infrared (1064 nm) laser excitation it is observed the
second harmonic generation and intense up-conversion photoluminescence in the visible
spectral region. For compounds containing Cl atom, when passing into a crystalline state,
only up-conversion photoluminescence is observed, which can be considered as twophoton
excited. Possible mechanisms of intramolecular charge transfer and collective
interactions in investigated molecular structures under one- and two-photon excitation
are considered.
|
| issn |
1560-8034 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/119048 |
| citation_txt |
Optical properties of π-conjugated donor-acceptor systems with controlled hyperpolarizability / Yu.P. Piryatinski, М.М. Sevryukova // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2008. — Т. 11, № 3. — С. 292-302. — Бібліогр.: 21 назв. — англ. |
| work_keys_str_mv |
AT piryatinskiyup opticalpropertiesofπconjugateddonoracceptorsystemswithcontrolledhyperpolarizability AT sevryukovamm opticalpropertiesofπconjugateddonoracceptorsystemswithcontrolledhyperpolarizability |
| first_indexed |
2025-11-27T08:33:25Z |
| last_indexed |
2025-11-27T08:33:25Z |
| _version_ |
1850809072007249920 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
292
PACS 71.35.-y, 78.55.-m
Optical properties of π-conjugated donor-acceptor systems
with controlled hyperpolarizability
Yu.P. Piryatinski, М.М. Sevryukova
Institute of Physics, National Academy of Sciences of Ukraine,
46, prospect Nauky, 03028 Kyiv, Ukraine; e-mail: yupiryat@iop.kiev.ua
Abstract. For supramolecular compounds with donor and acceptor fragments, coupled
with each other by the π-conjugated methyn chain, it was ascertained an influence of
Cl and Br atoms on the π-conjugation and charge transfer in them in molecular and
aggregate states. Compound, which contains Br atom, crystallizes into a noncentro-
symmetric polar structure and under infrared (1064 nm) laser excitation it is observed the
second harmonic generation and intense up-conversion photoluminescence in the visible
spectral region. For compounds containing Cl atom, when passing into a crystalline state,
only up-conversion photoluminescence is observed, which can be considered as two-
photon excited. Possible mechanisms of intramolecular charge transfer and collective
interactions in investigated molecular structures under one- and two-photon excitation
are considered.
Keywords: J-aggregate, H-aggregate, up-conversion photoluminescence, two-photon
excited photoluminescence, molecular exciton, charge-transfer exciton.
Manuscript received 10.06.08; accepted for publication 20.06.08; published online 30.09.08.
1. Introduction
In a number of works [1-4] we informed on high
efficiency of transformation of infrared (IR) laser
(λe = 1064 nm) irradiation into the visible one by means of
two-photon excited photoluminescence (TPL) for some
molecular compounds of heterocyclic derivatives at their
transition into the aggregate state. At sufficiently low
intensities of the exciting IR emission, the up-conversion
photoluminescence (PL) becomes compared to the one-
photon excited one (λe = 532 nm) PL in its intensity.
In this work, we report the high efficiency of up-
conversion transformation for IR irradiation in two-
quantum processes, which related with TPL and second
harmonic generation (SHG), for two, almost identical as
to their molecular structure, organic compounds in the
aggregate state. The compounds with intramolecular
charge transfer (ICT) under investigation contain the
molecular donor (D) and acceptor (А) fragments,
coupled with each other by π-conjugated electron system
of methyn groups (Fig. 1). These D-π-A molecules are
capable to show great nonlinear optical activity due to
strong polarizability of π-electrons, which slightly
coupled with a nuclear skeleton [5]. It causes a large
interest to research their nonlinear optical properties,
search of methods to predict their hyperpolarizability
values and model the structures of molecules with the set
nonlinear optical properties.
We have already established that a small difference
in the molecular structure of two investigated D-π-A
compounds is related with the fact that replacement of
Cl atom slightly coupled with the methyn chain by Br
leads to crystallization of DA-Br compounds in the polar
structure, while DA-Cl compounds creates centro-
symmetric structure.
Embedding the Cl or Br atoms into the structure of
D-π-A molecule obviously influences its π-conjugated
system and through it provides effect on hyper-
polarizability. Being in nanoaggregate and crystalline
states, the DA-Cl compounds demonstrate a high
efficiency of up-conversion PL, while for nano-
aggregates and crystals of DА-Br – along with up-
conversion PL – the SHG is observed. The efficiency of
up-conversion PL in compounds under investigation
essentially depends on the quantities of nanoaggregates
and is related with demonstration of collective electronic
properties at optical excitation in them. The mechanism
of high efficient up-conversion PL for similar molecular
structures is not studied in detail.
2. Materials and experimental methods
As objects of researches, polar molecular compounds of
a dyes 1-(4-dimethylaminophenyl)-3-(4-bromephenyl)-
4-cyano-4-(2,4-diamino-3,5-dicyanopyridynyl)-1,3-buta-
diene (DA-Br) and 1-(4-dimethylaminophenyl)-3-(4-
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
293
chlorinephenyl)-4-cyano-4-(2,4-diamino-3,5-dicyanopy-
ridynyl)-1,3-butadiene (DA-Cl) were used. The struc-
tural formulas of the dyes are shown in Fig. 1.
The kinetics, steady-state and time-resolved PL
spectra of these compounds in various aggregate states
were studied. Solutions of DA-Cl and DA-Br with
various concentrations in acetonitrile solvent, crystals
and crystalline films were investigated upon one- and
two-photon laser excitation. Researches have been
performed at temperatures 295, 77 and 4.2 К.
To excite PL, we used the following radiations: the
radiation of a nitrogen laser (λe = 337.1 nm, exciting
pulse time tI = 9 ns, peak power N = 5 kW, pulse
repetition frequency f = 100 Hz), the fundamental
harmonic of an Nd:YAG laser (λe = 1064 nm, tI = 10 ns,
N = 0.05 – 1.00 MW, f = 50 Hz), and the second harmonic
of an Nd:YAG laser (λe = 532 nm, N = 0.1 – 0.5 MW,
f = 50 Hz). Time-resolved PL spectra upon one- and two-
photon excitation were recorded with a stroboscopic
system [6], in which the investigated signal from a
photodetector through a line of a delay acted on the
stroboscopic oscilloscope connected with a computer.
