J. Phys. Chem. 1980, 84, 3031-3039
3031
Laser Flash Photolysis Study of Substituted Azobenzenes. Evidence for a Triplet State in Viscous Media H. Gorner, H. Gruen, and D. Schulte-Frohllnde" InstlYut fiir Strahlenchemle Im Max-Planck-Instifut fiir Kohlenforschung, 0-4330 Miilheim a.d. Ruhr, West Germany (Rec:eived: June 24, 1980)
Laser flash photolysis of nine substituted truns-4-nitro-4'-(dialkylamino)azobenzenes(e.g., 2-bromo-4,6-dinitro-4'-(dimethylamino)azobenzene, 6) in viscous solutions gives rise to a transient species with absorption maxima in the vicinity of 400 and 700 nm. The relative yield and the lifetime of the transient increase on decreasing the temperature in glycerol triacetate (GT) and 2-methyltetrahydrofuran(MTHF)due to increasing viscosity. The lifetime of the transient ranges from about 10 ns for 1-3 in several viscous solvents at room temperature (and from 10 to 100 ns for 4-9 at lower temperatured to several milliseconds for 1-9 in MTHF below -175 "C. Formation and decay of the transient exhibit viscosity dependences similar to those of the lowest trans triplet state of stilbenes. It is suggested that the tranisient of the azobenzenes be assigned to the lowest trans (?r,a*)triplet state. A second transient with a lifetime from microseconds up to hours was observed in the region of absorption of the truns-azobenzenes, For 5-9,4-nitro-4'-methoxy- (lo),and 4-nitroazobenzene (ll),and probably also for 1-4, which have an acetylamino substituent ortho to the azo group, this transient is amigned to the cis isomer. Introduction Studies of aromatic noncyclic azo compounds are remarkable for the dearth of evidence for transients generated by optical excitation and amenable to monitoring by spectroscopic meth0ds.l-I Fluorescence emission has been observed from several substituted aminoazobenzenes and those with bulky alkyl groups in ortho position^.^^ At low temperature the fluorescence intensity was reported as being low and no quantum yields were cited.I For trans-azobenzene in cyclohexane fluorescence lifetimes of 25 and 1 5 ps have been reported recently for S1 and Sz, r e s p e ~ t i v e l y .Phosphorescence ~~~ emission has not been detected so far.4 In contrast to the well-characterized triplets derived from various stilbenes,1°-15several thioindigo dyes,16J7and 91,10-diazaphenanthrene18"short-lived" species from truns- lor cis-azobenzenes with triplet charPossible causes for acter have not been revealed so the elusive nature of the searched-for triplet states of azobenzenes are discussed by Rau4 The role of triplet states in the cis-trans photoisomerization of azobenzene is a subject of discussion at present. The thermal1JSz1and catalyticz2cis trans isomerization as well as the cis 2=! trans photoisomerization under senand d i r e ~ t ~excitation * ~ ~ - ~conditions ~ of azobenzene and its derivatives in solution has been extensively studied. Different mechanisrhs for cis-trans isomerization such as twisting about the N=N double bond or inversion (by variation of the N=N-C angle or simultaneous variation of both N==N-C angles) have been disc w ~ e d Clarificatioil . ~ ~ ~ of~the~ mechanism ~ ~ ~ ~ of cis-trans ~ photoisomerization ia hampered by a lack of knowledge of the lifetime, energy, and electronic configuration of the triplet states. The work presented here deals with a laser flash photolysis study of a seriles of substituted trans-azobenzenes in viscous solutions. Evidence for the assignment of the observed transient to the lowest trans triplet state is presented; 3(7r,7r*) character for this state is shown to be plausible.
