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Picosecond spectroscopy of photoisomerization in arylethylenes and thioindigo dyes
261
JoumalofMolecularStructure,1l4(l984~ 261-264 Elsevier Science Publishers B.V., Amsterdam -
PICOSECOND
SPECTROSCOPY
AMI
THIOINDIGO
S.A.
KRYSANOV
OF
Printed in The Netherlands
PHOTOISOhIERIZATION
IX
ARYLETHXLEMES
DYES
and
?dd,V.
ALFIMOV
Institute of Chemical Physics, Academy of Sciences of the USSR, 142432 Chernogolovka (USSR)
ABSTRACT
The excited states decay channels of the molecules suffering the cis - trans photoisomerization have been investigated using picosecond absorption spectroscopy. Transient absorption spectra of arylethylenes (AE) (molecular solution and the super-cooled liquid microspheres) and thioindigo dyes (TD) (solution) were assigned. The singlet mechanism for AE and the triplet mechanism for TD isomerization were proposed. The cis isomers kinetic behavior was interpreted in terns of the Plotnikov - Mayer mechanism of the radiationless processes induced by the photochemical reactions. INTRODUCTION The modern molecular spectroscopy tends to study the photochemically unstable molecular systems. This interest is due to the development of the picosecond laser spectroscopy which provides the direct information about the spectra and kinetics of the short-lived excited states. The first quantum-mechanical interpretation in 1932 (ref.1) of the photochemical cis-trans isomerization has stimulated the experimental efforts during the subsequent five decades and the pulse methods application has only recently revealed the photoprocesses in stilbene (ref.2) and some derivatives (ref.3). The elucidation of the photoisomerization mechanism in dependence of the molecular structure is of importance at present. Picosecond absorption spectroscopy is applied in the present work to assign the excited states decay channels involved in the direct photoisomerization of naphthylethylenes and perinaphthothioindigo dye. The main attention is paid to the short-lived excited states of the cis isomers which are the least studied. The heat explosion of the microemulsion drops under the picosecond light pulses is proposed for such studies.
OOZZ-2360/34/$03.00 0 1984 EZsevier SciencePublishersB.V.
262
METHODS The apparatus used to measure picosecond transient absorbance spectra is described in detail elsewhere (refs,4,5). Briefly, it includes a mode&locked Xd:phosphate glass laser and amplifier system to produce single.6 ps TEZJoo pulses at 1055 am. The harmonics 528 and‘352 nm were used as an excitation pulses. The time scanning of the probing picosecond continuum generated in D20 by the 1055 nm pulse was accomplished by the.double-beam mirrors system (ref.5) which permits us to record the sample absorbance changes A in the large time interval 5 ps - 5 ns for one laser shot, RESULTS AND DISCUSSION The arylethylene trans isomers were investigated in our previous work (ref.5), Briefly, S, - S, trai ,itions in the trans isomers of I-phenyl-2-(1-naphthyl)ethylene (Ph-7N) and l-phenyl-2(2-naphthyl)etbylene (Ph-2N) peaked at 510 nm were assigned by picosecond absorption spectroscopy. The S, state lifetimes of both conformers (refs.6,7) of Ph-2N were shown to depend upon solvent viscosity, demonstrating the singlet mechanism of naphthylethylenes photoisomerization and their stilbene-like behavior (ref.2). No transient absorption could be detected with the cis isomers in hexane solutions within the 400-650 nm range at any delay time that is in agreement with the stilbene data (refe,8,9). We have
_a_-__,-
Time,
M
Fig. 1. The amorphous microspheres of cis-1,2-di-(l-naphthyl)eth.ylene in water in 1 mm cell at 298K. Absorbance kixetics at 440 and 600 nm after excitation-with a 352 nm picosecond pulse. Fig. 2. Potential energy diagram of the arylethylenes states as a function of rotation angle and dynamics c,f the cis S, state decay.
