2028
J. Phys. Chem. 1982, 86,2028-2035
Triplet States of Phenylethylenes in Solution. Energies, Lifetimes, and Absorption Spectra of 1,I-Dlphenyl-, Triphenyl-, and Tetraphenylethylene Triplets Helmut Gorner Max-Planck-Institof fik Strahlenchemie, ~7-4330 Mulheim a d . Ruhr, West Germany (Recelved: September 4, 1981; I n Final Form: December 7, 198 I )
Excitation of a number of high-energy triplet donors by laser flash photolysis in solution in the presence of 1,l-diphenyl-,triphenyl-, and tetraphenylethylenes led to the appearance of transients absorbing in the 360400-nm region (lifetimes ( T ~ ) :30-200 ns at 25 "C). These transients were assigned to the lowest triplet states of the phenylethylenes. Triplet energies ranging between 50 and 60 kcal/mol were estimated from the rate constants for energy transfer using various triplet donors. The effects of phenyl substitution of the ethylene molecule on T~ and on the rate constant for oxygen quenching of the observed triplet indicate the existence of a rapidly established confiiational equilibrium between the planar and perpendicular tripleb and deactivation to the ground state via the perpendicular triplet, similar to the case of stilbene. The low yields for intersystem crossing and the temperature dependence of the fluorescence quantum yields suggest that the radiationless deactivation of the excited singlet state occurs predominantly by internal conversion involving twisting about the central double bond.
Introduction cis-Stilbene and other sterically hindered phenylethylenes exhibit substantial effects of temperature and viscosity on their fluorescence proper tie^.'-^ Twisting about the C=C double bond and out-of-plane bending motions give rise to radiationless transitions in competition with fluorescen~e.~~~J Whether the twisting process occurs in excited singlet or triplet states depends strongly on the substitution pattern of the ethylene molecule.g14 It is well documented that the cis trans photoisomerization of stilbene occurs by twisting in singlet states."'O Twisting in an upper excited triplet state is important for trans-4bromostilbene at low temperatures,l1J2and twisting in the lowest triplet state is the dominant process for trans cis photoisomerization of 4-nitro~tilbenes.'~J~ Triplet states of stilbene and related compounds were first observed in viscous media.11J"17 It is only recently that phosphorescence of stilbenes has been detectedls and
some information on the kinetic properties of triplet states of simple arylethylenes in solution has been gained.'+24 Since the quantum yields of intersystem crossing are, in general, low and the lifetimes of arylethylene triplets at room temperature are in the nanosecond rather than millisecond range, energy-transfer techniques using pulsed lasers are to be favored for time-resolved triplet spectroscopy. Triplet lifetimes in the 100-ns range have been reported for ~ t y r e n e sstilbene,21~22 , ~ ~ ~ ~ ~ and l-phenyl-2-(2naphthy1)ethylene (2-NPE).%a The triplet states observed for styrenes have been assigned to the twisted configuration (3p*)19820 whereas for stilbene and 2-NPE establishment of an equilibrium between 3p* and the trans configuration (3t*)has been ~ u g g e s t e d . ~Upon ~ - ~ ~going from frozen to fluid solutions, the absorption maximum (A,-) of the lowest trans-stilbene triplet is shifted to shorter wavelengths precluding measurements of ,A, under sensitized excitation c o n d i t i o n ~ . ' ~ J " ~ ~ ~ ~ ~ In this work spectroscopic observations and kinetic investigations of the triplet states of three phenylethylenes in solution are presented. The interactions of various triplet donors with 1,l-diphenyl-, triphenyl-, and tetraphenylethylenes (DPE, TRPE, and TEPE, respectively) were studied by time-resolved absorption spectroscopy. Absorption spectra, lifetimes, and energies of the observed triplets of the phenylethylenes were determined, and the effect of phenyl substitution on the triplet behavior was studied. A mechanism describing decay of the first excited singlet and the lowest triplet state is proposed.
