-
7144
J . Phys. Chem. 1989, 93, 7144-7152
Trans Cis Photdsomerizatlon, Fluorescence, and Relaxation Phenomena of trans-4-Wltro-4’-(diakylamlno)stilbenes and Analogues with a Nonrotatable Amino Groupt Henry Gruen* and Helmut Gorner* Max-Planck-Institut fur Strahlenchemie, 0-4330 Mulheim an der Ruhr. Federal Republic of Germany (Received: January 9, 1989; In Final Form: April 26, 1989)
The steady-state fluorescence properties of five 4-nitro-4’-aminostilbenes (4-nitro-4’-aminostilbene (1); 4-nitro-4’-(dimethy1amino)stilbene(2); 4-nitro-4’-(diethylamino)stilbene (3); and 1-(4-nitrophenyl)-2-(2,3-dihydro-N-R-indol-5-yl)ethylene, R = H (4) and R = CH3 (5)) were studied in various solvents and as a function of temperature. The quantum yield of fluorescence for the trans isomer (Of), the Stokes shift, and the quantum yield of trans cis photoisomerization (aI,) depend , decreases strongly, markedly on the nature of the solvent. When the polarity is increased, the Stokes shift increases, O and aPr at first rises and then also decreases strongly. The five trans isomers show similar effects of solvent polarity and, generally, only small differences for a given solvent. This shows that the reduction in in solvents of moderate and higher polarity is not caused by deactivation of the first excited trans singlet state (It*) via rotation of the dialkylamino group since in 4 and 5 it cannot rotate whereas in 1-3 it can. Activation energies in m-xylene, toluene, and dioxane, obtained from a,(range: 7-1 3 kcal/mol) and from Of measurements (3-4 kcal/mol), differ significantly. It is concluded that the substantial decrease of a,, on going from nonpolar to slightly polar solvents is due (i) to relaxation to the trans ground state of a transoid excited state (A*) that is not identical with the solvent-relaxed spectroscopic It* state and (ii) to the presence of an energy barrier along the pathway of trans cis photoisomerization (following the step It* A*), which cannot be overcome in more polar solvents.
-
-
-.
Introduction It has recently been shown that the direct cis trans photoisomerization of 4-cyano-4‘-(dimethylamino)stilbene (CDMAS) occurs by twisting about the central double bond in singlet states.’ The quantum yields of fluorescence for the trans isomer, and of cis and cis trans photoisomerization (CPf, ,,PC and trans respectively) are substantial for CDMAS at room temperature and are little influenced by the polarity of the solvent.* Steady-statez and time-resolved’ measurements did not indicate the involvement of a fluorescing twisted intramolecular chargetransfer (TICT) state. On the other hand, relaxation of excited trans-CDMAS via a TICT state has recently been When the cyano group is replaced by the nitro group, CPt* decreases drastically with increasing solvent polarity.6-” trans-4-Nitro-4’-(dimethylamino)stilbene(trans-2), the nitro analogue of trans-CDMAS, has been the subject of numerous investigations.6-18 Over 30 years ago, Lippert and his group demonstrated that the large dipole moment of trans-2 in the excited singlet state gives rise to an extremely large Stokes red shift.I3-l5 For 4-nitro-4’-aminostilbene (1) and 2, Schulte-Frohlinde first proposed that the triplet state is involved in the trans cis photoisomerization.6 Fischer and co-workers have added to the knowledge about 2 by investigating the effect of temperature on ,,a , and A mixed singlet-triplet mechanism for the trans cis photoisomerization has been advanced for 2 and 4-nitro-4’-(diethylamino)stilbene(3) in nonpolar solventsI0 and a ”double activated mechanism” for 2 in toluene solution]’ (see also Discussion). In order to account for the small at+ values of 2 in polar solvents, it has been proposed that internal conversion not involving twisting about the central double bond should be ~ignificant.~ The question arises as to the origin of this internal conversion step. One conceivable possibility would be a rotation of the dimethylamino group into a highly twisted conformation (perpendicular relative to the phenyl ring). This work aims at illuminating the effect of solvent polarity on the deactivation channels of the excited trans singlet state (It*) of push-pull compounds like 4-nitro-4’-(dialkylamino)stiIbenes. Since nothing is known about Of of 1 and 3, and only little about
CHART I
*
-
-
1,R=H 2, R = CH3 3, R = CzHj
02N%NR / 4,R=H 5, R = CH3
6
-
-
Current address: Goethestrasse 4, D-4030 Ratingen, Federal Republic of Germany. Part 16: Cis-Trans Isomerization of Nitrostilbenes. Part 15: Reference 10.
