Impurity induced phosphorescence in anthracene crystal

Volume 30, number 1

CHEMICAL

PHYSICS

LETTERS

1 lunuary

IMPURITY INLWCED PHOSPHORESCENCE IN ANTHRACENE A. BRILLANTE*,D.P.

CUIG,

Rerearcl~

Sci~ool of Chernirrry,

Received

27 August

The phosphorescence

A.W.3.

Austmliorr

19i5

CRYSTAL

MAU and J. RAJIKAN

h’ario,ul

University,

Canberra,

A.C. T. 2600,

and crystals

containing

Australirr

1974

trace amounts of 2-methylanthracenc, fluorescence spectra. tn the ‘pure’ crystals delayed fluorescence and phosphorescence excited by 337 and - 400 nm radiation ore weaker by an order of magnitude or more than in crystals with traces of impurities. With 2-mcthyl- and 2-hydroxy-anihracene 3s dopants. the phosphorescence origin is shifted by 132 cm-l and 73 cm-’ respectively from the pure atithraccnc cxciton origin. In both the emission is characteristic of anthrncene host molcculcs acting as ‘X’ traps. 2-amino-anthracene as dopant _rivcs 2n emission with origin displaced by 311 cm-‘. In this case the dopant is the emitter, and thus acts as a chcmic21 trap. The two impurity induced ‘X’ traps are close in energy to traps reccntiy assigned to structural defects. However no significance is as yet plxsd on this result.

2-hydrosyanthracenc

spectra

of ‘pure’

nnthracenc

crystals

and 2-aminoanthracene ark reported, together with prompt and dclaycd

I. Introduction Excitation traps for singlet excitons in molecular crystals can be classified into (a) chemical impurities singly or in clusters with lower excitation energies than the host molecules, and (b) host molecules with excitation energies reduced from those in zones of perfect lattice structure by any one of several causes, including location at a surface or near a crystal dislocation, electronic perturbation or dispIacement’by chemical impurities (‘X’ trapping), or local lattice deformation following electronic excitation (self-trapping). Analysis in these terms of spectroscopic and other results has been confused by trace amounts of persistent impurities, so that supposed examples of(b) have often turned out to be (a). This is true even in crystaliine anthracene,

which is the best-studied system, and in which the most recent work (l-31 leads to reassignment to chemical impurities, notably 2-methyl-anthracene an,d 2-hydroxyanthracene, of what had been taken before to be defect levels. Traps for triplet excitons have been less studied..They can be classified in the same way, but ‘here are differences coming from longer radiative lifetimes and slower *

Permanent address: Laboratorio di Spettroscopia hlolecolare del CNR, 40126 Bologna, I?&.

migraticn, and from reduoed self trapping. There is also a wider scope in that if the triplets are produced fro-m singlets by intersystem crossing, the role of traps can be discussed in that process, as well as in the subsequent history of the triplet excitons themseIves. Where triplet excitons are produced directly by radiation, the first process is eliminated and the second stage can be examined separately. In this brief report we describe a study of the effects of impurities added as dopants on the prompt fluorescepce, delayed fluorescence and phosphorescence of anthracene crystal. The triplet molecuies are produced by intersystem crossing following excitation of the blue singlet system of anthracene. This study is thus complementary to the recent work of Goode et al. [4] in which the triplets were produced by irradiation into the T, level in the red.

2. Experimental Anthracene (A) (Fluka scintillation grade) was zone refined (200x) with added sodium. “Frio$’ quality anthracene (described as pure to I in 103 by Princeton Oiganics N.J.) was also used. Perdeuteroanthracene (Adlo) from Merck,‘Sharp and Dohme was purified by zone refining under N, (405). 2-me+ylanthracene _.

5

Volume

CHEMICALPHY~ICS

..

._

(a)

P-h;,

I’ ._

&TERS

‘.

