Unveiled optical properties of tetrapyrollic pigments in cryogenic environments

An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base (H₂Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser intensity. In all matrices the intensity of an emission band near 755 nm increased dras...

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Date:	2010
Main Authors:	Crépin, C., Shafizadeh, N., Chin, W., Galaup, J.-P., McCaffrey, J.G., Arabei, S.M.
Format:	Article
Language:	English
Published:	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2010
Series:	Физика низких температур
Subjects:	Nanostructures and Impurity Centers in Cryogenic Environment
Online Access:	https://nasplib.isofts.kiev.ua/handle/123456789/117046
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Cite this:	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments / C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, S.M. Arabei // Физика низких температур. — 2010. — Т. 36, № 5. — С. 563-570. — Бібліогр.: 21 назв. — англ.

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spelling	nasplib_isofts_kiev_ua-123456789-1170462025-02-09T22:02:24Z Unveiled optical properties of tetrapyrollic pigments in cryogenic environments Crépin, C. Shafizadeh, N. Chin, W. Galaup, J.-P. McCaffrey, J.G. Arabei, S.M. Nanostructures and Impurity Centers in Cryogenic Environment An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base (H₂Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser intensity. In all matrices the intensity of an emission band near 755 nm increased drastically when pumping the S₁←S₀ transition. This observation was assigned to stimulated emission in a four-level scheme involving a vibronic transition from the lowest electronic state to a vibrational level of the ground state. In the present work, we expose new similar results obtained with porphyrin molecules, i.e. tetra-benzoporphin (TBP). With free-base H₂TBP, stimulated emission was observed in Ar or N₂ matrices, but not in Xe matrices. A possible reason could be a fast inter-system crossing rate due to the heavy atom effect induced by Xe. We also report the observation of persistent burnt spectral holes, although the low efficiency of this process is not competitive with stimulated emission and no decrease of the stimulated emission with time was observed. With ZnTBP, no stimulated emission could be recorded; however the appearance of rather strong phosphorescence was noticed. Similar studies focusing on the stimulated emission phenomenon is that they should allow new insights into site effects and site selectivity. The assistance of Dr. Michel Broquier and Dr. Pierre Çarçabal for the design of the oven used for vaporization of the porphyrin molecules is gratefully acknowledged. The France-Belarus cooperation was supported by the CNRS/BRFFR no. 23181 agreement and grant BRFFR no. F09F-001. McC’s participation in this work was made possible with support from Science Foundation Ireland (SFI), Research Frontiers Programme (06/RFP/CHP012). 2010 Article Unveiled optical properties of tetrapyrollic pigments in cryogenic environments / C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, S.M. Arabei // Физика низких температур. — 2010. — Т. 36, № 5. — С. 563-570. — Бібліогр.: 21 назв. — англ. 0132-6414 PACS: 33.50.–j, 33.80.–b, 78.47.–p, 78.60.Lc https://nasplib.isofts.kiev.ua/handle/123456789/117046 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
institution	Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection	DSpace DC
language	English
topic	Nanostructures and Impurity Centers in Cryogenic Environment Nanostructures and Impurity Centers in Cryogenic Environment
spellingShingle	Nanostructures and Impurity Centers in Cryogenic Environment Nanostructures and Impurity Centers in Cryogenic Environment Crépin, C. Shafizadeh, N. Chin, W. Galaup, J.-P. McCaffrey, J.G. Arabei, S.M. Unveiled optical properties of tetrapyrollic pigments in cryogenic environments Физика низких температур
description	An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base (H₂Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser intensity. In all matrices the intensity of an emission band near 755 nm increased drastically when pumping the S₁←S₀ transition. This observation was assigned to stimulated emission in a four-level scheme involving a vibronic transition from the lowest electronic state to a vibrational level of the ground state. In the present work, we expose new similar results obtained with porphyrin molecules, i.e. tetra-benzoporphin (TBP). With free-base H₂TBP, stimulated emission was observed in Ar or N₂ matrices, but not in Xe matrices. A possible reason could be a fast inter-system crossing rate due to the heavy atom effect induced by Xe. We also report the observation of persistent burnt spectral holes, although the low efficiency of this process is not competitive with stimulated emission and no decrease of the stimulated emission with time was observed. With ZnTBP, no stimulated emission could be recorded; however the appearance of rather strong phosphorescence was noticed. Similar studies focusing on the stimulated emission phenomenon is that they should allow new insights into site effects and site selectivity.
format	Article
author	Crépin, C. Shafizadeh, N. Chin, W. Galaup, J.-P. McCaffrey, J.G. Arabei, S.M.
