Magnetic resonance studies of the deuteration effect on intersystem crossing in the anthracene molecule

CHEMICAL PHYSICS LETTERS

Volume 12, number 1

MAGNETIC RESONANCE STUDIES OF THE DEUTERiiTION EFFECT ON INTERSYSTEM CROSSING IN THE ANTHRACENE MOLECULE-f R.H. CLARKE $ Depmtment of Chemistry and Enrico FermiInstitute, University of Chicago, Chicago, Illinois 6063 7, USA Reuzived 24 September

The intersystem crossing decay constants by phenzzine crystal have been determined

1971

from the 3Bzu state into the ground state of anthrauxeil~o magnetic

resanmae

techniques nt lS°K

both

nt hisI_

magnetic

in a field

and, by a parameterization procedure, at zzo magnetic fie!d, A comparisonof the anthracenetilazero4icldresuIts

with thosefor anthracene-hlo show the effects of deuterium substitution to be largest for the in-planespin levelsof the anthraene

triplet

state.

1. Jntroduction The Gbservation of stimulated microwave emission in the EPR spectrum of photoexcited triplet states of organic mo!ecules at low temperatures (T< 4.2%) crossing mechanisms, arises from selective intersystem with rates

comparable

to relaxatioq

rates among

the

triplet spin levels [l-4]. Such a situation allows an investigation of the dynamics of the intersystem crossing process for each of the individual at high field and, by suitable extrapolation,

spin levels at zero

magnetic field. In general the decay of the observed EPR signal is a complicated function of radiative and non-iadiative transitions into and out of the triplet state and relaxation mechanisms among the three spin levels. In favourable cases, however, intersystem crossing rate information may be directly estimated from measurements of the decay of the EPR signal. Such

casesarisewhen t This research has been supported by the United States Ato,mic Energy Commiss.&~ Frequency counting equip ment used in this work was supplied by the Advanced Research Projects Agency. kcknowledgement is made to the donors of the Petroleum Rekrcb Fund, administered by the American Chemical Society, fir pkrtial support of this

research. $ Present address: Department of Chemistry, Boston Universit-y, Bosron, Massachusetts 02215, USA.

(i) the intersystem crossing rate is Larger than the spin-lattice relaxation rate and (ii) the intersystem crossing mechanism may be separated into either a predominantly radiative transition or a predominantly non-radiative transition;and simplifying assumptions may be made which allow quantitative estimates of intersystem crossing rates. In a previous publication [4] we had presented intersystem crossing data for the depopuIation of the triplet state of the anthracene-i~10 molecuie in a phenazine host crystal; in the present.note we report data for anthracene-dlo in phentie. These experiments provide an opportunity for investigation of the deuteration effect on intersystem crossing for the individual spin levels of the 3&, state of anfhracene, a molecule of central importance in both experimental and theoretical studies of the dynamics of excited states. Although there has been much work devoted

to the dynamical propWies of spin.aiigned rriplet states of nitrogen heterocyclic and halogenated arrqmaticsystems [S-7], there has been !ess progress in studies of unsubstituted planar aromatic systems, whose intersystem crossing mecha&ms are of central importance to theories of excited state dynamics of orga+

systems [8,9]. It is qf particular

interest,

therefore, to compare these results on the antbracene triplet state with recent theoretical work bn radiation157

Volume :2, number

-less transitions

in

1

CHCMICAL PHYSICS LETTERS

; .,

i Deeember 1971

planar aromatic systems [IO, 1 I] ;

-2. Treatmentof experimentalresults ‘The methods involved in the present magnetic resonance experiments performed on the lowest triplet state of the anthracene-d10 molecule in a phenazine host crystal are the same as those previously reported for anthracene-hl,-, [4]. Decay of the photoexcited triplet state EPR signals was measured along three orthogonal axes of anthracene at =lS°K and also in one case (HIl$) at 4.2”K. The EPR results on the spin-alimed triplet state of antbracene-drc at ~1.5% areindicatedin fig. 1. The strong fie!d decay constants for the three orientations of the laboratory magnetic field are given in table 1. They were obtained in the following manner: (1) Decay of the EPR signal obtained after cessation of light excitation was obtained and stored by signal averaging techniques. (2) Several decay curves were taken for each field orientation,

