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Comparison of resonance cars spectra of chrysene in the lowest excited singlet and triplet state
Volume 73. number 1
CEHMICAL
PHYSICS
1 July 1980
LETTERS
COMPARISON OF RESONANCE CARS SPECTRA OF CHRYSENE lN THE LOWEST EXCITED SINGLET AND TRIPLET STATE H -J. WEIGMANN, K LENZ, A. LAU, M. PFEIFFER Zentrahnsntut Recewed
fur OptiLk und
19 February
Spektroskopie.
Akademie
and W. WERNCKE
der lVissen.whaften der DDR.
1199 Berim. DDR
1980
The resonance coherent antIStokes Raman scatte,mg spectra generated The observed differences m wavenumber and mtenslty electromc states
of chrysene in the fust exctted singlet and triplet state aEe pomt out a changed molecular structure in the excited
1. Introduction Coherent antl-Stokes Raman scattering (CARS) generated under resonance conditions yields Raman spectra of molecules with h& detection sensitivity. This provides a posslbllity to observe CARS spectra of molecules in excited electromc states. Recently we reported a resonance CARS spectrum of chrysene m the first excited singlet state [l] . To obtam spectra from the tnplet T, state which 1s populated about 50 ns after smglet excitation [2] a delayed CARS technique was apphed. The mtenslty of the CARS signal obtamed 40 ns after excitation IS very low although the enhanced absorption of a 575 MI test signal (T,T, absorption) shows an increased triplet population. Thus demonstrates that the applied preresonance conditions have not been sufficient; it is necessary to brmg the CARS pump rarhation in strong resonance to the T, -T, absorption thus attaining a noticeable resonance enhancement for chrysene molecules in the tnplet state. The results are described m the following sections.
2. Experimental Fig. 1 shows the experimental apparatus. The radiation of the first nitrogen laser (200 kW, 3 Hz, 2.3 ns) was used to populate the tnplet state of chrysene. The second nitrogen laser (800 kW, 1 Hz, 4.5 ns) pumped two dye lasers producing the CARS pump rahtlons.
Fig. 1. Experunental apparatus.
One of the dye lasers worked at a constant wavelength (580 MI), the wavelength of the other one was in +fie range of 590 to 643 nm. The dye lasers are constructed as described in ref. [3] _ Cbrysene was imbedded into PMMA and cooled to a temperature of about -100°C [4]_ From earlier measurements, it is known that under these conditions the hfetime of the tnplet state is about 1 s [S] _Therefore a repetition rate of 3 Hz yields a sufficient steady-state population of the triplet state without synchronisation of the two N2 lasers. The CARS signals were spectralIy dispersed by a three-prism spectrograph,f= 27 cm (VEB Carl Zeiss, Jena), and detected photographically.
Volume
73, number
1
CHEMICAL
PHYSICS
3. Results and discussion
Table 1 CARS spectra of lfferent
Fig. 2 shows the resonance CARS spectrum (photometer curve) of triplet chrysene molecules Due to resonance enhancement many more Raman hnes occur m contrast to the ground-state spectrum, which shows only the strongest Raman hne at 1378 cm-l under these conditions In table 1 the observed Raman lmes of chrysene in the ground state, in the first excited singlet state and m the tnplet state are compared. Generally, the CARS frequencies of the two excited states are nearly identical, but quite different from the ground-state values. This IS verified by the Raman hnes near 15.50 cm-I, 1367 cm-1 and 1336 cm-1 In the excited state spectra compared with 1575 cm-l, 1378 cm-’ and 1360 cm-1 m the ground state, respectively Wlthout a specia1 vibrational analysis this leads to the conclusion, that the structure of the moIecule 1s changed after electronic
excltatlon,
t
Ef50 wovenumber
176
2. Resonance
CARS spectrum
-f450 (id]
-f550 -
of trIplet chrysene.
