J. Phys. Chem. 1994,98, 5431-5434
5431
Two-Laser, Two-Color Photochemistry from Upper Triplet States of 2-Bromonaphthalene and 9-Bromophenanthrene in Benzene J. C. Scaiano,*J B. R. Arnold,**+and W. G . McGimpsey**$ Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K I N 6N5,and Department of Chemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Received: September 28, 1993; In Final Form: January 13, 1994"
The photochemistry of 2-bromonaphthalene (BrN) and 2-bromophenanthrene (BrP) in benzene was examined using two-laser, two-color techniques. These experiments center on the photochemistry of the strongly absorbing triplet states of these molecules. Triplet photobleaching quantum yields in benzene are 0.12 and 0.15 for BrN and BrP, respectively. It is believed that these yields coincide with C-Br bond cleavage to yield in-cage radical pairs. Their magnitude reflects excited-state partitioning between C-Br bond cleavage and return to the lowest triplet surface. Bromine atoms were detected taking advantage of ?r-complex formation in benzene with A,, 540 nm. Less than half of the bleaching events result in the formation of detectable bromine atoms. Geminate radical recombination of the radical pair to regenerate starting material must compete with cage escape, which accounts for the remaining bleaching events.
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B'
Introduction It has been well established that the upper triplet states of aromatic organic molecules undergo many photochemical and photophysical processes in addition to simple internal conversion to their lowest excited state. Reverse intersystem crossing occurs in a number of systems, including anthracenes and chlorophyll.I4 Intramolecular processes involving upper excited triplet states have also been observed; for example, several aromatic substrates transfer energy from upper triplet states (presumably from T2) to biphenyls.5 The upper triplet states of benzophenone can be deactivated in benzene, presumably by energy transfer.6 Bond cleavage from upper triplet states has been detected in cases where the cleavage is slow and inefficient from the lowest triplet;' these processes are called "reluctantn cleavages. For example, triplettriplet absorption of benzil leads to Norrish type I fragmentation yielding two benzoyl radicals, a process that does not occur from the lowest triplet state.& Molecules that undergo cleavage to give free radicals exclusively under conditions of two-photon excitation could find technical applications as laser-specific free radical photoinitiators. It is then desirable to be able to study the excited-state photochemistry in detail and to quantify some of these upper excited-state processes. In particular, bromine atom sources could act as photoacid generators following hydrogen abstraction by the initially formed atom.9 We have chosen the debromination reactions of 2-bromonaphthalene (BrN) and 9-bromophenanthrene (BrP) as possible candidates for the study of this type of reluctant cleavage. The TI states of these two molecules are largely photostable, although some activated decomposition is known to take place in the case of BrN.Io In both cases triplet-triplet excitation enhances dramatically the yield of radical products, ensuring that these systems do in fact undergo reluctant upper excited-state cleavage with reasonably high quantum yield. In this paper the details of a study of the two-photon photochemistry of BrN and BrP are reported; in particular, two-laser, two-photon techniques are used to determine the quantum yields for upper triplet-state processes in benzene. To whom correspondence should be addressed. + University of Ottawa. t Worcester Polytechnic Institute. 0 Abstract published in Aduance ACS Abstracrs, March 15, 1994.
0022-3654/94/2098-543 1$04.50/0
Experimental Section Materials. 2-Bromonaphthalene (BrN) and 9-bromophenanthrene (BrP), both from Aldrich, were refluxed in hexane in the presence of charcoal and recrystallized twice. Benzophenone (Aldrich), BP, was recrystallized from methanol before use. Aberchrome 540 (Aberchromics Ltd., Cardiff, U.K.) was used as received, as were the solvents cyclohexane (BDH, spectrophotometric grade) and benzene (BDH, spectrophotometric grade). All laser dyes were purchased from Exciton. Laser Photolysis Systems. A full description of the laser flash photolysis system, including modifications to adapt the system for two-laser, two-color experiments, has been reported elsewhere.I'J2 Two separate laser systems have been used during the courseof this study. The first employed a Lumonics TE860-2 excimer laser charged with a Xe/HCl/He gas mixture (308 nm, -6 ns, 5 3 0 mJ/pulse) to supply the synthesis pulse (to generate the triplets) and a Candela flash lamp-pumped dye laser as the photolysis laser. The dyes used were Stilbene 420 (broad-band output centered at 433 nm for irradiation of BrN) or Coumarin 480 (broad-band output centered at 488 nm for irradiation of BrP) in 50% v/v aqueous methanol (-250 ns, 100 mJ/pulse). The second system is similar to that already described except the synthesis pulse was supplied by a Lumonics EX 5 10 (308 nm, -8 ns, 25 mJ/pulse), and a short pulse Lumonics HD 500 dye laser pumped by a Lumonics EX 530 excimer laser supplied the photolysis pulse. The dyes used were Stilbene 420 (tuned to 435 nm for irradiation of BrN) or Coumarin 460 (tuned to 465 nm for irradiation of BrP) in methanol (-5 ns, 6-10 mJ/pulse). Regardless of the system used, the excitation geometry was always the same. The angle between the synthesis and photolysis laser pulses was approximately 5O. Both beams propagated in the same direction through the sample and were perpendicular (excimer) and at 85O (dye) to the probe beam. Data acquisition was performed using a Tektronix-2440 digitizer GPIB interfaced to Macintosh-ci or -cx computer operating with LabVIEW 2.2
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0 1994 American Chemical Society
Scaiano et al.