;’

1 J~,uary

,‘:. .-

+2MeA-h12

-/ I

I

,

218

214

210

1

I

206

202 xlO’cm

(bl

A-h,,+20rlA-h,o

N, + a,

impurities

/

(cl

(e) A-h,,+

A-d,z+2MeA-d,,

Cf, Pure

A-h,,,

nielt

grown

J?-k_y 250

246

242

238

i

I

2 34x!OZcm

246

250

,

242

,

,238. ‘,

234xlO’cm .’

Fig. 1. Prompt fluorescence (upper) and delayed fluorescence flower) of dspcd and meltqown puke anthracene crystal;. The dop;nts UC (II) 2-methytanthkene, (b) 2llydroxyhnthraccne, (c) 2-methylantiuacene-d~2, (d) 2-aminoanthracene, (a) unidentified imp&i., .. ties in sodium-rich zone refined sample,.’ Inteniity scales arbitrary..

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Volume 30, number 1

CKEhilCAL PHYSICS LETTERS

(2Mt,4) (Koch-Light) was purified thro&h its phctodimer and then vacuum sublimed, 2.hydroxy-anthracene (20HA) was prepared via reduction of anthraqtikone-2sulphorLic acid, and 2-amino-a&hTacene (2.NH,A) (Fdrich) was crystallised and sublimed. Delayed fluor&cence and phosphorescence wkre measured by standard niethods described elsewhere [5]. Prompt fluorescence measurements were made in a Jarrell-Ash

3.4m Ebert spectrophotometer.

Excitation

.Table 1

Vib:ational atilysis

of prominent bands in anthracene

Displt. b) from

Guest 2hle~

20HA

2NHz A

14665 14606

14427C)

XI -391

14054

has

03-373 Q-625

13920

757 818

13443 13407

13346 13265 13203

2 hydroxy- and 2-amino-anthracene, shown hereafter as 2MeA, 20HA, and 2NH,A. Figs, 1e and 2e are spectra containing unknown impurities occ-erring in the sodium-rich parts of the zone-refining tube. The major vibrational maxima for the first three,dopants are ana-

13049

are less well resolved but are similar in overall band envelope to the, corresponding PF spectra, and with!n:the

x2-391

03-507

R&i

13503

grown crystal of high purity (figs. If and 3_r) and.others with low concentrations of dopants, viz., 2-methyl-,

Xl or&in X2 ori$n 03 ori+

_’ 13.981

3. Results

73 132 3r1

13854

Figs. la, lb and Id in ‘the PF results are close to those already referred to [l-3], with frequencies reproduced to 2 cm-l. The DF spectra, excited by 337 or 400 nm light,

14738 cm-’

464 523

14274

those of other

lvsed in table ! _

Assignment d)

~___

1421.5

We report the prompt fluorescence (PF) and delayed fluorescence (DF) (fig. 1) and phosphorescence (fig. 2) near 6K of siu anthracene crystals including a melt-

mixed

crystal pIlosphorescencez)

.‘(front surface) was either by a 337 nm N, laser or in the 390-410 nm range of q (r-NPO {2(l-&aphthyl)Sphenyl oxazole] dye laser. Wave numbers quoted incIuding autho.‘s are corrected to vacuum.

i Jhnuwy 1975

13109 13011

1235 1295 1331 1392 1473 1535’ 1629 1689 1727

X2-752 X1 -1162 X2-1163 XI-L258 X2-1260 X~-1400 x2-1403 XL-!556 x2 -1.557 03-1416

a) Wave numbers in fist three columns ‘corrected to vacuum. Air vnlucs about 4 cm-’ greater. b, Displacement from the a-polariscd or&in of anthracenc triplet exciton. ‘1 Strongest site origin. Others at 145L9, 14392, 14330, 14234 cm-‘. d, Ground state frequencies of anthraccnc appearing in crystal fluorescence [l] include 391,621.752, 1165, 1260, 1SC4, 1558 cm-l.

