Electronic structure and intersystem crossing in 9,10-diphenylanthracene

CHEMICAL

PHYSICS

LETTERS

ELECTRONIC

1 (196i)

336-336.

NORTH-HOLLAND

STRUCTURE AND INTERSYSTEM IN g,lo-DIPHENYLANTHRACENE CHEN-HANSON Inslitd

J?ir Pkysikalisch

e Ckemie

Received

9,10-Diphenylanthracene (DPA) is an interesting compound because its fluorescence yield is unity independent on solvent, temperature. concentration and many other factors [I]. This exceptional property makes DPA an ideal material for the calibration or standardization of fluorescence apparatus to measure absolute fluorescence yields of other compounds. It had been shown conclusively that the fluorescence yields of anthracene and its derivatives increased as the temperature was lowered [2,3]. The thermal activation energy of the radiationless intersystem crossing process was correlated closely to the red shift of the 1La band [4] in the substituted anthracene to that of anthracene [3]. DFA would have very high activation energy because of its high fluorescence yield. However, the shift of its 1~ band is only 1200 cm-l, much smaller tha;l 1700 cm-l observed in 9,10dichloroanthracene, which has an activation energy of 1500 cm-l_ The electronic structure of DPA has to be very different from that of anthracene and the problem is best attacked by theoretical consideration. The electronic structure of DPA was calculated with the nowadays conventional Pariser-Parr method [5]. The electronic structure of anthracene was also calculated for comparison. All resonance integrals were taken as -2.35 eV except the ones between phenyl rings and anthracene, B’ = -1.84 eV. The electronic repulsion integrals were approximated with the formula y(R) = = a/(R+b eacR). The constants a, b, and c were chosen so that the repulsion integral fitted the empirical value at K = 0 and the theoretical when

R is large.

The

states of DPA and anthracene October

I?67

low

energy

CROSSING

1Jnitwvsitiil

Slrrlfgari

1967

(DPA) is calculated with Pariser-Parr from the energy shifts of triplet states

m&hoi. relative

The *‘:

1. The results in anthracene do not agree precisely with those by Pariser [5] because of the choice of parameters. Nevertheless. the general features are the same. The deviations are not important because only the differences between DPA and anthracene are of present interest. In connection with the intersystem crossing process, the calculation shows two impkrtant trends when anthracene is substituted to form DPA : (1) The red shift of the lLa band as expected, and (2) the blue shift of all the triplet states. The result of these opposite shifts is that in DPA al! the triplets except the lowest 3La have higher energies than the lLa state. The triplets which are the most important ones for intersysym+ crossing in anthracene [S] tre thle :Bsu and Blu states [7] * just below the La [ Blu) state. These triplets in DPA have much higher energies than the IL, state, and they are no longer effective in deactivating the lLa state. Since the %a state is separated from the lb state by a large energy gap, the intersystem crossing between them is also very inefficient [6]. Therefore, the intersystem crossing process in DPA is completely suppressed. The fluorescence process be comes the only way of deactivating ILa state, and the yield would naturally be unity as observed. fig.

* The symmetry symbols .are confusing in the Dzh group. The Mulliken convention adopted here is different from that used by Pariser [Sl. The short axis, a-axis according to Platt 141, belongs to the Blu rcpresentation. and the long axis b belongs to the Bpu representation, For clarity, PIaff’s notations are

excited

are compared

AiiSTERDAM

COMPANY.

TING dev

1-l September

The electronic structure of 9.10-dillhen2-!nnthraccnc unique luminescent behavior of DPA can be understood the -1La state.

values

PUBLISHING

followed

in 335

as much

zs possible

in this

note.

CHEN-HANSON TING

336

8‘2” rBb-

(2.92)

B;@b (0.79) ‘X (0.18)

:

R *“I

B;,,Lb _ BA,L, -

-6-

(0.30)

Lc -

Triplets

B:,,L

(0.96)

The physical reason of the blue shifts of triplets is that in DPA the size of the conjugated system is greatly enlarged and the exchange interaction which separates singlet and triplet is reduced. The calculation was not entirely satisfactory because the geometry of DPA was not known exactly and the choice of the resonance integral ,3’ was tentative. The calculated energies and OScillator strengths for the ‘La and lBb states can be improved if the phenyl rings are twisted out of plane and p’ is further reduced. More thorough The author thanks Prof. interest carrying

is in progress.

Th. Fdrster

for his

in this work, and Mr. Udo Sommer for out the numerical calculations. This

B2;rLb

-

Singlets 9,10-

Triplets

Diphenylanthracene

Fig. 1. Singlet and triplet excited states in DPA and anthmcene. Numbers allowed transitions.

in this direction

%

-

Zu, 8 b

Anthracene

investigation

%,L,

---%I

Singlets

-

in parentheses

are oscillator

work is supported by the Deutsche meinschaft, Bad Godesberg.

strengths

for

Forschungsge-

REFERENCES [l] B. Berlman. Handbook of Fluorescence Spectra of Aromatic Molecules (Academic Press, New York, .^“_. lYb3J.

[2] z: J. Bowen and J. Sahu, J. Phys.

Chem.

63 (1959)

[3] c. C. Lim, J. D. Lapose +nd J. M. H.Yu, J. Molec. Spectr. 19 (1966) 412. I4! J.R.Platt. J. Chem. Phvs. 17 11949) 484. i5j R.Pariser, J. Chem. Phys. 24‘(1%%) 256. [S] J. P. Byrne, 3. F. McCoy and I. G. Ross, Au& J. Chem. 18 (1965)1589. [7] R.S. Mulliken, J. Chem.

Phys.

23 (1955) 1997.