The measurement technique allowed to write down
forms of PL pulses and the laser radiation reflected from
the investigated sample, at an identical optical way.
From the analysis of convolution of these signals it was
possible to define the PL life-time. The time resolution
of the system was about 0.1 ns for determining the PL
life-time and 0.7 ns for measuring PL spectra. To record
PL spectra, we used the MDR-12 monochromator. The
spectral slit width for measuring the PL spectra was 0.2-
0.4 mm. Spectral setup was used to record the spectra of
absorption, reflection R, and PL excitation. Used spectral
setup allowed also to perform researches of PL spectra,
absorption and reflection in the wide range of
temperatures 4.2–300 K.
3. Results
3.1. Absorption and one-photon excited
photoluminescence of solutions
Researched D-π-A molecular compounds (DA-Cl,
DA-Br) have clearly distinguished atom groups with
electron-donor (dimethylamino group –N(СH3)2) and
acceptor (cyano groups –CN) properties combined
with each other by the conjugated π-electron system of
methyn groups. In solution investigated D-π-A
molecules have already polar in the ground state due to
partial ICT from donor D at accepting A fragments.
Essential role in forming absorption and PL spectra
investigated chromophores plays 4-N,N-dimethyl-
aminophenyl C6H5N(CH3)2 fragment. Atom grouping of
dimethylamino group can be able to change its geometry
in ground and excited states and to have an influence on
distribution electron density of the whole molecule.
Intramolecular interaction of non-separated pair of
electrons (NSPE) of heteroatom dimethylamino group
with π-conjugated system of D-π-A chromophore (nπ-
conjugation) are defined by conditions of overlapping
orbitals of NSPE heteroatom with π-electron system of
neighboring aromatic ring and depended on
conformational changes in dimethylaminophenyl
fragment [7]. The conformations of C6H5N(CH3)2
molecular fragment are defined by a competition of two
factors: pπ-conjugation (it is maximum for flat sp2-
hybridization, when angle φ between the plane of
benzene ring and the plane of dimethylamino group was
equal 0) and steric difficulties (it is minimal for
orthogonal conformation, when φ = 90°). According to
contribution of these effects, which determines the
barrier of N(CH)3 group rotation relatively to the
benzene ring, conformations with some angle
0 ≤ φ ≤ 90° may be realized. Consequently, for φ = 0
(sp2-hybridization) both of D and A fragments in DA-Cl
and DA-Br molecules connected with each other by π-
conjugated methyn bridge that must not have an
essential influence at transfer of electrons from D to A.
For φ ≠ 0 both of D and A fragments are weakly
conjugated between themselves. Practically, we have to
do with supramolecular system [8] D-PhS-A, for which
molecular components (including π-conjugated system
of methyn groups with attached benzene ring with atoms
Cl or Br) between terminal D and A fragments may
perform the function of photosensitizer (PhS) of
electrons, which influences on directed ICT. Depending
on conformation of dimethylaminophenyl molecular
fragment the degree of conjugation of investigated
supramolecular system will be varied, and consequently
– its hyperpolarizability, too.
Fig. 2 shows absorption spectra of molecular (c =
2·10-5 mol/l, Т = 295 K) solutions DA-Br (1) and DA-Cl
(2) in acetonitrile solvent under Т = 295 К. In the
presented spectra, we may distinguish three spectral
regions (λ < 300 nm, λ = 300 – 350 nm and λ > 350 nm),
differing with ICT degree in the ground state.
Absorption spectrum of solution DA-Br (curve 1) in the
spectral range 250-350 nm is typical for molecular
solutions of 4-N,N-dimethylaminophenyl [9] with sp3-
hybridization of NSPE heteroatoms. The absorption
band with its peak at 260 nm correlates with local (local-
excited (LE)), without transfer of charge, absorption of
N
H3C
CH3
R
NC
R = Cl, Br
N
NH2
CN
CN
NH2
Fig. 1. The structural formulae of DA-Cl and DA-Br
molecules.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
294
4-N,N-dimethylaminophenyl fragment. In the absorption
spectrum of DA-Cl molecular solution, side by side with
the band of local absorption, the band at 325 nm appears
in addition. Appearance of this band may be related with
ICT from the heteroatom to the neighbouring aromatic
ring (ICT-1) in dimethylaminophenyl fragment with
conformation close to flat sp2-hybridization. In such a
way, it follows from the absorption spectra that supra-
molecular systems DA-Cl and DA-Br have different
degrees of conjugation of π-electron systems, and the
molecule DA-Cl is more flat.
In the longwave range of absorption spectra, DA-
Br and DA-Cl bands of ICT-2 at 426 and 456 nm,
accordingly, can be distinguished. Appearance of the
bands at 426 and 456 nm in longwave range of absorp-
tion spectra for solutions DA-Br and DA-Cl confirms the
supposition about supramolecular D-PhS-A structure of
compounds under investigation. Observed bands reflect
chromophore absorption of PhS, which has different
lengths of π-conjugation for supramolecular DA-Br and
DA-Cl systems. For DA-Cl this length is longer, and,
accordingly, the band of absorption PhS is strongly
moved to the longwave side. Supramolecular structure of
compounds under investigation is also confirmed by
researches of PL spectra. Fig. 3 shows the scheme
totality of states that are realized under optical excitation
of investigated supramolecular DA-Cl and DA-Br
systems. This scheme reflects directed intramolecular
transfer of charge in researched supramolecular struc-
tures and optical transitions in them.
Fig. 4 presents steady-state (curves 1, 4, 6) and time-
resolved (curves 2, 3) PL spectra of DA-Cl solution in
acetonitrile solvent under room temperature and 77 K
(curves 4, 5). The quantum yield of PL of DA-Cl
solutions under room temperature is very low (< 10-4), and
it rises essentially under 77 K. Steady-state PL spectra of
DA-Cl solutions in acetonitrile solvent (curve 1)
represents superposition of fast and slow components of
irradiation, related with conformational changes in DA-Cl
molecules. There can be distinguished bands peaking at
403, 540 and 660 nm in this spectrum. In the spectrum of
250 300 350 400 450 500 550
0
2
4
6
2
1
O
pt
ic
al
d
en
si
ty
, a
. u
.
