Fluorescence spectra and non-radiative processes of biphenylene vapor

Chemical Physics 47 (1980) 389-394 0 North-Holland Publishing Company

FLUORESCENCE

SPECTRA AND NON-RADIATIVE

Nobuhiro OHTA, Masahisa FUJITA,

Hiioaki

PROCESSES

OF BIPHENYLENE VAPOR

BABA

Divisionof Chemistry, Research Institute of Applied Electricity, Hokkaido University, Sopporo 060, Jopall

and Haruo SHIZUKA Department of Chemistry, Gunma Uhhsity, Received 5 September

Kiryu, GUWR 376, Japan

1979

Fluorescence spectra of biphenylene vapor were obscrvcd under collision-free conditions with excitation into different vibrational levels of the lowest excited singlet state (SI) and of the second excited singlet state (Sp). The fluorescence spectrum resultins from escitation into the vibrational level of S1 shows a distinct structure, while the one from cscitationinto the vibrational level of S2 is diffuse and slightly red-shifted, indicating that the fluorescence is emitted from a vibratioxd level of S1 even when the biphenylenc molecule is excited to Sz. The fluorescence quantum yield is as low as 3.3 X lo4 at the highest, and it decrcascs with increasing escitation energy. The unusually low fluorescence quantum yield of biphenylene, compared with those of other alternant hydrocarbons, is attributed to fast S1 -p So internal conversion caused by a large configurational difference between S, and So. The vibrotic couplingis discussed which allows the forbidden S1 c* So transition to occur.

1. Introduction Biphenyleneis simiiarin structure to naphthalene and anthracene, but its emissioncharacteristicsare unusual and have been the subject of considerablecontroversy. Once Hilpern [l] reported anomalous fluorescence from the second excited singlet state (S2) of the biphenylene molecule. Birkset al. [2] also observed the anomalous fluorescence in addition to the ordinary fluorescence from& first excited singlet state (S,). Later, however, it was proved by Hochstrasserand McAlpine[3] that these fluorescence emissionsoriginate from an impurity. After cautious purification they could not observe any emission which was attributable to biphenylene. On the basis of an unpublishedstudy by Walker [4a] on the fluorescence of biphenylene in cyclohexane, Birks pointed out that biphenylene appears to emit normal “S,” + So fluorescence [4b]. It is to be noted, however, that the state which was named “S,” by Birks is actually

the one which should be assignedto S2. Recently Shizukaet al. (51 have observed the fluorescence spectrum of biphenylene in some solvents. They concluded from the result of a fluorescence polarization experiment with crystal powder that the fluorescent state should be S,. If the fluorescence in solution were to be emitted mainly or exclusively,from Sz, biphenylene would provide a very exceptional case in molecular emissionspectroscopy; since the energy separation between its S2 and St states is rather small (=X00 cm-‘). In view of these facts, the fluorescence of biphenylene has been reexamined in the present study. Fluorescence spectra were measuredin the vapor phase at pressureslower than 500 mTorr. The use of biphenylene vapor has an advantagein that the weak fluorescence can be observed without being contaminated by an impurity emission,which may often originate from the solvent when a solution is used for fluorescence measurement. Furthermore, at a pressure lower than

390

M

Ohta

et alfFhorescence

500 mTorr, the biphenylene molecule in an excited singlet state can be shown to be free from co!.!isions during the lifetime of the excited state. Under these conditions, one may readily see how the nature of fluorescence varies with the vibronic level into which the molecuIe is initially excited [6,7]. The fluorescence and excitation spectra of biphenylenc vapor obtained in this study indicate indubitably that the fIuorescence always originates from S, _The low fluorescence quantum yield of biphcnylenc is ascribed to a geometrical change caused by escitation.

