External heavy atom effect on the emission of carbazole

Journal of Luminescence 33 (1985) 377—390 North-Holland, Amsterdam

377

EXTERNAL HEAVY ATOM EFFECT ON THE EMISSION OF CARBAZOLE G.K. MALLIK, T.K. PAL, S. LAHA, T. GANGULY and S.B. BANERJEE * Optics Department, Indian Association for the Cultivation of Science, Jadavpur, Calcutta — 700032, India Original manuscript received 2 October 1984 Revised manuscript received 6 July 1985

External heavy atom induced quenching of luminescence emission of carbazole in n-hexane and methylcyclohexane in the presence of chloro- and bromoacetic acids at 300 K and 77 K is described. A straight line relation between fluorescence quenching rate constant and the acid dissociation constant of chloroacetic acids, which is related to electron affinity, is observed. Different photophysical parameters at 77 K of unperturbed and perturbed fluorescent carbazole in ternary solutions have been determined and suitable reaction mechanism for quenching has been proposed. Inference has been drawn about complex formation in the triplet state of carbazole from the biexponential nature of the phosphorescence decay curve.

1. Introduction Heavy atom induced quenching of fluorescence is believed to be caused by spin—orbit coupling [1] and consequent breakdown of spin selection rules which facilitates large intersystem crossing. According to McGlynn et al. [2] an essential condition for external heavy atom effect is charge transfer interaction between the fluorophore and the heavy atom perturber. Depending on relative ionization potentials and electron affinities of fluorophore and quencher, a quencher may behave as electron acceptor or donor. Several instances of fluorescence quenching via intermediate contact exciplex of charge transfer nature have been reported [3—13].Observations have been made in dilute solutions as well as in the vapour phase [11,12]. Klein et a!. [14] have proposed a correlation between overall bimolecular quenching rate constant and the exponential of the measure of free energy change associated with formation of exciplex. Good Klein correlation has been shown to hold in the case of a number of halocarbons [11]. *

To whom all correspondence should be addressed.

0022-2313/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

378

G.K Mallik et al.

/

External heavy atom effect

Fluorescence quenching of carbazole in dilute solutions by trichloroacetic acid and other compounds at room temperature through charge transfer interaction has been reported by Johnson [15]. In the present work the luminescence spectra of carbazole in n-hexane at .300 K and in methylcyclohexane (MCH) at 77 K, in the presence of mono-, di- and trichioroacetic acid and mono- and tribromoacetic acid used as quencher were investigated. One of the objectives was to measure the quenching rate constant and to study how it changes with the electron affinity or associated physical property of the quencher in order to test the postulate of exciplex formation. Another point of interest was to look for possible complexation of carbazole in the excited triplet state with the chloroacetic acid (CA) and bromoacetic acid (BA) molecules. It was also intended to determine various photophysical parameters and to examine suitable reaction schemes for the quenching process.

2. Experimental Pure (95%) carbazole supplied by Fluka, Switzerland was used as fluorophore after vacuum sublimation. Trichloroacetic acid (E. Merck) and monochloroacetic acid (Riedel, Germany) were recrystallized twice from light petroleum—petroleum ether (b.p. 60—80°C) mixture and are kept in a vacuum desiccator which was further evacuated by a vacuum pump each time, just before the compounds were taken out for use. Tribromoacetic acid (Fluka, Switzerland) and monobromoacetic acid (Fluka, Switzerland) were purified by the same procedure. Dichioroacetic acid (Reidel, Germany) and acetic acid (E. Merck) were used after distillation under reduced pressure. Solvent n-hexane of spectroscopic grade purchased from E. Merck was used as such. Methylcyclohexane (MCH) obtained from BDH company, London (A.R. Grade) was treated with a mixture of conc. H2S04 (35% by vol. of MCH) and cone. HNO3 35% by vol. of MCH) to remove aromatic compounds e.g. benzene, thiophene, etc. and again it was treated with conc. H2SO4 (70% by vol. of MCH) and washed with distilled water until it was freed from acid. Anhydrous sodium sulphate was poured into MCH and kept for 24 hours. Finally, the sample was distilled at 101°Cin atmospheric pressure. The haloacetic acids and the other solvents were tested for any possible interfering emission when excited at 325 nm either at room temperature or at 77 K. The concentration of carbazole was fixed at 2.96 x io~ mol 1’ approximately and 6.43 x 1O~mol l~ at room temperature and 77 K, respectively, for all the experiments performed in emission. The concentrations of the perturbers (chioroacetic and bromoacetic acids) varied between 0 and 10~1 1at 77 K. MCH was used mol at room temperature 0 and 10~ mol Y~ as anl..1 inert solvent at 77 K toand obtain a glassy matrix. The fluorescence and excitation spectra were recorded with a Perkin—Elmer