0022-3654/89/2093-7144$01 SO/O
CDMAS the effect of environment on CPf of a systematic study appeared to us to be desirable. For comparison and in order to probe for (1) Gamer, H. J. Photochem. 1980, 13, 269. (2) Gruen, H.; Gorner, H. 2. Naturforsch., A: Phys., Phys. Chem., Kosmophys. 1983, 38, 928. (3) Safarzadeh-Amiri, A. Chem. Phys. Lett. 1986, 125, 272. (4) Gilabert, E.; Lapouyade, R.; Rulliere, C. Chem. Phys. Lett. 1988, 145, 262. (5) Rettig, W.; Majenz, W. Chem. Phys. Lett. 1989, 154, 335. ( 6 ) Schulte-Frohlinde, D.; Blume, H.; Giisten, H. J . Phys. Chem. 1962, 66, 2486. (7) Gegiou, D.; Muszkat, K. A,; Fischer, E. J . Am. Chem. Soc. 1968, 90, 3907. (8) Gorner, H.; Schulte-Frohlinde, D. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1102. (9) Gomer, H.; Schulte-Frohlinde, D. J. Photochem. 1978, 8, 91. (IO) Gorner, H. J . Photochem. Photobiol., A 1987, 40, 325. (1 1) Gorner, H.; Schulte-Frohlinde, D. J. Mol. Struct. 1982, 84, 227. (12) (a) Bent, D. V.; Schulte-Frohlinde, D. J . Phys. Chem. 1974, 78,451. (b) Schulte-Frohlinde, D.; Gorner, H. Pure Appl. Chem. 1979, 51, 279.
0 1989 American Chemical Society
Trans
-
Cis Phenomena of trans-4-Nitro-4'-aminostilbenes
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7145
possible involvement of a TICT analogues of trans-2, in which the alkylamino component is part of a "stiff" dihydroindole ring (trans-4 and trans-5), were synthesized and investigated. The Stokes shift and values for afand at- are presented for trans-1-trans-5 in various solvents and as a function of temperature. The character of the absorption and emission spectra, the magnitude of af,and its temperature dependence give no evidence for the involvement of an additional fluorescing state (e.g., a TICT state) for the three stilbenes and the two "stiffened" compounds. The effect of solvent polarity on the deactivation pathways of excited trans-4-nitro-4'-(dialkylamino)stilbenes will be discussed.
Experimental Section Apparatus and Procedure. The corrected emission spectra were recorded on a Spex Fluorolog that was equipped with a photoncounting detection system using a photomultiplier (RCA C3 1034) and a unit that corrects for the spectral sensitivity of the photomultiplier and the lamp.21 afvalues were determined with either 9,lO-diphenylanthracene (af= 0.9) or rhodamine 101 (@f = 1.0) both in argon-saturated ethanol at 24 OC as standardsZZand the typically 0.2-0.4. Corrections were made same absorbance at bXc, for the temperature dependence of the extinction coefficient. A correction of af for the refractive index of the solvents did not seem necessary in view of the magnitude of this effect relative to that of the change in solvent polarity. In nonpolar solvents where trans cis photoisomerization could contribute, the samples were irradiated for the shortest periods possible (the conversion was less than 10%). The experimental error of small afvalues is typically f30% and &IO% for Of1 0.2. The effect of oxygen on af was measured by monitoring the intensity (If) on a Perkin-Elmer Model LS-5 spectrofluorometer and repeated purging with argon and oxygen; it was found that If(02)/If(Ar) equals ado,)/@dAr). The absorption spectra were recorded on a spectrophotometer (Perkin-Elmer, Model 554). For at- measurements, a 1000-W high-pressure xenon-mercury lamp and a monochromator (Schoeffel; bandwidth 12 nm) were used. The incident light intensity was measured either by the Aberchrome 540 actinom, for 4-nitrostilbene in benzene.24 eterZ3or by comparison of ,,a All solutions (typical concentration, 40 pM) were purged with argon, and the procedure for determination of at, values was the same as described elsewhere:',2J0 error f20% for a,, 2 0.1 and &40% for the smaller values. In general, the samples were freshly dissolved and handled under red light. Materials. Trans isomers of 1 and 2 were prepared and purified as previously reported: and trans-3 was prepared by an analogous method. (If not indicated otherwise, 1-5 denote the trans isomers.) The precursors of 4 and 5, 5-formylindoline and l-methyl-5formylindoline, respectively, were synthesized as follows. The latter compound was prepared by a Vilsmaier-formylation reaction; a known procedure25was followed, but a lower ratio of phosphorus