author_facet	Crépin, C. Shafizadeh, N. Chin, W. Galaup, J.-P. McCaffrey, J.G. Arabei, S.M.
author_sort	Crépin, C.
title	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments
title_short	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments
title_full	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments
title_fullStr	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments
title_full_unstemmed	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments
title_sort	unveiled optical properties of tetrapyrollic pigments in cryogenic environments
publisher	Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
publishDate	2010
topic_facet	Nanostructures and Impurity Centers in Cryogenic Environment
url	https://nasplib.isofts.kiev.ua/handle/123456789/117046
citation_txt	Unveiled optical properties of tetrapyrollic pigments in cryogenic environments / C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, S.M. Arabei // Физика низких температур. — 2010. — Т. 36, № 5. — С. 563-570. — Бібліогр.: 21 назв. — англ.
series	Физика низких температур
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first_indexed	2025-12-01T06:23:28Z
last_indexed	2025-12-01T06:23:28Z
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fulltext	© C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, and S.M. Arabei, 2010 Fizika Nizkikh Temperatur, 2010, v. 36, No. 5, p. 563–570 Unveiled optical properties of tetrapyrollic pigments in cryogenic environments C. Crépin, N. Shafizadeh, and W. Chin Institut des Sciences Moléculaires d’Orsay, Bât. 210 J.-P. Galaup Laboratoire Aimé Cotton, Bât. 505 CNRS & Université Paris-Sud 11, 91405 Orsay cedex, France E-mail: jean-pierre.galaup@lac.u-psud.fr J.G. McCaffrey Department of Chemistry, National University of Ireland, Maynooth, Ireland S.M. Arabei Belarusian State Agrarian and Technical University, Minsk, Belarus Received February 15, 2010 An unexpected phenomenon was revealed in the laser induced fluorescence spectra of free-base (H2Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices under a moderate increase in the laser in- tensity. In all matrices the intensity of an emission band near 755 nm increased drastically when pumping the S1 ← S0 transition. This observation was assigned to stimulated emission in a four-level scheme involving a vi- bronic transition from the lowest electronic state to a vibrational level of the ground state. In the present work, we expose new similar results obtained with porphyrin molecules, i.e. tetra-benzoporphin (TBP). With free-base H2TBP, stimulated emission was observed in Ar or N2 matrices, but not in Xe matrices. A possible reason could be a fast inter-system crossing rate due to the heavy atom effect induced by Xe. We also report the observation of persistent burnt spectral holes, although the low efficiency of this process is not competitive with stimulated emission and no decrease of the stimulated emission with time was observed. With ZnTBP, no stimulated emis- sion could be recorded; however the appearance of rather strong phosphorescence was noticed. Similar studies focusing on the stimulated emission phenomenon is that they should allow new insights into site effects and site selectivity. PACS: 33.50.–j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem cross- ing, internal conversion); 33.80.–b Photon interactions with molecules; 78.47.–p Spectroscopy of solid state dynamics; 78.60.Lc Optically stimulated luminescence. Keywords: tetrabenzoporphin, cryogenic matrices, stimulated emission, fluorescence, phosphorescence, spectral hole-burning. 1. Introduction Recent luminescence work [1] on phthalocyanine-doped cryogenic matrices revealed the occurrence of stimulated emission for specific vibronic transitions. This phenome- non was characterized by a drastic intensity increase of a specific vibronic fluorescence line with only a moderate increase of the laser pump intensity. The most concerned transition was assigned as the one taking place between the first excited electronic state and a vibrational level of the ground state located at approximately 1550 and 1525 cm–1 in H2Pc and ZnPc, respectively [1]. One important question to address in the development of this work is whether the stimulated emission observed for the phthalocyanines in cryogenic matrices is a more C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, and S.M. Arabei 564 Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 general behavior occurring for other molecules in other environments. With this question in mind we are therefore in search for other systems, molecules and/or matrices, in which similar behavior might be observed. In present pa- per, we are looking for similar processes in the case of porphyrin molecules belonging to the tetrabenzoporphyrin family. Indeed, tetrabenzoporphin (TBP) molecules have the closest structure to the phthalocyanines (Pc), the most noticeable change being the replacement of the four bridg- ing nitrogen atoms belonging to the internal ring by carbon atoms (Fig. 1). Therefore, similar vibronic structures are expected between both species. The first absorption spectra of matrix-isolated por- phyrins and phthalocyanines were reported early by Baje- ma et al. [2,3]. More recently Waluk and co-workers re- ported on the spectroscopy [4] and isolation [5] of por- phyrins and related molecules in rare gas matrices. Other groups [6–8] analyzed the vibronic structure on the S1 state of several porphyrins and phthalocyanines in Shpol’skii mat- rices. A complete spectroscopic study and vibrational analysis of H2TBP in a Shpol’skii matrix of n-octane (n-C8) was early reported by Arabei [9]. The same author also studied the influence of temperature on the electronic spectra of H2TBP in n-octane [10]. However, the behavior of this important class of molecules isolated in the solid rare gases needs still to be more thoroughly analyzed and charac- terized. Laser-induced fluorescence spectroscopy is very well adapted for studying these molecules because most of their absorption bands in the visible region closely match the working range of tunable dye lasers. While undertaking such a spectroscopic study of matrix-isolated free-base phthalo- cyanine (H2Pc) and zinc phthalocyanine (ZnPc), by means of emission-excitation spectra using pulsed dye lasers for excitation, an unusually intense vibronic band was observ- ed with slightly increased laser power. This novel solid state effect, which had never been reported before, was the subject of our paper [1]. The band involved corresponded to the transition from the first excited electronic state to a vibrational level of the ground state at approximately 1550 cm–1. The drastic intensity enhancement was attri- buted to the occurrence of stimulated emission (SE) on the basis of the following observations: 1) the laser power de- pendence of the phenomenon exhibits a well-defined thre- shold, 2) there is also a threshold as a function of the con- centration of dopants, 3) the emission band is narrow com- pared to the spectral width of the standard fluorescence, even in the case of broad bands in the absorption spectrum, 4) its lifetime is strongly shortened in comparison with other vibronic transitions of the emission spectrum and exhibits a perfect synchronism with the exciting laser pulses [1]. A complete analysis is complicated by the different possible sites occupied by the guest molecules in the crys- talline host; however, one important point noticed was that the SE line appeared as a narrow line from the most popu- lated family of sites. In the case of H2Pc/N2, where differ- ent sites were clearly observed in the absorption band, SE exhibited several lines, correlated with the main sites. Ex- citation spectra highlighted energy transfers between the families of sites. In the case of H2Pc/Ne, a spectral hole- burning (SHB) phenomenon was also present, and entered in competition with SE, resulting in a decrease of the SE intensity with the total irradiation time [1]. These studies of the SE phenomenon should allow new insights into site effects and site selectivity. Possible reasons for the differ- ent observations made depending on the different hosts studied have still to be discussed. To extend the range of guest molecules beyond those belonging to the phthalo- cyanine family, it is easy to be convinced of the strong similarity, both in terms of the molecular structure and the spectroscopy, between phthalocyanines and tetrabenzopor- phyrins (Fig. 1). The aim of the present paper is to present new and preliminary results obtained with molecules be- longing to benzoporphyrins in different cryogenic envi- ronments. 2. Experimental details The tetrabenzoporphyrins, free-base H2TBP as well as ZnTBP, studied in this work were not commercial pro- ducts, however, we can mention that Frontier Scientific Inc. can also make these molecules available. The mole- cules we used were synthesized according to the procedure described in [11] and carefully controlled by chromato- graphy. For sublimation of the dye powder, a specially designed oven whose schematic is shown in Fig. 2, was used in the experiments. It consisted of a solid, stainless steel cylinder into which a hollow screw, containing the dye powder was fitted. The top of this screw was posi- tioned to emerge at right angles to a 2 mm opening passing through the length of the cylinder. This opening was con- nected to the gas inlet line. Resistive heating of the cylind- Fig. 1. Molecular structure of free-base phthalocyanine, as com- pared to that of free-base tetrabenzoporphin. The most noticeable difference