G. K. Mallik et a!. / External heavy atom effect

379

model MPF 44A fluorescence spectrophotometer provided with corrected spectra unit and the absorption spectra with a Shimadzu UV—VIS spectrophotometer model 210A. Carbazole was excited at 325 nm when the fluorescence spectra and phosphorescence spectra were recorded both at 300 K and 77 K. For studying phosphorescence decay the emission wavelength was chosen at 435 nm. After the phosphorescence intensity achieved a steady state, the exciting radiation was cut off to obtain the dark decay on the recorder. The relative fluorescence quantum yield of carbazole and fluorescence lifetime (T) in n-hexane solvent were measured by the procedure described elsewhere [16].

3. Results and discussion The electronic absorption and fluorescence spectra of carbazole in n-hexane at 300 K are not affected when acetic acid is added to the binary solution. This implies that no complex is formed between acetic acid and carbazole in the ground and excited states or in general there is no intermolecular interaction of significance between the molecules. When monochloro-, dichloro- and trichloroacetic acid (MCA, DCA and TCA) or mono- and tribromoacetic acid (MBA and TBA) are used in the place of acetic acid, the absorption spectrum of carbazole does not undergo any change. But when concentrations of the chloro or bromoacetic acids are increased in steps, the intensity of fluorescence emission of carbazole in n-hexane falls off gradually (figs. 1(a), 1(b), 2(a) and 2(b)). This combined with the observation that acetic acid does not produce any change in the fluorescence intensity leads to the inference that the quenching is induced by external heavy atom chlorine or bromine in the haloacetic acid molecules CA or BA. Additional support for this inference is provided by the fact that the changes induced by MBA and TBA are larger than those induced by MCA and TCA respectively (table 1). Absence of any change in the absorption spectrum of carbazole shows that there is no intermolecular interaction between CAs or BAs and carbazole in the ground state and so the Stern—Volmer equation (1) will be valid for those systems, f0/f=1+K~~[Q].

(1)

Here, f0 and f stand for relative fluorescence intensities of carbazole in n-hexane, without and with the quencher molecule whose concentration is [Q]; ~ is the Stern—Volmer constant which is equal to Kqro where Kq is the bimolecular rate constant of dynamic fluorescence quenching and ‘i-0 is the fluorescence decay time in the absence of a quencher. From the slopes of the curves obtained by plotting f0/f versus [Q](figs. 3 and 4) ~ is determined. The values of Ksv and Kq for the three CAs and the two BAs are shown in the table 1. It is seen that while for TBA, TCA and

380

G.K. Mallik et a!.

/

External heavy atom effect

(b)

(a) 0 0 2 3

325

340

355

370

\

385 325

355

385

(nm)

5 mol 1_I) at 300 K Fig. 1. (a) Fluorescence spectra of carbazole in n-hexane (2.96x10 (excitation wavelength = 325 nm). Concentration (mol 1_I) of TCA in 0,0; (1) 3.23 >< iO~ (2) 6.45 x i0~ (3) 1.29x 10_2; (4) 2.58 x 102; (5) 6.45 >< 10~2.(b) Fluorescence spectra of carbazole in n-hexane (2.96 x 10 mol 11) at 300 K (excitation wavelength = 325 nm). Concentration (mol l~’) of MCA in 0,0; (1) 1.79X103 (2) 3.94X103 (3) 5.37x103 (4) 6.96X10~3.

DCA the Kq values are comparable with the diffusion controlled rate constant Kd for MCA the Kq value is somewhat smaller. From table 1 it is observed that for carbazole in CAs, the quenching efficiency Kq/Kd increases in the order TCA> DCA> MCA and for Carbazole—BA systems the increase is in the order TBA> MBA. Clearly, the quenching efficiency increases with increase in the number of chlorine or bromine atoms in the quencher molecules, i.e., their electron affinities. Therefore, a reasonable inference is that heavy atom quenching is a consequence of formation of contact exciplex with charge transfer character, the molecules concerned in this case being carbazole acting as electron donor and CAs or BAs as electron acceptors. Klein et al. [14] has proposed the following relation for exciplex of charge transfer nature: Kq cr exp[ (ID EA C P hPe)/KT], (2) where Kq is the overall bimolecular quenching rate constant, ‘D the ionization potential of the donor, EA the electron affinity of the acceptor, C the Coulombic stabilization energy, P the polarization energy of the separate —