λ, nm
Fig. 2. Absorption spectra of DA-Br (1) and DA-Cl (2)
solutions in acetonitrile at the low concentration (2·10-5 mol/l)
and Т = 295 К.
Fig. 3. Schematic diagram displaying the optical transitions and intramolecular charge transfer in the supramolecular D-PhS-A
system. Designated by dotted arrows are the radiationless transitions.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
295
fast PL component that can be obtained if measuring the
time-resolved PL spectra with the time delay td = 0.7 ns
(curve 2), there observed bands at 403 and 660 nm. The
PL life-time within these bands is about 0.6 ns. The PL
band at 660 nm is also observed under excitation of DA-
Cl solutions by helium-cadmium laser with λe = 441.6 nm
(curve 6). The spectrum of the slow component can be
obtained by subtracting fast component of radiation from
steady-state spectrum or measuring the spectra with the
time delay td = 10 ns. In measurements of PL spectra with
the time delay td = 10 ns (curve 3), the PL intensity in
maxima of bands at 403 and 660 nm essentially decreases,
but the wide non-structured band peaking at 540 nm
increases within the range between them. The PL life-time
within this band is close to 3.3 ns.
It follows from offered results that for DA-Cl
molecule in the excited state the conformation of
dimethylamino fragment at room temperature is not
completely flat. It may be related with fluctuations in π-
conjugation under the influence of solvent both in
dimethylamino fragment and in the methyn chain at room
temperature. If the conformation of the DA-Cl molecule is
strictly flat, π-conjugated fragment (PhS) would not
influence on ICT between D and A in the excited state.
Irradiative recombination of charges after optical
excitation would occur between terminal D and A groups
through π-conjugated system between them within very
short time, as it is presented in Fig. 3c. In the PL spectra
(Fig. 4, curves 1, 4, 6), it would be observed only the
longwave band at 660 nm with a very short life-time.
Really, PL band with the peak at 660 nm and life-time
0.6 ns may be observed both under excitation of helium-
cadmium laser λe = 441.6 nm (curve 6) in ICT-2 band and
under ultra-violet (UV) (λe = 337.1 nm) excitation in a
more high vibronic state. Still, along with the band of
660 nm under UV excitation, one can observe radiation at
540 nm with longer life-time, which may be related with
weak perturbation of conjugation in D-PhS-A. The
mechanism of this ICT between D and A in the excited
state is shown in Fig. 3b. Shortwave PL for λ < 420 nm
may be explained by ICT transitions, presented in Fig. 3a.
When reducing the temperature of DA-Cl solutions
down to 77 K, the influence of solvent on the π-
conjugated system D-PhS-A is decreased. Because of
steric difficulties, conformational changes in DA-Cl
molecules in its ground and excited states are
embarrassed; terminal D and A fragments of molecules
are well conjugated with each other, and under optical
excitation it is observed ICT transfer via the mechanism
presented in Fig. 3c. In PL spectra, observed are the
bands at 396 and 593 nm (Fig. 4, curve 4), where the
band at 593 nm essentially surpasses in its intensity the
band at 396 nm. It is also seen from Fig. 4 (curve 5) that
within short times of delay td = 0.7 ns in the spectral
region λPL > 390 nm, one can observe only the band at
593 nm. The life-time of the band at 593 nm and 77 K is
3.3 ns. A shortwave displacement of the longwave band
from 660 nm at room temperature down to 593 nm at
77 K may be related with freezing the solvation effects
of solvent on PL spectra at low temperatures.
In Fig. 5, steady-state (curves 1, 4) and time-
resolved PL spectra of DA-Br molecular solutions in
acetonitrile solvent under 295 K (1-3) and 77 K (4) are
presented. For all the spectra within the ICT-2 range, the
band at 477 nm is observed. The life-time of PL in this
band at room temperature equals to 4.3 ns. When
lowering the temperature down to 77 K, it increases up
to 5.1 ns. As shown in Fig. 5, the band at 477 nm has the
wing moved into the longwave spectral range. Measured
at the wavelength of 560 nm, the life-time of PL at room
temperature equals to 4.3 and increases up to 8.2 ns
under 77 K. One can observe essential differences in PL
spectra of DA-Br and DA-Cl molecular solutions, which
may be related with conformational differences in region
of dimethylamino fragment. Owing to the non-flat sp3-
hybridization of heteroatoms’ NSPE, the π-electron
360 400 440 480 520 560 600 640 680 720 760
0.0
0.2
0.4
0.6
0.8
1.0
6
5
4
3
2
1
P
L,
a
.u
λ, nm
Fig. 4. Steady-state (1, 4, 6) and time-resolved (2, 3, 5) PL
spectra of DА-Cl solutions in acetonitrile solvent with the
concentration 2·10-5 mol/l: td = 0.7 ns (2, 5); td = 10 ns (3);
λe = 337.1 nm (1-5); λe = 441.6 nm (6); Т = 295 К (1-3, 6);
T = 77 K (4, 5).
400 450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
43
2
1
P
L,
a
. u
.
λ , nm
Fig. 5. Steady-state (1, 4) and time-resolved (2, 3) PL spectra
(λe = 337.1 nm) of DА-Br solutions in acetonitrile solvent with
the concentration 2·10-5 mol/l: td = 10 ns (2); td = 0.7 ns (3);
Т = 295 К (1-3); T = 77 K (4).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
296
conjugation of D-PhS-A system of DA-Br molecule is
essentially broken, and there is a potential barrier for
ICT between D and PhS. It is seen from the absorption
spectra (Fig. 2, curve 1) that at λe = 337.1 nm molecular
absorption of DA-Br is practically absent, and we can’t
excite PL within the shortwave range of the spectrum.
