9299
J. Phys. Chem. 1993,97, 9299-9303
Spectroscopic Studies of the Singlet and Triplet Excited States of Some Substituted Biphenylenes Mauricio Suarez, Chelladurai Devadoss, and Gary B. Schuster' Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received: March 26, 1993; In Final Form: July 12, 1993'
The steady-state and time-resolved spectroscopy of a series of substituted biphenylenes was probed to assess their possible application as photoreceptors in a liquid crystal-based optical switch. Their singlet-singlet and triplet-triplet absorption spectra were recorded, and the rates of their radiative and nonradiative relaxation processes were measured. In no case could a triplet biphenylene be detected following direct irradiation even when the substituent is expected to enhance intersystem crossing. Consequently, these compounds will not be suitable for application as photoreceptors that require triplet-state formation.
Introduction The resolution of chiral compounds into enantiomers by irradiation with circularly polarized light is a clearly understood phenomenon.' If the two enantiomers of a compound are stable in the ground state but are interconverted in an electronically excited state, then irradiation of the racemic mixture with circularly polarized light can lead to the enrichment of one enantiomer. The degree of enrichment (enantiomeric excess, ee) and the rate at which the photostationary state is established depend on the Kuhn anisotropy factor (gA = AeAlcA). We have been searching for photoresolvable compounds with large values of gA so that their irradiation with circularly polarized light in liquid crystalline media will generate an ee sufficiently large to convert a nematic to a cholesteric phase.* Irradiation of such a cholestericliquid crystal with unpolarized light will racemize the chiral compound and regenerate the nematic phase. Thus, this liquid crystalline material may be switched between cholesteric and nematic phases by alternate irradiation with circularly polarized and unpolarized light.' If compounds suitable for this function are discovered, they may form a basis for development of an optical switch. The ee required to generate a detectable cholesteric liquid crystal depends upon the helical twisting power ( 8 ~of) the chiral material. Our analysisof unorientedand homeotropically aligned nematic liquid crystals indicates that the maximum pitch that can be readily sensed is ca. 100 pm.2 With typical values for BM, a minimum value of lo-' for gA is thus required to produce a sufficiently large ee at the photostationary state for optical detection of a cholesteric phase.4 Since the magnitude of gA decreases inversely with ex, the molar extinction coefficient and directly related to AEA,we seekchiral compounds with small values of q, at appropriate wavelengths. Our attention so far has been focused on chiral biaryl compounds and on substituted 4-arylylidenecyclohexanes (see Chart 1).2 The biaryls may be photoresolved (photoracemized) by rotation about the single bond connecting the aryl groups, typically in the triplet state, and the cyclohexanes are photoracemized by rotation about the double bond in the excited state to an intermediate achiral structure. Chiral exciton coupling theory predicts that both the chiral biaryl and cyclohexane structures will exhibit split Cotton bands in their circular dichroism spectra.5 Our desire to maximizegA in this band led us to consider biphenylenyl-containingstructures 1-5. The spectroscopy and photophysicalproperties of biphenylene ( 1 ) have been extensively studied. Transient absorption spectra and time-resolved fluorescence and steady-state spectroscopic methods have been used to probe the nature and the mode of Abstract published in Advance ACS Abstracts, September 1, 1993.
decay of its singlet excited states. Despite intensive analysis, there is still not complete agreement on the properties of these states. The absorption spectrum of biphenylene has been studied in the vapor in rigid glasses?,* in mixed crystals? in solution,lOand in stretched polymer films." Recently, Bich and co-workers12 measured the two-photon absorption spectrum of biphenylene in cyclohexaneat room temperature in the spectral region 250-370nm. All analyses of the biphenylene absorption spectrum agree with Hochstrasser's conclusion6~9that the SO SItransition at 395 nm is symmetry-forbiddenbut observed due to vibronic coupling. The extinctioncoefficient of this band),e( is ca. 100 M-l cm-l. The symmetry-allowedSO SZtransition has an intense 0-0 band at 355 nm in the vapor (358 nm in solution). These observations have been confirmedby subsequent spectroscopicstudies'O including linear dichroism and magnetic circular dichroism and theoretical calculations.12-15 These optical properties make use of the biphenylene chromophore as part of the chiral absorber attractive in a liquid crystal-based optical switch of the kind described above. The propertiesof electronicallyexcited biphenylene differ from those of other aromatic hydrocarbons such as naphthalene and anthracene. Biphenylene is very weakly fluorescent, and the first experiments16J7seemed to indicate that it was a member of the rare class of compounds that violate Kasha's rule and emit from an upper excited state. Subsequent study by Hochstrasser and McAlpine6a9and later Munro and co-workers's showed that the anomalous emission assigned to S2 actually originates from an impurity. More recently, Shizuka and co-workers19,20studied impurity-free