—

—

—

—

G.K. Mallik eta!. / External heavy atom effect

k

.~

325

340

(a)

381

(b) ~fz

355

370

385 325

340

355

370

385

~ (n m) Fig. 2. (a) fluorescence spectra of carbazole in n-hexane (2.21 X 10 mall~’)at 300 K (excitation wavelength = 325 nm). Concentration (mol 1_I) of TBA in 0,0; (1) 9.13 X ion; (2) 4.56 X ~o 3; 3 (4) 8.22x303 (5) 1.88x102. (b) Fluorescence spectra of carbazole in n-hexane (3) 6.84x10 (2.19)< 10 mol 11) at 300 K (excitation wavelength = 325 nm). Concentration (mol 1 1) of MBA in 0,0; (1) 2.35 X io~(2) 4.71 >< i0~ (3) 7.07x103 (4) 9.42X103 (5) 1.88X102 (6) 3.76x102.

charges and hVe the excitation energy of the first excited state. For investigating CT exciplex formation where there is fluorescence quenching but no change in the shape of fluorescence band, one of the methods usually adopted is to vary EA by changing the number of electron acceptor atom or group in Table I Fluorescence quenching constants for carbazole-acetic acid, and carbazole-chioroacetic acid and carbazole-bromoacetic acid pairs in n-hexane at 300 K (the fluorescence decay time T 0 of carbazole in n-hexane is 23 ns) 1 Kq/l moI’ s—~ Kd a)/l mol1 s—~ Kq/Kd Ks~/lmo1 Carbazole + acetic acid No quenching Carbazole + MCA 42( ±3) 1.82( ±0.14)X i09 2.25 X 10’° 0.08 Carbazole + DCA 112( ±10) 4.86( ±0.47) x i09 2.25 x 1010 0.20 Carbazole + TCA 149( ±8) 6.47( ±0.40) X i09 2.25 X 1010 0.30 Carbazole + MBA 90( ±6) 3.91( ±0.25)x i09 2.25 x 1010 0.17 Carbazole + TBA 168( ±6) 7.31(±0.24) x i09 2.25 x iO~° 0.32 The standard deviation values are given in parentheses. The terms are explained in the text. ~ Kd = 8RT/300th 1

382

G.K. Mallik et al.

0

I

0

0.01

I

0.02

0.03

/ External heavy atom effect

0.04

[a] in ML~ Fig. 3.

f0/f

vs. [Q]relations for (1) carbazole—TCA. (2) carbazole—DCA and (3) carbazole—MCA systems in n-hexane.

the quencher, keeping ‘D of donor fixed. In the case of carbazole—CA systems, EA of acceptor CAs is not known. However, the electron affinity is directly related to the acid dissociation constant Ka which is in turn, related to the pK value. The Ka values were calculated from the relation pK = log Ka utilizing reported values of pK in benzene [17]. The plot of In Kq vs. Ka yields a straight line as shown in the fig. 5. This is in accord with the proposition that an encounter exciplex of CT nature is formed between carbazole and CA. —

o.bi

0

[Q]in Fig. 4.

f0/f

vs.

[Q]relations

0.02

0.03

ML’ for (1) carbazole—TBA. (2) carbazole—MBA systems in n-hexane.

383

G.K. Mallik eta!. / External heavy atom effect

26

,~.

24 22 20 0

0.05

0.1 Ka

0.15

0.20

Fig. 5. Dependence of the observed fluorescence quenching rate constant Kq of carbazole on acid dissociation constant Ka of chloroacetic acid quenchers.

Exciplex formation between carbazole and bromoacetic acid molecules may be logically expected. The increase of quenching efficiency Kq/Kd with the number of bromine atoms in BAs shows a similar trend as for chloroacetic acids. However, since values of acid dissociation constants were not available, Klein-type plot could not be studied for carbazole—BA systems. For carbazole—CA or BA systems the following reaction scheme appropriately describing the complete process of quenching may be considered: K 1 K_

1 (A*~ ...Q~)*-s

1

3(A*Q)~~s3A*+Q-s 1A+Q,

(3)

K1~~

where K1 and K_1 represent, respectively, constants thefluorescent formation 3A* the the rate triplet state offorthe and dissociation of the complex, and compound. As the quenching of the fluorescence proceeds via the CT encounter complex, as shown in the scheme, the Stern—Volmer constant K 5~can be written as, K1 K15~ Ksv KqT0 K + 1’ISC v T~j, (4) —1 where K 15~is the rate constant of heavy atom induced intramolecular intersystern crossing. According to Berlmann [18] the fluorescence quenching constant (Kq) is composed of two independent rate constants: (a) Rate constant KT for intermolecular singlet—triplet energy transfer (chromophore to quencher): (b)Rate constant KqISC for induced intramolecular intersystem crossing (in the chromophore). Since the quenchers MCA, DCA, TCA, MBA and TBA do not have any =

=

384

G.K. Mallik ci al.