range of excitation wavelengths the delayed fluorescence intensity shows the expected quadratic dependence on. intensity. It is known from the earlier work [l-3] or from new measurements that in each of 1a-ld ,the dopants act as chemical traps, with singlet trap depths now measured as: 2McA 194 cm-‘, 20HA 930 cm-‘, .2MeAd12 in Ad,, 201 cm-‘, 2NH,A j286 and 3565 cm” (values from ref. [2] ). In. the purest samples of

-depends on the growth, cooling, and perhaps other as- 3 pects of sample history. Tine particuIar spectrum shown, though among the strongest from pure samples, is less intense by an order OFmagnitude than ‘Lheothers in fig..1 and cannot yet be assigned to physical or chetical traps. The phosphorescence spectra df the same mixed crystals are given in frg. 2. In phosphorescence the ref-

A, in which’the amount of 2MeA and other impurities, is below the k-nit detectable in prompi fI&es&nce the delayed fluorescence (fig. If, melt-grown Crystal) is tit

Brence fre,quency is that of the lower component (a polarkd) of a Davydov doublet ofpure anthracsne oriE:aLly measured by .Avakian et at. [-9] at 147 1G cm , then by Smith [7] at 14736 cm from the phos

least an,order of magnitude weaker than in samples wi?h detectable impurities. DF from the-purest samples (If) . ,’

phorescence, excitation’spectrum,

and by Clarke and

:~01ume30,number

1 ‘.

:

..

.

.CI-IEMICAL PHYSICS !.JTJTER~.-

.-

j

: (a) A-h,;+

2MeAlh;z

. td)

‘,

A-h,i+2NH2A-h,,

(e) ,A- h,,+

‘If) Pure

(c) A-d,o+2MeA-d,;,

(yI+N2)

/l-h,,,

No

L-__ )I,-

148

~L_L_JLA-

144

140

Fig. 2. Phosphorescence

136

,g

‘&periments

,I’

::

with 20HA

.:.

::,

,,

unknown

melt

detectable

impurities

grown

phosphorescence

1 I

132x102cm~

from doped and’meltgrown

.Hochstrasser [8] at 14738 cm-’ from Zeeman studies. The value in AdI0 is 14790 cm-’ [5], the deuterium is_otope shift of 52 cm-l being close to the 59 cm_’ found [9]: For the lowest excited singlet. A measurement by Fersson and Msu [5] of the, spectroscopic origin in the phosphorescence spectrum on a sample of Print anthracene, with excitation into the anthracene singlet system, gave 14606 cm”. They deduced the existence of a trap, at that time unidentified, of 130 cm-l. ~xarnina~ion of the flI?qeSCenCe 6f the same Prinz hnthracene in the light of [l-3] shows trace amouts Of 2MeA, and likewise traces of 3MeAd,i in Mbreover experiments with A dc-. the sample ofAd,,. liberate/y doped with 2Me.4 show that the phqsphoresc&c from the origin at 14606 cni-’ increases with its doncerhdon: A similar,result holds for 2MeAd12 in AdlO.

‘: 1 ‘Jnnuary 1975 ‘..

; ;.

,’

and 2NH2_kindicate

148

pure snthracen:

I

I

I

:44

140.

132 XIOzcrr

136

crystals. Dopants as given for fig. 1.

Table ?

Singlet and triplet trap depths in anthmccne

(cm-‘)

An thracene~z i o

Anthmcene-d~ ,J

singlet

triplet

sillglct

triplet

194 a) 1443,e)

132b) _-

2513,e)

-

201 a,c)

132b)

930 al

7?b)

987 a,e)

201 WI

2Ni-I n-h 3286 c) 1 I1 3565”)

S15”)

Guest

2MeAh ,? 2hI~A-?~2 ~OHAJI ,.

a) a,emii

t13PS.

