Energy transfer between coronene and rhodamine 6G in PMMA matrices

15 November 1977

CHEMiCAL PHYSICS LETTERS

Voh~mc 52. number 1

Reuven KATRARO,

Arza RON and Shammai SPHSER

Deparrment o/Chen~istty, ‘Iscfwion-Isracl hstitute of Technology. Haifa. Israel Received 21 June 1977 Revised rnanuscnpt rcceivcd 1 August 1977

The l3~vrc5ccncc Iifetmle of coronene and the rate of dipole-dipole energy transfer from coroncne to rhodJmine 6C in PMMA matrices were found to bc tcmpcraturc dependent. Fvr both these photophysical processes an activation energy of about 500 cm-* is obbmed. The energy transfer rcgults can be annly7ed in terms of B model involving thermally activated energy transfer from excited states of coroncne.

The radlationless electronic excitation transfer bctwecn an excited donor molecule D* and an acceptor

dently R, can be calculated

molecule A is a well studied phenomenon [ I]. The process can bc represented by the bimolecular quenching process D* + A kzD+A*.

(1)

Several different mechanisms may contribute to k,. Long range transfer is determined by the dipole-dipole interaction between D* and A, which results in transfer rate constant kD, related to k, by [1,2] : k,,

= k2 [Al

- (l/71))

(&-J/-R)~ ,

(2)

where rD is the donor’s fluorescence lifetime, R is the distance between the D and A molecules, and RO is the critical transfer radius for which kDA = l/rD. According to Forstcr’s theory 121 X0 can be deduced experimentally by measuring the critical concentration [A01 , i.e. the concentration of A for which, in the case of pure exponential decay, k,, = l/rD. It can be shown that R, = (7.35/[Ao])‘/3

,

(3)

where Rg is in A units and [A01 in moles/K. Indepeni

I3:tsed in part on a ‘Thesis to bc subrnittcd by R.K. to the Senate of the Technion in partial fulfdment of the requirements for the MSc.

16

degree.

using the Fiirster formula

FD@)E#)~

-4dc

_

(4)

fiereGD

is the donor’s fluorescence quantum yield, is an orientation factor for the D and A dipoles and n is the refractive index of the medium. The critical transfer radius depends on the overlap between the normalized fluorescence specrrum F(U) of the donor, and the spectral distribution of the acceptor’s extinction coefficients e*(F). When D and A are irnbcdded in a rigid medium, diffusion processes are excluded, and the only possible means of long-range energy transfer is via dipole-dipole interaction. Averaging over a distribution of R ‘s yields the following expression for the normalized fluorescence signal P(t) following a short pulse excitation: K

f(t)

exp(-f/rD)

exp {-$T~/~ [A] Ro(t/r~)1/2) .

(5)

According to cq. (5) tliore is a linear relation between In 6 0) 2 In [P(~)/cxP(-~/TD)] and t1j2 from which RO can be extracted. Recently energy transfer-based dye lasers have been investigated and some success has been achieved in extending the Iasing spectral region [3-7]_ In order to differentiate between the various mechanisms contributing to energy transfer in dye laser systems we first

investigated a model donor-dye-acceptor couple. Coronene was chosen as the donor since its relatively long fluorescence lifetime (=300 ns) facilitates decay time measurements. Rhodamine 6G (R6G) is a natural candidate for a dye-acceptor, cspccially since its absorption spectrum overlaps the coroncne emission spectrum. Applying eq. (4) to the coronene-R6G system, we found that R. = 31 a. In order to ensure a pure dipole-dipole interaction, experiments were carried out in degassed polymcthylmethacrilate (PMMA) matrices. The samples were excited by a nitrogen laser (10 ns pulse). The fluorescence signa behaved as predicted by eq. (5) and the analysis of the 295 K decay curve (fig. 1) yielded K, = 48 A. The difference between the experimental and the calculated value of R, is much higher than that encountered in other systems studied under similar conditions [8,9]. One possible explanation of this discrepancy is a contribution to the transfer rate from thermally populated higher excited states of coronene. In several molecules which, like coroncnc have a relatively small S2SI energy gap, a temperature dependent fluorescence lifetime was observed [ lo- 121. Moreover, gas phase studies indicate the existence of radiative ccupling between S2 and the ground state S, in these systems [13]. For coronene the quantum yield for fluorescence originating from the S2 (or S3) + So transition is 0.03 [ 131. Therefore, our study concentrated on measuring the temperature dependence of the energy trans-

0

CORONENE

.