The Journal of Physical Chemistry, Vol. 98, No. 21, 1994
5432
t r
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w
5000 0 300
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Wavelength (nm) Figure 1. Triplet-triplet absorption spectra for BrN (0)and BrP (0) obtained by 308-nm laser excitation in benzene. software. In some experiments a Molectron UV-24 nitrogen laser (337 nm) was used for excitation. The experiments were carried out employing a flow system that allowed deaeration of the sample solutions by bubbling nitrogen, and the samples were flowed through a specially designed 7 X 7 mm Suprasil flow cell. The photochemical products were identified using gas chromatography (GC) and GCMS methods by comparison with authentic samples. GC measurements employed a Perkin-Elmer 8320 gas chromatograph with a capillary 15-m DB-5 column. The relative product yields were determined using GC techniques with a trace of n-dodecane added as an internal standard. The yields in benzene required the addition of 1% hexane as a hydrogen donor to scavenge the radicals produced. Under these conditions the yields computed from the loss of starting material or from hydrocarbon formation were, within experimental error, the same.
Results Characterization of Intermediates. Triplet-triplet absorption spectra of BrN and BrP are shown in Figure 1. The energytransfer method using benzophenone as a donor was used to determine the absolute values of the extinction coefficients (EA) where c523 of triplet benzophenone was taken as 7800 f 800 M-l cm-1.13 The signal due to triplet benzophenone before addition of acceptor and that due to the triplet arene acceptor when almost complete quenching of the benzophenone triplet is achieved are compared to give CA of the acceptor triplet. The concentrations of BrN and BrP required to reduce the lifetime of triplet BP from 1.5 ps to 95%quenching) lead tolargeacceptor absorptions at 308 nm so that their direct absorption is important. To overcome this difficulty, these sensitization experiments were performed using 337-nm laser excitation; under these conditions acceptor absorbance is negligible. This method leads to extinction coefficients at the maxima of 17 000 f 2000 M-l cm-l for BrN triplet and 17 500 f 2000 M-1 cm-l for BrP triplet at 425 and 485 nm, respectively, in benzene as solvent. The reaction scheme proposed is shown in Figure 2. Those processes that are of interest to this study are intersystemcrossing from the lowest excited singlet into the triplet manifold, Tz T1 internal conversion, upper triplet-state quenching, and the yield of separated radicals. The analysis of each of the individual process is given below. Process 1: Intersystem Crossing Yields. In simple cases, a plot of the optical density change due to transient formation (AOD) versus the laser dose will be linear where the slope is dependent upon the product of the quantum yield transient formation (O~scin this case) and the extinction coefficient of the transient absorption at the monitoring wavelength (EA) according toeq 1.
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AOD = a(@Is,)(c,)dose
(1)
Benzophenone triplet, produced with a quantum yield of one, could in principle be used as an actinometer with 2525 = 7800
Figure 2. Proposed photochemical reaction scheme for the bromoarenes BrN and BrP. The numbered processes are (1) intersystem crossing from the singlet into the triplet manifold, (2) upper triplet internal conversion, (3) upper triplet quenching by triplet energy transfer to benzene solvent,(4a) C-Br bond fragmentation, (4b) radical pair escape from the solvent cage, and (4c) geminate recombination of the radical pair.
TABLE 1: Quantum Yield Data for Triplet Processes of 2Bromonaphthaleneand 9-Bromohenanthrenein Benzene process n 0 . O process BrN BrP 1
2 3
4a 4b 4c a
0.9 f 0.1 0.88
@ISC @IC
0.8 f 0.1
0.85
@SI SBr