between these two molecules involves replacement of the four bridging carbon atoms by nitrogen ones, as highlighted by the circles. N N N N N N N N H H Free-base phthalocyanine (H Pc)2 Tetrabenzoporphin (H2TBP) NN NN H H m � Unveiled optical properties of tetrapyrollic pigments in cryogenic environments Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 565 er allowed temperatures of 300 °C to be reached whereby the dye vapor was entrained in the flow of the host gas and the mixture deposited on a sapphire window at cryogenic temperatures (20 K typically). Gas flows in excess of 10 mmol/h were required to achieve isolation of monomer dye molecules. Details of the cryogenic setup were described previo- usly [12]. Typical sample thicknesses were about 250 µm. Isolation conditions for the dye molecules in the matrix, as well as sample concentrations were estimated from ab- sorption spectroscopy [1]. Either DCM, Rh610 or Rh6G dye pumped with a frequency doubled pulsed YAG or a XeCl excimer laser were used for exciting either the QX and QY bands of H2TBP or the Q band of ZnTBP. Fluo- rescence was analyzed with a HRS 600 Jobin-Yvon spec- trometer and recorded on a fast iCCD (ANDOR, model DH-720) camera. All results were measured at 8 K. Before presenting and discussing our results, we want to emphasize one point concerning the sample preparation. The method used for introducing the guest molecules into the gas flow, then condensed as a solid on a low tempera- ture substrate, makes it quite difficult to accurately control the final guest concentration in the crystal host. To ensure that we have worked on low concentration samples, there- by preserving the isolation of the guest molecules, we made previously absorption spectral measurements at dif- ferent guest concentrations. A very useful indicator we have found in this regard is the width of absorption bands. In case of aggregate formation, absorption bands appeared quite broad and unstructured [13]. Fluorescence and other experiments were then done on samples with the narrowest or highly structured bands. 3. Results and discussion In this work, results obtained for H2TBP isolated in Ar, Xe and N2 and for ZnTBP isolated in Ar and N2 are pre- sented and discussed. Absorption spectra Following the procedure previously mentioned, we can ascertain that the absorption spectrum of H2TBP/Ar shown in Fig. 3,a clearly corresponds to well-isolated guest mole- cules. The two main bands appearing at about 654 nm (15284 cm–1) and 584 nm (17132 cm–1) belong to absorp- tions of the split Q band, frequently labeled as the QX and QY bands, respectively, in order of increasing energy. The two corresponding bands in the H2Pc/Ar absorption spec- trum are located at about 677 nm (14771 cm–1) and 635 nm (15748 cm–1). The shift of the absorption bands observed between H2TBP and H2Pc results from the change in energy of the molecular orbitals due to the re- placement of the interpyrrolic carbons Cm by the nitrogen atoms (Fig. 1). As a consequence, the H2Pc Q bands are shifted to the red while the B bands in the Soret’s region are oppositely blue shifted. Fig. 2. Simplified scheme of the oven, especially designed by Dr. Michel Broquier and Dr. Pierre Çarçabal, for introducing dye molecules in the gas flow for deposition on the cooled glass substrate. Electrical Feedthrough Stainless Steel Crucible Heating Coil Thermocouple Threaded Cavity Containing Sample Fig. 3. Absorption bands in the QX and QY regions of free-base H2TBP. The asterisks indicate residual absorption lines belonging to H2Pc impurities (a). Absorption spectrum of ZnTBP showing the unique Q band (b). Absorption spectra were obtained with Ar as the host crystal at about 8 K. Wavelength, nm Wavelength, nm Wavenumber, cm –1 Wavenumber, cm –1 O p ti ca l d en si ty O p ti ca l d en si ty 17500 17000 16500 16000 15500 15000 0 0 0.2 0.4 0.6 0.8 1.0 580 600 620 640 660 17500 17000 16500 16000 15500 15000 0.2 0.4 0.6 0.8 580 600 620 640 660 Q QY QX a b 5 8 3 .7 6 3 4 .5 6 5 4 .3 6 1 2 .9 C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, and S.M. Arabei 566 Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 From the analysis of the different absorption spectra we have recorded for benzoporphyrin in several hosts, we can extract the following results about the QX–QY splitting. For H2TBP, we report values of about 1848 cm–1, 1655 cm–1 (average value for the two main sites observed) and 1851 cm–1 for Ar, Xe and N2 hosts, respectively, as com- pared to the splitting values of 982 cm–1, 958 cm–1 and 1000 cm–1 found for H2Pc in the same hosts, respectively. It is important to mention that the values of the QX–QY split- ting are calculated by simply taking the difference between the maximum of the intense QY band associated to the lo- calization of the