Luminescence from a sandwich dimer of anthracene

.‘.

Received20 December 1974 :

Luminescence of a sandwich pair of anthraccne, prod&d by photolyk cleavageof ~~~~~~~~e, was investigated in methylcycIohcxane &ss at 77 K, The polarization of fluorescence indicates #&the lowest energy excimer s&giet state is derived from the ‘& molecHar exciton state, while the absence of a monomer-like phorphorescence suggests that the triplet state is v&y prbbabiy bound in the sandwich pair. The cizangiin~ polarization of the fluorescence t~ou~out its spectrum appear& to show differing dependence of chqe resonzce and excitation resonance co~~butions upon the intermolecular separations. .

onance (A+A-*A-A+) effects. Although polarizations of “excimer-like” fluorescence in pyrene and i?eryIene crystals [?I are consistent with the L, origin of the excimer state alternate a~~~rnents cannot be ruled out, and have in fact been proptied for several aromatic hydrocarbons [S]. A detailed theoretical study on &I anthracene sandwith pair [9) did not lead to r definitive assignment of the monomeric state from which the lowest energy excimer singlet state is de&d. It is therefore somewhat surprising to find that no measurement of the polarizatiorj of excimer fIuorescence from sandwich pairs ofaromatic hydrocarbons ha been reported, EX= cept for one very inconclusive, and doubtful, study on anthracene [lo]. There have be& several ex&mentaI reports of broad structureless F~~osphores~en~e from concen-, trated solutions of aromatic hydiocarbons, whicfr have been ass@ed to excimer’ phosphorescence, but the validity of these assignments&as been questioned at

1. Introduction The &ndwich pairs qf aromatichy&ocarbons;obtained by% photocl~emical cleavageof cowlent&

bonded dimers in rigid matrices have been proved tiy ChdIoss and co-workers [I ,ZJ to be extremely valuable for the understanding of excimer fobnation in ‘aromatic molec~es. The unique values of these Sp& ties lie in the fact that they allow the study of polarization.ofexcimer fluorescence ~thout.concen~ation depolariz&ion (high concentrations of monomers are otherwise needed td form excimers) and allow the ‘measuiements of phosphorescence of the molecular pair in rigid glasses (phosphdkscence is difficult to detect in-fluid media in which the encounter between ‘triple& gnd ground state molecules is possible;w~e, ., in rigid matrices, for which the detection of phpsphorescence is usual.ly straightforward,~it is difficult to’ ,‘. obtain mole&lar pairs of suitable geometry). The polarization of extiimer flti0rrscen.c.e is impoftant for L the understanding oFthe nature of the tionomer singlet state from which the lowestenergyexcimersingfet state is derived, $&ile the spectral ~h~racte~tics of phosphorescence of the molecular@aircan giveinformation on the stabi&,of triplet exdmers. : :. ‘. Current theoties on excimers E3-9 generalIy’c& : cluck that the bind$~g energy of the species is due to ,‘,. a’combinatio? of excitalion resontiije @*A++A*), involving t&‘& state (Piatt,notatitin), and charge r&: ._

:

least once

‘..

‘.

TEe’ main prbblem

[llj.

is the inherent

difficulty of measuring phosphorescence from fluid media and the~dif~culty of obtaining molecular pairs of suitable geometry in rigid matrices. Cai&.lations by ,Chandra a+ &im [12?] suggz2 that,triplet excimers of aromatic h~~oc~bons are susqeptible to dissociation into a monome; tripIqt +3 a monomer ground state, ig the int&planai sepa&ions and geom&ies favorable” f6r triplet excimers are very si+,& to those for singlet.;; ~x&&~,.~ _. _ _’ .. 503 : : .’ ,’ ,’

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.’ Volume 32, number 3.

CHEMICALPHYSICS LETTERS,

1 May 1975

measurements on a sandwich dimer of anthracene. of -fluorescence indicates that the emitting state is derived from the L, molecu!ar exci-

The most definitive exp&ime&I work on,excimer is that of Chandross and Dempster 1111 who studiep,the emission spectra of a sandwich .pti of naphthalene molebules, produced from the photochemical cleavage of dinaphthylpropafie. Their ‘results shqw that the phosphorescence spectrum of .the molecular pair is structured and monomer-like, despite the fact that the fluorescence of the sandwich dimer. is a broad structureless emission of excimer type. Although these results strongly suggest instability of the triplet excimer it is not clear to what extent the conclusion can be extended to other molecules. .From theoretical points of view, the. main contribution to the binding energy of triplet excimer is expected to come from charge resonance effects, since the contribution of exditation resonance is ne&ible due to the spin-forbidden nature of singlet-triplet transition. The binding energies of triplet excimers may therefore vary considerably with the difference between the ionization potential and electron affinity (1-d) of the molecule. Also, the interplanar separatioti favpred by triplet excimers may not find close phospZl+rescence

The bolar’,tition

ton state, WI& the absence

of a monomkr-like

phos-

phoresceE.ce suggests that, the triplet state is very probably bound in the sandwich pair. The changing polarization of the fluorescence throughout its spectrum appears to show differing dependence of charge resonance anil excitation resonance contributions upon the intermolecular separation.

