Time-resolved fluorescence studies on covalently linked porphyrin—nitroarene complexes. Conformational control of photoinduced electron transfer reactions

1. Photo&em. Phorabiol. A: C/rem., 81 (1994) 139-150

139

Time-resolved fluorescence studies on covalently linked porphyrin-nitroarene complexes. Conformational control of photoinduced electron transfer reactions Bhaskar

G. Maiya +rtt, S. Doraiswamy,

N. Periasamy+

and

B. Venkataraman

Chemical Physics Group, Tatu Instihcte ofFundamental Research, Homi Bhabha Road, Bombay 400 005 [India)

V. KrishnantTtt’ Department of Inorganicand Physical Chemistq Indian Institute of Science, Bangalore 560 012 (India) (Received

August

11, 1993; accepted

January

18, 1994)

Abstract Time-resolved fluorescence studies were carried out on a series of free-base and zinc(l1) derivatives of mesw tetraphenylporphyrins covalently linked to either 1,3-dinitrobenzene (DNB) or 1,3,5+rinitrobenzene (TNB) acceptor units. These acceptor units were linked at different sites (at the ortho, meta or para positions of one of the phenyl groups of meso-tetraphenylporphyrin) to the donor porphyrins such that the resulting isorneric intramolecular donor-acceptor complexes exhibit different centre-to-centre (ctc) distances and relative orientations. Biexponential tluorescence decay profiles observed for several of these covalently linked complexes were rationalized in terms of the presence of “closed” and “extended” conformers. Detailed analyses of the fluorescence decay data have provided a comprehensive understanding of the photoinduced electron transfer (PET) reactions occurring in systems containing zinc(B) porphyrin donors. It is observed that although DNB-linked zinc(B) complexes follow the trends predicted for the efficiency of PET with respect to donor-acceptor distance, the TNB-linked zinc(B) porphyrins exhibit a behaviour which is dictated by steric effects. Similarly, although the thermodynamic criteria predict a greater efficiency of charge separation in TNBlinked complexes compared with DNB-linked complexes, the reverse trend observed has been attributed to orientational effects. In the complexes containing free-base porphyrin donors, PET is expected to be less efficient from a thermodynamic viewpoint. In a few of these cases, fluorescence quenching seems to occur by parallel mechanisms other than PET.

1. Introduction In recent years, a variety of covalently linked donor-acceptor (D-A) complexes have been synthesized and their intramolecular charge transfer reactions studied [l-lo]. The central aim In these studies has been to elucidate the parameters that restrict or favour the course of energy or electron transfer processes. Of particular interest in these intramolecular D-A systems are those in which an aryl hydrocarbon or a porphyrin donor is linked to a variety of electron acceptor units. Maiya and Krishnan [ll, 123 have synthesized a series of D-A complexes containing either the free-base mesotetraphenylporphyrin (H,TPP) or its zinc(E) de+Authors to whom correspondence should be addressed. “Permanent address: School of Chemistry, University of Hyderabad, Hyderabad 500 134, India. “?Also associated with the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 5&l 012, India.

lOlO-6030/94/$07.00 0 1994 Elsevier SSDI 1010-6030(94)03791-R

Science S.A. All rights reserved

rivative (ZnTPP) linked to either 1,3_dinitrobenzene (DIVES)or 1,3,5trinitrobenzene (TNB) acceptor units. The solution state structures and physicochemical properties of these complexes, as determined by various spectroscopic and other techniques, have also been reported [ll, 121. Specifically, steady state fluorescence and on-line irradiation/electron spin resonance (ESR) spectroscopic results have indicated that an intramolecular photoinduced electron transfer (PET) reaction occurs in these systems. However, in the absence of time-resolved studies, it was not possible to evaluate the influence of the D-A distance and orientational factors on the rates of PET reactions. This is performed in the present study using the picosecond time-resolved fluorescence technique. The molecular structures of the D-A complexes and their nomenclature are given in Fig. 1. An examination of the structures suggests that linking of an acceptor unit via the conformationally flexible

140

the details of the PET reactions linked D-A systems.