-
(1)Lamola, A. A.; Hammond, G. S.; Mallory, F. B. Photochem. Photobiol. 1965,4 , 259. (2)Stegemeyer, H.Ber. Bunsenges. Phys. Chem. 1968,72,335. (3)Gegiou, D.;Muezkat, K. A.; Fischer, E. J. Am. Chem. SOC.1968, 90,12. (4)Sharafy, S.;Muezkat, K. A. J. Am. Chem. SOC.1971,93, 4119. (5)Klingenberg, H. H.; Lippert, E.; Rapp, W. Chem. Phys. Lett. 1973, 18,417. (6)Fischer, G.;Fischer, E.; Stegemeyer, H. Ber. Bunsenges. Phys. Chem. 1973,77,685. Fiecher, G.; Seger, G.; Muszkat, K. A.; Fischer, E. J. Chem. SOC.,Perkin Trans. 2 1975,1569. (7) Barbara, P. F.; Rand, S. D.; Rentzepis, P. M. J. Am. Chem. SOC. 1981,103,2156. (8)Saltiel, J.; D'Agostino, J. T. J . Am. Chem. SOC.1972,94,6445. (9)Saltiel, J.; Marinari, A,; Chang, D. W.-L.; Mitchener, J. C.; Megarity, E. D. J . Am. Chem. SOC.1979,101,2982. (10)For a review, see: Saltiel, J.; Charlton, J. L. in 'Rearrangements in Ground and Excited States"; de Mayo, P., Ed.; Academic Press: New York, 1980;Vol. 3, p 25. (11)Gorner, H.; Schulte-Frohlinde, D. J. Phys. Chem. 1979,83,3107. (12)Gorner, H.; Schulte-Frohlinde, D. J.Am. Chem. SOC.1979,101, 4388. (13)Bent, D.V.;Schulte-Frohlinde, D. J.Phys. Chem. 1974,78,451. Schulte-Frohlinde, D.;GBrner, H. Pure Appl. Chem. 1979,51,279. (14)Goner, H.; Schulte-Frohlinde, D. Ber. Bunsenges. Phys. Chem. 1977,81,713;J . Phys. Chem. 1978,82,2653. (15)Heinrich, G.; Blume, H.; Schulte-Frohlinde, D. Tetrahedron Lett. 1967,4693. Heinrich, G.; Giisten, H.; Mark, F.; Olbrich, G.; SchulteFrohlinde, D. Ber. Bunsenges. Phys. Chem. 1973,77, 103. (16)Herkstroeter, W. G.;McClure, D. S. J. Am. Chem. Soc. 1968,90, 4522. (17)Heinrich, G.; Holzer, G.; Blume, H.; Schulte-Frohlinde, D. Z. Naturforsch. B 1970,25,496.
0022-3654/82/2086-2028$0 1.25/0
Experimental Section Apparatus and Procedures. For excitation at 353 and 265 nm, the third and fourth harmonics of a Nd laser (energy I50 mJ, pulse width 10 ns)11J4and, for excitation at 308 nm, an excimer laser (Lambda Physik, type EMG (18)Saltiel, J.; Khalil, G.-E.; Schanze, K. Chem. Phys. Lett. 1980,70, 233. (19)Bonneau, R.J.Photochem. 1979,10,439. Bonneau, R. J. Am. Chem. SOC.1980,102, 3816. (20)Caldwell, R.A.; Pac, C. Chem. Phys. Lett. 1979,64,303.Caldwell, R. A.; Cao, C. V. J. Am. Chem. SOC. 1981,103,3594. (21)Sumitani, M.; Yoshihara, K.; Nagakura, S. Bull. Chem. SOC.Jpn. 1978. 51. 2503. (22)Gorner, H.; Schulte-Frohlinde, D. J.Phys. Chem. 1981,85,1835. (23)Saltiel, J.; Eaker, D. W. Chem. Phys. Lett. 1980,75,209. (24)Gorner, H.; Eaker, D. W.; Saltiel, J. J. Am. Chem. SOC.1981,103, 7164. @
1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 2029
Triplet States of Phenylethylenes in Solution
TABLE I : Rate Constants for Quenching of Various Triplet Donors by DPE, TRPE, and TEPEa 10-9h,,b M-' s" no.
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 a
sensitizer xanthone acetophenone propiophenone butyrophenone benzophenone thioxanthone anthraquinone 9-bromophenanthrene 2-acetonaphthone 2-nitronaphthalene chrysene l-nitronaphthalene fluorenone pyrene benzanthrone acridine anthracene
E T , kcal/mol
-
14 14 73.5 12 68.5 65.5 62.4 61 59.3 56.8 56.6 54.8 53.3 48.1 47 45.3 42.5
In deoxygenated benzene solutions at 25 "C,, , A,
A,
630 5400 5400 5400 535 650 460, 630 480 430 500 580 600 460 420 460, 500 430 4 20
-
= 353 nm.