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Experimental Section Laser Flash Photol,ysis. Excitation was carried out by a ruby laser (A,, = 694 nm, pulse width 20 ns) as well as by a neodymium laser (A,, = 353 and 530 nm, pulse
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width 10 ns).loJ1 Transient absorption was monitored by using the light beam of a pulsed xenon lamp (XBO 150 W, pulse time 2 ms) set at right angles to the laser beam; for longer times (2 ms-1 s) the pulser was switched off. The absorption signal was recorded with a photomultiplier (Hamamatsu R 955) on a transient digitizer (Tektronix R 7912) after passage through a monochromator (Applied Photophysi~s).~~~~~ The concentration (10410-3 M) of the azobenzenes was adjusted such that approximately 90% of the laser light was absorbed in the monitored volume (1 X 1 X 10 mm3) of a standard fluorescencecell (l-cm analyzing pathlength). For low temperature measurements a Suprasil cell cooled by nitrogen was employed as described e1~ewhere.l~ The rate constant for (decayof the transient was obtained from the time-dependent change of the optical density by using a PDP 11computer. Except where indicated the samples were degassed by purging with argon. Absorption and Emission Measurements. Steady-state irradiation measurements were carried out by using a 1000-W mercury-xenon lamp and a monochromator (S~hoeffel).'~ The samples were degassed by purging with argon. Absorption spectra at low temperatures were recorded on a spectrophotometer (Bruckl, HRS 4001 C) and on a Cary Model 17 or 219 spectrophotometer a t room temperatbe and at -196 "C. Fluorescence spectra at -196 "C were recorded on a Spex-Flu~rolog.~~ Materials. Compounds 1-4 were of commercial origin (BASF, Ludwigshafen) and were purified by column chromatography on silica gel with toluene-ethanol as eluant followed by recrystallization of the key fraction from toluene. Compounds 5-9 were prepared by diazotization of the appropriate amine and coupling with dimethylaniline by standard methods, cf. ref 19 and 35. Purified specimens were obtained by column chromatography and recrystallization from toluene. Trans isomers of azobenzene, 4-nitroazobenzene (1l),4-nitro-4'-methoxyazobenzene (lo), and compound 8 were the same as in ref 19 and 35. The numbers 1-11 refer to substituted azobenzenes as listed in Table I. Homogeneity of the purified azobenzenes was checked by TLC, employing, for example, silica gel TLC plates with a mixture of toluene and acetone (1:l)as eluant. A difference of less than *5% for the molar extinction coeffiN
0022-3654/80/2084-3031$01.00/00 1980 American Chemical Society
3032 The Journal of Physlcal Chemlstry, Vol. 84, No. 23, 1980
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TABLE I : Substituted trans.4-Nitroazobenzenes
28
2L
-
Gorner et al.
O X I O - ~ (cm-'I
20
12
16 -1
a
11
I 2' 3'
"4
position of substitution compd 2 6 2' 4' 5' l b NO, Br NHCOCH, Ra OCH, 2b CN Br NHCOCH, R OCH, 3b NO, C1 NHCOCH, R OCH, 4b NO, H NHCOCH, R OCH, 5' NO, H H R H ' 6 NO, Br H N(CH3)2 7' Br H H N(CH,), H 8d NO, H H N(CH3)2 9 ' H H H N(CH,), H loe H H H OCH, H lle H H H H H a R represents the group (HOCH,CH,)N( CH,CH,CN). Samples were of commercial origin (BASF). Samples were synthesized. Sample was the same as in ref 35. e Samples were the same as in ref 19.
cient (E-) of the maximum in the visible absorption band was recorded on a Cary Model 17 spectrophotometer in solutions of di-n-butyl terephthalate (DNBT) for concentrations of azobenzenes of and M, using cells of 0.01- and 1-cm path lengths, respectively. Mass-spectral data were consistent with the assigned structures. Di-n-butyl terephthalate was prepared by esterification of terephthalic acid with an excess of 1-butanol in the presence of sulfuric acid. The reaction product was purified by vacuum distillation followed by vacuum fractionation, employing a Fischer HMS 500 concentric tube column. Purity as determined by GC analysis was better than 99.8%. The analysis was carried out on a 25-m Carbowax 20-M glass capillary column, 120-250 "C at 4 "C/min, carrier gas H2 (0.5 atm) on a Hewlett Packard 427 11. Poly(methy1 methacrylate) (PMMA) matrices were similar to those in previous work.14J7 1-Phenylethanol was zone refined (purity by GC > 99.8%); glycerol triacetate (GT, Fluka purum), 1,3-dibromobenzene (Fluka purum), and diphenyl (Eastman Kodak, 99.5%) were used as supplied. Diphenyl ether (Fluka) and o-terphenyl (Monsanto) were zone refined (purities by GC > 99.7%). Benzene, methylcyclohexane, toluene, isopentane, ethanol, and methanol were purified by fractional distillation; cyclohexane, diethyl ether, and 2-propanol were of Merck p.A. grade; acetonitrile (Merck) was either uvasol spectroscopic or p.A. grade and purified by column chromatography over activated neutral alumina; 2-methyltetrahydrofuran (MTHF, Merck) was purified by fractional distillation under argon in the presence of potassium (purity by GC > 99.8%). Acridine (Fluka) and anthracene (Merck) were zone refined; ferrocene (Merck) was recrystallized; azulene (Aldrich) and all-trans-retinyl acetate (Sigma) were used as supplied. Tetraphenylporphine (TPP, Fluka) and zinc tetraphenylporphine (ZnTPP, Fluka, purum) were purified by column chromatography; examination by TLC showed only one spot. N,N,N',N'-Tetramethyl-p-phenylenediamine (TMPD, Aldrich) was purified by zone refining; 7,7,8,8-tetracyanoquinodimethane (TCNQ, Aldrich, 98% ) and 1,4-diazabicyclo[2.2.2]octane(Dabco, Aldrich, 99%) were used as supplied.
