Further results on the triplet—triplet fluorescence of anthracenes

Volume 63. number 2

FURTHER

RESULTS

Gregory D. GILLISPIE* Department

of Chemictry.

CHEMICAL

PHYSICS LETTERS

ON THE TRIPLET-TRIPLET

15 hlay

FLUORESCENCE

1979

OF ANTHRACENES

and EC. LIM Wayne State

University. Detroit.

Received 15 November 1978; in final form 9 February

Michigan 48202. USA

-

1979

Triplet-triplet fhrorescence spectra have been measured following excitation into the St state for eight anthraccnes For those molecules in which near-infrared 3Bau - 3B,o absorption has been reported, the @-0 band wavelength in absorption and fluorescence agrees well. Hoaever. the quantur?i yield of triplet-triplet emission in anthracene is more than a hundred times weaker than that predicted from independent measurements of Ta - Tt radiative and non-radiative decay rates. In general. halo-anthracenes exhibit considerably stronger triplet-triplet fluorescence than do the phenyl and methyl derivatives; the origin of this apparent interval heavy atom effect is unclear. The suggestion is made that more than one higher tripIet state is intimateIy involved in the St -T intersystem crossing of anthracenes.

I_ Introduction Over the past twenty years there have been numerous investigations of the S, + T intersystem crossing process in anthracenes. For the most part these studies have attempted to elucidate the role ofhigher triplet states, T,, (n > 2), in the non-radiative decay from S1 _ It is well known that the ISC rate in meso-anthracenes (i-e., those substituted at the 9- and/or lo-positions) is strongly temperature dependent_ In contrast, the ISC of side-substituted anthracenes is essentially temperature independent_ In the mid-19603 Kellogg [I], Bennett and McCartin [2], and Lim et al. [3] independently proposed that the energy of the T2 state relative to that of S, controls the photophysical behavior of anthracenes. The key assumption was that in sidesubstituted anthracenes T? is slightly below S, whereas in mesoanthracenes S1(rLa) drops below T, owing to the extension of conjugation along the transverse (short in-plane) axis_ When T, is below S, ICC through T2 is possible at alI temperatures; if T2 is above S, thermal activation in S, is necessary to permit the IX_

The main points of evidence supporting this inter* National

Science Foundation Energy Related Postdoctoral Fellow, 1975-77; University of Michigan Junior Fellow, 1975-78; present address: Department of Chemistry, SUNY

at Albany, Albany. New York 12222. USA.

pretation are the following: (1) The T, state has been located by triplet&iplet absorption (TTA) in anthracene [1,4], 2-methylanthracene [4], and 1 ,S-dichloroanthracene [5] and the T7(3B1g) state is found to he below Sr by 600,600, an;d 900 cm-l, respectively_ (2) From S, + So absorption spectra in solution, the Sl state is known to shift by = 1000 cm-l to lower energy (relative to So) for each meso-substituent. (3) Arrhenius type fits to the KC rate in meso-anthracenes yield activation energies (usually equated with the S, -T-, energy gap) of 900 to 1800 cm-l [6] _ In kter years other workers have additionally invoked the participation of another higher triplet state, T3, in the ISC process [7-g]_ No clear conclusion could be drawn whether T2 is 3Blg and T3 is 3Bju. or vice-versa. Perhaps the weakest link in all of these arguments is that the energy of the key higher triplet state(s) in meso-anthracenes had never been experimentally demonstrated to be above that of S, _ This deficiency was partially removed by the report by Gillispie and Lim [lo] of triplet-triplet fluorescence (TTF) folIowing excitation into S, _ A single band in the near infrared was found superimposed on the structureless tail of SI + So fluorescence for 9-bromoanthracene (9-BA) and 9,lOdrbromoanthracene (9,10-DBA). The wavelengths of these maxima are 841 mn for 9,10-DBA and 867 run for 9-BA. T, + So phosphorescence spectra 355

Volume 63. number 2

CHEMICAL

PHYSICS LETTERS

were also measured for these compounds in dilute, degassed heptane solutions at room temperature. The near infrared TTF was assigned as T2CBle) + T1(3B& and energies of E(T2) =E(St ) + 300 cm-’ for 9-BA and EO;) = E(SI) t 800 cm-* for 9,10-DBA were deduced fr;m the St + So fluorescence, T, *So phosphorescence, and T2 + T, fluorescence spectra. At that time neither the TTF of anthracene nor vibrational structure other than the 0 -0 band for bromoanthracenes could be detected owing to a sharp drop in photomultiplier sensitivity for h > 875 nm. We then had an opportunity to use a developmental photomultiplier tube with appreciabIe sensitivity to A = 1050 nm_ With its aid we were abIe to extend our TTF measurements to anthracene itself and several other substituted anthracenes. However, these new data indicate previously unsuspected complexities: in fact, we are unable to give an interpretation totally consistent with all available esperimental data. Thus, this paper ls more of the nature of an interim progress report of the results with a brief discussion of their irnplications.