S2←S0 transition. Indeed, this is not en- tirely true as in doing this, the Fermi resonance [9,14] which influences the exact position and intensity of the QY(0-0) band is not considered here. Therefore, the posi- tion as well as the intensity of the QY(0-0) band are not precisely defined in this work. Another comparison with previously reported spectra of H2Pc in the same host allows us to make the following comments: absorption lines recorded for H2Pc/Ar in simi- lar conditions looked narrower that those reported in Fig. 3. The broadening observed is probably not due to a strong contribution from aggregates, even if we cannot completely exclude it. Indeed, H2TBP/Ar spectra show some structured features that very likely arise from a great- er number of separated sites in the matrix. From a more detailed view of the QX absorption band of H2TBP/Ar, at least 7 sites can be distinguished. This point will be more explicitly clarified in a later paragraph concerning the ob- servation of spectral hole burning. Note that the presence of multiple sites would also explain the width of the bands. The absorption spectrum of ZnTBP/Ar at 8 K is shown in Fig. 3,b. ZnTBP belongs to the D4h point group, while H2TBP belongs to D2h and for symmetry reasons, the two bands QX and QY collapse in only one Q band located at about 613 nm in the argon host, i.e. in between the posi- tions of the QX and QY bands of H2TBP. From spectros- copic measurements of ZnTBP in supersonic expansion [15], the S1 state was located at 16579 cm–1 (603.2 nm) above the ground state, a value slightly blue shifted com- pared to 16316 cm–1 (612.9 nm) found here at 8 K. The present matrix value is in perfect agreement with the pre- viously reported one as 16313 cm–1 for ZnTBP/Ar at 20 K [2]. While the absorptions in nitrogen matrices are quite similar to those in argon hosts, the absorption spectra of H2TBP in Xe clearly exhibits «doublets» structures pro- bably corresponding to two major sites. A comparison be- tween the energy locations of the S1 ← S0 and S2 ← S0 transitions for H2TBP and ZnTBP in different environ- ments at low temperatures is presented in Table 1. Emission spectra α-fluorescence. Singlet emission spectra recorded for H2TBP and ZnTBP in the Ar host are shown in Fig. 4. Both fluorescence spectra shown for H2TBP and ZnTBP are presented with the same horizontal scale, with the posi- tion of the 0-0 region made coincident to allow comparison of their vibronic structure. For H2TBP, the non-resonant fluorescence spectrum is shown (Fig. 4,a). The higher energy band shown at about 654.2 nm corresponds to the 0-0 zero-phonon origin band. Its structure suggests the pre- sence of several marked sites as also noticed from the ab- sorption spectrum. Besides the rather intense electronic origin region, the most intense band which appears in the H2TBP/Ar spectrum is that centered at 731.9 nm, reveal- Table 1. Experimental spectral positions of the Q bands for H2TBP and ZnTBP in different environments. Guest Host Transition, band Peak energy cm–1 (nm) References H2TBP Ar, 8 K S1 ← S0, QX S2 ← S0, QY 15284 (654.3) 17132 (583.7) This work Xe, 8 K (2 main sites) 1S′ ← S0, QX 2S ′ ← S0, QY 1S′′ ← S0, QX 2S ′′ ← S0, QY 14986 (667.3) 16631 (601.3) 15205 (657.7) 16869 (592.8) This work N2, 8 K S1 ← S0, QX S2 ← S0, QY 15305 (653.4) 17156 (582.9) This work n-C8, 4.2 K S1 ← S0, QX S2 ← S0, QY 15175 (659.0) 16600 (602.4) [9,10] ZnTBP Ar, 8 K S1,2 ← S0, Q 16316 (612.9) This work N2, 8 K S1,2 ← S0, Q 16329 (612.4) This work Ar, 20 K S1,2 ← S0, Q 16313 (613) [2] n-C8, 4.2 K S1,2 ← S0, Q 15954 (626.8) [6] He, supersonic expansion S1,2 ← S0, Q 16579 (603.2) [15] Unveiled optical properties of tetrapyrollic pigments in cryogenic environments Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 567 ing, like in the case of H2Pc, the existence of stimulated emission. Stimulated emission observed in the case of H2TBP will be discussed in detail in a paragraph to follow. The fluorescence emission from ZnTBP/Ar is shown in Fig. 4,b. A complete analysis of this fluorescence spectrum is complicated by the probable presence of impurities. How- ever, undoubtedly, the major band at 681.2 nm is assigned to the same active mode at approximately 1636 cm–1 which gives rise to stimulated emission in H2TBP/Ar. Nevertheless, no evidence of stimulated emission was ob- served in that case, whatever the laser intensity was. A ne- gative result concerning the observation of stimulated emission was also obtained in the case of ZnTBP/N2. β-stimulated emission. As previously indicated, the in- tense and narrowed line at 731.9 nm (13663 cm–1) is due to stimulated emission. While collecting data about this stimulated emission, we have qualitatively noticed a de- pendence upon a threshold effect even if we did not cha- racterize