Because of presence of the potential barrier, electron
transfer between D and PhS using the ICT mechanism in
accord with the scheme Fig. 3a cannot be realized. Due
to presence of the potential barrier between D and PhS
under optical excitation, for DA-Br solutions at room
and low temperatures the mechanism ICT-2 by the
scheme Fig. 3d is predominated, and in PL spectra the
band at 477 nm is observed. The observed longwave
wing in PL spectra with the life-time 8.2 ns may be
related with some endowment of ICT by mechanism
illustrated in Fig. 3d.
When increasing the concentration of DA-Cl and
DA-Br molecules in acetonitrile solutions from 2·10-5 up
to 5·10-3 mol/l, the changes that may be related with
aggregation of molecules was observed in PL spectra.
These changes show that aggregation types of DA-Cl
and DA-Br molecules are different. For DA-Cl solutions
in acetonitrile at room temperature and under increasing
of the concentration from 2·10-5 up to 5·10-3 mol/l,
in PL spectra (Fig. 6, curves 1-5) in shortwave range
(λ < 500 nm) a decrease in the intensity of PL bands is
observed, but in the longwave (λ > 600 nm) – their
increase. Besides, when the concentration of DA-Cl
molecules grows, the band at 660 nm (curve 1) moves to
667 nm (curve 5), and the PL life-time in it decreases
from τ = 3.7 ns to τ < 0.1 ns. At 4.2 K in PL spectra of
solutions, one band is observed. At small concentrations
(2·10-5 mol/l) the maximum of this PL band is at 593 nm
(Fig. 6, curve 1a). The PL life-time in this band is 3.3 ns.
With increasing the concentration from 2·10-4 up to
5·10-3 mol/l, the maximum of the band shifts from 613
up to 661 nm (Fig. 6, curves 3a, 5a), and the PL life-time
changes from τ = 2 ns down to τ < 0.1 ns.
350 400 450 500 550 600 650 700 750 800
0.0
0.1
0.2
0.3
1
2
3
4
55a3a1a
5
4
3
2
1P
L,
a
.u
.
λ, nm
Fig. 6. Steady-state PL spectra of DA-Cl solutions in
acetonitrile solvent at 295 K (1-5), at 4.2 К (1а, 3а, 5a) and at
various concentrations – 2·10-5 (1, 1а), 6·10-5 (2), 2·10-4 (3, 3a),
6·10-4 (4), 5·10-3 mol/l (5, 5а).
Researches of PL spectra of DA-Cl solutions
showed that at concentrations of DA-Cl molecules in
solution up to 10-3 mol/l, they are similar to absorption
spectra. At large concentrations, above the wavelength
600 nm a new band appeared. These experimental facts
testify that at concentrations higher than 10–3 mol/l one
part of DA-Cl molecules in acetonitrile solvent are in
predimer states, and another one – in aggregated states,
with linear packing. Absorption spectra of predimer
states are similar to molecular, and emission spectra – to
exiplex ones [10]. Exiplexes, like to excimers, are
excited complexes of definite stoichiometric constitution
(usually 1:1) and are formed under interaction of excited
molecule with one or few other neighbouring molecules
in their ground state. But exiplexes, unlike excimers, are
polar formations. They are intermediate product of
reaction of intermolecular electron’s transfer in the
excited state from D component of one molecule to A
component of another. The absorption spectrum of
exiplex is like to the molecular one, and PL spectrum
has considerable the Stokes shift. Maximum of DA-Cl
exiplex irradiation (Fig. 6, curve 3) is at 667 nm.
Appearance of absorption in the longwave range
further than 600 nm for DA-Cl solutions at
concentrations >10-3 mol/l may be related with linear
aggregation of molecules. Linear aggregates are known
as J-aggregates [11]. For J-aggregates, longwave
absorption and narrowing their absorption and PL bands
during aggregation are typical. Decreasing life-time of
aggregate PL as compared to the molecular one is also
typical. All these characters distinguish J-aggregates
from DA-Cl molecules at their concentration 5·10-3 mol/l
in acetonitrile. Researches of low-temperature PL
spectra in DA-Cl nanocrystals have shown that DA-Cl
molecules are ordered by J-type with centrosymmetrical
packing in them. Under optical excitation, collective
electron properties are pronounced in them.
Fig. 7 shows PL spectra of DA-Br solutions in
acetonitrile solvent for different concentrations and
temperatures. Attention is attracted by the fact that at
room temperature for all concentrations of DA-Br
molecules in acetonitrile solution (from 2·10-5 to
5·10-3 mol/l) the PL spectra are like to the molecular ones
(Fig. 7, curves 1, 2). At the concentration of DA-Br in
solution 2·10-5 mol/l, the peak of molecular radiation is
located at 477 nm. When concentrations become higher
than 2·10-4 mol/l, the PL peak shifts to the longwave side
and is observed at 500 nm. The PL life-time in this band is
decreased to 1.2 ns. At 77 K in PL spectra of concentrated
DA-Br solutions, fast and slow components of emission
can be distinguished. At small time delay td = 0.7 ns,
molecular emission is observed in time-resolved PL
spectra (curve 3). At longer time delays, the band peaking
at 590 nm is observed in the PL spectrum (curve 5). The
PL life-time in the band of 500 nm at 77 K is almost the
same as under room temperature and equals 4.9 ns. For
the band at 590 nm it increases up to 18 ns. Absorption
spectra of DA-Br solutions at room temperature and
different concentrations are similar to the molecular ones.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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350 400 450 500 550 600 650 700 750
0.0
0.2
0.4
0.6
0.8
1.0
1.2
6 5 4321
PL
, a
.u
.
λ, nm
Fig. 7. Steady-state (1, 2, 4, 6) and time-resolved (3, 5) PL
spectra of DA-Br solutions in acetonitrile solvent at 295 K
(1, 2), 77 K (3-5) and 4.2 К (6) for the concentrations
2·10-5 (1), 2·10-4 (3-5) and 5·10-3 mol/l (2, 6): td = 0.7 ns (3),
td = 10 ns (5).