2. Experimental The T-T fluorescence spectra were measured with a photon counting spectrofluorimeter constructed in our laboratory_ The major differences in the setup over that employed for an earlier study of anthracene TTF {IO] are the follcwing(1) The soIution and glass filters and/or 0.25 m singIe monochromator used to seIect the exciting light were replaced by a Spex model 1672s 032 m double monochromator_ The reduction of stray light by the doubIe monochromator was such that the background couid not be reduced further with solution and glass tllters. (2) The RCA C3 1034A PMT sensitive to = S75 nm was replaced by a Varian-LSE 164 PMT with appreciable sensitivity for X < 1060 run. A dark counting rate Of== 15 s-l was attained when the phototube was cooied to -60°C by passing cold nitrogen gas through the housing..

The apparatus is described in detail eLsewhere [ 1 I ] _ The TTF spectra must be acquired fairly rapidiy because of the existence of photochemical degradation of the sample. This decomposition gives rise to a broad 356

IS May 1979

structureless impurity emission in the region h > 600 nm_ After = 20 min of irradiation of a sample under the usual experimental conditions, the weak TTF becomes signi&antly obscured by the impurity emission in the region of the 0 -0 band_ We found that this problem was best combatted by using high sample concentrations but maintaining normal excitation of the square fused silica cuvette and right angle detection_ The e-mission is thereby concentrated in a small sample volume which improves colIection efficiency and reduces counting time. Sample concentrations were adjusted to an optical density of 1 s/mm at the excitation wavelength (~350 nm). No attempt was made to correct for the small amount of concentration quenching at these concentrations, = IO-3 AI. Re-absorption effects of the near-infrared emission by the solvent were shown to be negligible_ The =F quantum yields were determined relative to the strongest emitter, 9,10-DBA. @~-T(9,10-DBA) was in turn measured relative to the S, + S, fluorescenceof9,lODBA(G+ =0-l) [Il>],whichserved as an internal standard_ Corrections were applied for PMT and monochromator response, but not for changes in the solvent refractive index with wavelength_ A more diIute soIution, = low4 M, was used for this determination_ In this manner we found @Z-T (9,10-DBA) = 1 X IOH6, estimated to be accurate to within a factor of 2. AI1 samples were purified by vacuum sublimation except anthracene-d,o, which was used as received_ The methylcyclohe.uane solvent was spectroquality and gave no detectabIe emission in the near infrared_

3. Results The TTF spectra of anthracene, anthracene-d,*, and four halogenated anthracenes are shown in fig_ I _ The smooth tail of S, + So fluorescence, reduced 5 to 6 orders of magnitude from its ma_ximum at -400 nm, appears as a background luminescence in each case_ The spectra were taken in undegassed methylcyclohexane soIutions at room temperature to eIimLnateany contribution from T, -*SO phosphorescence_ AI1 of the spectra are structured, but it is difficult to say whether the TTF is a good mirror image of the near-infrared TTA. The TTA of anthracene and A-d10 in a ptastic matrix [I] and in alcoholic solution [4] at 1 I3 K shows a strong O-O band and features at 0 f

CHFXICAL

Volume 63, number 2

15 May 1979

PHYSICS LETTERS

9,10-DBA

9-CA

9-BA

i-

~~ 6oo

875

I

950

1025

820

I

895

I 970

I

1045

1

320

I

895

I 970

A-%

I. 5-DCA

I

320

I

895

I

970

I

1045

A-40

I

1045

I

920

I

895

I

970

I

1045

Fiig. I. Uncorrected T-T fluorescence spectra ofanthncrnes m methb lq clohexanr at 15*C_ The absclsa is wa\elen$h in nm and the ordinate relativeintensit) .The inrensitrrs of the indhidIral>Tectm are not directly comparable because different countmg tune5 (0; to 4 s) acre used in each case. The spectral resolution is 2.5 nm.

