The influence of molecular geometry on the fluorescence spectra of biphenyl and the polyphenyls

Volume

34, number

CHEMICAL PHYSICS LEl-TEk

2

1.5 July 1975

THE iNFLUENCE OF MOLECULAR GEOMETRY ON THE FLUORESCENCE SPECTRA OF BIPHENYL AND THE POLYPHENYLS K. RAZI NAQV!, J. DONATSCH and U.P. WILD Pirysical Cllemisrv Laborarory. CH-8006 Zurich, Switzerland

Swiss Federal Imth~e

of Techrrology,

Received 23 April 1975

The shapes of the fluorescence

spectra of rigid solutions

Cm ethanol at 77 K) of biphenyl, p-terphcnyl, and pquaterlight: the spectral resolution enhances with increasing wavclength of excitation. These spectra are interpreted in terms of a model wherein the equilibrium conIiguration of the fluorescent state is planar, but that of the ground state deviates from planarity. Some other results reported in the Zterature are aiso discussed and it is concluded that the Found states of the singly and doubly charged ions of these molecules, as well as the lowest t:iplet states of the neutral molecules, are nearly planar.

phenyl are shown to depend on the wavelength of the exciting

The fading that the electronic absorption spectra of solutions of biphenyl and the polyphenyls do not display, even at low temperatures, any vibrational structure, but their luminescence spectra are moderately structured even at room temperature, is explained as follows: When in its ground state, a bi(poly)phenyl molecule prefers a non-planar configuration, but in the first excited singlet or triplet state it prefers to be planar; fig. 1 is a schematic representation of the foregoing statement [ !] Electronic transitions occur without

change in geometry; molecules with varying degrees of non-planarity act as absorbers, and a structureless spectrum results from the overlapping of a multitude of (presumably structured) spectra displaced with respect to one another, each spectrum corresponding to molecules with a particular geometry. An excited molecule relaxes to a (nearly) planar configuration before emission; since the upper curve is significantly steeper than the lower, luminescence ensues from a sample in which departure from equilibrium configuration is less pronounced than in the absorbing sample, and vibrational structure is not smeared out. Guided by the spectroscopic mkm that the occurrence of a long vibrational progression in an electronic band betrays the vibrational mode responsible for changing the geometry of the initial state into that of the final state involved in the transition, Lim and Li [2]

made an attempt to estabiish the planarity of the fluorescent and phosphorescent states of Siphenyl (BP). Unfortunately, their attempt was foiled by an incorrect DEVIATION FROM_ PLAJdARITY

Fig. 1. Schematic potential energy curves For deviations from the planar geometry of a biphcnyl or polyphenyl molecule. The dotted curve refers to the ground state; the solid curve, to the first excited singlet or the lowest triplet state. The lefthsnd end of the horizontal asis represents a planner molecule.

assignment for the frequency of the torsional mode [31,and their analysis cannot be used to buttress fig. 1. Theoretical investigations of the ground state geometry of BP seem now to be converging towards the dotted curve of fig. 1 [4] . Though several calculations predict th,e lowest excited singlet or triplet state to be planar, the agreement between the calculated and ob-

setied s&&a is far frorp satisfactory ;.and the ad hoc ‘. argument given in the fmt par&graph remains more -cogent than any calculation, reported so far. The result‘s 6f ttio calculatiofis ala:, suggest that the ground state of the BP rnononegatirle ion is nearly planar 15, ‘61. Jndeed, the spectrum of’ the BP anion is considerably sharper than that of the neutral molecule, and at 77 K vibrational structure can be clearly seen [7] ; this is true also for the mononegative and the dinegative ions of p-terphenyl (TP) and pquaterphenyl (QP). Though these results could be accommodated in a model in which the ground state of the anion has a very deep minimum for a non-planar configuration, it iz more reasonable to conclude, on the basis of theoretical calculations [5,6] and some arguments to be presented

later, that the ground states of the anions

of BP, TP, and QP.are almc-st planar. Fluorene, a rigid analogue of i3P, is known to be planar in the ground state [7] ; since it exhibits wellstructured

absorption

and rl-luorescence spectra

which

obey the usual mirror-image relationship, it is natural to assume that the first excited singlet state is also planar.