it quantitatively. This stimulated emission was al- so observed in N2 matrices around 731 nm under pulsed la- ser excitation of QX and/or QY, but not in Xe crystals. Such a situation is quite similar to what we reported already in the case of free-base phthalocyanine, for which stimulated emission was also observed in Ne, Ar, Kr but not in Xe hosts [1]. Stimulated emissions in H2Pc were assigned to specific vibronic transitions involving a vibration mode at about 1550 cm–1 assigned to the stretching vibration CαN bonds of the C–N–C bridges of the central ring [16,17], whereas the corresponding mode in the case of H2TBP is around 1620 cm–1. As was noted in [9,18], the 1620 cm–1 vibration of H2TBP corresponds to the Ag stretching vibra- tion of the CαCm bonds, i.e. to the vibration which is loca- lized in the CαCmCα bridges in H2TBP (Cα — carbon atom in the α-position of the pyrrole rings, Cm — carbon atom in the meso-position (CαCmCα bridges) as shown in Fig. 1). Upon increasing up the pump power intensity by a fac- tor of about ten, from tens of μJ/pulse to hundreds of μJ/pulse, the emission band at about 732 nm gained enor- mously in intensity while the others remained unchanged or diminished slightly. Moreover, the linewidth of this emis- sion band was reduced considerably while its intensity in- creased. The width of the band is approaching that of the exciting laser, decreasing from approximately 8 cm–1 to 2 cm–1, a value surely limited by the resolving power of the monochromator. Figure 5 shows a two-dimensional (2D) excitation/emis- sion plot of the stimulated emission observed for free-base H2TBP isolated in solid Ar at 8 K. The plot in Fig. 5,a de- picts the sites emission detected with excitation in the 0-0 region of the S1 ← S0 transition (QX band) while Fig. 5,b depicts that produced with excitation of the S2 ← S0 transi- tion (QY band). Both 2D excitation/emission plots were recorded while monitoring the SE mode centered around 732 nm. For H2TBP/Ar under pulsed excitation in the QX region, around 654 nm, doublet SE lines (separated by 3 cm–1) were recorded giving rise to similar excitation spectra (separated by about 5 cm–1). An analysis of the SE signals generated under selective excitation at 654.43 nm and 654.25 nm inside the broad QX absorption band of H2TBP/Ar at 8 K revealed the existence of 2 stimulated emissions, which are assigned to two dis- tinct sites (Fig. 5,a). These 2 sites look independent. In contrast to the case of H2Pc, no phonons are excited, but a vibration at about 215 cm–1 from the origin band is ex- cited, as evidenced by looking at the absorption spectrum (doted line) shown superimposed to the excitation spectra. Clearly excitation spectra recorded for the SE signal pro- vide greatly enhanced spectroscopic information over what can be extracted from either conventional fluorescence excitation or absorption spectroscopy. Fig. 4. Fluorescence spectrum of free-base H2TBP recorded un- der non selective laser excitation at about 583 nm (a). Fluores- cence spectrum of ZnTBP under resonant excitation wavelength at 612,9 nm (b). Both fluorescence spectra were obtained with Ar as the host crystal at about 8 K. The horizontal scales have been shifted adequately to make coincident the singlet-singlet electron- ic origin regions. The asterisk indicates the fluorescence line due to residual free-base phthalocyanine H2Pc. 15500 15000 14500 14000 13500 0 0.2 0.4 0.6 0.8 1.0 660 680 700 720 740 16500 16000 15500 15000 14500 0 0.2 0.4 0.6 0.8 1.0 610 620 630 640 650 660 670 680 a 0 -0 0 -0 b Wavelength, nm Wavelength, nm Wavenumber, cm –1 Wavenumber, cm –1 In te n si ty , ar b . u n it s In te n si ty , ar b . u n it s 6 5 4 .2 + 1 6 2 0 cm – 1 7 3 1 .9 6 8 1 .2 + 1 6 3 6 cm – 1 6 1 2 .9 C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, and S.M. Arabei 568 Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 The situation is more complicated in the case of H2TBP/N2 (not shown). Indeed, 4 sites giving rise to SE signals at 13678 cm–1 (731.1 nm), 13687 cm–1 (730.6 nm), 13694 cm–1 (730.3 nm) and 13701 cm–1 (729.9 nm) were identified, with possibilities for exciting several sites at one time because of possible transfers from one site to another as reported already for H2Pc [1]. In this case, the excitation spectra of SE lines exhibit quite intense phonon bands al- lowing the simultaneous excitation of all the sites. Upon excitation of the QY band of H2TBP/Ar, the ob- servation of two sites is still revealed by the occurrence of stimulated emission, but the excitation spectra recorded were found to be more complex, due to vibronic coupling between vibrational levels from the lower QX state and the electronic QY state. While monitoring one or the other site, the corresponding excitation spectra were