Time-resolved spectroscopy of phenol-triethylamine charge transfer complexes in polar and nonpolar solvents

Journal of Luminescence 43 (1989) 301—308 North-Holland, Amsterdam

301

TIME-RESOLVED SPECTROSCOPY OF PHENOL-TRIETHYLAMINE CHARGE TRANSFER COMPLEXES IN POLAR AND NONPOLAR SOLVENTS PB. BISHT, H.B. TRIPATHI and D.D. PANT Photophysics Laboratoiy, Kumaun University, DSB Campus, Nainital— 263 002, India Received 16 November 1988 Revised 13 March 1989 Accepted 30 March 1989

Hydrogen-bonded complexes of phenol-triethylaniine have been studied in different polar and nonpolar solvents. The ground-state equilibrium constants and the excited-state Stern—Volmer quenching rates have been calculated. Excitation spectra and time-resolved emission spectra in nonpolar solvents show that the ground-state hydrogen-bonded complex (GHBC) gives its emission slightly red-shifted to that of free phenol. However, in polar solvents no emission corresponding to GHBC or exciplex is observed.

1. Introduction Effects of intermolecular solute—solvent interactions including hydrogen bonding, on the electromc absorption and fluorescence spectra of molecules have been discussed by many authors [1—8, 20 26]. The changes in electronic spectra depend on the relative strength of the interactions in the ground and excited state of the molecules. Generally speaking, when aromatic hydroxy (proton donor) and amine (proton acceptor) cornpounds enter into hydrogen bond with the proton acceptor (donor) molecules, the equilibrium constant in the excited state (Ke) is larger than the ground-state equilibrium constant (Kg). This resuits in a red shift of the electronic spectra and quenching of fluorescence yield and shortening of lifetime. The latter has received wide attention [20 26] and several probable mechanisms have been suggested but, as yet, there is no clear-cut interpretation of this phenomenon. Nagakura et a! [7,8] have studied the effect of hydrogen bonding on the absorption spectra of phenol (PH) in nonpolar solvents with different proton acceptors. On the basis of the results obtamed, they concluded that the hydrogen bonding in PH is not purely electrostatic in its origin but

the charge transfer or delocalization mechanism contributes it to a considerable extent. In a previous work [6] we studied the hydrogen bonding interaction between 2-naphthol (2NP) and triethylamine (TEA). For the first time it was found that while in polar solvents, the exciplex and the ground-state hydrogen-bonded complex (GHBC) on excitation give the same emission; the exciplex in nonpolar solvents is totally nonemitting. The purpose of the present work is to present quantitatively the steady state and timeresolved results of the PH—TEA system in order to get a better insight into the behaviour of GHBC and the exciplex in different polar and nonpolar solvents.

2. Experimental PH (BDH, AR grade) was distilled in vacuum and kept under dry nitrogen atmosphere in the dark. Spectrograde solvents, viz. cyclohexane (CH). hexane (HEX), ethylalcohol (EtOH), acetonitrile (ACN), dimethyl sulfoxide (DMSO) and TEA were subjected to vacuum distillation prior to use. All these solvents showed no fluorescence when excited at wavelengths above 265 nm. TEA, being an

0022-2313/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

302

PB. Bishi eta!.

/ Phenol—triethylamine charge transfer complexes

exception among the saturated compounds, has been shown to fluoresce with high quantum yield in hydrocarbon solutions [9—11],and hence the excitation wavelength was chosen as 276 nm (unless otherwise specified) with a narrow (3 nm) band pass to avoid any spectral contamination of its fluorescence. Before measurements, the solulions (PH conc 1.4 x 10 “ M) were prepared under dry and warm conditions and then kept under dry nitrogen in a dark place. All measurements were made at right angles to the pump beam with 1 cm2 quartz cells to minimise the scattered light. The absorption and luminescence spectra were measured with the help of a Beckman DK-2A spectrometer and Spex-fluorolog model 1902 respectively and were found to be in agreement with those reported earlier [7,8,12]. Decaytime measurements were made with the help of Edinburgh model 199 ns fluorimeter under single-photon counting conditions [13,14], and data analysis was done with a PDP 11/2 microcomputer by reconvolution method using a leastsquares fitting program. The goodness of fit was