/

External heavy atom effect

0

(a)

5

~

(b) 0

1

> 4-, C)

I

____________

330

360

390

400 430 k~(nm)

I

460

Fig. 6. Fluorescence (a) and phosphorescence (b) spectra of carbazole in MCH (6.43 x iO~ mol 1_I) at 77 K (excitation wavelength 325 nm). For (a): Concentration (mol l~) of MCA in 0,0; 4.For (b): Concentration (mol l~1) of MCA in 0,0; (1) 1.23 x io~(2) 6.18 X io~(3) 1.85 >< i0~

(1) 1.23>< iO~:(2) 1.24X104.

triplet state lying lower with respect to the singlet state of carbazole K~T may be neglected and only contribution of K 1~SC need be considered. To test the proposed mechanism (3), kinetic analysis of different photophysical parameters of singlet and triplet states of carbazole in MCH at 77 K in the presence of haloacetic acids has been carried out. It is seen that in every case there is an increase of intersystern crossing rate constant (tables 2—6) and consequently of the triplet yield. At 77 K, gradual addition of MCA or DCA or TCA or MBA or TBA in the binary mixture of carbazole and MCH causes fall of intensity of both fluorescence and phosphorescence emission keeping the shape of the spectra unchanged (figs. 6 and 7). The phosphorescence decay curves of carbazole in MCH quenched by CAs and BAs presented in figs. 8 and 9 are seen to become linear in semilog plot and identical with the unperturbed carbazole phosphorescence decay after a

G.K. Mallik et aL / External heavy atom effect

385

0

>~.

~

a,

I

4-,

a, >

a

330

360 390

400 )~(nm)

430

460

Fig. 7. Fluorescence (a) and phosphorescence (b) spectra of carbazole in MCH (5.13 x 10 mol 1_I) at 77 K (excitation wavelength = 325 nm). Concentration (mol I_I) of TBA in 0,0: (1)

6.38 x

io~~ (2) 8.52 x ion;

(3) 1.06 X i0~.

considerable time. By subtracting the exponential decay component from the observed phosphorescence decay function, the phosphorescence decay function of the perturbed molecule is obtained as shown in the same figure. In case of each haloacetic acid quencher, phosphorescence lifetime (i-n’) of carbazole becomes smaller (tables 2—6) due to external heavy atom perturbation as the concentration of quencher molecules increases. The phosphorescence decay curves for carbazole perturbed by the haloacetic acids CAs or BAs are thus

composed of two exponential decays, one is due to unperturbed and another due to perturbed carbazole molecules indicating complex formation in the triplet state. Determination of kinetics of the perturbed and unperturbed carbazole

386

G.K. Mallik et aL

/

~

External heavy atom effect

3 \\

0.05-

\

\\\

~

\~ \\

0.02-

\

~b

3\\ 4I

0

\

8

~.

12I

15

Time in sec. Fig. 8. Phosphorescence decay functions of carbazole in MCH at 77 K (6.43x iO~ mol I_l)

without the perturber MCA molecules (0): with MCA molecules of concentration (mol I -.1) in (1) 1.23 x i0°: (2) 6.18 x 10~ (3) 1.24 > 10g. Dotted curves 1, 2 and 3 (below) represent phosphorescence decay functions of the perturbed carbazole molecules by MCA.

molecule is based on the following rate-determining reactions which were considered by Lessard and Durocher [19] for indoles after the formation of the excited perturbed (‘A’) and unperturbed (‘A) molecules. K,

‘A K,

‘A

K,

‘A0 1A

+ h Pf,

~,

1A

‘A’ ‘A’

0 K.

‘A 0,

=s

3A,

K

+ h vi.,

K.

‘A=~ 3A ~ ‘A 3A

~

~.

3A’ ~ 1A

~,

‘A 0+hv~,

3A~~1A0+hp~,.