600 cm-l and 0 t 1350 cm-l_ Anthracene exhibits very weak TTF only sli&Iy more intense than the S, + SI? background_ It does appear, however, that the band displaced = 600 cm -I from the origin is signifi-

cantly weaker in fluorescence than in absorption. Nevertheless, the wavelengths of the 0 -0 bands agree to within 50 cm-1 for TTA and TTF in anthracene (as well as 1,5-DCA and 2-MeA) and hence the fluorescence is assigned as ‘B, + 3B2U_ The TTF of XleA

(not shown) is also too weak to provide a definitive test of mirror image symmetry_ The TTA of 1.5~IX-4 [S] shows three well defined bands at 955, S50. and 760 nm, whereas only a single ma_Gmum is seen in emission at 959 nm. if the fluorescence were a mirror

image of the absorption, the second maximum would be at lOS5 nm, which is beyand the detection limits of the PMT. An attempt was made to obtain a more highly re-

357

CHEMICAL PHYSICS LEITERS

Volume 63. number 2 Table 1 T,r-Tt ZPC

~IUOXSCCXICSZ

of anthracenes in methylcycbhexane at

Molecule

WFX

108

5 5 - a>

anthncene anthracenr-dto Z-phen) Ianthmcene 2-methyhnthracene 9-phenyhnthracene

9-meth~hnthracene 1 J-dichloroanthncene 9chIoroanthracene 9.iOdichtoroanthr~cene 9-bromo~nrhrzcene 9,lO-dtbromoanthrne

~o-o(nm) 891 888 -

4

896

t2 70 b) 20

917 9.59 860 -

30 100

867 841

aj None deCKi&. Ruorescznce assumed to be mirror image of T-T sorption.

b)T-T

ab-

solved ‘ITF spectrum of anthracene in a n-heptane Shpol’skii rnAtrix at both 77 and 43 K. However, the TTF is so weak that it is almost completely masked by the T, -+ So phosphorescence despite the relatively low quantum yield (@p = :! X 10M4) 1131 of this Iatter emission_ The resuIts for ail of the anthracenes studied are in table I - With the exception of 9,lOdichioroanthracene

rrr,rr,,,lrr,,I,

Fig_ 2_ T-T

U

IL E tar-’ L IO?

IO

fluorescence spectra of 9-BA and 9,10-DBA cor-

rected for photomultiplier response and emission monochromator grating response and slit function

358

IS nray 1979

(9,10-DCA), the halogenated anthracenes exhibit substantially stronger TTF than do anthracene and its phenyl and methyl derivatives. For 9,10-DCA the S, --t So fluorescence is quite strong (+p = 05) 1141 and red-shifted relative to anthracene; consequently, the background of nor;nal fluorescence is large in the near-infrared. We can say with certainty that the quantum yield of TTF in 9,lODCA is at least a factor of ten smaller than in 9,lODBA. Corrected TTF spectra of 9,10-DBA and 9-BA are shown in fig_2, where corrections for P&IT response and emission monochromator response and slit function have been made_ The contniution of the underIying S1 + So fluorescence tail has also been subtracted_

4. Discussion Let us first try to analyze the data of table I within the framework of the model which assumes that ISC from St takes pIace through the Tt state. In this case the quantum yield of TTF is given by

(1) where QE is the quantum yield of S1 + T ISC, kFz_T1 is the radiative decay rate from T2(‘B&) to T,(3B2,), and TTT is the Tt lifetime_ For all of the antluacenes studied ersc is between 03 and 1 .O at room temperature [&6,9,14]. The radiative decay rate can only be obtained from the integrated TTA spectrum_ Thus, we = 2 X IO5 s-l, 2 X lo5 s-l, and 6 X IO5 fmd +#j s-* for anthracene [4], Z-methylanthracene [4], and 1,5dichIoroanthracene [S]. respectively_ The near-infrared TTA has not been reported for any other anthracenes- FinalIy, there are two estimates of the T, lifetime: (I) the photochemical sensitization studies of Liu and co-workers [l s-18) give rTz =Zsocl ps for several meso-anthracenes; (2) an empirical energy gap correlation of internal conversion rates by Gillispie and Lim 1191 also suggests ==200 ps for the T, lifetime. These estimates for the parameters of eq. (I) lead to a v&e of greater than 10e5 for the quantum yield of TTF_ However. even for the strongest observed TTF in 9,10-DBA, GITF is an order of magnitude lower. Moreover, for the parent compound and its phenyl and methyl derivatives, the discrepancy is on the order of lOOO! There is also an indication of some type of internal

volume 63, number 2

CHE~IICAL PHYsrcs LETTERS

hea-vy atom effect. With the exception of 9,10dichloroanthracene, the halo-anthracenes exhibit substantially stronger TTF than do the non-halogenated derivatives. The source of this heavy atom effect is not at all obvious, since increased spin-orbit coupling would primarily affect apIsC, which does not vary much. In our opinion the most logical way to rationalize the extreme weakness of the TTF is to invoke the participation of a second higher triplet state in the ISC process. The Platt notation 3Lb and 3Bb states (both 3B3u in the D,, point group) are possible candidates. For example,ff T2 were actually 3B3,, and Tj were

