J . Phys. Chem. 1993,97, 5054-5057
5054
Electron-Transfer Quenching of Excited l&Diphenylhexatriene Cation Radicals Z. Wang and W. G. McGimpsey’ Department of Chemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Received: January 19, 1993; In Final Form: February 18, I993
The photochemistry of the all-trans- 1,6-diphenyl-1,3,5-hexatrienecation radical, ‘DPH+*, in air-saturated acetonitrile solution at room temperature has been investigated by two-laser flash photolysis. In the presence of anisole, 1,2-dimethoxybenzene, and biphenyl, 590-nm photolysis of ‘DPH+* resulted in electron-transfer quenching. In the presence of anisole and 1,2-dimethoxybenzene, this quenching was indicated by the absence of the trans cis isomerized cation radical, CDPH+’,which is the primary photoproduct in the absence of electron donors. In the presence of biphenyl, the biphenyl cation radical with ,,A 375 nm was observed. A Stern-Volmer analysis using the absorbance of the biphenyl cation radical and an estimate of the rate constant for electron transfer from biphenyl to excited ‘DPH+’ allowed a determination to be made for the ‘DPH+’ excited-state lifetime: 7 > 430 ps.
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Introduction Recently, there has been considerable interest in the photochemistry of ions and ion radicals. Early studies in solution have shown that photoexcitationof aromatic dianions and anion radicals causes electron ejection.’-I0 When a suitable electron acceptor is present, irradiation of anion radicals may also result in electron transfer. For example, the fluorescence emission from excited anthraquinone, pyrene, and perylene anion radicals was quenched by electron transfer to a variety of acceptor^.^ Since most cation radicals react efficiently with trace amounts of water and with other nucleophiles, it is difficult to generate concentrations sufficientlylarge to perform photochemicalstudies. Thus, reports of cation radical photochemistry in solution are less ~ o m m o n . ~ - ~Eriksen J J ~ and co-workers’ have reported the electron-transfer quenching of thianthrene cation radical fluorescence by aromatic donors. We have recently reported a two-laser nanosecond flash photolysis study of the photochemistry of all-truns- 1,6-diphenyl1,3,5-hexatriene cation radical (‘DPH+*) and its octatetraene analogue (‘DPO+*)in acetonitrile solution at room temperature.12 One laser (UV) was used to generate ‘DPH+’ by a biphotonic mechanism. Before it could decay, a second laser, tuned to the ‘DPH+’absorption band was used to photoexcitethecation radical. It was determined that excitation resulted in efficient trans cis isomerization. The steady-state fluorescenceemission of excited DPH+*,which is denoted by *DPH+’, has recently been reported in samples of ‘DPH adsorbed in ze01ites.I~Since the conformational changes associated with isomerization are restricted by the zeolite cage, emission would be an efficient excited-state deactivationpathway. Unfortunately, while the zeolite environment does stabilize +DPH+*, it does not allow bimolecular processes to occur efficiently. Thus, we have used the two-laser technique to study the photochemistry of *DPH+’ in solution. We have found that in the presence of a number of electron donors, electron-transfer quenching occurs. We have also used Stern-Volmer analysis to estimate the lifetime of the excited cation radical.
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Experimental Section Materials. All chemicals were obtained from Aldrich unless otherwise noted. Acetonitrile (MeCN) was spectrophotometric grade and was used as received. all-trans- 1,6-Diphenyl-l,3,5hexatriene (‘DPH) (98%) was recrystallized from benzene. Anisole (99%), 1,Zdimethoxybenzene (DMB) (99%), and 1,2,4trimethoxybenzene (TMB) (97%) were distilled prior to use.
Biphenyl (BIP) (99%), and decafluorobiphenyl (DFB) (99%) were recrystallized twice from hexane. 1,4-Dinitrobenzene(98%) was recrystallized from hexane. Laser Flash Photolysis. The two-laser flash photolysis system has been described in detail e1~ewhere.l~Briefly, samples were irradiated first with the pulse of either a Lumonics EM5 10 XeCl excimer laser (308 nm; 30 mJ/pulse; 8 4 s duration) or the frequency-tripled output of a continuum “Surelite* Nd/YAG laser (355 nm; 50 mJ/pulse; 6 ns). In the two-laser experiments, the transient species produced by the first pulse were excited after a short (ca. l-rs) delay by a pulse from a Candela SLL 250 flashlamp-pumped dye laser (590 nm; 430 f 60 ps.
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Acknowledgment. We acknowledge the financial support of the US.Army Natick Research, Development and Engineering Center. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work and to Acton Research Corporation for loan of equipment.'
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