-
( 1 3) (a) Lippert, E.; Moll, F. Z . Elektrochem. 1954,58, 718. (b) Lippert, E. Z . Naturforsch., A : Astrophys., Phys. Phys. Chem. 1955, 10, 541. (c) Lippert, E. Z . Elektrochem. 1957, 61, 962. (14) Lippert, E.; Liider, W.; Moll, F. Spectrochim. Acta 1959, 10, 8 5 8 . ( I S ) Lippert, E.; Luder, W.; Moll, F.; Nagele, W.; Boos, H.; Prigge, H.; Seibold-Blankenstein, I. Angew. Chem. 1961, 73, 695. (16) Baumann, W. Z . Naturforsch., A : Phys., Phys. Chem., Kosmophys. 1981, 36, 868. (17) (a) Warman, J. M.; De Haas, M. P.; Hummel, A,; Varma, C. A. G. 0.;van Zeyl, P. H. M. Chem. Phys. Lett. 1982,87, 83. (b) De Haas, M. P.; Warman, J . M . Chem. Phys. 1982, 73, 3 5 . (18) Kobayashi, T.; Ohtani, H.; Kurokawa, K. Chem. Phys. Lett. 1985, 121, 356. (19) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. Nouv. J . Chim. 1979, 3, 443. (20) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971, and references cited therein. (21) Holzwarth, A. R.; Lehner, H.; Braslavsky, S . E.; Schaffner, K. Justus Liebigs Ann. Chem. 1978, 2002. (22) (a) Karstens, T.; Kobs, K. J . Phys. Chem. 1980, 84, 1871. (b) Heinrich, G.; Schoof, S.; Gusten, H. J . Phorochem. 1974/75, 3, 315. (23) Heller, H. G.; Lagan, J. R. J . Chem. Soc.,Perkin Trans. 2 1981,341. (24) Gorner, H. Br. Bunsen-Ges. Phys. Chem. 1984, 88, 1199.
: ;
0
'
30
I
25
...
I/ 20
I
15
t - V ~ ~ Icm") O - ~ Figure I . Absorption, fluorescence excitation, and corresponding emission spectra (dotted, dashed, and full lines, respectively) of trans-1 (a) in toluene and (b) in n-pentane and emission of trans-1 (c) in MTHF. Conditions: 24 O C * xexc = 366 n m ~ as in I.
oxychloride (0.15 M) to 1-methylindoline (0.135 M) was employed. The reaction product, containing the mono- and diformylindolines as well as other unknown compounds, was purified by column chromatography on basic alumina with a mixture of cyclohexane-ethyl acetate (7:3) as eluent. The first fractions were recrystallized from a mixture of cyclohexane-isooctane (20:l) to yield almost colorless fine needles (mp 37-39 OC; purity by G C analysis 99.5%). TLC on silica gel with the former solvent mixture showed a single yellow band. Absence of diformylindoline was indicated by comparison with a fraction isolated in the chromatography and shown to be a dialdehyde.26 5-Formylindoline was obtained by a Fries rearrangement of 1-formylindoline in the presence of phosphorus oxychloride as described for homologous compounds.2s The M S and N M R data, as well as IR and UV absorption spectra, are consistent with the assigned structures. Compounds 4 and 5 were prepared by the method of Pfeiffer26 but in chlorobenzene as solvent. 1-Methyl-5-formylindoline (3 g), 4-nitrophenylacetic acid (3.5 g), piperidine (1 mL, dried over CaCI2), and chlorobenzene (15 mL) were heated to 136-140 OC, maintained at this temperature for 1-2 h, and then allowed to crystallize. The precipitate was filtered off, washed with chlorobenzene followed by cyclohexane, and subsequently dried. A second fraction could be isolated on prolonged standing (yield of the combined fractions, 40%). The reaction product was recrystallized several times from ethanol. The clusters of rectangular crystals rearranged to needles at 172-176 OC and melted at 180-181 OC (uncorrected) (purity by G C analysis 98%). TLC on silica gel with cyclohexane-ethyl acetate (7:3) showed only one spot. Compound 4 was synthesized in an analogous fashion (35% yield). The recrystallized product also showed a solid-state structural change between 174 and 176 "C and melted at 182-184 "C. The structures assigned to compounds 4 and 5 were confirmed by MS, NMR, IR, and UV-vis spectra. The integrity of the indoline ring in 4 and 5 is supported by the reaction with 2,3dichloro-5,6-dicyanobenzoquinone as oxidant. Facile hydrogen transfer occurs in dioxane with formation of the corresponding (25) (a) Terent'ev, A. P.; Ban-Lun, G.; Preobrazhenskaya, M. N. J . Gen. Chem. USSR (Engl. Transl.) 1962, 32, 1311. (b) German Patent No. 645880, 1937. (26) Pfeiffer, P. Ber. Dfsch. Chem. Ges. 1915, 48, 1796.