3B k, the f-oIIowing could be the case: the quantum yield of 3Big+3B2u (T3 + T1) fluorescence could be quite small owing to very fast T3 -+ T7 internal conversion_ Because T2(3B3u) + T,(zB,,j radiative decay is electric-dipole forbidden this emission might either be too weak to be detected or occur at Ionger wavelengths than the detection limits of even the Varian PhlT. However, this scenario does not provide a ready interpretation of the apparent internal heavy atom effect_ Moreover, in 9-BA the fluorescing triplet state is only = 300 cm-l abobe S1 _If this fluorescing triplet state is jB1, (as it almost surely must be) then there is only this very narrow energy range in which to place 3B3u. (If 3B3u were below S, then it is expected that ISC could occur at all temperatures, whereas a, = 1 .O for V-BA at low temperature [s]. On the other hand, can it be possible that Sl + T ISC is only fast when both T2 and T3 are slightly below S,? To the best of our knowledge the assumption that the existence of a single higher triplet state below S, is sufficient for rapid ISC is just that - an assumption. This point is discussed in greater detail elsewhere [Xl] .) The discussion we have presented here is of a very general nature owing to the complexities described above. Although a more detailed analysis could be given, it would have to rely on a number of speculative assumptions_ We therefore feel it best that any such attempt be deferred until more detailed experimental data are available-

15 May 1979

Acknowledgement This work was supported in part by the National Science Foundation and the Department of Energy. We gratefully acknowledge the Varian Corporation for their cooperation in providing the developmental photomultiplier tube used in this study.

References

111RX. Kellogg,o.J. Chem. Phys -t4 (1966) 411. JZJ R-G_ Bennett and P-J. McCartin, J. Chem. Phys. 44 (1966) 1969. 131 E-C_Lim, J.D. Lapon and J.M.H. Yu, J. Mol. Spectry. 19 (1966)

412.

141 Y.H. Meyer, R. Astier and J.M. Leclercq, J. Chem. Phys. 56 (1972) 801. 151 J.P. Roberts and RX Dixon, J. Phqs. Chem. 75 (1971) 845_ I61 E-J. Bouen and J. S&u, J. Phi s Chem. 63 (1959) 4. 171 T-F. Hunter and RX‘. W1 att, Chem. Phys. Lerters 6 (1970) 211. 181 R-P. Widman and J-R. Huber, J. Phrs. Chem. 76 (1972) 1524.

[91 X. Kearve!l and F. N dkinson, J. CInm. Phys.. SpeciaI

Edition Transitions Non Radiatives Dans les lIoI&uIes (1970) p_ 125. [ 101 G.D. Gdlispie and EC. Lim, J. Chem. Phys. 65 (1976) 1022. [ 111 G-D. Gillispic and E C_ Llm, J. Chem. Phi s. 68 (1978) 4578. [ 121 W.H. hlelhuish, J. Phqs. Chem 65 (1961) 229[ 13 ] J. Langelaar_ R.P_H_ Rettschnick and G-t. Ho) rink. J_ Chem_ Phqs_ 54 (1971) I_ [ 141 W-R. Ware and B.& Bnldain. J. Chem. Phys. 43 (1965) 1194. 1151 R.S.H. Liu and J.R. Edmsn, J. Am. Chem. Sot. 91

(1969) 1492 [I61 R.S.H. Liu and R.E. Kellogg, J. Am. Chem. Sot_ 91 (1969) 750. 1171 CC. Lad\\is and R.S.H. Liu, J. Am. Chem_ Sot. 96

(1974) 6210. R 0. Campbell and R S H. Liu, J Am. Chem. Sot. 95 (1973) 6560. fl91 G-D. Gdhspie and EC_ Lim, Chem. Phys. Letters 63 1181

(1979) 193_ [201 G D. Gillespie and

E C. Lim, to be published.

359