Modulation excitation spectra of some aromatic hydrocarbons at high concentration, evidence for triplet excimer formation

Volunie 25f;‘iumber

LETTERS

CHEMIC+PHYSiCS

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.MOD WATTON EPCCFFATItiN. SpECTti OF SO& ARQMATK HhiOCARBOMS PT~G~~~~N~~NTRATI~N.EVID~~CEFOR~'RIPLETEXCLII~ERF~R~~~ATION : ;. 1M.A.SLIFKIN anndA.0. AL-CHALA~I* ~Depnrtm’entof Pure and Applied Physics. The SalfordM5 4WT, UAI

Univtirsity,

Received ‘26 April 1974 Mo.duhtion

&citation

spectra of 1:2_benzantIuacene

and 1:2:3:~Idibemnthracene

at very high concentration

are characterized by the appearance of new brood bands, decreased lifetimes and a marked demmse in the quantum yield of the triplet state. These tie interpreted as arising frdm triplet excimer formation.

it was discovered a long time ago [I] that at high concentration the.norrna! fluorescence spectra of certain aromatic hydrocarbons, pyrene for example, were quenched and replaced by a new emission spectrum attributed to the formation of an excimer, i.e., an unstable complex fortied between the singlet excited molecule and a ground sttie’molecule. Much more recently the pyrene excirner has been observed in absorpiion using moduIation excitation spectrophotometry [2]. We have now looked for the triplet cxcimer using the same techniqtie by observing very 1 highly concentrated solutions of some very soluble polybenzenoid aromatic hydrocarbons in toluene at room {emperature. me triplet excimer has been postulated to explain the concentration quenching of phosp&oresc$nce of fluid sclutions of various aromatic hydrocarbons [?I. The triplet excimer is the tmrisient dimer formed between an,excited triplet state molecule and a ground ,state molecule, viz., 3M*.+ 1M.A 3D”.. The methods anh techniques of modulation spectrophotometry have been reviewed elsewhere as has our i&r&lent [4, 51. In figs...l_and 2 are shown the modulation.exciiation spectia of 1:2:3:‘$dibenzantiacene and 1:2&enzanthracene in deoxygenated toluene at room fRe$nt

address: SchooLof Physics,University Coventry, UK.

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temperature in a 1 nun pathlength,cell. A comparison of these spectra with the triplet-triplet spectra at much lower concentration shows certain differences, [6,7]. The major difference in both cases is the appeara;lce at the higher concentrations of a broad structureless band in the red, although the rest of the spectrum is similar. These bands occur at 612 nm in the exe of the dibenzantluacene and at 618 nm for the benzanthracene. Atother striking difference can be seen in fig. 1 in which the ratio of the triplet-triplet transition at ca. 490 nm to the ground state depletion level at ca. 410 nm is at least an order ofmagnitude smaller than that obtained at 10m4 M concentration 161. This confirms that a high proportion of the molecules excited from the ground state are not in the triplet state :>ut have been converted to some other transient species. Yet another difference is that the lifetimes of the. tripIei states at these concentrations are in the range of 10-s to 10e6 s which is‘less than the lifetime at ldwer concentrations which are of the order’of ... 10-4 s [7]. These obsetiations clearly-demonstrate the quenching of the triplet state at high co&entrationS and show the aljsorption.spe$urn of the new transienf-species. The most likely explanation is that’this,new species is the triplet excimer. As’s footnote we might mentipn that other molecules we h&e examined which do not ‘. .’

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Fig. 1. Modulation excitation spectrum of 1%benzantiacene (10-l M) in toluene at room temperature. Pathlength 1 mm, modulation Gequency 800 Hz.

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Pi& 2. Modulation excitation spe-ctrum of 1:2:>.44benzanthracene (IOm2 Mj in toIucne at room temperature. Path~lcrgth 1 mm, modulation frequency 800 Hz.

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khibit‘ these effects at hgi~ corlcentrafions are pyre,ne;l: 12-be?ioperyl&e and’:be&o (b) chrytine.. lJdyi.fer, it b possible that’the ljroad 'Structuieless band seen in highly concentrated pyrene in cycle; :’ hexane at caI.490 e. [Z] in contradistinctiori to tk :, stru+ured band seen in toluetie and poly(kethylY methacrykte) [8] ..is also ,a triplet ex&n&. .

qHYSICS LETTERS

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1 Novemter

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[I] Th. :Fo;st& arid K.Kas-pu, ,.. .- 976..

2. Elektroch&

[2] M.A. Slifkin and’A.0. Al-ChaQi;Chem. iQ (2973) 211.

j9, (l,F$)

Phys. Letters.

[3] J. Lngelaar, Thesis, University of Amsterdk, me Netherlands (1969). [4] M.A. Slifkin, @ys. Eull. 24 (1973) 431. I.51 M.A. SIitIcin and R.H. Walmsley, J. Phys.‘E3 (1970) 160. [6] MA. Slifkinand RH. WaLqsie$Photochem. Photobiol. 13 (:!9?1) 57. .’ [7] v. Labha& Helv. G-k. Acta.47. (1964) 2279. -. [B] M.+.SLifkin and A-0. AiChalabi, Chem. Rhys. Letters, to kc published:

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,One of us (AD. A.-C%)wishes to acknowledge tenure of a-university of Salford research studentsliip; .Th~‘modulation

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sPectrophotorrieter was. from the Cancer Research

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