-

- ,

..-

bj ‘X’ trap: -ribrationk stnrcture charactcktic of host.’ c, Host isotope shift 57 cn-‘. Directiy m~sured value 59 WI-~ [9]:

..

d, In 3 cryst31 also contining 2hled~,,l~. .’ “! Not a mwurcd v&.‘Deduccd tising isotope shift of 57 cm-‘. . . ..’ ..;. ..

Voiume 30,

number I

. .

‘,

CffEhftCAL

PFtYStCS

traps of 73 cti-’ and 3 11 cm” respectively (table 2). At first siglit there are two possibilities, i.e., that the phosphorescence is from dopants as chemical trap&or that it isfrom hpst anthracene moIectiles.disturbed by the dopant molecules and acting as phosphorescence X-traps: It is e&y demonstrated that the emission from anthracene doped either with 2MeA or 20HA is ndt chhracteristic cf the dopant, but of host anthracene itself. The measured vibrational intervals in table 1 are close to those measured for anthracene in non-crystalline &vironments [5], and differ unmistakably from thGse for 2MeA in fluorescence [2]. Tlk contrary resuit is founb. with ZNH,A dopant, which is itself the emitter, acting as a chemical trap, as in fig. 2d and ’ table 1. In this case the main origin is at 14427 cm-’ . Thus as triplet traps 2NH2A is type (a) z+d 2MeA and 20HA are type (b). The polarizkion properties of the various emissions have been measured. All PF and DF emissions have polarization ratios b/a =Z 4/l, in good agreement with expectatiori for the short in-plane axis transition direction for the singletWmsition. The phosphorescence from 2MeA doped A is polarized b/a = I/4, consistently with that for an out-of-plane polarised ‘BzU-lAn transition (talc. b/a = l/3,6). There is evidently no s&Xcant misorientat~orl of the emitting host molecules. Finally we note that with 337 or 400 nm excitation of the singlet system no phosphorescence couId be detected from the purest crystals. Its intensity is at least two orders of magnitude less than that from the doped crystals. Also neither phosphorescence noi DF was detected in a crystal doped with naph~~ene, of which both the low singlet and triplet lie above the lowest .’ excited singlet and lowest triplet respectively of anthacene. Comparison with the phosphorescence arid DF from anthracene under red kktination studied by Goode et al. [4] shows near coincidences between their 129 cm-’ structural imperfection trap’ and the 132 cm-’ ZMeAinduced trap, and between their 81 cm-’ structural i;ap tiA%o and the.73 cm-l 20HA-induced trap. Neither of these coincidences is necessarily significant; in particular the 129 cm-I structural trap of Goode et al., is secsitive to mechanical stress, suggesting that it is not impurity induced though it remains possible that a stteSsed crystal would strain p~eferenti~ly &i zones adjacetit to ~purities.

1 r2nuxy

LETTERS.

4. Discussion

15175

..