Using the obtained results, we may suppose that
under increasing the amount of DA-Br molecules in
solution they are in predimer states like to DA-Cl
molecules. But contrary to DA-Cl molecules, predimer
states of DA-Br molecules are formed as the H-type with
sandwich packing. In the excited state, they are capable
to near to each other, but in consequence of structural
difficulties at room temperature they cannot reach the
Van-der-Waals distance and to form exiplex. By this
reason, at all the concentrations and room temperature in
PL spectra molecular radiation prevails. Intermolecular
interaction in sandwich structures leads to that the
potential barrier for ICT of NSPE of heteroatom
form D to PhS lowers a little, and we can observe ICT
transitions between them. Besides, because of steric
difficulties in the excited state, molecules in predimer
pairs cannot get closer to Van-der-Waals distance. But at
high concentrations in solution they can be at such a
distance, and at low temperatures the molecular transfer
of charge between neighboring molecules is possible. It
is expressed in appearance of the band at 590 nm (Fig. 7,
curves 5, 6) with the life-time τ = 18 ns in the low-
temperature PL spectrum of concentrated solutions and
recombination radiation with the life-time in some
seconds.
3.2. One- and two-photon excited photoluminescence
spectra of DA-Cl and DA-Br crystals and crystalline
films
DA-Br monocrystals were grown from supersaturated
polar solutions under slow vaporization of them. DA-Br
crystals are polar and have a needle form. Their length is
about 0.5-1 cm, width is b = 0.1 cm, and thickness is
d = 0.2 cm. Propensity of DA-Br molecules to aggregate
by H-type with sandwich packing leads to that in
crystals DA-Br molecules form stacks with noncen-
trosymmetrical packing (Fig. 11a). Distance between
planes of neighbouring molecules in crystal is less in
stacks than in predimer DA-Br pairs in solution. As
researches of reflection spectra showed, in polarized
light DA-Br molecules are parallel in stacks, and their
long axis are perpendicular to direction of crystal
growing along a-axis. Such an arrangement of molecules
in the crystal leads to high values of overlapping integral
SDA of orbitals D and A neighboring fragments of
molecules. In the excited state, DA-Br molecules in the
crystal can be located closer to each other at the distance
when their collective electronic properties and transfer
of heteroatom NSPE along the stack are developed.
Under such arrangement of the molecule, a numerical
value of the oscillator strength for excitons with charge
transfer (CT-excitons) takes its maximum value and is
proportional to (SDA)2·z2, where z is the distance between
D and A fragments of two neighboring molecules in the
stack. These electronic properties were observed in
crystalline complexes with weak transfer of charge
[12, 13], in which in excited state side by side with
molecular Frenkel excitons (FE) CT-excitons arise in the
crystal, and wave functions of them are delocalized within
limits of two or more molecules. Energetic adjacency of
FE and CT-excitons leads to their strong mixing
(hybridization). States mixing for systems with n-π
transitions is very important on molecular level already. If
n-orbitals are p-orbitals “clear” localized on heteroatoms,
then n-π transitions in molecules are forbidden. And these
transitions are allowed if n-orbitals are s-orbitals or hybrid
ones with a large part of s-orbital. Model of exciton
mixing for crystals is presented in the research work [14].
A high degree of delocalization in CT-crystals puts them
into an intermediate class between ordinary molecular
crystals, localized excited states of which describe FE,
and semiconductor crystals where delocalized excited
states are described by Wannier excitons. Delocalization
of CT-excitons in DA-Br crystals must reduce to high
nonlinear optical properties. For comparison of optical
properties and developing in them collective electronic
properties, we have researched amorphous and crystal
DA-Br structures.
Fig. 8 shows the steady-state (curve 3) and time-
resolved (td = 0.7 ns) (curve 4) PL spectra of the DA-Br
amorphous film at 77 K and at one-photon excitation
λe = 337.1 nm. DA-Br amorphous films got from melt that
was exposed to fast cooling in quartz cuvette 60 µm thick.
The temperature of melting DA-Br crystals is 458 K. In
these amorphous films, the predimer structure dominates
(Fig. 11b) [15], and small regions of crystallinity are
observed. This is confirmed by that in PL spectra of
amorphous film (Fig. 8, curves 3, 4), like to spectra of
concentrated solutions, we can distinguish wide
structureless bands of 560 and 595 nm. The PL lifetime of
amorphous film is essentially high than the molecular one.
At 77 K lifetime in the band of 560 nm is 6.8 ns, and in
the band of 595 nm – 10 ns. Crystal emission displays a
weak shoulder at 602 nm. At pulse excitation of these
films by IR laser (λe = 1064 nm) locally, in places of
crystallinity, SHG was observed (Fig. 9, curve 4).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
298
520 540 560 580 600 620 640 660 680 700 720 740
0.0
0.2
0.4
0.6
0.8
1.0
6
5
4
3
2
1
PL
, a
. u
.
λ, nm
Fig. 8. Steady-state (1, 3, 5) and time-resolved (2, 4, 6) PL
spectra of DA-Br monocrystals (1, 2), amorphous films
(thickness d = 60 µm) (3, 4) and crystalline DA-Cl films (5, 6).
T = 77 K; td = 0.7 ns (2, 4); td = 20 ns (6). λe = 337.1 nm.
Under transitions from the amorphous (Fig. 11b)
state into the crystal one (Fig. 11a), the predimer pairs of
DA-Br molecules were ordered and formed stacks. This
immediately influenced on PL spectra. In the PL spectra,
intense narrow band with the peak at 602 nm appears
(Fig. 8, curves 1, 2), which is related with charge transfer
NSPE of heteroatom along the stack. Because of
participation NSPE of DA-Br in excitation, the lifetime in
the band of 602 nm is long and equals 30 ns under 77 K.
In consequence of electronic excitation’s collectivization
this time is lower than that in the band of 565 nm, which
may be related with exсiton molecular excitations in DA-
Br crystal. The lifetime in the band of 565 nm equals
25 ns. Because of energetic adjacency of molecular and
CT-exсitons in DA-Br crystal such states mix [14] and, as
a result, their lifetime are not essentially different.
For DA-Br monocrystals under IR excitation
(λe = 1064 nm, N = 100 kW) the intensive up-conversion
PL with the peak at 610 nm is observed (Figs 9, 10,
curves 2). It is observed both in steady-state and in
measured PL spectra with subnanosecond resolution.