2. Experimental

M

position

@=

‘&Oz

@) =

NOz

Nlo* 2H

Zn

pN02

H,TPP-p-DNB

para meta

H,TPP-p-TN6 H2TPP-m-TNB

ortho

H2TPP- o-TN5

H,TPP-m-DNB H,TPP-o-DNB

para

ZnTPP-p-TNB

ZnTPP-p-DNB

meta ortho

ZnTPP- m-TNB ZnTPP-o-TNB

ZnTPPZnTPP-

Fig, 1. Molecular structures and nomenclature complexes investigated in this study.

m-DNB o - DNB of the D-A

C-O-C spacer linkage at the ortho, meta or para positions of the porphyrin phenyl group changes the D-A distances as well as their mutual orientations. Thus these linked porphyrin-nitroarene systems provide an opportunity to examine the time-resolved fluorescence properties as a function of D-A distance and orientation. Furthermore, the thermodynamic criterion for excited state electron transfer in these systems is varied in a subtle fashion either by appropriate choice of polynitroaromatics exhibiting different electrochemical reduction potentials or by the presence or absence of a metal ion in the porphyrin crevice. Thus it is possible to examine the PET reaction rates of these complexes as a function of the free energy of the excited state electron transfer reaction (AGO). In this paper, we report the picosecond timeresolved fluorescence decay results for 12 covalently linked systems in five different solvents. As this study reveals, the fluorescence decay profiles of several systems are biexponential and the results can be analysed in terms of the presence of both “extended” and “closed” forms of D--A pairs. The time-resolved fluorescence data of intermolecular quenching of H,TPP and ZnTPP by uitroaromatic acceptors are also reported. Results of both intermolecular and intramolecular quenching experiments are shown to be helpful in understanding

occurring

in these

details

meso-tetraphenylporphyrin The free-base (H,TPP) and its zinc(B) derivative (ZnTPP) were synthesized using the method of Rothemund and Mennotti 1131. The u-, m- and p-hydroxyphenylsubstituted porphyrins were synthesized by mixed condensation of pyrrole (four equivalents), benzaldehyde (three equivalents) and either a-, m- or p-hydroxybenzaldehyde (one equivalent) [14]. The D-A porphyrins were synthesized by the reported procedures [ll, 121. Briefly, meso-triphenylmono(hydroxyphenyl)porphyrin (having the hydroxy group at either the ortho, meta or para position of the phenyl unit) and either l-chloro2,4-dinitrobenzene or 1-chloro-2,4,6-trinitrobenzene were condensed together in dimethylformamide using potassium carbonate as a base. The products were purified by extensive column chromatography on alumina (eluent, CHCl,) and were obtained in nearly 50%-60% yield. Fresh samples (air-saturated) taken from a chromatographic column were used for the fluorescence decay experiments. The integrity of the structures and the purity of the samples were checked by comparing the proton nuclear magnetic resonance (IH NMR) and UV-visible spectra of these complexes with the reported spectra [ll, 121. The solvents used {toluene, chloroform, methanol, acetonitrile and propylene carbonate (PC)) were either spectrograde or analar grade purchased from BDH (India) or Aldrich Chemical Company {USA) and were free from fluorescent impurities. 2.1. Methods UV-visible absorption spectra and steady state fluorescence spectra were recorded with a Hitachi model 3400 UV-visible spectrophotometer and a Hitachi model 650-50 spectrofluorometer respectively. Details of the picosecond fluorescence lifetime spectrometer utilized in this study have been reported elsewhere 1151. The sample was excited by 4 ps laser pulses at a repetition rate of 800 kHz. The fluorescence was detected at the magic angle (54.7”) with respect to the polarization of the incident laser beam by a cooled XP2020Q photomultiplier at a rate of less than 2X lo4 s-‘. Fluorescence decay data were collected at either 42 ps per channel (for ZnTPP-based systems) or 160 ps per channel (for H,TPP-based systems)