2. Experimental

Biphenylene was synthesized and purified by repeated recrystallizations from ethanol, followed by zone melting. The sample was degassed by the usual freeze-pump-thaw cycles on a vacuum line. During the spectral measurement the sample vapor was maintained at a particular pressurebetween3 and 500 mTorr by keeping the c-e11containing an excess of the sumple at a definite temperature between 30 and 100°C. The vapor pressure was estimated through the absorption intensity on the assumption that the value of the molar estinction coefficient at the O-O band of the S2 + So transition in the vapor phase is equal to the corresponding value in a cyclohexane solution, 9.9 X 103. (See section 3 for the position of the O-O band.) A Cary 15 spectrophotometer was used to obtain absorption spectra. Fluorescence and excitation spectra were measured with a high-sensitivity emission spectrophotometer based on the photcn counting method and equipped with a cooled Hamamotsu R585 or R649 photomuItiplier [S]. The fluorescence spectra were corrected for tile spectral sensitivity of the detector system by using a solution of quinine bisulfate. The excitation spectrum was also corrected fur ihe spectral quantum iatensity distribution of the exciting Ii&t and for the inner filter effect by using a solution of rhodamine B. The absolute quantum yield was dcterminL:d by comparing the corrected fluorescence spectrum with that of quLGne bisulfate in a 0.1 N sutfuric acid [9]. A nitrogen-laser-pumped dye laser (Molectron Spectroscan 10) was used to measure the fluorescence decay with a single-photon counting apparatus equipped with a time-to-amplitude converter.

of bipkenylene

vapor

3. Results and discussion Two different electronic transitions were confirmed between 310 and 390 nm in the absorption spectrum of biphenylene from the solvent effect and the vibrational structure [3,10]. The polarized absorption spectrum ofa biphenylene-naphthaIene or a bi-

phenylene-biphenyl mixed crystal shows that both transitions are polarized along the long molecular axis [3,11]. Zanon [lo] supported this conclusion on the basis of the rotational structure of the bands belongingto these eIectronic transitions. The intense bands in the region between 310 and 360 nm were assigned to the SZ(Bju) + So&) absorption [12]. Note that the long and short mol&ular axes are taken asx and J’ axes, respectively; the absorption spectrum is shown in fig. 2a below. By comparison with theory, the weak but sharp bands in the region between 360 and 390 nm were assignedto the S, (Blp) + SO(A,) absorption [ 121. This, along with the polarizati& data, shows that the St + So transition becomes allowed through vibronic interaction, involving a bzu vibration, between S, and a B3u singlet state; the nearby S, state is the most probable candidate for such a Bjn singlet. Fig. I shows the fluorescence spectra of biphenylene obtained at 9O’C by excitation at 382 and 354 nm under a pressure of ~450 mTorr. The band at 354 nm in the absorption spectrum was assigned to the O-O band of S2 + So [IO]. The assignment of the absorption band at 382 nm has not been established, though it must be one of the vibronic bands of the S, + So transiticn [IO,12]. Both spectra in fig. 1 were obtained with an excitation bandpass of 2 nm and an emission bandpass of 3 nm. The former bandpass is narrower than the halfwidths of the bands at 382 and 354 run, 3.5 and 5.2 nm, respectively. The fluorescence spectra obtained with excitation at 382 and 354 nm will hereafter be denoted by spectrum I and spectrum II, respectively. It may be noted that when biphenykne w2s excited into Sl, no emission was detected in the frequency region just adjacent to the S2 + So absorption spectrum where the S, + So fluorescence is expected to appear if it occurs at all. The lifetime of biphenylene fluorescence was too short to be determined by means of our pulsed dye laser with a duration of =5 ns, so that it-must be

N. Oh ta et ai.jFi’uorescence of biphenyhe

55u

500

450

391

vapor

bUL)

WAVELENGTH (nm)

Fig. 1. Fluorescencespectraof biphenylcnc vapor at 90°C obtained by excitation at 382 nm (-_) and at 354 nm Fe). The spectra were correctedas describedin the test; the relativeintensity representsrelativequanta per unit wavelengh interval. For 382 nm escitation,

the scattered exciting light is superposed

on the short-wavclcngth

portion (380-420

nm) of the spectrum. The

bands discussedin the test arc indicated by arrows.