2. Experimental

’

correspondence to the intermolecular separation~JI the excimer singlet state. In this paper we report the results of luminescence

A sandwich dimer of antluacene was prepared in methylcyclohexane glass ai 77 K by photoGherniGa1 cleavage of dianthracene, following the procedure of Chandross et al. [ 1,2]. The dianthracene sample was a gift (in 1966) of Dr. D.E. Appleqnist (and subsequently purified), while perdeuterated dianthracene was prepared by ultraviolet (h > 280 m-n) irradiation of a deg;.sse.d solution of perdeuterated anthracene. Measurements of the polarization of fluorescence spectrum (and its excitation ~~ctrum) were based Oll the photoselection method, and they were run on an

1I

.: :\eq 304

350 WAVELENGTH

(nm)

100

.,-

500

Fig. 1. Polarizauon’of flu&esc&e ;

he;;and ghs .‘,.

,504. :_,. .I

.y

and that of fluorescence excitation spectrum for ant&dcne it.77.5.. Wavelengths of excitation or obse~tibn are indicated, in the figure. ,,, :

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sandwich dinier in methyl-cyclo:

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L hfay 1975

CHEMICALPHYSICSLEZTtiRs

Volume 32, number 3

Am&o SPF-lOOc!speetrofluorometer

using Glan prisms as.poIarizers. The polarization of the ,ernission is expressed as, p = (I,, -I-L)/(III +I,), where f,[ and f,, .’ are the intensities of the emitted right polarized parai-. iel and perpen~cular to the electric Yector of the exciting light, respectively. The degree of polarization p can be related to the directional cosine of the transition moment in a straightforward manner. The emission spectra were studied with a I meter Czemy-Turner gratmg spectronieter, equipped with an RCA C3 1034 photomultiplier tube. (having relatively fiat response downto about 930 run), and the fluorescence lifetimes were determined using the timecorrelated single photon apparatus described previously 1131; All measurements refer to the samples (low4 - IO-’ M) in methylcyclohexane glass at 77 K.

WAVELENGTH

2 Directional &stibution of the transition moments, as represented by the plot of directional cosine YP~SIES emission

Fig.

w=cr~n,&. X,Y. =d 2:repreSentlong&s, short-a,& and out-of-plane axis of the cdhracene moIecuIe,resPWScl)-.

3. Results

part of the fluorescence s_pectrum. Wh incre&ng wavelength of the emission the degee of poIarization obtained by the ‘Bzu excitation falls off steeply, so that the discrepancy between the two polarization

Fig. 1. presents the polarization of fluorescence and*ti-tat of the fluorescence excitation spectrum for anthracene sandwich pair in methylcyclohexane glass ai 77 K. As noted by Chanriross [2] the absorption spectrum (or fluorescence ex$tation spectrum) of the sandwich ‘pair closely resembles that of anthracene, and the bands at about 382 and 245 nm can.be assign: ed to the short-axis polarized ‘Al, 3 ‘BZu (‘LJ and. the long-axis polarized ‘Alg -+ ‘B3,, (I%) transitions, respective!y. The weaker absorption at about 260 nm may be ascribed to the forbidden exciton component of”Aln + ‘13ju (lE$,) absorption 1143, made allowed by de~ation of the diner conformation @n-&e ground The polarization of &oresstate) from D 2-r sy~et~. cence excitation spectrum indicates that this absorp. tion is pdlarized less along the long-axis of the molecule than the allowed component at 245 nm. The polarization data in fig. 1 show that the ex&mer-like fluorescerxk of tie sandwich pair is p&&d more.

