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Azobenzene photoisomerization: Two states and two relaxation pathways explain the violation of Kasha's rule.
Femtochemistry and Femtobiology M.M. Martin andJ.T. Hynes (editors) © 2004 Elsevier B .V. All rights reserved.
45
Azobenzene photoisomerization: Two states and two relaxation pathways explain the violation of Kasha's rule. T. Schultz^'\ S. Ullrich", J. Quenneville", T.J. Martinez', M.Z. Zgierski\ A. Stolow' ^ Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Canada ^ Present address: Max-Bom Institute for Non-linear Optics, Berlin, Germany; ^Department of Chemistry, University of Illinois, Urbana, USA; 1. INTRODUCTION Azobenzene is a photochromic molecule, which undergoes trans-cis isomerization upon irradiation in the near-UV. The photoisomerization reaction is ultrafast, efficient and clean and has attracted much attention for its possible application in molecular electronics, data storage and nonlinear optics [1]. In violation of Kasha's rule, the isomerization yields vary anomalously with the excitation wavelength [2]: the symmetry forbidden and weak transition to Si(n7r*) at 440nm yields up to 25% c/5-azobenzene, while the strongly allowed S2(7i7i*) state at 350nm yields only 12% c/5-azobenzene. Two possible reasons have been proposed to explain the violation of Kasha's rule. Based on the investigation of quantum yields in substituted azobenzenes, Rau suggested a large deformation of the molecular structure along the torsional coordinate in S2, which would quench the isomerization reaction proceeding along the inversion coordinate in Si [2]. Fujino et al. could not fmd evidence for torsional motion in time-resolved fluorescence and resonance-Raman spectra but observed ultrafast S2 —> Si internal conversion [3]. To explain the violation of Kasha's rule, Fujino assumed that additional relaxation pathways for high vibrational levels in Si could quench the observed quantum yield after excitation of S2. We studied the photoisomerization reaction by timeresolved photoelectron spectroscopy (TRPES) and propose a new relaxation pathway, which can reconcile the two conflicting models. First results were recently published in a communication [4]. Here we present an improved data analysis and a detailed discussion of the experimental data. 2. EXPERIMENT AND RESULTS TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5]. Traw^-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy.
46 2.0
-^^ 2.01 1.5]
1.5 •o 0)
1.0
1.0 I
2 0.5 1 Q. dl E 0.0:
g. 0.5 0.0
0.0
0.5 1.0 1.5 electron energy (eV)
2.0
0.0
0.5 1.0 1.5 electron energy (eV)
2.0
Fig. 1. TRPES of azobenzene with 330nm excitation and 200nm ionisation (left) and globalfit(right). The two-dimensional spectrum (contour plot) shows the photoelectron spectra along the x-axis (integrated spectrum shown at bottom) and the evolution with the pump-probe time delay on the y-axis (integrated decay trace shown on the left). Energy calibrafion of the photoelectron spectra and determination of the laser crosscorrelation were performed by measuring TRPES traces for NO and butadiene. We found Gaussian shaped cross-correlafions with a full-width-at-half-max of 120-190fs, depending on the wavelength. In Fig. 1 we show a TRPES trace of azobenzene and a corresponding global fit. The two dimensional data were fit with fiinction (1) to extract the excited state population dynamics Pi(t) and the corresponding photoelectron spectra Ii(E) for each ionization channel /. We assumed exponential rise and decay for the population dynamics and a Gaussian timeprofile g(t) for the laser cross-correlation. The simultaneous fit of spectra and dynamics in the two-dimensional data allowed the reliable extraction of dynamics even for overlapping bands.