estimated by x2~distribution of residuals, standard deviations, the autocorrelation function and the Durbin Watson parameter (DW) [13]. For a good fit, x2 was around one, residuals distributed evenly on both sides of the fitted curve and the standard deviations less than 5%. In nonpolar solvents, where the standard deviation exceeds that due to small amplitudes or spectral overlap, self-consistency of the lifetime data was a check. In all the solvents, since a TEA concentration greater than 0.30 M gives an unreasonable value of x2 (probably due to the formation of higher complexes), the concentration of TEA was kept below this level. The decay-time measurements were carried out at intervals of 10 nm, with 3 nm band pass for the emission monochromator. The decay data were corrected for variation of photomultiplier response with Aem. We measured the emission wavelength dependence of fluorescein in 20N H 2S04, obtaining a fixed lifetime of (3.85 ± 0.08) ns after applying the correction. The reliability of the instrument was checked using the fluorescence standards, viz. anthracene in cyclohexans and rosebengal in ethanol.

08

3. Results and discussion

8

06

7 04

I

Figure 1 shows the absorption spectra of PH with varying [TEA] in CH. In nonpolar solvents, the shift in the absorption spectra of PH caused by hydrogen bonding with TEA is towards the red, and isosbestic points are obtained indicating the formation of a ground-state complex. The Kg values are calculated in the manner suggested by Belletête et al. using the modified Benesi—Hildebrand equation [15].

0 02

0 ______________________________ 270 275 280 285 290 WAVELENGTH/nm Fig. 1. Absorption spectra of PH caused by 1: 1 hydrogen bond with M; (2) 0.0008 M; (3) 0.002 M; (4) 0.02 M; (7) 0.07 M;

showing isosbestic points TEA in CH. [TEAl (1) 0 0.007 M; (5) 0.014 M; (6) (8) 0.14 M.

Kg[Qfl]_~~A, where Q is the concentration of TEA, n (1) the

number of TEA molecules involved in the corn-

plex, A° and AF respectively are the absorbances at a particular wavelength in (1) absence of TEA, and (2) presence of TEA molecules sufficient to complete the complexation in the ground state (A is the absorbance of a solution with varying TEA

PB. Bishi et a!.

/ Phenol—triethylamine charge transfer complexes

303

Table 1 PH—TEA system in different solvents Solvent

Dielectric constant

(mn) Free PH Acm

PH-TEA

FWHM (cm 1) Free PH PH-TEA

complex CH 2.06 288 302 HEX 1.90 288 302 ACN 37.5 302 DMSO 46.0 310 EtOH 24.55 305 a) r is the lifetime of free PH. b) k 8 is the ground-state equilibrium constant. kq

d)

kD is the diffusion rate constant.

2.34 2.12 4.90 2.33 4.60

80 60 1.5 0.5 0.2

)

(

1

s

)

kD (M

kq/kD

s

complex 2087 3132 3888 3239 4012

5210 5196

concentration). Since a straight line is obtained for n 1 which confirms 1 : 1 complexation between the molecules of PH and TEA. The Kg values thus obtained are given in table 1 and are in agreement with those available in the literature [7,8]. In polar solvents, the change in the absorption spectra is not so large and the Kg values are much smaller than in nonpolar solvents probably due to solvation of molecules of PH as well of TEA which hinder the hydrogen-bonding interaction. In any case, however, knowledge of Kg as well of c ‘/e, the ratio of extinction coefficients at the excitation wavelength of PH and the PH—TEA complex (GHBC), respectively, is required, in order to calculate the fraction [16] =

=

kq~

b)

(M

9 2.66x10 0.52x109 0.15x109

1.7x101° 3.2x109 6.lx iø~

0.156 0.163 0.025

is the quenching rate in the excited state as calculated from the lifetime.

‘~

a

kg

a)

(ns)

~Kg• [Q]/1

+

—K5[Q]

(2)

of the directly excited complex. The fraction a even at 0.014 M TEA in ACN is only 2% of the free PH. Therefore, hydrogen bonding in the ground state in polar solvents, for [TEA] under study, can be neglected. Table 1 lists the A max for the emission of PH and GHBC in different solvents. The dielectric constant has a small effect on Amax of free PH. The FWHM values of emission bands have been obtained (1) in all solvents without TEA, and (2) in nonpolar solvents, with optimum concentration of TEA such that the emission of free PH is negligible (see figs. 1 and 2). The emission of GHBC is broad and the shift is small, which

results in the overlap of two spectra and a double-exponential decay even at 280 nm. The quenching constants in polar solvents have been calculated with the help of lifetime of PH by varying [TEA]. In order to understand the fluorescence decay we consider the usual hydrogen-bonding interaction k, (A)

(DH*)~

~ (DHA”)

(3)

k,’ + kd’

+

K

(DH)

+A~

(DHA)

(DH) is the proton donor, A is the proton acceptor, the asterisk indicates the excited states, (DHA) the hydrogen-bonded complex, kf and kd are radiative and radiationless decay rates for (DH *) as ~ and kd’ for (DHA*). Let kr << k~(A), the decay curves will follow ‘(DH *) (t) = A0 e (4) 1/Ti

and I(DHA.)(t)

A0k1(A) [e a a’ where

t/~-2

—

e1/’Th]

a=

=

Ic) (A)

+

B e °

—

+

k~+ kd,

l/T2

(5)

304

PB. Bisht et al.