Now, on the basis of the model shown in fig. 1,

one would expect that, other things being equal, the fluorescence spectra of fluorene and BP would be nearly superposable. In fact, ths two spectra show iittle similarity, and other things are evidently not equal. One must remember, for instance, that the two phenyl rings of fluorene are asymmetrically aligned with respect to the coannular bond [8]. Whatever the reason(s) for the gross difference between *he fluorescence spectra of the two compounds, comparison with fluorene is of no help in establishing the planarity of the first excited singlet state of BP. However, such comparison does provide an argument
‘_

triplet States of TP and QP, we decided to undertake an extensive investigation’; some of our results will be described in this Letter. Remembering that the spectra of tetracene and pentacene corroborated tile prediction, made by Hoytink [ 141, that the electronic absorption spectrum of an alternant hydrocarbon in its lowest triplet state should resembIe, and be blue-Gifted with respect to, the spectrum of its dinegative ion, we began by comparing the triplet-triplet spectra of TP and QP, recorded by US and others [9,15] , with the published spectra of the corresponding dinegative ions [7]. The comparison

confirms

Hoytink’s

cates that the geometry

prediction

of the lowest triplet

and indi-

state of is nearly the same as the geometry of the ground state of the corresponding dinegative ion. Our observation of the likeness of the triplet-triplet spectra of TP and two of its rigid analogues [ 16,171 leads finally to the deduction that Lhe triplet state of TP, and consequently the ground state of the TP dinegative ion, is planar; it seems reasonable to extend this conclusion to the ground state of the dinegative ion of QP and the lowest triplet state of the neutral molecule. For ascertaining the planarity of the polyphenyls, a new stratagem was necessary. The curves of fig. 1, drawn under the assumption that the solvent does not significantly restrain the torsional motion of the excited molecule, gave us the clue to an alternative approach: it follcws from fig. 1 that the more nearly planar an unexcited molectile, the longer the wave!ength corresponding to the onset of its S1 + So transition; consequently, the fluorescence spectra of highly viscous solutions of the polyphenyls shou!d become increasingly structured as the wavelength of excitation (A,,,) is moved progressively towards the red edge of ‘he absorption spectrum. Our results, shown in figs. Z-4, confirm this prediction and corroborate the model depicted in fig. 1. The fluorescence spectra in figs. 2 and 3 indicate that the ri@d environment (ethanol, 77 K) does not prevent the excited molecule from relwing to a more nearly planar geometry; otherwise the fluorescence spectrum would have been as structureless as the absorption spectrusn, and its energy would have varied appreciably with the viscosity of the medium. That the &ape of the fluorescence spectrum is sensitive to A&C implies that the rigid solvent does impose some constraint on the degree of planarity attained by the exTP or QP

VQbIie

34, number 2

CHEMICAL

PHYSICS

LETTERS

cited molecule excited

during

its lifetime.

state population

becomes

As A&increases,

the

richer in planar or

nearly plana: conformers, and the fluorescence specm trum looks increasingly like that of 3 planar aromatic hydrocarbon having an Jlowed radiative transition. To complete the argument, we show, in’ fig. 4, the fluerescence

spectra of TP xlutions at 300 and 200 K; at the former temperature, the shape of the fluorescence

spectrum WAVELEWGTH

(mm)

Fig. 2. Uncorrected fluorescence emission spectra of TP (ca. I 0e5 hi in ettunol, 77 K). Escitation wavelengt’n: 280 nm, --310 mm, .._ 31.5 nm;excitation bandwidth: 4.5 nm.

remains practically

independent

of hexc; at

2OG K, A,,, did affect the shape of the fluorescence spectrum, but the effect is negligible in comparison

with that seen at 77 K. When we did a similar experiment with BP solutions, we found that the resolution of both the fluorescence and the phosphorescence spectra increased with increasing \,,. However, the effect of A,_,,, on the shape

of the emission spectra was much smaller than

WAVELENGTH

(nm)

Fig. 3. Uncorrected fluorescence spectra of QP (ca. low5 hi in athanoi, 77 K). Excitation wavelength: -- 320 nm, --330 nm, . . . 335 nm; excitation bandwidth: 4.5 nm.

1181 and more recently y-irradiated

WAVELEtiGTH

(nm)

Fig. 4. Uncorrected fluorescence spectra of TP (cn. 10S5 hl in ethanol) at 300 K (---) and 200 K (-aar.d . ..). Excihtion waveler&~: --and - 310 nm. _._ 28s nm;excitntion bendwidth:

4.5 nm.

that

seen in TP and QP. Even with the longest Xerc employed in our work, the shape of the fluorescence spectrum did not become similar to those of TP and QP spectra; this is in keeping with Berlman’s conclusion that the fluorescent state of BP is of a different nature from that of TP [13]. Berlman has noted that compounds planar in both t.he ground state and the first excited singlet state are more prone to excimer formation and/or concentration quenching than non-planar compounds [ I] - Thus, despite the planarity of its fluorescent state, biphenyl is neither expected nor seen, in prompt fluorescence studies [I] , to form excimers, because the other component of the excirner - the ground state monomer is non-planar. Now, it was argued above that the ground states of radical. ions of BP and the polyphenpls are planar; if this assertion is correct, recombination of radical mono-cations and mono-anions of BP, TP, and QP should generate excimer fluorescence. Excimer emission was indeed observed in a low temperature pulse radiolysis study of BP solutions in isopentane in the thermoluminescence

of

BP solutions

in squalane [ 191 . Since the triplet states of BP and the polyphenyls are believed to be planar, triplet-triplet annihilation should also engender (delayed) excimer fluorescence. Taken individually, the results of the theoretical calcu!ations or any of t!!e foregoing arguments based on the spectral charactetistics may be called in question, but t&en together they leave little room for doubting the view that the ground states of BP, TP, 237