263
used the water microemulsion in the study of the cis photoprocesses. The microemulsion was prepared by the addition drop by drop the ethanolic solution of cis-1,2-di-(1-naphthyl)ethylene (IN-la) (1o-2 M) to water. This method ensures the formation of the spherical lN-1N particles of the 10-4-10-5 cm size (ref.10). Excitation of a sample with the 352 nm pulse resulted in the strong transient absorption throughout the visible which is decreased with the wavelength increasing. The kinetic curve (Fig. 1) has the induction time of about 400 ps. We ascribe the transient absorption to the increased scattering of the probing light by the growed IN-1N particles as a result OS an evaporation (the heat explosion) after the high-power laser pulse absorption. The dynamics of the electronic energy transformation into the heat is shown in Fig. 2. The energy gap in the cis isomer is of 30000 cm-l and the direct (0=180°) internal conversion can be neglected (Ermolaev-Sveshnikova rule, refs.11, 12). The internal conversion rate is greatly increased at the intermediate angles (Plotnikov-Eager mechanism, ref,13). The rotation is restricted in the microemulsion particles that results in the delay (induction) between the excitation and heating instants. 2.0
-I 1.2
0
aA 0.8
0-D.
400
500
600
700 Wavelength,
800 nm
Fig. 3. Dye PNT in chloroform et 293K. Left-hand scale: groundstate absorption spectra of the pure trans and cls isomers. Righthand scale: transient absorbance spectra of trens isomer measured at 10 ps delay (curve 1) and 5 ns delay (curve 2) and of cis isomer measured at 400 ps delay (curve 3) after excitation with a 528 nm picosecond pulse.
264
Perinaphtho-thioindigoid dyes (PIiT) having a large difference in the trans and cis isomer absorption spectra in the visible are one of the most appropriate compounds for the reversible photoisomerization study.. The transient spectra observed are shown in Fig. 3. An excitation of the trana isomer with a 6 ps, 528 nm pulse to the singlet state S, (the peak of Sn-S,) absorption at 700 nm, curve 1) results in the intersystem crossing (the peak of Tn-Tl absorption at 730 nm, curve 2) with the quantum yield of 0.7 and rate constant 1.5-109 s-1. An excitation of the cis isomer results also in the intersystem Crossing (the peak of T -T absorption at 670 nm, nlols-1 . We ascribe the large curve 3) with the rate constant 4.10 difference between these rates to the barrierless rotation in the cis S, state. The non-planar structure increases drastically the spin-orbit coupling (ref.12). This picture of the preferential cis intersystem crossing differs somewhat from that of model of Memming et, al. (ref.14) where the possibility of the large rotation angles in the cis S,state is assumed. ACKROWLEDGEXlERT We gratefully acmowledge stimulating discussions.
Dr. Vladimir F. Razumov for
REFERENCES 1 2 3 4 5 6 7
R.S. Mulliken, Phys, Rev,, 41 (1.932) 751-758. R.M. Hochstresser, Pure Apgl. Chem., 5:! (1980) 2683-2691, D. Schulte-Frohlinde, H. Garner, Pure 4pp1, Chem., 51 (1979) 279-297. S.A. Krysanov, M-V. Alfimov, Chem. Phys. Lett., 76 (1980) 221-224. Sj;.1;8ysanov, M.V. Alfimov, Chem, Phys. Lett., 98 (1983)
Y&B. Sheck, N.P. Kovalenko, M.V. Alfimov, J. Luminescence, ;5 ;;2797) 157-168. G. Fischer, E. Fischer, J. Phys. Chem., 82 (1978) 1238-1643. 8 M. Sumitsni, N. Nakashima, K. Yoshihara, Chem. Phys. Lett-, 68 (1979) 255-258. 9 B.I. Greene, R.M. Hochstrasser, R.B. Weisman, Chem. Phys., 48 (1980) 289-298. IO V.F. Razumov, personal communication. 11 V.L. Ennolaev, E.B. Sveshnikova, Opt. i Spectr, 16 (1964) 587. 12.V.G. Plotnikov, Uspekhi khimii, 49 (1980) 327-361. 13 V-G.-Plotnikov, G.V. Xayer, Opt. i Spectr,, 47 (1979) 113-120. 14 R. Menzning; K, Kobs, Ber. Bunsenges, Phys. Chem., 85 (1981) 238-242.