/ Phenol—triethylamine charge transfer complexes

Table 2 Lifetimes and amplitudes for the PH—TEA system in CH (c c (M)

Probe (nm)

0 0.0008

Single-exponential fit T Stand

300 280 330 280 330 280 290 300 310 320 330 340 280 340

0.0013 0.0021

0.0070

(ns)

dev. (ns)

2.39 2.26 2.15 2.23 1.93 2.12 2.04 2.03 1.79 1.66 1.61 1.41 1.92 0.86

0.03 0.01 0.02 0.01 0.02 0.02 0.01 0.04 0.01 0.04 0.02 0.01 0.02 0.02

,~2

1.01 1.44 2.53 1.40 4.43 1.21 1.39 2.22 2.90 4.13 3.97 5.32 1.92 1.54

concentration of TEA, T

Double-exponential fit Stand. r~

200

C)

Stand,

(ns)

dev. (ns)

(ns)

dev. (ns)

2.35 2.45 2.35 2.43 2.35 2.34 2.35 2.39 2.37 2.37 2.40 2.37 2.58

0.01 0.02 0.02 0.02 0.03 002 0.13 0.08 0.03 0.03 0.09 0.02 0.11

0.29 0.60 0.55 0.39 0.68 0 59 0.43 0.54 0.59 0.47 0.64 0.73 0.46

0.04 0.13 0.05 0.09 0.15 0 OR 0.10 0.03 0.04 0.03 0.01 0.05 0.02

a

1

a2

0.82 0.56 0.77 0.33 0.64 0 55 0.48 0.32 0.27 0.23 0.17 0.45 0.04

0.18 0.44 0.23 0.67 0.36 045 0.52 0.68 0.73 0.77 0.83 0.55 0.96

0.99 1.07 1.07 1.13 1.17 1 08 1.13 1.03 1.12 0.98 1.01 0.94 1.11

~ Normalised to 1.0.

and a’

1 =

—

—

~

+

emission from (DH *) should decay exponentially while that of (DHA*) should show a risetime provided the two emissions do not overlap. In case of a spectral overlap of (DH *) and (DHA*), the

kd’.

2

A0 is the instrumental factor and B0 is the coefficient due to direct absorption of (DHA) from the ground state. As evident from eqs. (4) and (5),

decay curve at any emission wavelength can be written as [17] I~(t)

=

a1

e

r1

+

a2 e

1

(6)

T2

where \

\

>-

\

\

‘I)

A0 l+y

\

\‘o,~

a2=

\

I-

\

o 210

300

AOkI(A) ,1+ y)(a a )

+

—

(7) B0(a—a’) AO/CI(A)

,

(8)

and y ‘h,,,, (DH * )/IXcrn (DHA*) is the ratio of normalised intensities for (DH *) and (DHA*) at the wavelength of detection. The fluorescence lifetimes as a function [TEA] in CH are given in table 2. In the presence of TEA the decay curves do not fit a single-exponential function. Apart from a fixed decay time corresponding to free PH (2.34 ns), a fast component (300 600 ps) is observed. The amplitude of the =

\ \

260

(1

k1(A) (a—a)

320

340

~

WAVELENGTH/nm Fig. 2. Normalised spectra for PH TEA system (pump: 275 nm). 0 M TEA, ; 0.14 M TEA, o o .

latter increases both with increasing the probe wavelength and [TEA]. Figure 3 shows the decay curves in CH (0.0021 M TEA) with a double-

P.B. Bisht et a!.

/ Phenol

triethylamine charge transfer complexes

305

~

>.

5

J.

4 .

1 iii. ~

Chisgat.17

TIm,/IS sec

~

.