200, 6 >6 26 6 5.5 4.5 1 0.2 0.15 0.04 0.025 0.0015 rO.OOO1 ~0.0001
TEPE
6
6
I
4 5
4.5
2.8 2.2 1.8 1.2 0.15 0.06
2 2 0.9 0.8 0.3 0.1
0.01 0.006 ro.001
Experimental error of +30%.
hydrofuran, isopentane, acetonitrile, and tert-butyl alcohol). Requirements for the Sensitizers. For observation of acceptor triplet states when using the third harmonic of a neodymium laser for excitation, the sensitizer should have the following properties: (1)a moderate molar extinction coefficient at 353 nm; (2) a steep gradient in the ground-state absorption spectrum around 350 nm in order to permit transient absorption measurements at wavelengths 1 360 nm; (3) a sufficiently high triplet energy since energy transfer at diffusion-controlled rates is necessary to make triplet energy transfer faster than decay of a "new transient absorption"; (4) a very low fluorescence intensity; detection of a short-lived transient may be hindered if substantial emission of the donor occurs at the same wavelength (during the laser pulse). According to these requirements the triplet donors 1-10 of Table I were selected. A number of aromatic ketones fulfill conditions 1-4. In order to probe for the nature of the transients in the ketonephenylethylene systems, other sensitizers are also necessary. Unfortunately, many sensitizers not carrying carbonyl groups are unsuitable since they do not fulfill condition 4. With triphenylene, phenanthrene, or chrysene, for example, observation of transients with lifetimes shorter than 100 ns was not possible in the 360-400-nm region since the photomultiplier was saturated for about 100 ns under our conditions because of the exceedingly large fluorescence intensities.
Results Transient Absorption Spectrum. On excitation of various high-energy triplet donors in solution at room temperature, the lifetime of the donor triplet was reduced M) and a new in the presence of DPE ([DPE] 1 1 X transient absorption was observed in the 360-400-nm region within the rise time of the laser (-5-10 ns). These transient absorption spectra were identical on excitation of xanthone, benzophenone, or 9-bromophenanthrene in benzene solutions at 353 nm (Figure 1). The transient of DPE was clearly observable (at 40 and 70 ns) after the laser pulse when the sensitizer triplets were fully quenched. Transient absorption spectra ranging between 360 and 400 nm were also observed with TRPE and TEPE using several high-energy triplet donors (Tables I and 11). The observable parts of the absorption spectra, obtained under benzophenone-sensitized excitation in benzene solutions, are shown in Figure 2 (full lines). The TRPE and TEPE transients were also independent of the nature of the
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
2030
Giirner
02
A
I
0
b-
"..
I
350
400 -450
500
55OF6O0
300
650
hlnmi
LOO
350
h Inm)
I
150
Flgure 1. Time-resdved transient absorption spectra of D E (1 X lo-* M) under sensitized excitation at 353 nm in deoxygenated benzene solutions at 25 "C using (a) xanthone, (b) benzophenone, and (c) 9-bromophenanthrene as sensitizers (1 X M). The DPE transient (at 360-400 nm) and the sensitizer triplets (at X > 450 nm) were measured at the maximum optical density, Le., 10 ns (full line), 40 ns (dashed line), and 70 ns (dotted line) after the laser pulse.
Flgure 2. Absorption spectra of (a) TRPE and (b) TEPE: ground state (dotted line) and transient (full line, 10 ns after the laser pulse) under benzophenone-sensitlzed excitation (1 X M, A,, = 353 nm) in deoxygenated benzene solutions at 25 "C; transient absorption spectrum of TRPE in EPA at -196 "C (dashed line; 100 ns after the = 308 nm) and of TEPE in n-pentane at 25 "C ((-a -) laser pulse, bXc at the maximum ( 10 ns) of the laser pulse,,,A = 308 nm).