revealed, demon- strating that they have the same structure simply shifted with respect to the one another by ~4 cm–1, the exact ener- gy spacing between the two sites. From this important ob- servation, we can conclude that the vibronic coupling between QX and QY is quite similar for the two sites, and that no phonon modes are contributing (Fig. 5,b). For QY in the case of H2TBP/N2, the situation (also not shown) needs to be clarified as disturbances due to excitation of phonon modes from the matrix are also assumed to contribute. Un- der QY excitation, only the two main SE lines located at 13678 cm–1 (731.1 nm) and 13687 cm–1 (730.6 nm) are clearly observed, allowing the identification of two main sites separated by about ~9 cm–1. In the case of the fluorescence spectra of ZnTBP/Ar under pulsed laser excitation, no stimulated emission en- hancement could be observed whatever the incident laser intensity was. A similar observation was also reported for H2TBP/Xe. A possible explanation of the absence of sti- mulated emission in the case of ZnTBP and H2TBP/Xe could be due to the heavy atom (Zn or Xe) effect which in- creases the probability of the intersystem crossing rate from the excited singlet S1 state level to the lower lying triplet T1 state. This non-radiative relaxation mechanism is then competing with the fluorescence and thereby reduces the chance of the excited singlet state population for reach- ing the threshold condition. As a consequence of the en- hanced intersystem crossing the triplet state is efficiently populated, leading to the observation of phosphorescence signal. c-phosphorescence. In the case of ZnTBP/Ar, for which no stimulated emission could be monitored, conversely a rather strong phosphorescence signal was recorded with a lifetime longer than 30 ms. The corresponding phospho- rescence spectrum is shown in Fig. 6. The recorded spectrum is not easily understandable, and the situation is even unclear after attempting to analyze the associated excitation spectra (not shown). However, in- spection of Fig. 6 shows that a «doublet» structure is clear- ly apparent in the phosphorescence spectra of ZnTBP/Ar and ZnTBP/N2. A similar «doublet» structure was earlier reported for ZnTBP/n-C8 by Arabei et al. [6]. The interpre- tation given by the authors was as follows: due to a non correlation between S1–S0 and T1–T0 transition energies in the site structure, a site selective excitation in the S1–S0 re- gion can induce the phosphorescence from different sites with distinct T1–S0 transition energies. However several questions remain unsolved still and we consider that the analysis of the phosphorescence spectra of ZnTBP in vari- ous cryogenic matrices require further study. Amongst these, one should clarify if an efficient intersystem transfer, as as- 728 730 732 734 736 0 0.2 0.4 0.6 0.8 1.0 a 6 5 6 6 4 2 6 4 4 6 4 6 6 4 8 6 5 0 6 5 2 6 5 4 0 0 .2 0 .4 0 .6 0 .8 1 .0 728 730 732 734 736 642 644 646 648 650 652 654 656 728 730 732 734 736 580 582 584 586 588 728 730 732 734 736 0 0.2 0.4 0.6 0.8 1.0 5 8 0 5 8 2 5 8 4 5 8 6 5 8 8 0 0 .2 0 .4 0 .6 0 .8 1 .0 b Optical density Optical density O p ti ca l d en si ty O p ti ca l d en si ty Fig. 5. 2D emission-excitation spectra of free-base H2TBP/Ar at 8 K recorded under selective laser excitation. Both QX and QY regions were studied, emission spectra are shown on top, excita- tion spectra, together with the absorption spectra (dotted lines) are on the left. Analysis of the 2D spectrum under SE conditions of QX region around 654 nm (a). Analysis of the 2D spectrum under SE conditions of QY region around 584 nm (b). Unveiled optical properties of tetrapyrollic pigments in cryogenic environments Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 569 certained by the observed phosphorescence, is the major factor for the non-observation of stimulated emission in ZnTBP. Spectral hole-burning With H2TBP/Ar, persistent spectral changes could be detected under selective laser excitation at 8 K. These spectral hole-burning changes were observed when looking for stimulated emission conditions under selective excita- tion of one or the other site. At low temperature, the spect- ral hole-burning phenomenon is quite well known in the case of free-base porphyrins [19]. It is then not unusual to observe it with H2TBP. Figure 7 shows an expanded view of the absorption spectrum corresponding to the QX ab- sorption band before excitation and then, after excitation of one site and then of the other one. The inset of Fig. 7 presents a more expanded view to show the changes ob- served close to the band maximum. Typical burning times needed to observe this persistent spectral change were in the order of one hour, under a laser energy of ∼ 40 µJ/pulse at 10 Hz. Among the different sites which are spectrally identi- fied