S(£,o = ZA(^)-[^(0®g(0]
(1)
A minimum of 3 ionization channels with distinct dynamics were necessary to reproduce the measured data at all excitation wavelengths. The fitted photoelectron bands (Fig. 2a) allowed the determination of ionization potentials as indicated for band 8i. For an easy interpretation of the partially resolved bands 82 and 83, we defined an effecdve ionization potential IP50 at half the band height. The fitted decay spectra (Fig. 2b) gave lifetimes of 155fs, Ofs and 420fs for band 81, 82 and S3 respectively, which corresponds to the lifetimes of the intermediate excited states. The lifetimes varied little with excitation wavelength in the investigated range. None of the bands displayed a delayed rise (for all bands we found Xrise ~ 0 ± 20 fs) and we assumed direct laser-excitation for all observed excited states. The measured ionization potentials depended on the excitation energy and are plotted in Fig. 3a. For bands 81 and 83, the measured ionization potentials increased linearly with the excitafion energy. This is expected from Franck-Condon considerafions if vibrafional excitation is preserved in the ionisation step. From the observed ionization potentials and using Koopmans' correlations, supported by ab initio calculadons, we assigned band 81 to the ionization channel S2(TI7I*) -> DO(TC"') and band 83 to the ionization channel 83/4(7171*) -> 1)2/3(71"'). In S2 and Do, the respecfive 7t-orbital is delocalized over the molecule and leads to reduced bonding between the nitrogen atoms. This is in contrast with a phenyl-ring localized character of the corresponding Tt-orbital in S3/4 and D2/3. Comparing the band shapes with those observed in He^-photoelectron spectra [6], we find good correspondence between band
47
8.0
8.5 9.0 9.5 Ionization potential (eV)
0 500 1000 1500 2000 pump-probe delay (fs)
Fig. 2. Integrated photoelectron spectrum (a) and selected decay traces (b) obtained by the global fit shown in Fig. 1. Despite a considerable overlap of the photoelectron bands, the fit was very robust and gave reproducible band shapes and lifetimes of 155, 0 and 420fs for bands 8i, 82 and S3. 81 and Do, but band 83 is both wider and shifted to higher energies than the corresponding signal for D2/3. From Franck-Condon arguments, we can thus assume that the structure of S2 resembles that of the ground state, while that of S3/4 may be somewhat deformed. We did not observe a photoelectron band with delayed rise, which might be expected from Si after internal conversion from 82. We ascribe this to the fact that the corresponding ionization potential for Si(n7i*) -> Di(n^) is shifted out of our detection window due to the large amount of vibrational excitation (>1 eV) in Si. The ionization potential for band 82 showed no clear correlation with the excitation energy, but varied with the probe wavelength at high excitation energies. This indicated an inverted process with the probe beam exciting a high-lying and short-lived excited state, which is subsequently ionized by the pump beam. The dependence of IP50 on the probe wavelength was less pronounced near and below the threshold for nn* excitation (-3.65 eV) and the shape of band 82 changed markedly. Here we assume that an additional 'pre-resonant' pump-probe ionization process contributed to band 82, i.e. the cross-section for non-resonant two-photon ionization was enhanced by near-resonance of the pump beam with the TITX* states. Similar observations were made in resonance-Raman experiments [7]. The signals from the probe-pump process and non-resonant ionisation have no impact on the analysis and interpretation of the low-lying TITI* state dynamics. In the excitation spectrum (Fig. 3b), we observed near-degeneracy of S2 and S3/4. This observation was supported by high-level ab initio calculations, which predicted three close-lying nn* excited states, two of which have considerable oscillator strength [4]. 3. DISCUSSION Our observations clearly show the existence of two near-degenerate photoactive TITT* states S2 and S3/4 with lifetimes of approx. 150fs and 420fs. The different lifetimes indicate distinct relaxation channels and allow a simple model for the violation of Kasha's rule: As reported by Fujino et al. [3], S2 shows ultrafast internal conversion to Si without large amplitude motion along the torsional coordinate. This is supported by our observation of a short lifetime and a narrow photoelectron band resembling that observed for ground-state ionization. We expect subsequent relaxation of Si similar to that observed for direct Si excitation, leading to
48
(b) 0
•
IP50(^l)
•
IPv(si)
•
IP50(^2)
^
IP50(^3)
•a
0.8
0.03.6 3.8 4.0 4.2 4.4 excitation energy (eV)
\ •
0.6-
^ F 0.4 ^ a? 0.2-
• ..