G.K. Mallik et al. / External heavy atom effect

387

1.0

0.5

0.2

~ ~

> 0.1a ~

0.05

~

-

0.02-

0.01

0

3

2

4

2

3

6

8 10 12 Time in sec.

14

16

18

20

Fig. 9. Phosphorescence decay functions of carbazole in MCH at 77 K (9.09 X 10 mol I’’) without the perturber MBA molecules (0); with MBA molecules of concentration (mol l’) in (1) 4 (2) 8.92x104 (3) 2.12x103. Dotted curves 1, 2 and 3 (below) represent phos6.37x10 phorescence decay functions of the perturbed carbazole molecules by MBA.

Kf and K~ are the radiative fluorescence and phosphorescence decay rate constants; K, and K 11, are S~—+ T~ and T3 =s S0 intersystem decay rate constants; and K~is the S1 S0 internal conversion rate constant. The phosphorescence intensity which is shown to be composed of two exponential decays is represented by [19] I~(t)/J~=/3exp(—t/T~)+(1—/3)exp(—t/T~.), (5) and I~(t)denoting stationary state and time-dependent phosphorescence intensity. In figs. 8 and 9, I~(t)/I~has been plotted as a sernilogarithmic curve against time t. Extrapolation of the long-lived portion of the phosphorescence decay curves to t 0 yields log $ as intercept and values of f~obtained therefrom are given in tables 2—6. All the photophysical parameters given in the tables 2—6 are derived from the relations used by Lessard and Durocher [19]. =~

=

388

G.K. Mallik et al. / External heavy atom effect

Table 2 Influence of MCA concentration on fluorescence and phosphorescence photophysical parameters of carbazole in MCH at 77 K (the carbazole concentration is 6.43x10° mol L’) MCA conc. (moll’)

F

5 1.23x10 6.18x10° 1.24x104

0.57 0.33 0.30

‘F

‘P

0.46 0.12 0.11

/3 0.73 0.60 0.34

~

Ti,.

(s)

(s)

6.78 6.78 6.78

2.45 2.31 1.88

a determined from the relation [19]. a/$

a~

K,. ~

0.34 0.07 0.04

4.25 5.87 5.80

K~. 0.35 0.10 0.11

K,,.

cu,,

4.10 5.68 6.18

1.51 1.57 1.58

~,.

0.85 0.88 0.88

=

Table 3 Influence of DCA concentration on fluorescence and phosphorescence photophysical parameters of carbazole in MCH at 77 K (the carbazole concentration is 6.43x105 mol 1’) DCA conc. (moll’)

‘F

F

F

1.21 x10° 6.15x105 1.21 x104

0.55 0.34 0.25

0.38 0.28 0.22

‘P

~

0.76 0.70 0.60

~

ri,.

(s)

(s)

6.78 6.78 6.78

3.03 2.88 2.80

a

K,.

K,,.

0.28 0.20 0.13

3.91 9.36 12.56

0.19 0.15 0.15

K~,,. ~ 3.78 4.00 4.20

d.ti,.

(J)~.

1.49 1.63 1.66

0.83 0.91 0.93

Table 4 Influence of TCA concentration on fluorescence and phosphorescence photophysical parameters of carbazole in MCH at 77 K (the carbazole concentration is 6.43x10° mol 1’) TCA conc.

‘F

1,,

(mol1~) 5 6.42 1.28x10 x 10 1.28x i0’

~‘ij 0.53 0.32 0.22

Jil

/3

it,,

i~,,.

0.46 0.41 0.33

(s) 6.78 6.78 6.78

(s) 2.16 1.80 1.44

a

K~.

K,,.

K 11,.

0.25 0.13 0.10

0.12 0.05 0.03

3.03 5.53 8.54

0.34 0.19 0.21

5.24 6.43 8.07

~,,

1.42 1.57 1.63

cu,. 0.80 0.88 0.91

Table 5 Influence of MBA concentration on fluorescence and phosphorescence photophysical parameters of carbazole in MCH at 77 K (the carbazole concentration is 9.09x105 mol 1’) MBA conc.

‘F

‘P

(mol I ~‘)

jij

F

4 6.37 x iO~ 8.92x10 2.12x l0~

0.25 0.42 0.09

0.57 0.80 0.24

$

0.38 0.45 0.29

T 1,

T1,.

(s)

(s)

6.78 6.78

1.89 2.02 1.73

a

K,.

K,,.

~

0.22 0.36 0.07

45.64 18.26 82.25

0.93 1.00 0.40

5.55 5.12 6.51

(J)~.