7146 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Gruen and Garner
TABLE I: Maxima of Absorption and Fluorescence Spectra'
A,b nm
A,, nm
solvent n-pentane'
1
2
370
cyclohexaneC
carbon tetrachloride m-xylene di-n-butyl ether toluene benzene diethyl ether dioxane MTHF TH F acetic acid ethyl ester chloroform dichloromethane acetone DMF acetonitrile ethanol
3
4
465 490 520 sh
470 502 532
470 493 515
5 485 515 545 sh
445 468 500 sh
470 502 535 sh
478 509 545
475 504 540
495 525 560
514 544 556 554
548 570 562 583
556 580
559 590 585 600
578 605 590 619
558 590 598 633 [5821 647 648 682 694 740 830 >850 >850 [5901
596 602 620 655 ~5861 670 674 735 770 800 850 >850 >850 [5901
595 602 62 1 658 [6021 670 680 785 820 830 >830 >850 >850 [bo51
4
5
1
41 1
3 418
400
410
440 460 490 sh
380
(412) 417
(420) 427
(403) 408
(413)d 420
390 398 398 399 (402) 398 402 404 410
(418) 423 428 428 428 (429) 429 422 424 425
429 434
(408) 412 419
(419)d 426 430 432 433 (435)d 437 428 432 436
41 1 406 399 400 410 412 406 404
426 424 434 435 433 440 435 428
438 439 430 434 438 442 434 447 447 44 1 440
426 (424) 424 418 422 430
423
432
435 428 438 439 433 445 432 426
2
589
607 635 630 677 ~ 9 696 702 770
627 640 660 695 1 [620Ic 725 720 770 850 >850 >850 >850 >850 [620Ie
'In argon-saturated solutions at 24 OC. No deviations were found in the presence of air. bCorrected fluorescence emission maximum: A, = 366 nm. No difference was found in nonpolar and slightly polar solvents when excited within f 1 0 nm of A, [for observation of a second emission for 4 and 5 (E,(30) > 36 kcal/mol), see text]. cData for 1-3 are taken from ref 10. dValues in parentheses refer to the corrected fluorescence excitation maximum. CValuesin brackets refer to -196 OC. 2,3-dichloro-5,6-dicyanohydroquinoneand indoles, e.g., 1-(4nitrophenyl)-2-(5-N-methylindolyl)ethylene (6).Additional evidence for the presence of an N H group in 4 is provided by ready formation of an acetyl derivative with a strongly reduced fluorescence. 9,lO-Diphenylanthracene (EGA) and rhodamine 101 (Lambda Physik) were used as received. Solvents (Merck or Aldrich) were spectrograde, e.g., glycerol triacetate (GT), Uvasol (acetone, acetonitrile), and Gold Label [benzene, dimethylformamide (DMF)], purified by distillation [methylcyclohexane (MCH), toluene, 2-methyltetrahydrofuran (MTHF; free of peroxides), m-xylene], or purified by chromatography [tetrahydrofuran (THF), chloroform] and checked for absorbing or fluorescing impurities.