m. \ •
•
••
S-i
• 1
\ £o . ^ . - - ~ ^ - —-A-' . ^ 3 •- . £ 2 A''
3.6 3.8 4.0 4.2 excitation energy (eV)
Fig. 3. (a) Ionization potentials for bands 8i, 82 and 83 in dependence of the excitation energy. Full symbols refer to measurements with 207nm ionization pulse, open symbols to 200nm ionization pulse. The sum of the energy from excitation and ionization pulse is shown as black line. Stars denote the four lowest vertical ionization potentials of 8.4, 8.75, 9.3 and 9.3eV [6]. (b) Normalized excitation spectrum showing near degeneracy for the TTTT* states corresponding to bands 81 and 83. comparable quantum yields. To explain the violation of Kasha's rule, we propose that relaxation of S3/4 proceeds with reduced isomerization yield. Hence the violation of Kasha's rule is neither due to torsional motion in S2, nor to additional relaxation channels for hot Si but rather to the relaxation of state S3/4, which was not identified in previous absorption, fluorescence or resonance-Raman spectra. Compared to ground state ionisation, the photoelectron band for S3/4 ionization is broadened and shifted to higher energies, indicating a change in molecular geometry. We can speculate that this may be due to torsional motion preceding electronic relaxation. This would agree with observations reported by Rau [2] and others, who linked the reduction in isomerization yield to the torsional coordinate. The presented model may thus reconcile the apparently contradicting observations in earlier time- and frequency-domain experiments.
REFERENCES [1] N. Tamai, H. Miyasaka, Chem. Rev., 100 (2000) 1875. [2] H. Rau, Photochromism: Molecules and Systems; Diirr, H.; Buas-Laurent, H., Eds.; Elsevier: Amsterdam 1990, p. 165-191. [3] T. Fujino, T. Tahara, J. Phys. Chem. A, 104 (2000) 4203; T. Fujino, S.Y. Arzhantsev, T. Tahara, J. Phys. Chem. A, 105 (2001) 8123. [4] T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T. J. Martinez, S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z. Zgierski, and A. Stolow, J. Am. Chem. Soc, 125 (2003) 8098. [5] S. Lochbrunner, J.J. Larsen, J.P. Shaffer, M. Schmitt, T. Schultz, J.G. Underwood, A. Stolow, J. Electron Spectrosc. and Relat. Phenom., 112 (2000) 183; A. Stolow, Annual Reviews of Physical Chemistry, 54 (2003) 89; A. Stolow, International Reviews in Physical Chemistry, 22 (2003) 377. [6] T. Kobayashi, K. Yokota, S. Nagakura, J. Electron. Spectrosc. And Relat. Phenom., 6 (1975) 167. For a correct band assignment, please refer to: N.E. Petrachenko, V.I. Vovna, G.G. Furin, J. Fluor. Chem., 63 (1993) 85. [7] N .Biwas, S. Umapathy, J. Chem. Phys., 107 (1997) 7849.
45
Azobenzene photoisomerization: Two states and two relaxation pathways explain the violation of Kasha's rule. T. Schultz^'\ S. Ullrich", J. Quenneville", T.J. Martinez', M.Z. Zgierski\ A. Stolow' ^ Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Canada ^ Present address: Max-Bom Institute for Non-linear Optics, Berlin, Germany; ^Department of Chemistry, University of Illinois, Urbana, USA; 1. INTRODUCTION Azobenzene is a photochromic molecule, which undergoes trans-cis isomerization upon irradiation in the near-UV. The photoisomerization reaction is ultrafast, efficient and clean and has attracted much attention for its possible application in molecular electronics, data storage and nonlinear optics [1]. In violation of Kasha's rule, the isomerization yields vary anomalously with the excitation wavelength [2]: the symmetry forbidden and weak transition to Si(n7r*) at 440nm yields up to 25% c/5-azobenzene, while the strongly allowed S2(7i7i*) state at 350nm yields only 12% c/5-azobenzene. Two possible reasons have been proposed to explain the violation of Kasha's rule. Based on the investigation of quantum yields in substituted azobenzenes, Rau suggested a large deformation of the molecular structure along the torsional coordinate in S2, which would quench the isomerization reaction proceeding along the inversion coordinate in Si [2]. Fujino et al. could not fmd evidence for torsional motion in time-resolved fluorescence and resonance-Raman spectra but observed ultrafast S2 —> Si internal conversion [3]. To explain the violation of Kasha's rule, Fujino assumed that additional relaxation pathways for high vibrational levels in Si could quench the observed quantum yield after excitation of S2. We studied the photoisomerization reaction by timeresolved photoelectron spectroscopy (TRPES) and propose a new relaxation pathway, which can reconcile the two conflicting models. First results were recently published in a communication [4]. Here we present an improved data analysis and a detailed discussion of the experimental data. 2. EXPERIMENT AND RESULTS TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5]. Traw^-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy.