i(I = x, y, z), with

successively

lower

microwave power levels, until no changes were observed with further power decrease, to eliminate saturation effects. (3) Since the change in intensity of an EPR signal with time is determined by the change in the difference in populations with time of the two spin levels, m and m’, connected in the resonance experiment, the printed output from +he signal averager (400 data points) Gas fitted by computer to a function of the

pi& 1. Pints bf energy versus magnetic field strength for the triplet state of anthracenellre. The emissiveor absorptive charecters of the Am = 1 EPR transitions observed at 1S”K are indicated schematically, with solid arrows representing steady-state continuous optical excitation conditions and

dashed arrows after cessation of illumination. form

r,(t) =A exp(-R!,,t) - B exp(-Rfn,t) , Rh, Rfn, are the total decay constants of the two strong field spin levels involved in the EPR transition’, and _4 and B are the (relative) steady state populations of the two levels. Typically, this procedure yielded an overall standard deviation of=O.l%in the fit to the experimental decay curves. The total decay constants, R,, obtained

Table 1

Decay times, 7, (in mi.Ukcond.9, of the strong-field spin levels of anthracene4 10 mwsured from the Am = I EPR signals at 15°K along the molecular fine structure axes Skndard deviations in parentheses Hlli bw f=Id

trnnsition

2,

high

field transition

46.96(0.46) 203.42(2.94)

138

H II2

-

34X64(0.71)

164.26(1.18) ii3.79(i.l3)a)

56.36(0.49)

304.14(1.66)

70

T+~213.86(18.11)

‘a) Measured at 4.2%.

Herr"

71.51(0.57) 70.5~(2.20)a),

78.72(1.,11)

Volume 12, number 1 in this manner

contain

CHEYICAL PHYSICS LETTERS contributions

from all depo-

pulating mechanisms acting on each spin level, i.e., all radiative, radiationless and relaxation processes and

assumes only that such processes are all describable by a first-order kinetics scheme. From these strong field data for the anthracene-dt,, 3B2u state we can obtain values for the intersystem crossing rates, kj, for the three triplet sublevels j(i = x,y, z) at zero magnetic field. !f spin-lattice relaxation is assumed negligible, the strong field decay constants can be considered as simple linear combinations of the zero field intersystem crossing rate constants [ 1] , thus providing (from table 1) nine experimental measurements (not necessarily independent) of the three kj. The consistency of va!ues for ki calculated from the data in table 1 indicates that relaxation effects between spin levels are small. However, to obtain the most accurate set of kj the additional degrees of freedom were used to make a quantitative estimate of the contribution of spin--lattice relaxation

to the strong field decay constants measured in the EPR experiments. A set of relaxation parameters, wi, into which

were lumped

all spin-lattice

relaxation

phenomena nssociated with a given field orientation. j, without any consideration of the details of the relaxation mechanism, were included in a least-squares -;:ctcr determination of the kj from the strong field data in table 1. The relaxation parameter, wv, was set equal to zero, since the EPR decay curve associated with the high field H IIT transition showed Table 2 Zero-field depopulation rate constants (in sac-l) for the 3BzU state of anthracene in a phenazine host aystaL Standard deviations in parentheses

s

kz

anthracene-hto

84.90(0.88)

29.98(2.98)

7.82(0.19)

anthraccne-dlo

24.41(0.27)

11.07(0.60) 2.71(0.44)

3.7X0.46) 209(0.35)

ratio kj+/$

3.48(0.08)

3. Discussion The mechanism the anthraceneAIO

ture from 1.5% :o 4.2*Kj.. The “best” set of ki obtained for anthracenedIO

are given in table 2. Values

for the spin-lattice relaxation parameters obtained were (w,l=2.43 kO.45 set-* and Iw,i=O.53 F 0.48 see-I.

AIso included in table 2 are a “best” set

of values for anthracene-hi0 ously reported [4].

revised over those previ-

of population triplet

state

of the spin levels of in a phenazine

host

crystal is presumed to be the same as that previously proposed for the anthracene-h10 triplet state in phenazine, viz. a.transfer of energy from the host triplet exciton band to the guest triplet state, and the measured rates of population are consistent with this model. This is a reasonable

result, since the populating

mechanism is basically controolfed by intermolecular exchmge interactions, which would not be expected to be affected by deuteration 1121. The zero field intersystem crossing rate constants kj obtained