9 Sec

TIm.jI~

ChIS~I.Oi

Fig. 3. Fitted curves for the decay of PH—TEA system CH (0.0021 M TEA) at 280 am and 340 nm (see table 2).

exponential fit at the edges (280 nm and 340 nm) of the emission band. The decay-associated amplitudes a 1 and a2 (eqs. 7 and 8) can give more decisive information regarding the reaction kinetics. These amplitudes can be used to form the decay-associated spectra (DAS) [18,19]. In the case of ground-state heterogeneity and in the absence of any excited-state reaction (i.e. k1(A) = 0), a1 and a2 follow the same pattern as for steady-state emission of two separately absorbing and emitting species. With the help of the amplitudes we demonstrated the

270

260

310

330

350

WAVELENGTH /flm Fig. 4. Time-resolved emission spectra (TRES) for PH + 0.0021 M TEA in CH with time windows. (a) 0—0.5 ns (early); (b) 2.5—5 ns (late).

method of formation of DAS in the 2NP—TEA system in an earlier communication [6]. It can be seen from table 2 that the amplitudes a1 and a2 corresponding to the free PH and the GHBC. respectively, are reversed at the red edge of the emission band as compared to blue and finally at 0.007 M TEA, a1 becomes negligible at 340 nm. The excitation spectra were also taken with TEA concentrations 0 M, 0.0021 M and 0.14 M, respectively, on monitoring probe at 330 nm. The latter

260

280 300 320 340 360 WAVELENGTH (nm) Fig. 5. Emission spectra of PH TEA system in ACN (pump: 275 nm). [TEA] = (1) 0 M; (2) 0.007 M; (3) 0.014 M; (4) 0.021 M; (5) 0.036 M; (6) 0.07 M.

306

P. B. Bisht et a!.

/ Phenol

triethylamine charge transfer complexes

/

tern in CH taken with ‘early’ (0 0.5 ns) and ‘late’ (2.5 5 ns) time windows. As expected the ‘early’ TRES shifts towards red to that of ‘late’ TRES. A comparison to the steady state results (fig. 2, table 1) reveals the fact that the red-shifted emission is due to GHBC. Exactly similar behaviour is obtamed in HEX. The PH TEA system exhibits entirely different behaviour in polar solvents. The progressive changes of fluorescence spectra of PH caused by the addition of various concentrations of TEA in ACN are indicated in fig. 5. The fluorescence of PH is quenched without the appearence of any new structure. The excitation spectra do not show any change with variation of TEA. A similar behaviour is observed in DMSO and EtOH. In polar solvents the fluorescence lifetime of PH decreases significantly with the addition of TEA. The quenching efficiency can be measured by a Stern Volrner (SV) plot (fig. 6) based on the lifetime data, following the relation

/ / f

1.6

/ / / 7. / / J

1 4

1.2

~

/ /

0/

5

1•0

_______________________________________ 002

004

&06

008

0~tO

012

ITE AJ/M Fig. 6. Stern Volmer plots for PH—TEA system in ACN (0), DMSO (X) and EtOH (L

~

two spectra show a red shift of 600 cm This shift agrees well with the absorption spectra (fig. —

T

~.

where ‘r0 and T are the lifetimes in the absence and presence respectively, of TEA; ~ the SV constant, is equal to kqro where kq is the quench-

1). Figure 4 shows the time-resolved emission spectra (TRES) for the PH TEA (0.0021 M) sys-

D.W..i 71

D.W.c 1.73

__

2~~\

0 >. ~

4

.

~

8

12

,

t~me/ioSec

16~20

24

28

32

36

~chs~qo0. ~

0

~

4

8

12

16

20

24

,tIm~/I0~Sec

28

32

36

chi~Q.1’,I0

I_____

Fig. 7. Decay curves with single exponential fit at 280 nm and 340 nm for PH + 0.014 M TEA in ACN.

PB. Bisht et a!.

/ Phenol—rriethy!amine charge transfer complexes

ing rate constant. The kq values for different polar solvents are given in table 1. Moreover, kq values are compared with the values of the diffusion-controlled rate constant estimated by kD

=

8RT/3000~.

As is evident, the kq values are somewhat smaller than the diffusional rate constant (kD). This type of situation has been explained by Ikeda et al. in the 2-naphthylamine-pyridine system as being due to the solvent dependence of kq in polar solvents [20]. The quenching efficiency (kq/kD) changes about one order of magnitude due to the solvent effect upon interaction between TEA and excited PH. This study was limited to TEA concentration below 0.30 M as above this level the presence of the higher complexes (A max = 340 nm), having low quantum yield as well as lifetime, gave irrelevant results. The fitted curves for the decay of PH in ACN (0.014 M TEA) at the two extremes of the emission band are shown in fig. 7 along with the residuals and autocorrelation function. The decay data fit well with the single-exponential function and a trial for a double-exponential fit was not successful. Moreover, the absence of any change in the FWHM of the spectra (see fig. 6) indicates the absence of any contribution of GHBC or exciplex in the emission.