biphenylene in cyclohexane solution at room temperature and measured the quantum yield of fluorescenceto be 1.8 x 1V. In these studies no biphenylene phosphorescence was observed even in rigid glasses at low temperature. The absence of significant intersystem crossing to the triplet of biphenylene was confirmedby time-resolved spectroscopy. These observations indicate that SIof biphenylene undergoes an unusually rapid internal conversion reaction initiated by distortion of the excitedstate structure. Peradejordi and co-workers21reported that triplet biphenylene was not observed by flash photolysis under direct excitation. However, a transient species with a lifetime of 100 ps in cyclohexanesolution could be generated by energy transfer from triplet naphthalenewhich was assigned to triplet biphenylene. The first picosecond time scale spectroscopic study of biphenylene was reported by Rentzepis.2z Lin and ToppZ3reported that the lifetime of the biphenyleneS1 state is 240 f 20 ps. More recently, Elsaesser and co-workers24 measured the lifetime of singlet biphenylene by picosecond transient absorption spectroscopy and time-resolved fluorescencemeasurements and reported detection of an additional fast decay component with a lifetime of 8 f 3 ps. They assign the fast component to relaxation of the
0022-3654/93/2091-9299$04.00/0 0 1993 American Chemical Society
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9300 The Journal of Physical Chemistry, Vol. 97, No. 37, 1993
CHART I
Biaryl Compounds
X
-
Arylylidene Cyclohexanes
1, 2, 3 , 4
H, COCH,, Br, OMe
5
6
-
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S2 state. Nickel and Hertzberg8 confirmed these results and reported the observation of delayed SI SO and SZ SO fluorescence and delayed excimer fluorescence and phosphorescence from triplet biphenylene produced by energy transfer. Similarly, Samanta and co-worker~~~ studied the transient absorption spectrum of biphenylene excited state in the spectral range 400-760 nm. They seemed to observe the rapid decay of a sharp band with A, at 410 nm (time constant of 15-30 ps) and a slower decay of a broad band around 650 nm with a lifetime of 245 f 15 ps. However, additional experimental investigation by this group has revealed that both the 410- and 650-nm bands decay with essentially the same 245-ps lifetime.26 Very recently, Hirata and Okada2’ examined the excited singlet state of biphenylene by picosecond absorption spectroscopy and concluded, from the narrowing of the absorption band, that there is a fast decay component (1 5 ps) due to a vibrational relaxation in the SIstate. Presently, there is not complete agreement about the nature and mode of decay of the excited singlet state of biphenylene. The photochemistry of compounds containing a biphenylene chromophore has not been studied extensively. Biphenylene itself is apparently inert - there are no reports of its unimolecular photodegradation in solution. However, it seems that extended irradiation of relatively concentrated solutions in hexane gives a dimeric compound.28 This is consistent with the formation of triplet biphenylene under these conditions. Also, Fischer and co-workers report that diarylethylenes containing the biphenylyl group undergo reversible isomerization reactions related to the well-known analogous processes of styrenes and ~tilbenes.2~ This is a particularly important observation for development of a liquid crystal-based optical switch of the type described above since a related isomerization of a biphenylylidene cyclohexane might interconvert nematic and cholesteric phases. Since the spectroscopy and photophysical properties of substituted biphenylenes have not been reported, we initiated an investigationof these compounds in order to assess their suitability for application to liquid crystal-based optical switches. Herein we report a study of the time-resolved and steady-state spectroscopic behavior of biphenylene (1) and its derivatives, 2-acetylbiphenylene (2), 2-bromobiphenylene (3), 2-methoxybiphenylene (4), and 1,l’-bis(biphenyleny1) (5). Experimental Section Materials. Biphenylenewas prepared as described by Friedman and co-workers3O and purified by column chromatography on
Suarez et al. silica gel eluted with hexane and then by sublimation and recrystallization from ethanol.” 1.1’-Bis(biphenyleny1) (5) and 2-acetylbiphenylene (2) were prepared according to the method of McOmie and co-workersg2and purified by chromatography on silica gel, recrystallization from aqueous ethanol, and finally sublimation in vacuo. 2-Methoxybiphenylene (4) was prepared by methylation of 2-hydroxybiphenylene33 with CH,I/KOH in The product was purified by sublimation and recrystallization from low-boiling petroleum ether: mp 68 OC; ‘H NMR (CDC13) 6 3.70 (3 H, s), 6.05 (1 H, d), 6.30 (1 H, s), 6.47-6.68 (5 H, m). Anal. Calcd for C13Hlo0: C, 85.69, H, 5.53. Found: C, 85.91; H, 5.55. 