JoumalofMolecularStructure,1l4(l984~ 261-264 Elsevier Science Publishers B.V., Amsterdam -
PICOSECOND
SPECTROSCOPY
AMI
THIOINDIGO
S.A.
KRYSANOV
OF
Printed in The Netherlands
PHOTOISOhIERIZATION
IX
ARYLETHXLEMES
DYES
and
?dd,V.
ALFIMOV
Institute of Chemical Physics, Academy of Sciences of the USSR, 142432 Chernogolovka (USSR)
ABSTRACT
The excited states decay channels of the molecules suffering the cis - trans photoisomerization have been investigated using picosecond absorption spectroscopy. Transient absorption spectra of arylethylenes (AE) (molecular solution and the super-cooled liquid microspheres) and thioindigo dyes (TD) (solution) were assigned. The singlet mechanism for AE and the triplet mechanism for TD isomerization were proposed. The cis isomers kinetic behavior was interpreted in terns of the Plotnikov - Mayer mechanism of the radiationless processes induced by the photochemical reactions. INTRODUCTION The modern molecular spectroscopy tends to study the photochemically unstable molecular systems. This interest is due to the development of the picosecond laser spectroscopy which provides the direct information about the spectra and kinetics of the short-lived excited states. The first quantum-mechanical interpretation in 1932 (ref.1) of the photochemical cis-trans isomerization has stimulated the experimental efforts during the subsequent five decades and the pulse methods application has only recently revealed the photoprocesses in stilbene (ref.2) and some derivatives (ref.3). The elucidation of the photoisomerization mechanism in dependence of the molecular structure is of importance at present. Picosecond absorption spectroscopy is applied in the present work to assign the excited states decay channels involved in the direct photoisomerization of naphthylethylenes and perinaphthothioindigo dye. The main attention is paid to the short-lived excited states of the cis isomers which are the least studied. The heat explosion of the microemulsion drops under the picosecond light pulses is proposed for such studies.
OOZZ-2360/34/$03.00 0 1984 EZsevier SciencePublishersB.V.
262
METHODS The apparatus used to measure picosecond transient absorbance spectra is described in detail elsewhere (refs,4,5). Briefly, it includes a mode&locked Xd:phosphate glass laser and amplifier system to produce single.6 ps TEZJoo pulses at 1055 am. The harmonics 528 and‘352 nm were used as an excitation pulses. The time scanning of the probing picosecond continuum generated in D20 by the 1055 nm pulse was accomplished by the.double-beam mirrors system (ref.5) which permits us to record the sample absorbance changes A in the large time interval 5 ps - 5 ns for one laser shot, RESULTS AND DISCUSSION The arylethylene trans isomers were investigated in our previous work (ref.5), Briefly, S, - S, trai ,itions in the trans isomers of I-phenyl-2-(1-naphthyl)ethylene (Ph-7N) and l-phenyl-2(2-naphthyl)etbylene (Ph-2N) peaked at 510 nm were assigned by picosecond absorption spectroscopy. The S, state lifetimes of both conformers (refs.6,7) of Ph-2N were shown to depend upon solvent viscosity, demonstrating the singlet mechanism of naphthylethylenes photoisomerization and their stilbene-like behavior (ref.2). No transient absorption could be detected with the cis isomers in hexane solutions within the 400-650 nm range at any delay time that is in agreement with the stilbene data (refe,8,9). We have
_a_-__,-
Time,
M
Fig. 1. The amorphous microspheres of cis-1,2-di-(l-naphthyl)eth.ylene in water in 1 mm cell at 298K. Absorbance kixetics at 440 and 600 nm after excitation-with a 352 nm picosecond pulse. Fig. 2. Potential energy diagram of the arylethylenes states as a function of rotation angle and dynamics c,f the cis S, state decay.