4. Conclusions Spectroscopic evidence of the formation of a 1 : 1 complex due to hydrogen bonding in the ground state between PH and TEA in nonpolar solvents and SV quenching in polar solvents have been demonstrated by steady-state and transient results. GHBC shows a small spectral shift from the free PH in nonpolar solvents. The kq values in polar solvents have been compared with the diffusional rate constant and are found to show solvent dependence. The mechanism suggested in eq. (3) has, therefore, to be modified inasmuch as the exciplex (DHA*), formed by excited-state reaction, is non-emitting in polar solvents, i.e. a situation like the 2-naphthylamine-pyridine system [20], whereas no quenching is observed in nonpolar solvents. A

307

possible mechanism for the quenching process in the excited state may be the charge transfer interaction in the encounter complex (formation of short-lived exciplexes) leading to rapid hydrogen atom transfer, and conversion to the ground state as well as triplet state formation [25]. On the other hand, the emission of GHBC may be due to hydrogen bonding and proton transfer.

Acknowledgements We thank Dr. J.N. Pant, Department of Chemistry, for providing PH and Mr. H.C. Joshi for critically reading this manuscript. We also thank DST New Delhi, for financial assistance.

References [1] N. Mataga, Y, Kaifu and M. Koizumi, Bull. Chem. Soc. Japan 29 (1956) 373. [2] N. Mataga, Y, Kaifu and M. Koizumi, Bull. Chem. Soc. Japan 29 (1956) 456. [3] N. Mataga and S. Tsuno, Bull. Chem. Soc. Japan 30 (1958) 711. [4] N. Mataga, Bull. Chem. Soc. Japan 31 (1958) 481. [5] N. Mataga and Y, Kaifu, Mol. Phys. 7 (1963) 137. [6] P.B. Bisht, G.C. Joshi, H.B. Tripathi and D.D. Pant, Chem. Phys. Lett. 142 (1987) 291. [7] S. Nagakura and M. Gouterman, J. Chem. Phys. 26 (1957) 881. [8] 5. Nagakura and H. Baba, J. Am. Chem. Soc. 74 (1952) 5693. [9] A. Flalpern, Chem. Phys. Lett. 6 (1970) 296. [101Y. Muto, Y. Nakato and H. Tsubonura, Chem. Phys. Lett. 9 (1971) 597. [11] G. Köhlar, Chem. Phys. Lett. 126 (1986) 26. [12] R.F. Chen, Anal. Lett. 1 (1963) 35. [13] D.V. O’Connor and D. Phillips, Time-Correlated Single-Photon Counting (Academic Press, New York, 1984). [14] D.D. Pant, G.C. Joshi and H.B. Tripathi, md. J. Phys. 60B (1986) 7. [15] M. Belletéte, G. Lessard, J. Richer and G. Durocher, J. Lumin. 34 (1986) 279. [16] H. Beens, K.H. Grellmann, M. Gurr and A.H. Weller, Disc. Faraday Soc. 39 (1965) 183. [17] W.R. Laws and L. Brand, J. Phys. Chem. 83 (1979) 795. [18] L. Davenport, J.R. Knutson and L. Brand. Biochemistry 25 (1986) 1186. [19] J.R. Knutson, D.G. Walbridge and L. Brand, Biochemistry 21 (1982) 4671.

308

PB. Bisht et a!.

/ Phenol

triethylamine charge transfer complexes

[20] N. Ikeda, T. Okada and N. Mataga, Bull. Chem. Soc. Japan 54 (19811 1025. [21] K. Kikuchi, H. Watarai and M. Koizumi, Bull. Chem. Soc. Japan 46 (1973) 749. [22] D. Rehm, And A. Weller, Israel J. Chem. 8 (1970) 259. [23] K. Chatteijee, S. Laha, S. Chakravorti, T. Ganguly and S.B. Baneijee, Chem. Phys. Lett. 100 (1983) 88.

[24] J. Dresner and J. Prochrow, J. Lumin. 24/25 (1981) 539. [25] M. Gordon and W.R. Ware. eds.. The Exciplex (Academic Press, New York, 1975) P. 134. [26] M. Nakamizo, Spectrochim. Acta 22 (1966) 2039.