2-Bromobiphenylene (3) was prepared from the hydrocarbon with Br2 and thallium acetate as described by McKillop3S and purified by column chromatography and sublimation. The pale yellow solid obtained by this procedure [mp 61-62 “C; ‘H NMR (CDCl3) 6 6.95-6.72 (m, 3 H) 6.70-6.60 (m, 1 H)] contains ca. 5% of a dibromobiphenylene isomer as revealed by GC/MS analysis. Efforts to remove this contaminant were not successful. The presence of this compound is not expected to influence the spectroscopic results reported herein. Spectroscopy. Absorption spectra were recorded with a PerkinElmer 532 spectrophotometer and fluorescencespectra of samples with matched absorbances were recorded on a Spex Fluorolog spectrometer. The fluorescence quantum yields were calculated by reference to a standard and are generally accurate to &lo%. The picosecond transient absorption spectra were obtained by the pump-probe method with 355-nm laser pulses (18-ps fwhm at 5-Hz repetition rate) from an actively-passively mode-locked Nd:YAG laser (Quantel, Model YG 5014). The white light continuum, generated by passing the residual fundamental beam (1064 nm) through a cell containing a H20/D20 mixture, was used as the probe beam. The probe beam was split into two parts, one nearly collinearly overlapped with the excitation beam at the sample and the other part, the reference beam, passing through an unirradiated part of the sample solution. The probe beams were focused onto the slit of a spectrograph (Jobin-Yvon) and then to an intensifieddual-diodearray detector (ST-100, Princeton Instruments, Inc.). The time resolution was obtained by delaying the probe beam using a computer-controlled translation stage. The sample solution is flowed through a 2-mm-path length cell to avoid multiple exposures. A description of this spectrometer is given elsewhere.36 The nanosecond flash photolysis experiments were carried out with Lambda Physik FL3002 laser system. The excitation beam at 308 nm was generated with a XeCl excimer laser. A continuous or flash Xe lamp provided the probe light which passes through the sample perpendicular to the excitation beam and was focused onto the slit of a monochromator. Kinetic decay measurements werecaniedout with HP54111D transient digitizer. Thetransient absorption spectra over a spectral range of 300 nm were recorded at different delays with a gated dual diode array combined with an optical multichannel analyzer (Princeton Instruments, Inc.). Results
Steady-StateAbsorption and Emlssioo Spectraof Biphenylenes 1-5. The absorption spectra of compounds 1-5 are presented in
Figure 1. The spectra of the substituted biphenylenes are clearly related to biphenylene itself. The absorption spectrum of biphenylene shows a clear progression of vibronic bands, particularly at low temperature, corresponding to its symmetryforbidden lowest-energytransition. These sharply defined bands in the spectrum of 1 have been assigned to a ca. 365-cm-’ progressionof a skeletalvibration mode.9 The absorptionspectra of substituted biphenylenes 2-5 similarly show a low-intensity, low-energy band. The position and width of these bands are differently affected by the nature of the substitution. This is
Excited States of Some Substituted Biphenylenes
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9301
1
1.6
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0.15 0.4
tt
0.2 0 220
O.' 0.05 240
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320
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360
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I
Wavelength, nm
400
Figure 1. Absorption spectra of (a) bis(bipheny1ene) (5, 1.7 X l e 5 M), (b) 2-acetylbiphenylene (2,2.2 X l e 5M), (c) 2-methoxybiphenylene(4, 1.8 X l k 5 M), (d) 2-bromobiphenylene (3, 2.2 X lesM), and (e) biphenylene(1,1.8 X 1k'M) incyclohexanesolutionat room temperature. The inset shows the region from 320 to 450 nm, which displays the red shift and broadening of the lowest-energyabsorption band on substitution of biphenylene.
--I
700 f
r
nnn
I
400
'i
450
500
.u'
I
-
550
600
650
Wavelength, n m Figure 2. Fluorescence spectra of (a) bis(bipheny1ene) (9,(b) 2-acetylbiphenylene (2), (c) biphenylene (l),(d) 2-bromobiphenylene (3), and (e) 2-methoxybiphenylene (4) in cyclohexane solution at room temperature. The fluorescence for each of these compounds was excited by
irradiation at 350 nm. particularly clear for 2 and 4 where substitution with the acetyl or methoxy group causes significant red shifts of their lowestenergy absorption band. The spectral broadening and red shift may be explainedby the reduction in molecular symmetry which formally affects the allowedness of the symmetry-forbidden SO S1 transition. Interestingly, for bis(bipheny1ene) 5 both the SO S2 and SO SItransitions show some exciton coupling. Similarly, it is interesting to compare the absorption spectrum of 1,l'-bis(naphthy1) (6, Chart I) with that of bis(bipheny1ene) 5. The absorption spectrum of binaphthyl6 in fluid solution and in frozen medium is almost identical with that of na~hthalene.~'s~ In contrast, the absorption spectrum of bis(bipheny1ene) 5 differs significantlyfrom that of biphenylene 1. For 5, the SO S1 band is shifted to lower energy by ca. 1000 cm-1 and its extinction coefficient is increased. These spectral changes indicate considerableinteraction of the *-electron systemsof the biphenylenyl rings of 5. This difference in *-electron overlap is predicted by molecular mechanics calculations as described in the Discussion section. The fluorescence spectra of biphenylenes 1-5 at room temperature in cyclohexane solution are shown in Figure 2. The fluorescenceof biphenylene itself is remarkable due to its wealth of sharply resolved vibronic bands, particularly at low temperat~re.9Jo.z~The low-temperature spectra of compounds 2 and 3 lack the clearly defined vibrational progressions of 1, but their overallprofiles aresimilar to that of 1. Toour surprise, methoxysubstituted biphenylene 4 is much less fluorescent than the other compounds we investigated. Measurement of the fluorescence quantum yields (Table I) shows that for 4 +fl