46 2.0
-^^ 2.01 1.5]
1.5 •o 0)
1.0
1.0 I
2 0.5 1 Q. dl E 0.0:
g. 0.5 0.0
0.0
0.5 1.0 1.5 electron energy (eV)
2.0
0.0
0.5 1.0 1.5 electron energy (eV)
2.0
Fig. 1. TRPES of azobenzene with 330nm excitation and 200nm ionisation (left) and globalfit(right). The two-dimensional spectrum (contour plot) shows the photoelectron spectra along the x-axis (integrated spectrum shown at bottom) and the evolution with the pump-probe time delay on the y-axis (integrated decay trace shown on the left). Energy calibrafion of the photoelectron spectra and determination of the laser crosscorrelation were performed by measuring TRPES traces for NO and butadiene. We found Gaussian shaped cross-correlafions with a full-width-at-half-max of 120-190fs, depending on the wavelength. In Fig. 1 we show a TRPES trace of azobenzene and a corresponding global fit. The two dimensional data were fit with fiinction (1) to extract the excited state population dynamics Pi(t) and the corresponding photoelectron spectra Ii(E) for each ionization channel /. We assumed exponential rise and decay for the population dynamics and a Gaussian timeprofile g(t) for the laser cross-correlation. The simultaneous fit of spectra and dynamics in the two-dimensional data allowed the reliable extraction of dynamics even for overlapping bands.
S(£,o = ZA(^)-[^(0®g(0]
(1)
A minimum of 3 ionization channels with distinct dynamics were necessary to reproduce the measured data at all excitation wavelengths. The fitted photoelectron bands (Fig. 2a) allowed the determination of ionization potentials as indicated for band 8i. For an easy interpretation of the partially resolved bands 82 and 83, we defined an effecdve ionization potential IP50 at half the band height. The fitted decay spectra (Fig. 2b) gave lifetimes of 155fs, Ofs and 420fs for band 81, 82 and S3 respectively, which corresponds to the lifetimes of the intermediate excited states. The lifetimes varied little with excitation wavelength in the investigated range. None of the bands displayed a delayed rise (for all bands we found Xrise ~ 0 ± 20 fs) and we assumed direct laser-excitation for all observed excited states. The measured ionization potentials depended on the excitation energy and are plotted in Fig. 3a. For bands 81 and 83, the measured ionization potentials increased linearly with the excitafion energy. This is expected from Franck-Condon considerafions if vibrafional excitation is preserved in the ionisation step. From the observed ionization potentials and using Koopmans' correlations, supported by ab initio calculadons, we assigned band 81 to the ionization channel S2(TI7I*) -> DO(TC"') and band 83 to the ionization channel 83/4(7171*) -> 1)2/3(71"'). In S2 and Do, the respecfive 7t-orbital is delocalized over the molecule and leads to reduced bonding between the nitrogen atoms. This is in contrast with a phenyl-ring localized character of the corresponding Tt-orbital in S3/4 and D2/3. Comparing the band shapes with those observed in He^-photoelectron spectra [6], we find good correspondence between band
47
8.0
8.5 9.0 9.5 Ionization potential (eV)
0 500 1000 1500 2000 pump-probe delay (fs)
Fig. 2. Integrated photoelectron spectrum (a) and selected decay traces (b) obtained by the global fit shown in Fig. 1. Despite a considerable overlap of the photoelectron bands, the fit was very robust and gave reproducible band shapes and lifetimes of 155, 0 and 420fs for bands 8i, 82 and S3. 81 and Do, but band 83 is both wider and shifted to higher energies than the corresponding signal for D2/3. From Franck-Condon arguments, we can thus assume that the structure of S2 resembles that of the ground state, while that of S3/4 may be somewhat deformed. We did not observe a photoelectron band with delayed rise, which might be expected from Si after internal conversion from 82. We ascribe this to the fact that the corresponding ionization potential for Si(n7i*) -> Di(n^) is shifted out of our detection window due to the large amount of vibrational excitation (>1 eV) in Si. The ionization potential for band 82 showed no clear correlation with the excitation energy, but varied with the probe wavelength at high excitation energies. This indicated an inverted process with the probe beam exciting a high-lying and short-lived excited state, which is subsequently