Side by side with up-conversion PL in emission spectra
of DA-Br crystals under IR excitation SHG, at the
wavelength of 532 nm, is observed. When changing the
intensity of the exciting IR light, the intensity of up-
conversion PL and second harmonic of the crystal
change in proportion. We can make a conclusion about
two photon-excited character of up-conversion PL. For
DA-Br crystals TPL lifetime in the band of 610 nm at
room temperature is 7.3 ns, and for two-photon excited
PL – 8.7 ns.
Unlike monocrystals (Fig. 9, curve 2), for amor-
phous DA-Br films (Fig. 9, curve 4) the up-conversion
PL in the spectral range of 570–650 nm under the same
levels of IR excitation was not observed. In places of
crystallinity only SHG was observed (Fig. 9, curve 4).
The intensity of the second harmonic in partially
crystalline DA-Br films may be compared with that of
the second harmonic of resorcinol crystal powder [19]
under the same level of IR excitation.
520 540 560 580 600 620 640 660 680 700 720 740 760 780
0,0
0,2
0,4
0,6
0,8
1,0
43
21
PL
, T
PL
, a
. u
.
λ, nm
Fig. 9. Time-resolved PL spectra of DA-Br (1, 2) and DA-Cl
(3, 4) crystals: Т = 295 К; λe = 337.1 nm (1, 3) and
λe = 1064 nm (2, 4); td = 0.7 ns.
The appearance of TPL in noncentrosymmetric
DA-Br crystals is not related with generation and
reabsorption of the second harmonic in the volume of a
crystal. This is proved by that under excitation of
amorphous films with the second harmonic of Nd:YAG
laser TPL is absent. A difference between amorphous
and crystalline films consists only in ordering of
molecules. Under excitation of amorphous film by laser
radiation at the wavelength of 532 nm, like to
monocrystals, intensive one-photon PL with the peak at
606 nm is observed (Fig. 10, curve 3). For DA-Br mono-
crystals (curve 1) under excitation at the wavelength
λe = 532 nm, one-photon PL has its peak in the position
less than 610 nm.
Figs 8 (curves 5, 6) and 9 (curves 3, 4) show PL
spectra of polycrystalline DA-Cl film that was produced
by irrigation and vaporization of concentrated DA-Cl
solution on a silica substrate. In PL spectrum of DA-Cl
films at room temperature and one-photon excitation, the
band peaking at 655 nm (Fig. 9, curve 3) is observed.
The PL lifetime in the band 655 nm at room temperature
is shorter than 0.1 ns. After lowering the temperature
down to 77 K, PL spectrum shifts a little into the
shortwave side, and the peak of the most intensive PL
band is located at 651 nm (Fig. 8, curves 5, 6). The PL
lifetime in the band 651 nm equals 4.2 ns. There is a
shoulder at 622 nm at the shortwave side of the band
651 nm. The PL lifetime in this region is 1.9 ns.
For crystalline DA-Cl films under IR excitation
(λe = 1064 nm, N = 100 kW), intense up-conversion PL
is observed with its maximum at 662 nm and PL lifetime
less than 0.1 ns. Emission pulse of up-conversion PL of
crystalline DA-Cl films under IR excitation repeats a
pulse of the second harmonic Nd:YAG laser emission.
These testify about two-photon character of up-conver-
sion PL. Unlike DA-Br crystals, for crystalline DA-Cl
films SGH under IR excitation was not observed.
Absence of SHG under IR excitation of crystalline
DA-Cl films testifies that it is centrosymmetric. The
centrosymmetric unit cell from the pair of polar DA-Cl
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© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
299
molecules is formed due to their antiparallel sandwich
arrangement. In a crystallite, the DA-Cl molecules form
linear aggregates, growth direction of which coincides
with that of the long molecules’ axis (Fig. 11c). Each of
two antiparallel chains is characterized by strong
resonant dipole-dipole interaction of molecules, and
forms J-aggregate [17]. One-dimensional J-aggregates
are described by the model of Frenkel’s excitons. Local
transmitting symmetry in combination with interaction
of neighboring molecules in J-aggregate leads to
delocalization of excitation of one molecule along
conjugated place of aggregate, which consists of N
molecules. In consequence of this, molecules, which
belong to this place, coherently interact with emission.
Moreover, in the excited state two molecules in
neighbouring chains may strongly interact due to
formation and localization of interchain CT-exciton.
Strong mixing FE and CT-exciton states [14] of
neighbouring molecular chains leads to excitation like
excimer. In spectra of PL excitation both crystalline DA-
Cl films and DA-Br monocrystals, under registration in
region of CT emission intensive bands are observed,
which characterize FE excitation. It is obvious that
unlike molecular excimer state, where one molecule
absorbs and excitation is localized at two neighboring
molecules, in our case the number of interacting
molecules in two neighboring chains with linear J-
package is determined by the number of coherently
linked molecules in each of them. Research of one- and
two-photon absorption spectra, which are different with
their rules of selection, and PL linked with them under
intensive laser excitation in the visible or IR region,
allow extracting intermolecular CT-exciton transitions at
the background of intramolecular. If polar molecules in
aggregate are arranged symmetrically, so that their static
dipole moment is completely compensated, some of
dipole-forbidden exciton states under one-photon
absorption become resolved under two-photon absorp-
tion. This effect is important not only for investigations
of dipole-forbidden exciton states, but for gaining optical
nonlinearity.
550 600 650 700 750
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
4
3
2
1
PL
, T
PL
, a
.u
λ, nm
Fig. 10. Time-resolved PL spectra of DA-Br monocrystals
(1, 2) and amorphous films (thickness d = 60 µm) (3, 4):
Т = 295 К; λe = 532 nm (1, 3); λe = 1064 nm (2, 4); td = 0.7 ns.
As we can see from presented PL spectra for DA-
Br crystals and crystalline DA-Cl films (Fig. 9), their
TPL spectra shifted to the longwave side as compared
with one-photon excited PL, and that may be related
with differences in the character of one- and two-photon
excited PL.