~Vol&ne

34, number

2

CHEhlICAL

PHYSICS LETTERS

’ and.QP are non-planar, but the ground states of their

‘singly and doubly charged radidal ions are netily planar, a&are the first excited singlet and the.lowest triplet states of the neutral molecules. From the knowledge thsf neither triplet-triplet annihilation [ZO] nor elec. trogenerated cation-anion annihilation 1211 leads to excimer fluorescence in 9,lOdiphenylanthracene soluthat the ground states of its mononegtive and monopositive ions and its !owest tripiet stat& are all non-planar; the non-planarity of the monopositive ion has aiready been pointed out by Hamilton [22]. While discussing our fluorescence spectra, we tacitly assumed that the lowest energy band in the absorptions, we can also conclude

tion spectrum corresponds to one excited electronic state. In biphenyl, however, vhree L electronic transitions (with different oscillator strengths) seem to lie in the spectral region covered by the first absorption band [12] ; simila:ly, two or more electronic transitions WJJ comprise the first absor$ion band of TP and/or QP. The mutual separation of these close lying electronic levels will depend on the geometry of the molecule, as will the oscillator strengths for transitions from

the ground state of these states. Complete characterisation of the absorption and fluorescence spectra would thus require consideration of more than one absorption transition and internal conversion to the lowest excited state. The simplistic model of fig. i would, however, remain applicable if the energies of all the excited states (lying in the first absorption band) attain their minimaat the planar configuntion; our re-

sults indicate that this is true for bipheriyl and the polyphenyls. A full discussion of the problem will be pubhshed later. We gratefully acknowledge receiving financial sup-

15 !uly 1975

work from the Schweizerischer Nationalfonds zur Fijrderung der wissenschaftlichen Forschung.

port for this

References . [ 11 I.B. Berlman, J. ences therein.

Pi-~ys.Chem.

‘74 (1970) 3085, and refer-

E-C. Lim and

Y.H. L, J. Chem. Phys. 52 (1970) 6416. R.D. McAlpine and J.D. Whiteman, Phys. 58 (1973) 5078.

t:; R.hl. Hochstrasser. J. Chem.

141J. Almliif, Chem. Phys. L51A. Golebiewski and A. (1970)

6 (1974) 135. Parczewski, Z. Naturforsch.

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I61

A.D. McIxhlan, Mol. Phys. 3 (1960) 233. Buschow, J. Dieleman and G.J. Hoytink, hlol. Phys. 7 (1963) 1. !81 DM Burns and J. Iball, Proc. Roy. Sot. A227 (1955) 200. and H. Labhart, C’hem. Phys. Letters 4 [91 W. Heinzelmann (1369) 20, and references therein. [lOI P-J. Wager, J. Am. Chem. Sot. I39 (1967) 2820. [Ill J. Slispelter, Chem. Phys. Letters 10 (1971) 539, and rcferences therein.

I71 K.H.J.

[I21 LB.

Berlman,

J. Chem. Phys. 52 (1970) 5616. H.O. Wirth and O.J. Steingraber, J. Phys. Chem. 75 (1971) 318. Pure Appl. Chcm. 11 (i963) 393. [I41 G.J. Hoytink, and P.R. Hammond, J. Am. Chem. Sot. [ISJ T.G. PavIopouios 96 (1974) 6568. U.B. Rannldsr, H. Kinzig and U.P. Wild, J. Photochem. [lb! 4 (1975) 97.

[I31 1-B. Berlman,

I171 U.B. Ranalder, 1181 1191

J. Donatsch, K. Razi Naqvi and U.P. Wild, to be published. J.H. Basendale and P. Wardman, Chem. Commun. (1971) 202. B. Brocklehurst, D.C. Bull and Xl. Evans, J. Chem. Sot.

Faraday II 71 (197.5) 543. (1967) 744. [?Ol C._4. Parker and T.A. Joyce, Chem. Commun. J.W. Lfmgwarth and R.E. Visco, J. Am. 1211 E.A. Chandross, Chem. Sot. 87 (1965) 3259.

1221 T.D.S.

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