electromc
CARS spectra S, state a) (cni’ ) 1550 m-s 1502 m-s
l-1 state b,d) (cm-’ )
states of chrysene Spontaneous Raman spectrum (ground state)
Cl
(cm-’ 1
1555 s 1519 s
1575 w-m -
1453 m e,
1431 m 1382 s
1367 s
1369 m
1336 m
1335 s
1240 m 1164 w
1364 m 1330 w
1245 m 1180 s 1090 s
1228 w 1160 w 1136 w 1045 w 1019 m
but that there are
only small structure changes between the first excited singlet state and the lowest triplet state of chrysene. Wlule the frequencies of the two excited states do not show large differences there are obvious changes m mtenslty. The strongest hnes of the excited smglet state arise at 1550 cm-l and 1367 cm-l while m the tnpletstate spectrum the most pronounced line appears at 1335 cm-*. This may come from different symmetry conditions of the electronic transitions (S I -S,, or
Fg
1 July 1980
LETTERS
a) b, c, d,
Chrysene in PMMA. c = 5 X IO-’ hl Chrysene m PhIMA. c = 3 X IO? M. Chryscne powder. Accuracy, absolute + 10 cm-‘, rehtlve III the column cm’. e, DIsturbed by a PMMA lme at about 1450 cm-’ f) Raman lme of ground-state molecules
+5
T1 -T,) responsible for resonance enhancement in both cases. Furthermore, it 1s of some mterest to consider the Raman lmes in the regon of CC stretching vlbratlons (1450-1600 cm-l). In the excited-state spectra the appearance of a new Raman line at 1502 cm-l or 1519 cm-l (both values are identical withm wavenumber accuracy) is remarkable. In the ground-state spectra, only an infrared band exists at 1515 cm-l but no Raman hne. Thus is the same situation as observed m the case of excited rhodamme dyes [6]. After transition of the dye molecules mto excited electronic states some vibrations become Raman active which are forbidden III the ground state. It IS possible to explain this for chrysene as well as for rhodarnme dyes supposmg a lowered symmetry of molecules in the excited states. This IS in agreement Hnth results from phosphorescence measurements [7] which also lead to the conclusion that the symmetry
Volume 73, number 1
CHEMICAL
PHYSICS
of chrysene is lowered m the triplet state. The CC stretchmg vibration of the highest wavenumber is obviousIy lowered when gomg from groundstate molecules to the excited states. Thus points out a lowered force constant arising from a decreasing n-bond order between the nng atoms after excltatlon. Consequently, it IS more probable that the molecule leaves the planar configuration. In conclusion, the results presented above demonstrate that CARS spectra are well suited to obtain information about structure changes in molecules after electronic excitation.
LETTERS
1 July j980
References [I] [2) [3) [4] [S] [6] [71
W. Werncke, H.-J. Weigmann, J. Pgtzold. A. Lau, K- Lenz and M. Pfelffer, Chem. Phys Letters 61 (1979) 105. K.A. Hodglcmson and LH. Munro, Chem. Phys. Letters 12 (1971) 281. I. Sbonan, N. Danon and U. Oppenheim, J. AppL Phys 48 (1977) 4495. H. Kobwhlce and D. Leupold. Monatsber. Deutsch. Akad. Wlss. 9 (1967) 302. J. P&old, Dissertation A, m preparation. R. K&ig, A. Lau and H.-J. Weigmann, Chem. Phys. Letters 69 (1980) 87. A Olszowsk~ and 2. Kubiak. Bull. AC&. PoIon. Sci Ser. Math. Astron. Phys. 23 (197.5) 641.
Acknowledgement We wish to thank Mrs. R. Lendt and Mrs. B. Krause for technical assistance.
177
CEHMICAL
PHYSICS
1 July 1980
LETTERS
COMPARISON OF RESONANCE CARS SPECTRA OF CHRYSENE lN THE LOWEST EXCITED SINGLET AND TRIPLET STATE H -J. WEIGMANN, K LENZ, A. LAU, M. PFEIFFER Zentrahnsntut Recewed
fur OptiLk und
19 February
Spektroskopie.
Akademie
and W. WERNCKE
der lVissen.whaften der DDR.
1199 Berim. DDR
1980
The resonance coherent antIStokes Raman scatte,mg spectra generated The observed differences m wavenumber and mtenslty electromc states
of chrysene in the fust exctted singlet and triplet state aEe pomt out a changed molecular structure in the excited
1. Introduction Coherent antl-Stokes Raman scattering (CARS) generated under resonance conditions yields Raman spectra of molecules with h& detection sensitivity. This provides a posslbllity to observe CARS spectra of molecules in excited electromc states. Recently we reported a resonance CARS spectrum of chrysene m the first excited singlet state [l] . To obtam spectra from the tnplet T, state which 1s populated about 50 ns after smglet excitation [2] a delayed CARS technique was apphed. The mtenslty of the CARS signal obtamed 40 ns after excitation IS very low although the enhanced absorption of a 575 MI test signal (T,T, absorption) shows an increased triplet population. Thus demonstrates that the applied preresonance conditions have not been sufficient; it is necessary to brmg the CARS pump rarhation in strong resonance to the T, -T, absorption thus attaining a noticeable resonance enhancement for chrysene molecules in the tnplet state. The results are described m the following sections.
2. Experimental Fig. 1 shows the experimental apparatus. The radiation of the first nitrogen laser (200 kW, 3 Hz, 2.3 ns) was used to populate the tnplet state of chrysene. The second nitrogen laser (800 kW, 1 Hz, 4.5 ns) pumped two dye lasers producing the CARS pump rahtlons.
Fig. 1. Experunental apparatus.
One of the dye lasers worked at a constant wavelength (580 MI), the wavelength of the other one was in +fie range of 590 to 643 nm. The dye lasers are constructed as described in ref. [3] _ Cbrysene was imbedded into PMMA and cooled to a temperature of about -100°C [4]_ From earlier measurements, it is known that under these conditions the hfetime of the tnplet state is about 1 s [S] _Therefore a repetition rate of 3 Hz yields a sufficient steady-state population of the triplet state without synchronisation of the two N2 lasers. The CARS signals were spectralIy dispersed by a three-prism spectrograph,f= 27 cm (VEB Carl Zeiss, Jena), and detected photographically.