The interpretation ‘of these ejcperiments is concerned with t&o stages; intersystem crossing of the ekited singlet produded by the absorption into a’triplet state, ., and the subsequent.emission from the triplet state. As to the first stage, the fact that very pure crystals show extremely low levels {undetectable by us) df 337 qr 400 nm’excited phosphorescence and DF shows that intersystem crossing is very ineffkient in the absence of chemical &purities as is already obvious from the high fluorescence yield (= 0.99) at room temperature IlO] . . There is a similar sitdntion in naphthzlene [I 11 and biphenyl [ 121. The role ofirr,purities may hinge ofi the relative disposition @the first sin&et state (S,), directly populated by the radiation,‘and a triplet state, probably, the second (T,), nearly degenerate with it. There is a variety of evidence [ 10, 13, 141 that T, is higher by several hundred wave numbers. Thus ‘intersystem cro&ng S, -L T; does not occur at very low temperatures and the process S, --t T, is slow on account of the wide energy gap,and yields only a small standing population of triplets in competition with nonradi~tive phocesses the rates of which are indicated by the short phosphorescence? lifetime. The measured phosphorescence lifetimes at low temperature in 2MeA dbped A and the perdeuterated analogue are 35 and 130 ms resp&tiveIy, similar to those observed in solution. Ferguson and Mau [5] found from the excitation spectruti that anthracene phosphorescence in Shpolsltii rigid sokents shows a remrrrkable variation between different sites, the intersystem crossing rate being apparently much greater in certain of thein. It is evident in general that the Intersystem crossing Fate will be strongly influenced by the ordering of the S and T levels, but this is not the only factor. For example other experiments suggest that intersystem crossing may be faster in trapped than in free excitons. In the systems here discussed, in the presence of even trace amounts of 2MeA (singlet trap depth 194 cm-l), 2CIHA (933 cm-‘), and 2NH,A (3286 and 3565 cm-‘) [2] there is highly efficient trapping,ofthe singlet excitok, followed by intersystem crossing to T, which, in these substituted anthracenes, nay Iie beIow S, . Two results are possible according to the position of the lower (Tl) triclet trap level of the impurity compared vrith the-lowest host tripiet level. We believe thnt 2MeA ad !20HA are negative’ tripl!t traps, with levels above .,’

vi$&O.&tfe~i

I January 1975 ’ CtiEMiCAL PHYSICS LETTERS ..,. : ,: ; [i] J. Rajikan, to be published. ‘the lqkst a&able host triplet which be1011gs”toan ,, . ., i [4] D. ,Goo’d?‘Y. Lupien, W. Siebrand, D.F. Williams, J.hi. antiracene mqlecule displaced by the irnpu!lty and Thomns:md J.0. Williams, C!hem.‘Phys. Letters 25 (1974) &sk to it..Energy tr&,fer now od&rs’f@m the them308. ical itiptirity to the di$$ced host niolecule. ‘Ii%s .[5] 3. Ferysbn and A.W.-Hi Mau,‘Mol. Phys., to be published. .~&its.tith ~h~rac~crjstic energy shift from thehost. : [6] ‘P. ALakim, E: Abramion, R.G. Xeplcr and J.C. Caris, J. Chem, PiiYs. 39 (1963) 1127. triplet exciton level,- &having as. an ‘X’ trap. If the &II-. .’ [7] G.C.,Smith, Phys. Rev. 166 (1968) 839. purjty behaves as a &xitive triplet trap, as we belieye [ 8j R.H. Clarke pnd R.M. Hochstrasser, -1.Chem. Phys. 46 .2NH,A does,.phosphorescence emission occurs fro& (1967) 4532. the tr+ldt, level the chemical impurity, acting as 3 [9] E. Glockncr and H.C. Wolf, Chem, Phys. Lcttcrs 27’(1974) 161. chemical trap. Thus the behaviour observed depends [lo] R.E. Kctio&3. Chem. Phys.44 (1966)411. on the r&tive trapping ch&acteristics of an impurity1111 E.B. Priestley :ind G.W. Robinson, Mol. Phys. 26 (1973) molecule in its excited singlet a& triplet states. . . ‘. 159.’ [12] A. Brce :!nd R. Zwarich, Mol. Cryst: Liquid Cryst. 5 : ,. ..,’ (1969) 369. .. Y’ [13] J. Adolph and D.F. Williams, I. Chem. Phys. 46 (1967) .’

of.

References

.’

4248.

‘” [ 1’1L.E. Lj.0115 and ‘i-J. Warren,‘Austrdian J. Chem. 25 (1972) 1411. [ 21 N.J. Bridge and D. Vincent, Chem. Sot. Faraday II 68 (1972) 1522; 70 (1974) 1874.

1141 L.>I_ Lo~sn,~IJ(. hlunro, D.F. Williams and F.R. Lipsctt,

in: MoIe~:ularluminescence, cd. E.C..Lim (Benjamin, New York, 1969).

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