&ta becomessmakr at the longer waveIen$hregion of the fluorescence spectrum. The po~a~~tio~ data in fig. I were broken down into ~on~~butions from the short-axis, Iong-axis and out-of-plane polarized intensities, and the directional cosines of &he transition moments rire plotted in fig. 2. The most interesting feature of the pIot is the presence of sizeable out-of-plane intensity which increases with the increasing wavelength of the fluorescence band. The contribution of the out-of-piane intensity is in

fact so large that it is ou~t~pped by the shortaxis polarized intensity on& in the shorter wavelen& region of. the emission band. 3.2. Phosphorescence Under conditions in which the ~hosph~res~~n~e $ectrum of monbmeric arttticene was readily detected we were unable to observe any emission ofhz than fluorescence from the sandwich pair of anthracene:T& result is in agreement with the work of Ferguson et al. [24],‘which appeared after the completion of our experimerrti work. By comparing the upper l&i&of phosphorescence kterisity in the sand-

positive (or less negative) with respect to the excitation into the ‘B2u’statc than the excitation into the ‘&ju state. Thus, the transition moment-of excimer fluorescence is more p&l.leI to the tr+.ition moment of the “A, A LBzu band than that of the L&g 3 ‘B,, band for d wavelen&s of the emission’band. This difference is espetiially large in the short wavelength :

(nm)

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505

‘: Volume 32, number 3 ,.’ .”

CI-Eh@U+IYSICS

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“t‘The fluorescence lifetime of ihe sandwich pair bf ~$n&ra.c&e-h10 Was measured to be 185 ng while that

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sents the intxmo!ecuhx

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charge ~esonlnce cha+r on one transition energy on the other.

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the charge resonant

cont~ibu~ons.

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;eiathe contribution of th&e configurations is .) kxpedterj to depend on the.intermokcular separatiori, .. with the cI&ge resorience co~t~bution i&easing with .‘. decreasing &stake of separation 14-61. The increase .. .,in out-of-plane pol+rization ti;it.h increasing wavelength :of the fluores&ce’_band is cbnsisten’c with the en@cement qf the, charge resonance character wit& de _. .. .:. . : ,;$I?.- .‘,’ ,..- ..’ . . .L-. : (.,,. ._ .. i ” ,; ,,.‘.,C,.‘.. -, .‘. ,.I’,. ;,,: ‘_:.

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The diagram correlates band with the amount of hand, and the vertical

vcdon is not however, iike& k view of the estibfished ‘formation of the low&t .triplej statk in $he related

comes from the La excita-

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creasing interplarlar ‘skparation,ifit is assumed that the large bandwidth’of the excimer fluorescence is caused by radiative,,trakitions to 8 steep-rising, repulsive, ground state potentia! curve [ 161: This is illustrated in fii;. 3, in which the spectral region of the emission bznd is correlated with tie mount of charge resonance character, dn one hand, and the vertical transition energy, on the other. The n.on-observation’of monomer-like phosphorescence is an important negative res&t which has a bearing on !he &ape of the potential cube of the, triplet state. The IoW qu~tum yield of fluorescence. (a m 0.3) suggests that the triplet state’of the anthracene sandwich pail: is formed with hi@ efficiency or ,’ internal conversion to the ground state is a dominant photophysic ptocess.‘&h efficiency of interns con-

s&ectrum of the s&d&h ptistrengly suggests that thd lcwest ener& excimer singlet state is derived frdm ‘. the ‘L? (‘B&state of the monomer, as predicted by most theories on excimers. The observation of the sizhbk dut-of-pkne poIa~za~on in excimer fluorescence is.very likely.caused by deviation.of the excimer c?nforniaGon from D2h (a perfectly superimposed sandwich dimer) and certain other high symmetries, which can lead t,o the. out-of-plane intensity by virtue ofthe charge reso&nde cont~bu~on (to explain the,. observed polarization in terms of the 182u i: ‘AIg’ : ‘r@ative tr&sitibn, it is necessary to assume a substantial deviation from @arallel conformatibri)~ The mixed ‘, pckrizatibn of the puorescence is therefore consistent .with the conv&ntion.a_lnotion that the binding energy ,iif thii e&%tner tion’r~son~~

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f&zing&f the soI$on’ cont&ng $e sandwich pair, .: :we were able t9 conclude’ that the ph?s~h~r~s~~ce .‘_ quan~myieldofthedimerisnom&than$%’bf : :::thatofthemonomericanthramne(~~~x 10-4) : [I S].,This result is therefore in sharp contrast to the ,case of.the napM&ene sLtn&tich pair which exhibits :, monomer-like,p~ospho;escence.spectntm with mod,“. erkte intensjty [II]. :

., 1.