dent

for anthracene-dl,-,

depopu!stion

show the most effi-

route from the anthracene

tripIet

is via the x (long axis) spin state, as was the case for anthracene-hlo. The zero field depopuIa:ion rates in order k, > &,, > kZ are also in the same order as that observed for anthracene-hlo [4]. The ratios of the ki for anthracene-lzLO to those for anthracene-dtO p rovide particulariy interesting information about the deuteration effect on intersystem

crossing in planar aromatic systems. The short lifetimes of the anthracene ured rate constants

triplet levels indicate that the measare dominated

by non-radiative

transitions from the triplet to the ground state. Since a direct spin-orbit coupling interaction between the triplet excited state and the singlet ground state is forbidden for all three spin levels, it is necessary to invoke both a spin-orbit and a vibronic interaction for such a non-radiative transition to be allowed. Further, symmetry considerations of the spin-orbit coupling interaction show that for a TK* tripIet state, the in-plane spin levels (X and y) are expected to be :oupled to CW*singlet states and the out-of-plane (2) qin level can couple only to nrr* singlet states. Recent t The factor of ~2 difference (se table I) in decay rates for the high and Iow field EPR sknals at I.S”K with Kirj? (which

no temperature dependence on raising the tempera-

I December 1971

in a strong

field

approximation

should

bc

the

ume)

is as yet unexplained acd may involve unusual spin-lattice relaxation mechanisms.The low t?eld EPR transition with ii Il_?did show an appreciabie change in decay constant on changing the temperature.

Mowever. the ki vakes obtained

from the l.FK low field deay constant for HI!: were not consistent with those cdculated from decay constants for the other two lab field orientations, whereas the high field decay constant was consistent Therefore. on!y the H ilp high field dec3y constant’was used in the determination of the “best” set of kj. 159.

Volume 12, ntiber work

of Henry

1

CHEMICAL PHYSICS LETTERS

and Siebrand

[ 19, II]

on non-radi-

ative

transitions of triplet states of aromatig hydrpcarbons induced by spin-vibronic interactions has shown that iln* triplet levels spin-orbit coupled to an* singlet states are most efficiently depopulated via out-of-plane‘C-H vibrational modes, while TUT* triplet levels spin-orbit coupled tc,lirr* singlets are depopulated via in-plane C-C modes. Deuteration will, therefore, be expected to have a larger effect on non-iadiative transitions involving spin-orbit coupling to u7r* states. The k$$ ratios for the lowest triplet state of anthracene; given in tabie 2, show the deuteration effect to be largest for the x s in level, the effect being in the.order k,“/e > kH/ kJy > e/e. The observed deuteration effect wou Yd seem to support the calculations of Henry and Siebrand which would predict the largest detiteration effects to cccur for the in-plane spin components and confkins the central. role of C-H vibrations in radiationless transitions of triplet states of planar aromatic

systems

[io, 11, 133.

1 De&m&

1971

in whose laboratory these experiments were perforned, .4cknovjledgement is also due .to the National

Science Foundation and the Natiqnal Institutes of Health for Postdoctoral Fellowships.

References

’

[!I J.H.van der Waals and M.S. de Groat, The triplet state (Cambridge Univ. Press, London,

1967) p. 101.

121M.Schwoerer znd H.&l, Chem. Phys. Letters 2 (1968) 14. 131 M.Schwocrer and H.&l,

Z. Naturforsch 24a (1969)

952. t41 ts1

R.H.Clarke, Chem. Phys Letters 6 (1970) 413. J.Schmidt, W.S.Veeman and J.H.van der Waals, Chem.

Phys. Letters 4 (1969) 341. M.A.Ei-Sayed. J. Chcm. Phys. 54 (1.971) 680. I71 M.A.El-Sayed and C.R.Chen, Chem. Phys. Letters 10

[Gl

(1971) 313. [Sl W.Siebrand, J. Chem. Phys. 47 (1967) 2411. 191 J.Jortner, S.A.Rice and R.M.Hochsfxasser, Advan. Photochem.

1. (1969) 149.

[lOI B.R.Henry and W.Siebrand, Chem. Phys. Letters 1 (1970) 533.

Ackno&dgements

The author gratefully acknowledges the support and encouragement of Professor CA. Hutchison Jr.

1111 B.R.Henry and W.Siebrand, J. Chem. Phys. 54 (1971) 1072.

1122 J.Jorlner, S.A.Rice, J.L.Katz and S.Choi, J. Chem. Phys. 41(19&) 309. [13] W&brand, Chem. Phys. Letters 6 (1970) 192.