4. Discussion
As marked above, the crystals consisting of polar
molecules can be both centrosymmetrical and polar. In
the medium possessing the center of symmetry, the
ground state is nondegenerate and electronic states can
be divided into even (g) and odd (u) states. In the dipole
approximation, one-photon transitions from (g) in (g) or
from (u) in (u) are forbidden; however, two-photon
transitions between these states are allowed. Thus, using
two-photon absorption spectroscopy, one can probe new
electronic states that cannot be detected during one-
photon absorption. Also, it is possible to relate to such
states the lowest СТ-exciton states that are forbidden at
one-photon absorption and in immediate adjacency from
the allowed transitions.
In the dipole approximation, the probability W of
absorption of two photons with a frequency ω and
corresponding electric-field polarization e in the centro-
symmetrical medium has the form [16, 17]:
( ) ( )∑ ω−ωδω∝ω
f
fgfgFNW 2M,
222e ,
∑ ω−ω
=
i ig
igfi
fg
pp2
M ,
where g, i, f stand for the ground, intermediate and final
states of a two-photon transition, respectively; fgM is
the composite matrix element of transition between the g
and f states; pij is the matrix element of transition
dipole moment (projection onto direction e) between the
different i and j states; ħ ωij = Ei – Ej is the energy
difference between the i and j states; F is the exciting-
light intensity; and N is the density of molecules or unit
cells in the medium. It is usually assumed that the main
contribution to the probability of two-photon transitions
is determined to be made by the intermediate i states that
are close located to the levels g and f.
For crystals with a center of symmetry, SHG is
forbidden. If a system loses its inversion symmetry and
has a static dipole moment, as in case of DA-Br crystals,
SHG is not forbidden. For noncentrosymmetrical polar
crystals the ground state is degenerated and has no
definite symmetry [8]. In this case, transitions between
the ground and excited states for one- and two-photon
light absorption are not rigorously forbidden. fgM for a
molecular system without a center of symmetry can be
expressed, dividing the static iiµ and transition ijp
dipole moments, as [18]:
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
300
a b c
Fig. 11. Prospective packing of molecules in DA-Br crystals (a) and amorphous films (b), and in DA-Cl crystals (с).
∑ ω
∆
−
ω−ω
=
i
fgfg
ig
igfi
fg
ppp µ
2
2
M ,
where ggfffg µµµ −=∆ is the difference static dipole
moment (projection onto direction e) between the final
and ground states.
The coefficients of the one- and two-photon
absorption of a two-level system without an inversion
center can be written as
( ) ( )fgfgp ω−ωδ∝ωα 2 ,
( ) ( ) ( )fgfgfg
Fp ω−ωδ
ω
ω
∆∝ωα 2µ2 22
h
.
The TPL intensity is
( ) 22 II PL ωαη= ,
where η is the PL quantum yield.
The high quantum yield of TPL for DA-Br
monocrystals and absence TPL in amorphous films
under the same intensities of excitation light allows to
explain collectivization of CT-states in DA-Br crystals
and essential change of the static dipole moment fgµ∆
in the excited state.
For DA-Cl crystals, consisting of polar molecules,
under excitation by laser radiation λ = 1064 nm the SHG
was not observed. It will be coordinated with centro-
symmetric packing of molecular pairs in the unit cell
of a crystal. However, for them intensive two-photon
excited PL is observed. Contrary to DA-Br crystals
where higher intense TPL is achieved due to collec-
tivization of СТ-transitions, for DA-Cl crystals the
essential role is played by collectivization of FE-
transitions in a linear chain of these molecules. Two-
photon processes for the crystals with charge transfer
consisting of polar molecules and having the center of
symmetry are considered in the work [2]. It is shown
there that intermolecular charge transfer between
interacting molecules in the unit cell consisting of two
antiparallel polar molecules allows to have a nonzero
dipole moment in the excited state (Fig. 12). This dipole
moment brings the contribution into the dipole matrix
element of transition between the ground and excited
states of the unit cell with predimer arrangement of
molecules in it, and results in a resonance in the
coefficient of two-photon absorption. The coefficient of
two-photon absorption αTPA(ω), determined by an
imaginary part of a susceptibility of the third order, for
such crystals it is possible to express as [2]:
( )
( )
( )
( ) ( ) 22
224
22
0
2
2
Γ2
Γ
µρ
32
−
++
+
+ω−ω
∆∆ω×
×
ω
ω
ω
π
=ωα
gg
g
fg
TPA
pf
F
cn
N
hh
where n0 is the linear refraction index of material; f (ω)
is the coefficient of local field; c is the light velocity; ∆ρ
is the difference of occupancy of the g and f states; ω+g is
the frequency of optical СТ-transition between the
ground ψg and excited ψ+ states of the pair of molecules
in the unit cell, forbidden at one-photon absorption and
allowed at two-photon absorption; Γ+g is the decay rate
of the ψ+ state; pfg is the dipole moment of transition
between ground and excited states of molecules in
aggregate under one-photon absorption; ∆µ is the matrix
element of the dipole transition between excited states of
the pair of molecules that interact. ∆µ = 〈ψ+⎪µ⎪ψ-〉,
where µ is the static dipole moment of molecule.
The bipolar arrangement of molecules in the unit
cell of a crystal results in splitting the energy bands.
Researches of one- and two-photon excited PL spectra
allow to distinguish between the possible two packings
of polar molecules in the centrosymmetric linear DA-Cl
aggregate. At an antiparallel arrangement of molecules
in the unit cell, the transition allowed at two-photon
absorption is located below the transition allowed at one-
photon absorption. Thus, in the model of the linear
aggregate with predimer arrangement of molecules in
the unit cell at two-photon absorption of light the
emission occurs from the lowest excited СТ-state that is
forbidden by its symmetry. For molecular crystals, this
transition is possible with participation of intramolecular
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
301
Fig. 12. Energy level scheme for a physical dimer and the
optical transitions in it under one- and two-photon
excitations.
odd vibration modes [17]. In this case, the emission
spectrum under two-photon absorption always will
be a little shifted to the longwave side with respect
to PL spectrum under one-photon excitation (Fig. 9,
curves 3, 4).