shorter than 1 ns. This is consistent with the decay time of the lowest excited singlet state in hexane at 300 K, 24-Ops, obtained recently by Lin and Topp [13] with a new laser technique. The meantime between successive collisions of biphenylene molecules under a vapor pressure of 450 mTorr at 90°C is estimated to be 84 ns from the gas kinetic theory on the assumption of a 10 A molecular diameter. Accordingly, the present experimental conditions can be regarded as collision free, and the fluorescence is considered to be emitted from a level which has the same energy as the initially excited state. Spectrum I shows a sharp structure, whereas spectrum II is very diffuse (fig. I). These spectra are similar to each other in the general intensity distribution, but a slight red shift is noticed for spectrum II as a whole compared with spectrum I. In fact the intensity maximum at 470 nm in spectrum II corresponds to a maximum at 467 nm in spectrum I. Fig. 2a shows the corrected excitation spectrum obtained with an excitation bandpass of 1.3 nm by

monitoring the biphenylene fluorescence at 470 nm with a 19 nm bandwidth, together with the absorption spectrum obtained with a 1.3 nm bandpass. Apart from the intensity distribution, the complete agreement of the band positions in the excitation and absorption spectra indicates that the observed fluorescence is due to biphenylene itself. It is seen that, w!len excited to S,, the biphenylene molecule emits fluorescence from the vibrational level of S, with a relatively high efficiency. In the case of excitation to S2, the fluorescent state cannot be determined from the excitation spectrum alone. The fluorescence spectrum resulting from excitation to Sz, e.g. spectrum II, is very diffuse and slightly red-shifted compared with spectrum I. Therefore, on reference to vapor-phase fluorescence spectra of other polyatomic molecules under collision-free conditions [14], the fluorescence induced by Sz + So transition is found to be emitted solely from the highly excited vibrational level of S, _ In other words, excitation to S, is followed by very fast internal conversion to the l&h vibrational level of

A! Ohta et alfFluorescence of biphenylene vapor

39’

1(a)

WAVELENGTH(nm) 135 2. (3) The Iluorcsccncc-escit~tioll spectru~n (-) and absurprion spuctru~n (--) of biphcnylenc vapor at 90°C. T11e cxcitxtion spcctrtm ws corrected 3s dcscribcd in tlw text. For the absorption spectrum, tiw rchtivc intensity is proportional to the absorbrtncc. (6) The Iluorcsccnce quantuniyield of biphcnylcnc vapor ut 90°C as a function of cscitation \wcIcn:rtlL

S,, yielding spectrum II or its analogue. It is thus concluded that the tluorescencc of the isolated biphenylene m&cule origirlates from S, even if the molecu:le is excited into S,. The fluorescence quantum yield as a function of excitation wavelengthwasobtained by the use of the excitation and absorption spectra; the result is shown in fig. Zb. It is noticed that the quantum yield shows a remarkabIe excess energy dependence. The quantum