Volume
73, number
1
CHEMICAL
PHYSICS
3. Results and discussion
Table 1 CARS spectra of lfferent
Fig. 2 shows the resonance CARS spectrum (photometer curve) of triplet chrysene molecules Due to resonance enhancement many more Raman hnes occur m contrast to the ground-state spectrum, which shows only the strongest Raman hne at 1378 cm-l under these conditions In table 1 the observed Raman lmes of chrysene in the ground state, in the first excited singlet state and m the tnplet state are compared. Generally, the CARS frequencies of the two excited states are nearly identical, but quite different from the ground-state values. This IS verified by the Raman hnes near 15.50 cm-I, 1367 cm-1 and 1336 cm-1 In the excited state spectra compared with 1575 cm-l, 1378 cm-’ and 1360 cm-1 m the ground state, respectively Wlthout a specia1 vibrational analysis this leads to the conclusion, that the structure of the moIecule 1s changed after electronic
excltatlon,
t
Ef50 wovenumber
176
2. Resonance
CARS spectrum
-f450 (id]
-f550 -
of trIplet chrysene.
electromc
CARS spectra S, state a) (cni’ ) 1550 m-s 1502 m-s
l-1 state b,d) (cm-’ )
states of chrysene Spontaneous Raman spectrum (ground state)
Cl
(cm-’ 1
1555 s 1519 s
1575 w-m -
1453 m e,
1431 m 1382 s
1367 s
1369 m
1336 m
1335 s
1240 m 1164 w
1364 m 1330 w
1245 m 1180 s 1090 s
1228 w 1160 w 1136 w 1045 w 1019 m
but that there are
only small structure changes between the first excited singlet state and the lowest triplet state of chrysene. Wlule the frequencies of the two excited states do not show large differences there are obvious changes m mtenslty. The strongest hnes of the excited smglet state arise at 1550 cm-l and 1367 cm-l while m the tnpletstate spectrum the most pronounced line appears at 1335 cm-*. This may come from different symmetry conditions of the electronic transitions (S I -S,, or
Fg
1 July 1980
LETTERS
a) b, c, d,
Chrysene in PMMA. c = 5 X IO-’ hl Chrysene m PhIMA. c = 3 X IO? M. Chryscne powder. Accuracy, absolute + 10 cm-‘, rehtlve III the column cm’. e, DIsturbed by a PMMA lme at about 1450 cm-’ f) Raman lme of ground-state molecules
+5
T1 -T,) responsible for resonance enhancement in both cases. Furthermore, it 1s of some mterest to consider the Raman lmes in the regon of CC stretching vlbratlons (1450-1600 cm-l). In the excited-state spectra the appearance of a new Raman line at 1502 cm-l or 1519 cm-l (both values are identical withm wavenumber accuracy) is remarkable. In the ground-state spectra, only an infrared band exists at 1515 cm-l but no Raman hne. Thus is the same situation as observed m the case of excited rhodamme dyes [6]. After transition of the dye molecules mto excited electronic states some vibrations become Raman active which are forbidden III the ground state. It IS possible to explain this for chrysene as well as for rhodarnme dyes supposmg a lowered symmetry of molecules in the excited states. This IS in agreement Hnth results from phosphorescence measurements [7] which also lead to the conclusion that the symmetry
Volume 73, number 1
CHEMICAL
PHYSICS
of chrysene is lowered m the triplet state. The CC stretchmg vibration of the highest wavenumber is obviousIy lowered when gomg from groundstate molecules to the excited states. Thus points out a lowered force constant arising from a decreasing n-bond order between the nng atoms after excltatlon. Consequently, it IS more probable that the molecule leaves the planar configuration. In conclusion, the results presented above demonstrate that CARS spectra are well suited to obtain information about structure changes in molecules after electronic excitation.
LETTERS
1 July j980
References [I] [2) [3) [4] [S] [6] [71
W. Werncke, H.-J. Weigmann, J. Pgtzold. A. Lau, K- Lenz and M. Pfelffer, Chem. Phys Letters 61 (1979) 105. K.A. Hodglcmson and LH. Munro, Chem. Phys. Letters 12 (1971) 281. I. Sbonan, N. Danon and U. Oppenheim, J. AppL Phys 48 (1977) 4495. H. Kobwhlce and D. Leupold. Monatsber. Deutsch. Akad. Wlss. 9 (1967) 302. J. P&old, Dissertation A, m preparation. R. K&ig, A. Lau and H.-J. Weigmann, Chem. Phys. Letters 69 (1980) 87. A Olszowsk~ and 2. Kubiak. Bull. AC&. PoIon. Sci Ser. Math. Astron. Phys. 23 (197.5) 641.
Acknowledgement We wish to thank Mrs. R. Lendt and Mrs. B. Krause for technical assistance.
177