Results Absorption and Fluorescence Spectra. Absorption, fluorescence excitation, and emission spectra for trans-1 and trans-5 in npentane and toluene solutions at room temperature are shown in Figures 1 and 2. The corrected fluorescence excitation spectrum and the absorption spectrum of 5 coincide in n-pentane and dearly so for 1 and 5 in toluene. Similar results were found for 2-4 in these solvents. The fluorescence spectra show some structure only in nonpolarizable solvents, e.g., cyclohexane and n-pentane, and they are red-shifted on going t o toluene and 2-methyltetrahydrofuran (MTHF). The maxima of the absorption and fluorescence spectra (X, and AI, respectively) are listed in Table I. The pattern of a red shift of A, for trans-2 and an even larger Stokes shift on going to more polar (or polarizable) solvents is in good agreement with previous data from Lippert et al.I3-l5 Generally, the spectra in a given solvent are similar for the five trans isomers (Table I). Values for X, increase slightly and those for XF moderately in the sequence 1 , 2, and 3. A small red shift of X, on going from 1 to 4 and from 2 to 5 is in accord with enhanced *-electron delocalization in the ground and excited singlet states of the compound with nonrotatable dialkylamino groups ( 4 and 5 ) . The dependence of the Stokes shift (t, - tf) on the solvent polarity is illustrated in Figure 3 for 1, 2, and 5 . When the
U
-
25
20
15
~ ~ 1 0 - ~ ( ~ ~ - ~ ) Figure 2. Absorption, fluorescence excitation, and corresponding emission spectra (dotted, dashed, and full lines, respectively) of trans-5 (a) in toluene and (b) in n-pentane and emission of trans-1 (c) in MTHF. Conditions: 24 ' C , A,, = 366 nm, AI as in Table I.
empirical Dimroth parameter [ E d 3 0 ) ] used , as a measure for the solvent polarity,2' was increased, t, - tfincreases almost linearly for the five compounds in the range between 30 and approximately 44 kcal/mol. The plot may extend even above E d 3 0 ) = 44 kcal/mol (tfvalues smaller than 11 800 cm-' could not be mea(27) (a) Reichardt, C. Angew. Chem., Int. Ed. Engl. 1979, 18, 98. (b) Cowley, D. J.; Healy, P. J. J. Chem. SOC.,Perkin Trans. 2 1979, 484.
Trans
-
Cis Phenomena of trans-4-Nitro-4’-aminostilbenes
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7147 tt ( O C I
16
1L
-
12
6
L
12
8
1031~1~-11 Figure 4. Temperature dependence of the fluorescence maximum of trans-1 (squares), trans-2 (circles), and trans-5 (triangles) in MTHF: A,, = 366 nm.
30
35
LO
L5
ET(301 I k c a l l m o l l Figure 3. Stokes shift plotted vs the Ed30) polarity parameter for trans-1 (squares), rrans-2 (circles), and t r a n s 4 (triangles) in solution at 24 OC. For rrans-CDMAS (dashed line) see ref 2.
TABLE II: Quantum Yield of Fluorescencea ETW, solvent
n-pentane cyclohexaneb
kcal/mol 31.2
@f
1
2
0.004 0.002 0.002 0.2
0.14 0.33 0.30 0.4
3 0.28 0.34 0.36 0.5
4
0.12 0.20 0.3 0.4
S
0.38 0.40 0.2 0.5
sured under our conditions). Despite some scatter, the slopes of MCH these dependences are similar. Data in Figure 3 show a trend 32.5 carbon tetrachloride to a larger Stokes shift for 5 as compared to that of 2. Inter0.50 0.55 0.55 0.53 m - x y I en e 33.3 0.37 values for 1 are essentially compensated by estingly, the larger If 0.52 0.52 0.54 33.4 0.36 0.54 di-n-butyl ether larger I , values. For comparison, the Stokes shift is also shown 0.45 0.40 33.9 0.42 0.53 0.50 toluene’ for the cyano analogue trans-CDMAS;2 this slope is markedly 0.46 0.40 34.5 0.48 0.53 0.54 benzene‘ smaller than for the five nitrostilbenes. 0.33 0.45 34.6 0.47 0.55 0.60 diethyl ether The fluorescence spectra of 1-3 in toluene and MTHF solutions 0.25 0.35 36.0 0.38 0.32 0.55 dioxane 0.08 0.12 36.5 0.22 0.15 0.22 MTHFd are independent of the wavelength of excitation (e.g., &, between 0.04 0.05 37.4 0.17 0.11 0.15 THF 366 and 490 nm for 2). For 4 and 5 in MTHF, only one 0.03 0.03 0.06 0.11 38.1 0.15 acetic acid ethyl fluorescence band was found on excitation at the long-wavelength ester part (A,, = 490 nm). However, on excitation at shorter wave0.018 0.023 0.015 39.1 0.05 chloroform lengths (A,, between 313 and 436 nm), a second 10-fold weaker dichloromethane 41.1 0.038 0.008 0.014