yield in the spectral region of the S, +-So transition showsa monotonic decrease as the excitation energy increases,and it is smaller than the yield at any excitation waveIengthcorresponding to the S, + SOtransition. Becauseof the serious overlap between two absorption band systems for the St f SOand S, + So transitions, due to the small energy separation (QIOO cm-‘), the observed quantum yield at any band of the S, + So transition is not expected to represent the true quantum yield for the vibrational level of S, associatedwith that band. Nevertheless,the latter quantum yield seems to decrease with increasing excess vibrational energy, except for the level correspending to the first absorption band at 387 nm. As has already been shown, the fluorescence of biphenylene is emitted from a vibrational ievel of St irrespective of whether the molecule is initially excited into St or S1_Let it be assumed that the radiative lifetime is constant for all the vibrational levels of St. Then, aside from the fluctuations of the qumtum yieid value, the decrease of the quantum yield with increasingexcitation energy, as observed in fig. 2b, suggeststhat the non-radiative decay rate from the vibrational level of S, increaseswith increasingexcess energy. The fluorescence quantum yield does not change appreciably on changingthe excitation wavelength from 387 to 382 nm (fig. 2b). The reason for this may be that the weak absorption band at 387 nm is a hot band, and consequently the same emitting state is reached by using either of the 387 and 382 nm bands for excitation. Three kinds of non-radiative processes are considered to occur from S,, that is, the St + So internal conversion, S, + T, intersystem crossingand photochemical reaction. From a study of transient absorption spectra associated with a sensitized triplet state of biphenylene, Tetreau et al. [15] concluded that the S, -+ T, intersystem crossingis not the main non; radiative process from S, , The same is true for the photochemical reaction, t!le quantum yield of which was reported to be less than 10e5 [5]. Accordingly, the S, + S0 internal conversion is considered to be the main non-radiativeprocess starting from S1. As is seen in fig. 2b, the fluorescence quantum yield of biphenylene is only 3.3 X 1O-4 at the highejt which is the yield value for excitation at the 382 nr$

N. Ohta et al/Fluorescence of bipheny!ene vapor

band. The low fluorescence qrrantum yield of biphenylene; compared with other aromatic hydrocarbons, can be interpreted as due to unusually fast S, + So internal conversion.The excess energy dependence of the fluorescence quantum yield is ascribableto the changeof the St i S, internal conversion rate with excessenergy. In general, the rate of the S, + So conversion from a vibrational level of S, is related to the Franck-Condon overlap factor between that vibrational level and the isoenergeticvibrational level of So. The increase of the S1 + SOinternal conversion rate of biphenylene with increasingexcess energy may possiblybe attributed to the increase of the correspondingFranck-Condon factor. A large configurational difference between S, and So is suggestedby the fluorescence spectrum I, since the separation between.the intensity maximum of the Franck-Condon envelope of spectrum I and the position of the longest-wavelengthbands belongingto S, + So absorption is as great as 5000 cm-l, indicating the occurrence of a large Stokes shit. The unusually fast S, + So internal conversion may thus be attributed to a large Franck-Condon overlap factor which must result from the large configurational difference mentioned above. Next, the vibrational structure of the fluorescence spectrum I will be discussed.In fig. 1 is shown the displacementin cm-l of each of the vibronic bands from the position of the exciting light at 26170 cm-1 (382 nm), where a strong fluorescence band is probably hidden under the much stronger scattered exciting light. Although the assignmentof the absorption band at 382 nm has not been established,it may safely be assumedthat the O-cm-r displacementin spectrum I corresponds to the zero-point vibrationallevel of the ground electronic state, SO.The vibrational structure of spectrum I appears to have noteworthy features, but we are not in a position ti make a full analysisof the structure on account of the low spectral resolution. The discussion,therefore, will be confined to vibronic bands with reIatively small displacements. From an analysisof the absorption spectrum of biphenylene [lo], two vibrational frequencies, 748 and 1I30 cm-l, are known to exist in S,. They are assignedto aBvibrations which, according to the Raman spectrum [ 161,have frequencies of 765 and 1105 cm-l, respectively, in So. Two bands observed in spectrum I at distances of 738 and 1103 cm-*