For DA-Cl crystals, packing of molecules in an
one-dimensional chain is characteristic for the J-aggre-
gates located by the type “head-to-tail”. It can result in
substantial growth of the transition dipole moment
0pNp kfg = owing to collective absorption Nk of
coherently interacting molecules in the aggregate [20].
Here p0 is molecular dipole moment of transition. For
the unit cell to be centrosymmetric, two neighboring
J-aggregates should have an antiparallel orientation.
On the other hand, as well as in the case of
predimer pairs, between D and A fragments of two
neighboring chains the intermolecular electron transfer
(the formation of located CT-state with large static
dipole moment) is possible. As the result of influence of
these two factors for crystalline and nanocrystalline
DA-Cl forms, high cubic nonlinearity is observed.
The SHG intensity is specified by the polarizability
squared; for charge-transfer crystals, this polarizability
is [19]
[ ] ( )[ ]2222
22 1
ω)2(2
∆µ3
β
ωWWm
fWe
CTCT
fgCT
CT
hh
h
−−
= .
Here, βCT depends on the characteristic radiation
quantum energy ħ ω, the electron charge e and mass m,
the energy gap WCT, the oscillator strength f of the
transition to the CT-state, and the change in the dipole
moment fgµ∆ upon this transition. To achieve the
maximum value of βCT, it is desirable to have molecules
(crystals) in which an electronic transition has an energy
close to 2ħ ω and does not fall inside the absorption
band. Moreover, the oscillator strength should be high
and the dipole moment should change significantly
during this transition. These conditions coincide with the
conditions required for effective absorption upon two-
photon excitation of PL [16, 17]. However, TPL is
observed from states ETPL ≤ 2ħ ω. In our case, ħ ω is
related to the light-quantum energy for IR excitation.
Therefore, until the sizes of polar nanoaggregates for
which WCT > 2ħ ω are smaller than a critical size, only
SHG takes place upon the IR excitation of DA-Br
solutions. As the nanoaggregates grow, the ionization
energy of the molecular structure decreases (because of
possible molecular conjugation in the aggregates) and
the polarization energy increases (because of collective
intermolecular charge transfer in the crystalline
aggregates) [21] and WCT should decrease, too. When
WCT < 2ħ ω, TPL appears along with SHG.
References
1. R.T. Deck, A.Yu. Piryatinski, Yu.P. Piryatinski,
E.S. Repetski, Two-photon excited luminescence of
organic donor-acceptor crystals with intramolecular
charge transfer // Pis’ma zhurnal tekhnich. fiziki
22(1), p. 34-38 (1996) c
2. A. Piryatinski, R. Deck, Excimer states and
enhanced two-photon absorption in intramolecular
charge-transfer crystals // Chem. Phys. Lett. 269,
p. 156-160 (1997).
3. Yu.P. Piryatinski, O.V. Yatsun, V.G. Nazarenko,
Controlled electrochromic effect in a liquid crystal
cell // Pis’ma zhurnal tekhnich. fiziki 25(19), p. 6-
10 (1999) (in Russian).
4. Yu.P. Piryatinski, M.M. Sevryukova, Photoelectric
properties of a self-ordered molecular disperse
structure based on a polar-dye solution in a liquid-
crystal matrix // Fizika tverdogo tela 49(11),
p. 2096-2105 (2007) (in Russian).
5. K.Yu. Suponitsky, T.V. Timofeeva, M.Yu. Antipin,
Molecular and crystal design of nonlinear optical
organic materials // Uspekhi Khimii (Russian
Chemical Reviews) 75, p. 515-556 (2006) (in
Russian).
6. J.R. Lacovitcz, Principles of Fluorescence
Spectroscopy. Plenum, New York, 1983.
7. I.P. Romm, E.N. Gur’yanova, pπ-conjugation in
aromatic compounds of elements of V and V1
groups // Uspekhi Khimii 55(2), p. 225-250 (1986)
(in Russian).
8. J.M. Len, Supramolecular Chemistry. Nauka,
Novosibirsk, 1988 (in Russian).
9. V.L. Yermolayev, Luminescence of simple
benzene derivatives // Optika i spektroskopiya
11(4), p. 492-497 (1961) (in Russian).
10. J.A. Barltrop, Excited States in Organic Chemistry.
John Wiley and Sons, 1975.
11. T. Kobayashi, J-aggregates. Singapore, World Sci.,
1996.
12. M. Pope, C.E. Swenberg, Electronic Processes in
Organic Crystals. Clarendon Press, Oxford, 1982.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2008. V. 11, N 3. P. 292-302.
© 2008, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
302
13. V.M. Agranovich, P.M. Hohstrasser, Spectroscopy
and Dynamics of Excitation in Condensed
Molecular Systems. Nauka, Moscow, 1987 (in
Russian).
14. P. Petelenz, Mixing of Frenkel excitons with
charge transfer states in the neigbourhood of a
charged defect // Chem. Phys. Lett. 47, p. 603-605
(1977).
15. E.F. Sheka, Spectroscopy of electron absorption of
amorphous substances with molecular structure //
Uspekhi fizicheskikh nauk 160(2), p. 263-298
(1990) (in Russian).
16. Y.R. Shen, The Principles of Nonlinear Optics.
John Wiley & Sons, Inc., New York, 1984.
17. V.I. Bredikhin, M.D. Galanin, V.N. Genkin, Two-
photon absorption and spectroscopy // Uspekhi
fizicheskikh nauk 110, p. 1-43 (1973) (in Russian).
18. S. Abe, Two-photon probe of forbidden exciton
states in symmetric aggregates of asymmetric
molecules // Chem. Phys. 264, p. 355-363 (2001).
19. Nonlinear Optical Properties of Organic Molecules
and Crystals, Edited by D.S. Chemla and J. Zyss,
Vol. 1. Academic Press, Inc., 1987.
20. B.A. Malyshev, Localization length of 1D exciton
and temperature dependence of the radiative life-
time for freezed solutions of dyes with J-aggregates
// Optika i spektroskopiya 71(6), p. 873-875 (1991)
(in Russian).
21. A.V. Vannikov, A.D. Grishina, Photochemistry of
Polymer Donor-Acceptor Complexes. Nauka,
Moscow, 1984 (in Russian).
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