1.75 1.71 1.77

(J)~.

0.98 0.96 0.99

G.K. Mallik et al.

/ External heavy atom

389

effect

Table 6 Influence of TBA concentration on fluorescence and phosphorescence photophysical parameters of carbazole in MCH at 77 K (the carbazole concentration is 5.13>< 10 mol 1_I) TBA cone.

‘F

‘P

(moll’)

F

J~

4 8.52x104 6.38x10 1.06x103

0.56 0.20 0.13

0.74 0.30 0.18

~

~

0.72 0.52 0.48

(s) 6.78 6.78 6.78

a (s) 2.02 1.80 1.73

0.53 0.16 0.09

K,.

K,,.

K

~

7(

k

27.2 36.7 40.0

0.84 0.37 0.24

5.23 6.30 6.68

1~.

(lIp.

ID,.

~ 1.73 1.74 1.75

0.96 0.97 0.98

From the kinetic study at 77 K, it is seen that for carbazole-chloroacetic acid or carbazole-brornoacetic acid complexation, all photophysical parameters change with varying concentration of the quencher. The phosphorescence lifetime (Tn) of carbazole in MCH is 6.78 s (tables 2—6). On increasing the concentration of chloroacetic or bromoacetic acid quenchers the perturbed lifetime T~ decreases gradually, K~~/K 1and simultaneously ~ or increases (~is the triplet yield and prime sign denotes perturbed molecule). In calculating k, it is assumed [20] that ‘~I~ + k, 1, and the value of the fluorescence yield, reported in literature [20,31] is utilized. It is also seen from the tables that for the CA and BA perturbers the radiative phosphorescence rate constant (K,,/K~) gradually decreases with increase of quencher concentration while at the same time the non-radiative phosphorescence decay rate constant (K,~~/K~~) increases. As expected, the intensity of phosphorescence emission is lowered as the concentration of quencher increases. =

~,

References [1] M. Kasha, J. Chem. Phys. 20 (1952) 71. [2] S.P. McGlynn, T. Azumi and M. Kinoshita, Molecular Spectroscopy of the Triplet state (Prentice-Hall, Englewood Cliffs, 1969). [3] S. Lipsky, W.P. Helman and J.F. Merklin, in: Luminescence of Organic and Inorganic Materials, H.P. Kallmann and G.M. Spruch, eds. (Wiley, New York, 1962) p. 83. [4] S.P. McGlynn, T. Azumi and M. Kasha, J. Chem. Phys. 40 (1964) 507. [5] T. Medinger and F. Wilkinson, Trans. Farad. Soc. 61(1965) 620. [6] W.R. Ware and C. Lewis, J. Chem. Phys. 57 (1972) 3546. [7] G.. Das Gupta and D. Phillips, J. Chem. Soc. Faraday II. 68 (1972) 2003. [8] D. Saperstein and E. Levin, J. Chem. Phys. 62 (1975) 3560. [9] J. Bendig, M. Siegmund and S. Helm, Adv. Mol. Relax. Inter. Processes 14 (1979) 121. [10] M.E.R. Marcondes, V.G. Toscano and R.G. Weiss, J. Photochem. 10 (1979) 315. [11] HA. Khwaja, G.P. Semeluk and I. Unger, Can. J. Chem. 60 (1982) 1767. [121 HA. Khwaja, G.P. Semeluk and I. Unger, Can. J. Chem. 61(1983)1952. [13] F. Castano, S. Lombrana, E. Martinez and M.T. Martinez, Spectrosc. Lett. 16 (1983) 805. [141 J. Klein, V. Plazanet and G. Loustriat, J. Chim. Phys. Physicochim. Biol. 67 (1970) 302. [15] G.E. Johnson, J. Phys. Chem. 84 (1980) 2940.

390

G.K. Mallik ci al.

/

External heavy atom effect

[16] K. Chatterjee, S. Laha, S. Chakravorti. T. Ganguly and SB. Banerjee, Can. J. Chem. 62 (1984) 1369. [17] L.P. Hammett, Physical Organic Chemistry (McGraw-Hill, New York, London, 1940) p. 260. [18] lB. Berlmann, J. Phys. Chem. 77 (1973) 562. [19] G. Lessard and G. Durocher, J. Phys. Chem. 82 (1978) 2812. [20] J.E. Adams, W.H. Mantulin and JR. Huber, J. Am. Chem. Soc. 95 (1973) 5477. [21] J. Najbar and I.H. Munro, J. Lumin. 17 (1978) 135.