393

from the exciting light position can be related to these ag vibrations, provided that the experimental error is taken into account. A relatively intense band appearsat 1499.cm-1 displacement,but the frequency 1499 cm-r cannot be assignedto any fundamental in So. If the absorption band at 382 nm corresponds to the 1 6 0 band in a bzu vibration (hereafter referred to as vJ, the 1 -f 0 and 1 + 2 bands in ~a are expected to appear in spectrum I, since they are vibronically allowed. The 1 + 0 band must be located at O-cm-t displacement,and the I+ 2 band lies to the red of the 1 + 0 band at a distance of two quanta of V, in So. A b,, vibration’witha frequency of 751 cm-r is found in the infrared spectrum of biphenylene [17]. If the vibration V~is identified with this 751 cm-r vibration, the band in question at 1499. cm-l displacementcan reasonably be assignedto the 1 + 2 band in va. The rest&s of the present study can be summarized as follows: (1) Biphenylene vapor exhibits fluorescence which otiginates from S, irrespective of whether the biphenylene molecule is initially excited into its S, or Sz state. (2) The fast S, + So internal conversion resulting from a large difference in configuration between S, and So, along with the slow radiative decay, leads to an unusually low quantum yield of the biphenylene fluorescence. (3) The blu vibration with a frequency of 751 cm-t in So appears to participate in the vibronic couplingbetween Sl and SZ which induces the St * So transition polarized a!ong the long molecular axis. We have also measured the fluorescence of biphenylene in 2.methylpentane solution, and have found that the fluorescence spectrum is similar in the intensity distribution to those obtained in the vapor phase and that the fluorescence-excitation spectrum in solution follows closely the corresponding absorption spectrum in both the S, + So and S2 -+So absorption regions. These observations indicate that the fluorescence characteristicsof biphenylene in solution are essentially the same as in the vapor phase, except that in the former case the vibrational relaxation is completed before the fluorescence is emitted.

N. Ohra et al./Ruorescence

394

Acknowledgement The authors wish to express their gratitude to the Ministry of Education for one of the Government grants in aid of scientific researches,special researches (l), 1978.

References [ t ] J.W. Hilpcm, Trans. I%aday Sot. 6 1 (1965) 605.

[ 21J.B. Birks, J.M. de C. Contc and G. Walker, Phys. Lcttcrs 19 (1965) 125. 131 R.M. Hochstrasar and R.D. Mcr\lpinc, J. Chcm. Phys. 44 (1966) 3375. [4] (a) G. Walker, unpublisbcd muIts; (b) J.B. Birks, Photophysics of aromatic molecules (Wiley-lntcrsciuncu, New York, 1970).

[ 5j Ii. Shizuka, T. O$van,

S. Cho and T. liorita,

Phys. Lcrrcrs42 (1976) 311.

Chcm.

of biphenyiene vapor

[6] M. Stockburger, in: Organic moIecular photophysics, Vol. 1, cd. J.B. Birks (Wiley-Interscience, New York, 1973) p. 57. [7] J.B. Birks, in: Organic molecular photophysics, Vol. 2, cd. J.B. Birks (Wiley-Interscience, New York, 1975) p. 409. [S] K. Chihara and H. Baba, Bull. Cheti. Sot. Japan 48 (1975) 3093. [V] W.H. hlclhuish, J. Phyr Chem. 65 (1961) 229. [lo] I. Zanon, J. Chcm Sot. Faraday II 69 (1973) 1164. [ 1I] R.M. Hochstrasser, J. Chem. Phys. 33 (1960) 950. [ 121R.hi_ Hochstrasser, Can. J. Chem 39 (1961) 765. [ 131 H.B. Lin and M.R. Topp, Chem. Phys. Letters 64 (1979) 452. [ 141 J. Jortner, S.A. Rice and R.M. Hochstrasser, in: Advances in photochemistry, VoL 7, cdr W.A. Noyes Jr., J.N. Pitts Jr. and G.S. Hammond (Wiley-Intcrsciencc, New York, 1969) p. 149. [ 151 C. Tctrcau, D. Lavalette, E.J. Land and E Pcradcjordi, Chem. Phys. Letters 17 (1972) 245. [ 161 A. Cirlando and C. PcciIe, J. Chem. Sot. I’araday II 69 (1973) 813. [ 171 C. Pccileand B. Lunelli, J. Chem. Phys. 48 (1968) 1336.