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naphthaleno sandwich pair [ll]. Wk suspect, therefore ” that the lack of monomer-Fe phosphorescence in the anthratine Gndwichdimer is related to the presence ot: a potent& min$mum,ii the triplet state. Once the ‘: t+plet e&mer state is pop~~ted by ~ters~stem crossing from the singlet excimer s&e;..the molecular pair, shou!d.decay into-tie repulsive. ,. ground siate by r?diative kd -radi&onless &nsitions..The non-obser& ‘tion of bxciner phcsph&e&r& may 3; the result .of : .’ ‘;

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”

Volurp~ 32, number 3

~~~CAL:P~YSICS

1 May, 1975

LETTERS

:

trated, solu$oris of monomers (Q”p* 0.34 and TF = 220 ns inconcentrated toluene or xy:ene solutions of ,,9,lO-~rne~yl~~a~Re~ f8] it is tempting to conelude that the confo~mati~R and the radiative sources of excimeis in concentrated soIutions are very similar to those in sandwich dirners. ” : : Ac~~~ed~~e~t __

We u;ish to thqk Cheng-Schen &qr~g and Shigeo ,’ Okajima for the lifetime measurements znd ChBs _ Russell for a copy of her thesis. This workwas supported by the U.S. Atomic En&y Co~ssio~.

,R

(a1

Pig. 4. Schemtiio

potential

knergy diagrams, r~tio~~~~’

the emissian ~~a~te~tic~ of san~~vich pairsof napht~ene and mthrntine.

The dashed curwtq represent

tid whkh prevents the dissoci&on

solvent potenof the molecuku p,airinto

two moncmcrs.

.

‘.

an efficient ra~a~onle~ ~r~sition or it may be re._ Iated to the fact that the emission wavelength is be: yond the response range:of the detector (i.e., X > 930. an).. The assump~on of the, o~~u~en~ of a ~ote~ti~ mourn in the lowest triplet state of an ~t~a~ne sandwich pair;but not in the t&plet state of a naphtha-

[1] EA, Chandros~, k&xn.

Phys. 43 (196s) 417.5 J. Feryson and E.G. &Rae, 5. Chen. Phys. 45 (1966) 3546. [3] E. Konijnenbetg, Thesis, University of hsterdam

[Z] &A, Chandrks, (1963).

141 3.N. hiurkli and 3. Tanaka, bfot Phys. 7 (k964) 363. (51 ‘F. Azumi, A.T. Axmstrongzsd S.P. McCIynn,1; Chem. Phys. 41’(1964) 3839. 161.AK. c?ft.mdnad EC Lim, J. Ckx~.. Phys. 48 (L968)

lene sandwich pair, dqes not seem unreasonable, since the

charge-~~sfer,interact~on

.- molecules

is expcfed

between two epicene

to be stronger

than &at

2589.

between

two ~aph~ene mol.ecules. Potential energy diagrams comparing the einissian characteristics ofnaphthal(ene and antMcene sandwich dimers are shown in fig. 4. ‘III& radiativ&lifetime of excimer fluorescence, dedueed from the measured quorum yield (a 0.3 based ‘on ths monomer fluorescence yield) and lifetime of the emission, is about 600 ns. The magnitude of the di&ve lifetime is con&tent with the supposition that the emission is d Sykes-forbidden tr~~~tio~, made allowe@ by deviation of’the co~f~~ati~n fro& ‘I& synuatry [12]. In vieti of ttie closecorrespo~

[7] R.M. Hochstnsscr

and A.

h%IIiais,

I.

Che’m- Phys.. 4%

(1965) 2243:

[S] J.B. Birks, Plrotophysicsof aromatic molecufesO%.leYInterstienke,

[9] ,~~~4~~,

New York, 1970), and references therein Thesis, AustraLian National UnivefsitY

IlO] S:K. C&crab&i, Mot P&s. 18 (f97G] 275. [il] EA. Cftandross and C.J. Dumpster, I. Am. Chem, Sot. 92 (1970). 704. snd references

_’ [12] AK. an&a .’

dence of the spectial po@ion, quantum yield aitd ,’ lifetie.tif the s~d~ch pair (42, =Z0.33 and ~1;;= .. :’ 220 ns’ior t&e g-rne~y~~~acene sandwich dimer) .. 1141 with those of excimer fluoiescencc in concenT ‘, .. : .’ (.’ ” ,.

‘.

~e~e~en~es

,:,.

therein,

and EC Kim, I. Chem. Phys. 49 (1968)

[iif

~~?Lsi&i C-S,. Ha& znd EX &-I, 1. Chem. Phys. 60 (1974)‘4345. [I41 5; Ferguson, A!W,-HI hfau and J-K Mor& AWX* J. Chem. 26 (1973) 91. [lsl J. reangeiaar, RP.H, Rett&nickard G.J. Hortink, J. Ckin. Ph$s. 54 (197.l) 1. (161 B. Stevens and M.L’Be.n,T&s. Fatad& See. 60 (1964)

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