Results Transient Absorption Spectra. Transient absorption
I
I
O'
LbO
560
600 h(nm1-
700
800
Figure 1. Transient absorption spectra Immediately after laser excitation = 530 nm) and ground-state absorption spectra in DNBT at room temperature: (a) 1, (b) 2, and (c) 8 (vertlcal scale In Figures 1-3 valM only for transient absorption).
(A,
for 1 was recorded at room temperature in di-n-butyl terephthalate (DNBT) in the wavelength region around 400 nm and between 660 and 2850 nm as shown in Figure la. Very similar transient absorption spectra were observed in DNBT and in glycerol triacetate (GT) when the wavelength of excitation (Aexc) was changed from 694 to 530 and to 353 nm; absorption maxima (A,,=) are given in Table 11. Similar transient absorption spectra were found for 2 and 3 in DNBT (Figure 1,b and c) and for several trans-azobenzenes in poly(methy1methacrylate) (PMMA, Table 11)whereas 4 in DNBT and GT gave rise to a weak transient absorption (700-800 nm). No transient absorption at 600-900 nm was observed for trans-azobenzene and 5-11 in GT and DNBT at room temperature. Laser excitation of 1 in several moderately viscous media yielded transient absorption spectra at X > 660 nm with a similar band envelope and ,A, nearly the same as in DNBT but with significant differences in optical density (Table 11). The transient observed between 680 and 720 nm will now be designated as the "700-nm transient". The 700-nm transient of 1 was formed simultaneously with the laser pulse and decayed within about 20 ns. However, in 2-propanol and a mixture of o-terphenyl and diphenyl ether a second transient was formed within the decay time of the first transient. The second transient was observed in the same wavelength region (700-800 nm) as the short-lived transient but its lifetime was longer than several hundred nanoseconds. On decreasing the temperature in GT below -40 "C and in 2-methyltetrahydrofuran (MTHF) below -150 "C transients were observed for 1-9 (Table 111). Transient absorption spectra of 1 and 8 in MTHF at -170 "C and of 4 and 7 in GT at -75 "C are shown in Figures 2 and 3, respectively. The absorption spectra are similar in band shape with A, between 680 and 720 nm. Under suitable
The Journal of Physical Chemistry, Vol. 84, No. 23, 1980 3033
Laser Flash Photolysis Study of Substituted Azobenzenes
TABLE 11: Absorption Maxima, Decay Rate Constant, and Yield of the Transient of Substituted truns-Azobenzenes in Various Solvents at Room Temperature kobsd,
x 10' ODb 400, 695 (760)c 353 0.3 3.8 400, 700 (750) 530 4.0 710 (760) 694 0.1 10 400, 695 (780) 530 17 o-terphenyl-diphenyl ether (1:1) 8 0.1 400, 700 (780) 530 9 1-phenylethanol 0.07 12 420,700 (800) 4.7 530 diphenyl-diphenyl ether (1:3) 9 700 353 5 GT 0.05 8 530 400,695 (770) >7 694 710 0.05 700-800 8 530 2-propanol 2.9 0.2 390,700, (780) 10 530 2.2 1,l$-dibromobenzene 2 12 0.01 700-800 0.6 530 MTHF