TABLE 11: Lifetimes of t h e Phenylethylene Triplets in Argon-, Air-, and Oxygen-Saturated Benzene Solutionsa
in which the fluorescence intensity at A, was sufficiently small, is tentatively assigned to a S1 S, absorption (see Discussion). In EPA at -196 "C a transient absorption spectrum was observed for TRPE (Figure 2a). Assuming the same extinction coefficient as for triplet-triplet absorption of trans-stilbene,l'J6 a triplet yield of 50.05 could be estimated. Attempts to observe similar transients for TEPE and DPE in rigid media (EPA and ethanol at -196 "C and glycerol triacetate (GT) at -70 "C)failed. In GT at room temperature, transients (lifetime 2 10 ps) with weak absorption between 360 and -600 nm appeared after repeated flashing of TRPE or TEPE (A,, = 308 nm) or after irradiation at 313 nm. Involvement of a photoproduct is supported by changes in the absorption spectra after irradiation or repeated flashing. Triplet Energy Transfer. Decay of the sensitizer triplet in the absence of additives, measured at the maximum (AJ of the triplet-triplet absorption spectrum (Table I), displayed mixed first- and second-order kinetics in most cases; no attempt was made to evaluate the decay kinetics. In the presence of a phenylethylene as acceptor (A),however, decay of the sensitizer triplet was pseudo first order throughout. Time-dependent transient absorption signals at 370 and 640 nm are shown in Figure 3 for typical cases using xanthone-sensitized excitation. The fmt-order decay rate constant (kobd) at A, was found to be linearly dependent on [A] as shown in Figure 4 for typical cases. For DPE and TRPE the maximum observable rate constant for decay of the sensitizer triplet was limited to 1 X lo8 s-l by the laser pulse duration while for TEPE it was limited to 5 X lo7 s-l due to absorption by TEPE at 353 nm which prohibited the use of high [TEPE]. From the slopes of the linear plots of koM vs. [A] (Figure 41,the rate constants (k,) for quenching of the sensitizer triplet by the phenylethylenes were determined (Tables I and 111). When the triplet energy of the sensitizer (ET)increased, k, values increased more strongly for DPE than for TRPE and TEPE. From plots of log k , vs. ET (Figure 51, the triplet energies of the three phenylethylenes were determined (see Discussion). For high-energy triplet sensitizers the k , values were close to the diffusion-controlled limit. Measurements in several solvents at 25 "C reflect a dependence of k , on solvent viscosity (7)as expected from the Debye equation ( k , T/q). For example, k , values for quenching of triplet states of xanthone or benzo-
-
78,
compd
sensitizer
DPE
xanthone acetophenone propiophenone butyrophenone benzophenone anthraquinone 9-bromophenanthrene TRPE xanthone acetophenone propiophenone butyrophenone benzophenone 9-bromophenanthrene 2-acetonaphthone 2-nitronaphthalene TEPE xanthone acetophenone benzophenone 9-bromophenanthrene
70,
ns
4 0 b (38)c 40 40 40 41b (40) 38 42 105b (105) 100 110 105 105b (105) 105
ns
rex,
ns
25 10 26 11 25 Q l l 26 6 1 2 26 G12 25 11 58 56
58 6G 58 58 55
90 90 1 7 0 b ( 1 8 0 ) 100 170 95 180b ( 1 7 0 ) 1 0 5 180 100
-
21 19 625 624 22 21 25 40 40 40 40
At 25 "C measured a t 360-390 nm using [ A ] 1.5 X Samples were freed from oxy= 353 nm. gen by three freeze-pump-thaw cycles (1 X lO-'torr). Values in parentheses refer t o 7 5 "C.
lo-* M ; A,,
triplet donor (e.g., xanthone, benzophenone, and 9bromophenanthrene). The transient absorption maxima of the three phenylethylenes could not be determined with this method even under benzophenone-sensitized excitation because of the overlap of the absorption spectra of the transients with those of the ground state of the sensitizers below -370 nm (Le., A,, 5 360 nm). On acetone-sensitized excitation at 308 nm, however, A, 330 nm could be determined for the DPE transient in tertbutyl alcohol solutions. In the absence of a sensitizer, no transient absorption with a lifetime longer than 50 ns (10 ns in the absence of fluorescence) was found below 400 nm for any phenylethylene (e.g., DPE in acetonitrile, A,, = 265 nm, TRPE and TEPE in benzene, A, = 308 and 353 nm). As shown in Figure 2b for TEPE a transient absorption (A, = 405 nm, half-width 4000 cm-') appeared within the duration of the laser pulse (i.e., lifetime < 10 ns). This absorption, observed in those solvents (e.g., n-pentane and benzene)
-
-
-
-
-
-
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
Triplet States of Phenylethylenes in Solution
2031
TABLE 111: Lifetimes o f the Phenylethylene Triplets and Rate Constants for Quenching o f the Sensitizer Triplets in Several Solventsa compd
sensitizer
solvent
trans-anethole
benzophenone
DPE
xanthone benzophenone
n-pentane benzene tert-butyl alcohol n-pentane acetonitrile tert-butyl alcohol n-pentane acetonitrile tert-butyl alcohol acetonitrile tert-butyl alcohol
xanthone benzophenone
TRPE
xanthone benzophenone xanthone
TEP E
b b b 33 33 (35)C 42
12 6 2 14
9 2.5 13 6 1.7 4 1.4
24 24 31 56 50 56 100
110
-
85 ( 8 0 ) 100 200 ( - 2 0 0 ) 200
12 11 16 18 30 20 40 42
110
No discernible transient absorption at h > 355 nm.