ionized by the pump beam. The dependence of IP50 on the probe wavelength was less pronounced near and below the threshold for nn* excitation (-3.65 eV) and the shape of band 82 changed markedly. Here we assume that an additional 'pre-resonant' pump-probe ionization process contributed to band 82, i.e. the cross-section for non-resonant two-photon ionization was enhanced by near-resonance of the pump beam with the TITX* states. Similar observations were made in resonance-Raman experiments [7]. The signals from the probe-pump process and non-resonant ionisation have no impact on the analysis and interpretation of the low-lying TITI* state dynamics. In the excitation spectrum (Fig. 3b), we observed near-degeneracy of S2 and S3/4. This observation was supported by high-level ab initio calculations, which predicted three close-lying nn* excited states, two of which have considerable oscillator strength [4]. 3. DISCUSSION Our observations clearly show the existence of two near-degenerate photoactive TITT* states S2 and S3/4 with lifetimes of approx. 150fs and 420fs. The different lifetimes indicate distinct relaxation channels and allow a simple model for the violation of Kasha's rule: As reported by Fujino et al. [3], S2 shows ultrafast internal conversion to Si without large amplitude motion along the torsional coordinate. This is supported by our observation of a short lifetime and a narrow photoelectron band resembling that observed for ground-state ionization. We expect subsequent relaxation of Si similar to that observed for direct Si excitation, leading to
48
(b) 0
•
IP50(^l)
•
IPv(si)
•
IP50(^2)
^
IP50(^3)
•a
0.8
0.03.6 3.8 4.0 4.2 4.4 excitation energy (eV)
\ •
0.6-
^ F 0.4 ^ a? 0.2-
• ..
m. \ •
•
••
S-i
• 1
\ £o . ^ . - - ~ ^ - —-A-' . ^ 3 •- . £ 2 A''
3.6 3.8 4.0 4.2 excitation energy (eV)
Fig. 3. (a) Ionization potentials for bands 8i, 82 and 83 in dependence of the excitation energy. Full symbols refer to measurements with 207nm ionization pulse, open symbols to 200nm ionization pulse. The sum of the energy from excitation and ionization pulse is shown as black line. Stars denote the four lowest vertical ionization potentials of 8.4, 8.75, 9.3 and 9.3eV [6]. (b) Normalized excitation spectrum showing near degeneracy for the TTTT* states corresponding to bands 81 and 83. comparable quantum yields. To explain the violation of Kasha's rule, we propose that relaxation of S3/4 proceeds with reduced isomerization yield. Hence the violation of Kasha's rule is neither due to torsional motion in S2, nor to additional relaxation channels for hot Si but rather to the relaxation of state S3/4, which was not identified in previous absorption, fluorescence or resonance-Raman spectra. Compared to ground state ionisation, the photoelectron band for S3/4 ionization is broadened and shifted to higher energies, indicating a change in molecular geometry. We can speculate that this may be due to torsional motion preceding electronic relaxation. This would agree with observations reported by Rau [2] and others, who linked the reduction in isomerization yield to the torsional coordinate. The presented model may thus reconcile the apparently contradicting observations in earlier time- and frequency-domain experiments.
REFERENCES [1] N. Tamai, H. Miyasaka, Chem. Rev., 100 (2000) 1875. [2] H. Rau, Photochromism: Molecules and Systems; Diirr, H.; Buas-Laurent, H., Eds.; Elsevier: Amsterdam 1990, p. 165-191. [3] T. Fujino, T. Tahara, J. Phys. Chem. A, 104 (2000) 4203; T. Fujino, S.Y. Arzhantsev, T. Tahara, J. Phys. Chem. A, 105 (2001) 8123. [4] T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T. J. Martinez, S. Lochbrunner, M. Schmitt, J. P. Shaffer, M. Z. Zgierski, and A. Stolow, J. Am. Chem. Soc, 125 (2003) 8098. [5] S. Lochbrunner, J.J. Larsen, J.P. Shaffer, M. Schmitt, T. Schultz, J.G. Underwood, A. Stolow, J. Electron Spectrosc. and Relat. Phenom., 112 (2000) 183; A. Stolow, Annual Reviews of Physical Chemistry, 54 (2003) 89; A. Stolow, International Reviews in Physical Chemistry, 22 (2003) 377. [6] T. Kobayashi, K. Yokota, S. Nagakura, J. Electron. Spectrosc. And Relat. Phenom., 6 (1975) 167. For a correct band assignment, please refer to: N.E. Petrachenko, V.I. Vovna, G.G. Furin, J. Fluor. Chem., 63 (1993) 85. [7] N .Biwas, S. Umapathy, J. Chem. Phys., 107 (1997) 7849.