263
used the water microemulsion in the study of the cis photoprocesses. The microemulsion was prepared by the addition drop by drop the ethanolic solution of cis-1,2-di-(1-naphthyl)ethylene (IN-la) (1o-2 M) to water. This method ensures the formation of the spherical lN-1N particles of the 10-4-10-5 cm size (ref.10). Excitation of a sample with the 352 nm pulse resulted in the strong transient absorption throughout the visible which is decreased with the wavelength increasing. The kinetic curve (Fig. 1) has the induction time of about 400 ps. We ascribe the transient absorption to the increased scattering of the probing light by the growed IN-1N particles as a result OS an evaporation (the heat explosion) after the high-power laser pulse absorption. The dynamics of the electronic energy transformation into the heat is shown in Fig. 2. The energy gap in the cis isomer is of 30000 cm-l and the direct (0=180°) internal conversion can be neglected (Ermolaev-Sveshnikova rule, refs.11, 12). The internal conversion rate is greatly increased at the intermediate angles (Plotnikov-Eager mechanism, ref,13). The rotation is restricted in the microemulsion particles that results in the delay (induction) between the excitation and heating instants. 2.0
-I 1.2
0
aA 0.8
0-D.
400
500
600
700 Wavelength,
800 nm
Fig. 3. Dye PNT in chloroform et 293K. Left-hand scale: groundstate absorption spectra of the pure trans and cls isomers. Righthand scale: transient absorbance spectra of trens isomer measured at 10 ps delay (curve 1) and 5 ns delay (curve 2) and of cis isomer measured at 400 ps delay (curve 3) after excitation with a 528 nm picosecond pulse.
264
Perinaphtho-thioindigoid dyes (PIiT) having a large difference in the trans and cis isomer absorption spectra in the visible are one of the most appropriate compounds for the reversible photoisomerization study.. The transient spectra observed are shown in Fig. 3. An excitation of the trana isomer with a 6 ps, 528 nm pulse to the singlet state S, (the peak of Sn-S,) absorption at 700 nm, curve 1) results in the intersystem crossing (the peak of Tn-Tl absorption at 730 nm, curve 2) with the quantum yield of 0.7 and rate constant 1.5-109 s-1. An excitation of the cis isomer results also in the intersystem Crossing (the peak of T -T absorption at 670 nm, nlols-1 . We ascribe the large curve 3) with the rate constant 4.10 difference between these rates to the barrierless rotation in the cis S, state. The non-planar structure increases drastically the spin-orbit coupling (ref.12). This picture of the preferential cis intersystem crossing differs somewhat from that of model of Memming et, al. (ref.14) where the possibility of the large rotation angles in the cis S,state is assumed. ACKROWLEDGEXlERT We gratefully acmowledge stimulating discussions.
Dr. Vladimir F. Razumov for
REFERENCES 1 2 3 4 5 6 7
R.S. Mulliken, Phys, Rev,, 41 (1.932) 751-758. R.M. Hochstresser, Pure Apgl. Chem., 5:! (1980) 2683-2691, D. Schulte-Frohlinde, H. Garner, Pure 4pp1, Chem., 51 (1979) 279-297. S.A. Krysanov, M-V. Alfimov, Chem. Phys. Lett., 76 (1980) 221-224. Sj;.1;8ysanov, M.V. Alfimov, Chem, Phys. Lett., 98 (1983)
Y&B. Sheck, N.P. Kovalenko, M.V. Alfimov, J. Luminescence, ;5 ;;2797) 157-168. G. Fischer, E. Fischer, J. Phys. Chem., 82 (1978) 1238-1643. 8 M. Sumitsni, N. Nakashima, K. Yoshihara, Chem. Phys. Lett-, 68 (1979) 255-258. 9 B.I. Greene, R.M. Hochstrasser, R.B. Weisman, Chem. Phys., 48 (1980) 289-298. IO V.F. Razumov, personal communication. 11 V.L. Ennolaev, E.B. Sveshnikova, Opt. i Spectr, 16 (1964) 587. 12.V.G. Plotnikov, Uspekhi khimii, 49 (1980) 327-361. 13 V-G.-Plotnikov, G.V. Xayer, Opt. i Spectr,, 47 (1979) 113-120. 14 R. Menzning; K, Kobs, Ber. Bunsenges, Phys. Chem., 85 (1981) 238-242.