At 25 "C ( 2 6 "C for tert-butyl alcohol), he,, = 353 nm. in parentheses refer t o 7 5 "C. a
Values
A
I
t
t
m Z
w
n -I
a
2a 0
Time
50
.D
6
I
I
I
V
I
I
I
I d'.L1"
- *I ........ ...,.... ,....... .............. ..............
'0
0
40
I
*-A-
0
A-
0
1
-A0
0.
I
I
A
I
-
70
Figure 5. Plots of log k, vs. E, for DPE (0),TRPE (A),and TEPE (0) in benzene solutions at 25 O C obtained at & (Table I); the straight line corresponds to the condition A log k , = AET/(2.3RT).
h =380nm
b
60 E,lkcal/moll
Figure 3. Transient absorption signals obtained under xanthone-sensitlred excitation at 353 nm of (a) DPE (1 X lo-* M), (b) TRPE ( 3 X lo3 M), and (c) TEPE (5 X lo3 M) in deoxygenated benzene solutions at 25 O C measured at 640 nm (dashed line) and 365, 370, and 375 nm (full line) for DPE, TRPE, and TEPE, respectively.
Transient Lifetime. Decay of the transients of DPE, TRPE, and TEPE, measured in the 360-400-nm range under sensitized excitation conditions in solution at room temperature, was first order throughout (Figure 3). While plots of kobsd vs. [A] exhibited linear dependences at A,, curved plots reaching constant values were found at 360-400 nm (Figure 4). Since the curved plots level out, a self-quenching process of the phenylethylene transients plays no role. Below 400 nm k&,d approached constant values at concentrations of 0.3 X and 0.8 X M for TEPE and DPE, respectively, when xanthone-sensitized excitation in benzene solutions was used. When tert-butyl alcohol and n-pentane (more viscous and more fluid, respectively) were used, the respective concentrations were higher and lower. The lifetimes ( T = ~ k0bd-') of the three phenylethylene transients in benzene solutions, obtained at sufficiently high acceptor concentrations, i.e., k, [A] > T ~ - are ~ , listed in Table 11. Increasing the temperature to 75 O C or changing the solvent from n-pentane (nonpolar and fluid) to tert-butyl alcohol (viscous)or to acetonitrile (polar) had little influence on T @ Furthermore, the transient lifetimes are (within experimental error) independent of the nature of the high-energy triplet donors (Tables I1 and 111). Quenching by Oxygen and Ferrocene. In the presence of oxygen the lifetime of the transient observed below 400 nm was reduced; transient lifetimes in argon-, air-, and oxygen-saturated solutions ( T ~ T, ~and , T,,, respectively) are listed in Tables I1 and I11 for various combinations of
2032
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982
Wrner
TABLE V: Quantum Yields of Fluorescence and Intersystem Crossing at High and Low Temperatures compd DPE
0
.X
trans-stilbened
MTHF ethanol
TRPE
GT MCH-MCP
0'
I
c
solvent cyclohexane ethanol
L
111 I
P
/
c
42 0
ethanol 0
2
L
6
0
[OXYGEN] ( M x ~ O ' ~ )
Flgure 6. Plot of k,, vs. [OZ]for DPE (0),stilbene (O), TRPE (A), and T E E ( 0 )in benzene solutions at 25 OC under xanthone-sensitized excitation at 353 nm obtained at 370-390 nm; values for stilbene are taken from ref 22.
TABLE IV: Rate Constants for Quenching of t h e Phenylethylene Triplets b y OxygenQ
TEPE
EPA MTHF GT MCH-MCP ethanol EPA MTHF GT
t, "C 25 25 -196 -196 25 -196 --80 25 -196 25 -196 - 196 -196 - 70 25 -196 25 -196 -196 -196 - 70
0 fa
3
X