COROHENE

5d6 +

is shown

in fig. 2. Coroncne-R6G non-exponential

solutions

which

-_-

350

‘;:

behavior

_----

300-

2 5 c

CORONENE

5rld5M

I”

RIK8

250

,m t

0

300

IO0

_I

400

TPK)

Fig. 2. Fluorcscencc hfetimcs for coronene in PMhfA versus temperature. Tbc best fit calculated curve is drawn through tllc experimental points. 60

--.-_----------

1

YRCG

from coronene and the

coroncne-R6G pair in PMMAat various temperatures following a nitrogen Iaser excitation.

PMMA

is also temperature dependent. Applying eq. (5) to these decay curves yielded a temperature dependent transfer radius as exemplified in fig. 3.

exhibit

Y

1.26~16~

t hssc)

sign&

1977

fer from coronene and of its fluorescence lifetime. Typical fluorescence signals for coroncne (5 X 10M5 M) and for coroneneR6G (R6G 1.26 X 10B3 M) in PMMA recorded at different temperatures are shown in fig. l_ For coronene alone, exponential dccay is observed. However, the associated lifetime is temperature dependent. This temperature dependence

i

Fig. 1. Typical fluoresccncc

15 November

CHEMICAL PHYSICS LETTERS

Volume 52, number 1

T(OK) Fig. 3. Temperature dependence of the critical transfer radius for coronenc-R6G in PMMA. Tbc best fit curve IS drawn througil tbc experimental point.

17

Volume

52, number

CHEMICAL

1

The results can be accounted

for by formulating

PHYSICS

15 November

LETTERS

addition to the familiar transfer from state 2 described

a

U)l k&21) = [l/&O)1 [RoCWIRl6 3 by [see eq.

model where molecules, thermally popuhited to highly excited states are coupled to the ground state both radiatively and throug?t non-radiative energy transfer routes. The kinetic scheme presented in fig. 4 is based

(6)

a contribution

of direct transfer from state 3 is significant. If we assume that the same type of dipole-dipole

on the Imown coronene absorption spectrum. Here the close-lyiug excited states of coronene are states 2 and 3 separated by an energy gap AR and coupled by the rate constants kF3 and k&, where k 23 = k$f2 exp(-AR/ RT). Radiative coupling-to the Iround state 1 is detcrB and by the radiative mined by the donor’s lifetime r21 decay rate constant of state 3, kR_ In the presence of an acceptor, energy transfer occurs. We assume that in

interaction determines the transfer rate from 3 we imply thatthis rate is proportional to l/R6. In analogy with eq. (6) this proportionality reads kDA(3I)=(@

+

k~Dz)[RoW)/R16-

(7)

Employing the kinetic scheme of fig. 4 with the condition kp2 > kDA(3 1) yields the following expression

for the fluorescence ACCEPTOR

DONOR

1977

p(t) = exp ]-#I X

exp{-

signal of the donor

G?l

$3/2

[A] Ri [t/7;

(0)] *12) .

@I

Comparing eq. (8) with eq. (5) we note that the donor’s fluorescence lifetime is now temperature dependent = l/~f~(O)

l/r;,(T)

+ kFl exp(-AE/RT)

and that the effective critical

temperature Rg= &21)

L-------

_________

COUPLED

dcpcndcnt --.

-I

TRANSITIONS

enorgy transfer _.

parameters

7: (0) R

8 k31 D AE

-__--_----

exp(-AE/RT).(lO)

_--_-_

_-

~_-

- -

3.1 x 106 5-1 6

kp3 = kg

AE

+ ~~l(0)k$?2R$(31)

321 ns 32 A

(21)

k,,R,(31)

for coroncnc-R6Ga)

---

transfer radius is also

dependent

The best fitted parameters, summarized in table 1, were calculated for eqs. (9) and (10). The calculated curves together with the experimental points are presented in figs. 3 and 4. It is satisfying to note that the temperature independent transfer radius R. (21) is 32 a, in good agreement with the calculated value. The approximate activation energy for both r?I and RO is

Fig. 4. Kinetic scheme for describing thermal activation of an energy transfer process. The system is pumped by a short laser pump of intensity Ip at a rate oIp_

Table 1 Tcmpcraturc

7.2 X lOI

A”/s

exp(--aE/RT) 4 15 f 110 em-1/molecule

600 + 100 cm-l/molcculc _--.-- - .-_-_---_-

eq. (9) cq. (10)

- __ __ -- --_. D a) The value of k 31 varies between log s-r in the gas phase (based on the value kg/k?* FJ0.03 [ 131 to 10” s-l in condensed phncc (line widl measurements [ 141). Ti IC corresponding values for Ro(3 1) arc between 44 A and 14 A, values typical of dipole-dipole intcractron [ I]. The increased efficiency of energy transfer at higher temperatures is due to kDA(3 1) which is proportional to k3z R,6{31) [see eq.