in the absorption, only two specific lines showed se- lective persistent spectral changes. Burning at 654.25 nm induced a decrease of the absorption at this wavelength, while, a similar change was observed when burning the other site at 654.43 nm. It is interesting to notice that quite similar spectral changes are observed on the vibronic band located at about 15500 cm–1, assigned to the same vibra- tion mode as the one which gives rise to SE under laser excitation. This phenomenon could be explained by tauto- merization. In this model the two sites would be associated with the two tautomers of the porphyrin molecule, and the observed changes arise from the interconversion of one form to another, i.e. the flipping of the inner protons from one position to the other [20,21]. In the case of H2TBP/Ar, this process has a rather low efficiency and cannot reduce SE significantly. Such a situation differs from what we reported for H2Pc/Ne [1], for which a very efficient hole- burning phenomenon was observed in the 0-0 S1 ← S0 absorption band with even weak dye laser excitation. Con- sequently, as the excited family of sites disappeared during excitation, SE was not sustainable for long periods in this solid. 4. Conclusions The results obtained in the present work confirm that the phenomenon of stimulated emission firstly observed for phthalocyanines in cryogenic matrices under modest conditions of pulsed laser excitation is not an exception. With free-base H2TBP, stimulated emission was observed in Ar and N2 matrices, but not in Xe matrices. The vibro- nic transition involved is, like in the case of H2Pc, the one involving a mode at about 1620 cm–1, a particularly active mode corresponding to the stretching Ag-vibration of the CαCm bonds [9,18]. We also reported the observation of persistent spectral hole-burning; however the quite low ef- ficiency of the burning process can not affect SE. Finally, with ZnTBP, no stimulated emission was recorded; how- ever the appearance of rather strong phosphorescence was noticed. From all results we have obtained on SE in cryo- genic matrices, with H2Pc, ZnPc as well as H2TBP, we can reasonably predict that SE should also be observable for these molecules isolated in other solid materials that also produce narrow linewidths such as Shpol’skii matrices, like n-octane. Other possible interesting candidates could also be sol-gel or polymer hosts. Another important remark is the following: compared to what is obtained in normal fluorescence excitation scans, 15500 15400 15300 15200 15100 0 0.2 0.4 0.6 0.8 1.0 1.2 c b a Wavelength, nm 644 646 648 650 652 654 656 658 660 662 15290 15285 15280 15275 0.3 0.4 0.5 654.2 654.4 654.6 654.8 a b c Wavenumber, cm –1 O p ti ca l d en si ty Fig. 7. Extended view of the absorption spectrum corresponding to the QX absorption band of H2TBP/Ar at 8 K. a, b and c are the traces recorded before irradiation, after. irradiation at 654.43 nm and after irradiation at 654.25 nm. The inset shows evidence of persistent spectral changes (hole-burning effect) between two specific sites in the QX origin region. 12500 12400 12300 12200 0 0.2 0.4 0.6 0.8 1.0 ZnTBP/N2 ZnTBP/Ar Wavelength, nm Wavenumber, cm –1 800 804 808 812 816 In te n si ty , ar b . u n it s Fig. 6. Phosphorescence spectra of ZnTBP/Ar and ZnTBP/N2 recorded under selective laser excitation at about 612 nm. The spectra were recorded at about 8 K. C. Crépin, N. Shafizadeh, W. Chin, J.-P. Galaup, J.G. McCaffrey, and S.M. Arabei 570 Fizika Nizkikh Temperatur, 2010, v. 36, No. 5 greatly enhanced site selectivity is obtained for excitation scans recorded for the SE mode. This behavior stems from the increased resolution of individual sites whose lineshape change from featureless fluorescence excitation bands to a highly structured SE excitation band. The very well-resolv- ed spectra available in SE excitation scans allow detailed exploration of site occupancies in a given solid and from one solid to another. Added to our previous results already published [1], the results of the present study of SE should allow new insights into site effects and site selectivity. Acknowledgements The assistance of Dr. Michel Broquier and Dr. Pierre Çarçabal for the design of the oven used for vaporization of the porphyrin molecules is gratefully acknowledged. The France-Belarus cooperation was supported by the CNRS/BRFFR no. 23181 agreement and grant BRFFR no. F09F-001. McC’s participation in this work was made possible with support from Science Foundation Ireland (SFI), Research Frontiers Programme (06/RFP/CHP012). 1. N. Dozova, C. Murray, J.G. McCaffrey, N. 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Unveiled optical properties of tetrapyrollic pigments in cryogenic environments

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