J. Phys. Chem. 1994,98, 7022-7028
7022
Photochemistry of 1,2- Dibromoethy1 Arenes Bing Zhang, C. R. Pandit, and W. Grant McGimpsey' Department of Chemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 Received: March 14, 1994; In Final Form: May 2, 1994'
UV laser photolysis of 2 4 1,2-dibromoethyl)naphthalenein benzene and acetonitrile results in C-Br homolytic bond cleavage via the singlet and possibly the triplet manifolds. Cleavage produces a bromine atom, Br., and the 2-bromo-l-(2-naphthyl)ethyl radical. In benzene, the presence of Br' is indicated by formation of the Br'-benzene ?r-complex, while in acetonitrile with added B r , Br2'- is produced. In addition to these two transient probes, the formation of acidic solutions following photolysis in the presence of a H atom donor is also an indication of Br' generation. The debromination quantum yield was determined by quantifying the formation of both Br2'- @B~-= 0.87 f 0.10, and acid @H+ = 0.85 f 0.14. The difference between these values and those obtained for other vicinal dibromides ( @ ~ > p 2) which undergo photochemical debromination followed by facile thermal C-Br cleavage is attributed to the greater stability of the 2-bromo- 1-(2-naphthyl)ethyl radical relative to 2-vinylnaphthalene, the product formed by loss of the second Br'. Photolysis of the 2-bromo-l(2-naphthy1)ethyl radical using a second laser pulse leads to further C-Br cleavage resulting in enhanced production of 2-vinylnaphthalene. UV laser photolysis of 2-bromo-9-( 1,2-dibromoethyl)anthracene also causes C-Br cleavage resulting in the production of Br' and the 2-bromo- 1-(2-bromo-9-anthryl)ethylradical. The efficiency of debromination was lower in this compound (@B~* = 0.45 f 0.10; @I++ = 0.48 f 0.1 1) than for the naphthalene compound because of competition between cleavage from the singlet manifold and intersystem crossing to a low-energy unreactive triplet state. Like the naphthalene analog, laser photolysis of the anthrylethyl radical leads to further debromination. Both radicals exhibit unusual stability in the presence of oxygen.
Introduction
Efficient photochemical acid generation is of great interest to the photoresist and photolithography industries. To date, the most commonly used photoacid generators have been aryl diazonium, iodonium, and sulfonium with complex metal halide anions. When irradiated, these compounds produce very strong acids in generally good yields but also suffer from inconvenient absorption characteristics and/or poor thermal stability. Recently, photoacid generation has been reported for a class of organic compounds called vicinal dibromides.*-1° Direct photolysis of these compounds in the presence of a H atom donor results in homolytic C-Br bond cleavage to produce bromine atoms, Br'. H atom abstraction by Br' yields HBr, a strong acid. The potential advantages offered by the use of these compounds include greater thermal stability, tunability of the absorption characteristics, and a high acid quantum yield. For example, 1,2-dibr~moethane~ undergoes debromination (and, therefore, acid production) with a quantum yield of 2.3. We recently reported a similar quantum yield for trans-10,l l-dibromodibenzosuberone (I).10 0
cleavage which is driven by the instability of the intermediate radical relative to the product. For I, conjugation of the two phenyl rings provides a large driving force for the thermal debromination step. In addition to the high acid yield, I has an absorption spectrum which extends to the near UV, making it particularly useful in situations where deep UV irradiation is not desirable. We undertook the present study in order to (i) determine whether high debromination yields are a general characteristic of vicinal dibromides and (ii) show that debromination and acid production could be achieved in vicinal dibromides with extended aromatic chromophores, Le., compounds with absorption spectra in the visible range. This paper describes the fluorescence and phosphorescence spectroscopy, and the CW and transient photochemistry of 2 4 1,2-dibromoethyl)naphthalene (II) and 2-bromo-9-( 1,2-dibromoethyl)anthracene(III) in solution. Included in the transient photochemical results are determinations of the debromination and photoacid quantum yields as well as evidence for two-laser-induced debromination involving an intermediate radical species.
e R-
I In each case, the high debromination yield was explained by photoinduced C-Br cleavage followed by facile thermal C-Br *Abstract published in Advance ACS Absrracts, June 1, 1994.
0022-3654/94/2098-1022$04.50/0
II
III
Experimental Section Materials. Unless otherwise specified, the following materials were obtained from Aldrich. Acetonitrile(MeCN), cyclohexane 0 1994 American Chemical Society
Photochemistry of 1,2-Dibromoethyl Arenes
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 7023
TABLE 1: Emission Parameters for 11, III and Model Compounds SI(kcal/mol) i“1 (kcal/mol) I1
naphthalene III 9- bromoanthracene
84.1 91.5 70.6
13.2
60.8 60.9
-
-41
91 0.010
0.230 0.003 0.030
(CH), benzene (BZN), isopropyl alcohol (IPA), and ethanol (EtOH) were Spectrophotometric grade and were used as received. Bromine (99.5+%) was used as received. 2-Vinylnaphthalene (98%) and 9-vinylanthracene (97%) were recrystallized from ethanol. Benzophenone (BZP)(99%) was recrystallized from methanol prior to use. Anthracene (99.9%),9-bromoanthracene (96%),and tetra-n-butylammoniumbromide (TBAB)(99%) were recrystallized from hexane. Naphthalene (99+%) was recrystallized from ethanol. Azoxybenzene (98+%) was received from Lancaster Synthesis and was recrystallized from ethanol and water. Synthesiiof 2-(1,%Dibromoethyl)nphthakne (II). Toastirred solution of 2-vinylnaphthalene(3.08 g, 20.0 mmol) in CHC13 (8 mL) at 0 OC was added dropwise a solution of bromine (3.20 g, 20.0 mmol) in CHCl3 (5 mL). The reaction mixture was stirred for 1 h, followingwhich thesolvent was evaporated. Theresulting solid was recrystallized from aqueous EtOH to give the dibromide 11 (3.26 g, 52% yield), which was identified by NMR and GCMS: mp 78-79 OC. Anal. Calcd for C I ~ H I O BC,~ 45.90; ~: H, 3.21. Found: C, 46.04; H, 3.01. Synthesis of 2-Bromo-9-( 1,2-dibromoethyl)anthracene (III). Bromination of 9-vinylanthracenewas carried out exactly as for 2-vinylnaphthalene. In addition to bromination at the vinyl side chain,GC-MS analysis indicated incorporation of a third bromine. The l3C spectrum confirmed that substitution occurs at the 2 position. III was obtained as a yellow solid and recrystallized from 1:1 ethanol/methanol: 0.88 g, 42% yield, mp 142-143 OC. Anal. Calcd for ClaH11Br3:C, 43.83; H, 2.50. Found: C, 43.14; H, 2.24. Fluorescenceand Phosphorescence. Fluorescence spectra were obtained using a Perkin Elmer LS-50 spectrofluorimeter. Samples contained in quartz cells were outgassed with dry nitrogen and optical densities were