B.G. Maiya ef al. I Time-resolved jiuomcence

resolution on a Tracer Northern model TN 7200 multichannel analyser. The excitation wavelength ranged between 580 and 590 nm corresponding to the Q-bands of these porphyrins. The fluorescence was collected at 610 nm for ZnTPP-based systems and at 645 nm for H,TPP-based systems. Typical concentrations of the intramolecular complexes and the fluorophore in the porphyrin-acceptor intermolecular complexes ranged between lo-’ and 10e6 M. Concentrations of DNB and TNB used in the latter cases ranged between lo-’ and 0.1 M. Instrument response functions were obtained using a suspension of MgO in water. 2.2. Fluorescence decay data anaIysis The fluorescence decays of all the samples studied in this work were fitted to either a oneexponential or multiexponential decay function. The deconvolution analysis was carried out by the method of iterative reconvolution of the instrument response function and the assumed decay function [16-181. The goodness of fit of the experimental data to the assumed decay function was judged by the following standard statistical tests: (i) random distribution of the weighted residuals and the autocorrelation functions; and (ii) the values of reduced x2. A two-exponential decay function was used only if the one-exponential decay function did not give an adequate fit for the data. In a few cases, a three-exponential decay equation was also used as the fitting function. The error in the lifetimes of H,TPP and ZnTPP is less than 5% and this estimate is based on measurements spanning over a long period of several months. For the intramolecular D-A complexes, the error estimates of the lifetimes obtained by the one-exponential fits and of the mean lifetimes {defined as (7) = E&q, where CA,= 1) obtained by multiexponential fits are less than 10%. Although no specific attempt has been made to estimate the error limit on the relative amplitudes (Ai), we believe that it would be less than about *lo%. For this reason, and also for completeness, we have retained in Tables 2-5, (see Section 3.2), the entries for Ai values that are above 1%. In the intermolecular quenching experiments, the quenched fluorescence decays of H,TPP and ZnTPP (in the presence of DNB or TNB) are not expected to fit well to a one-exponential decay function, especially when the bimolecular quenching reaction is rapid and influenced by diffusion [15, 191 (a non-exponential decay function usually gives a better fit [El). However, we have, for this purpose, fitted the data only to a single-exponential

of pquhyrin-nitroarene

complies

decay and the x2 values obtained in these were high but no larger than about 1.6.

141

cases

3. Results 3.1. Intermolecular system Detailed studies on the electronic absorption and steady state emission spectral features of H,TPP and its metal(H) derivatives, carried out in both the absence and presence of nitroaromatic electron acceptors, have been reported by Chandrashekhar and Krishnan [20]. It has been observed that the porphyrin absorption bands are shifted and their extinction coefficients decreased in the presence of nitroaromatic acceptors [20, 211. Furthermore, the fluorescence intensities of the porphyrins have been observed to decrease in the presence of nitroaromatic acceptors and the extent of this decrease depends on the concentration of the acceptor. We have carried out a similar set of experiments using H,TPP and ZnTPP as donors and either TNB or DNB as acceptors and have observed analogous spectral changes. The fluorescence lifetimes of HJPP and ZnTPP in several solvents, measured using our experimental apparatus, are in good agreement with the values reported in the literature [22, 231. In the presence of DNB or TNB, the fluorescence lifetimes of H,TPP and ZnTPP decrease with an increase in concentration of the acceptor. Typical fluorescence and quenched fluorescence profiles of ZnTPP in CHCl, are shown in Fig. 2. The observed fluorescence quenching process follows the Stern-Volmer equation for dynamic quenching 7 -I=

r,,-lfkqCq

(1)

where r and T* are the lifetimes of the porphyrin singlet excited states in the presence and absence of the quencher, k, is the bimolecular reaction rate coefficient and C, is the concentration of the quencher. It should be noted that eqn. (1) is valid even for measurements of r,, and r in air-saturated samples, since the quenching term due to oxygen cancels out. When T& is plotted against C,, straight lines with unit intercept are obtained and representative plots for ZnTF’P-TNB and ZnTPP-DNB are shown in Fig. 3. The kq vahres obtained from the slopes of these plots are given in Table 1. Also given in Table 1 are the values of k,/kd, where k, is the rate constant for a diffusionlimited reaction in CHCl,. Furthermore, assuming an electron transfer reaction as the quenching mechanism, free energy changes for the photoin-

142

duced electron transfer reactions (A(Y)+ were calculated using the redox potentials++ of H,TPP, ZnTPP, DNB and TNB and the excited singlet state energies [26] of H,TPP and ZnTPP. The AG” values thus calculated are also given in Table 1. The k, values increase with decreasing AG” values indicating that PET reactions contribute to the quenching of these porphyrins by TNB or DN3.

TIME/ns Fig. 2. Fluorescence decay curves for the intermolecular quenching of ZnTPP by TNB in CHCI,. The concentrations of TNB are 0.00 M (a). 0.012 M (b), 0.024 M (c) and 0.048 M (d). Curve c is the instrument response function.

1

1

Zn TPP 3-

o

DNB

.

TNB

I

I

1

CHCL,

P

1

I

I

I

I

I

0.02

0.04

0.06

0.08

0.1

I

Cq/M Fig. 3. Plot of 7&r vs. concentration CHCl,: 0, DNB; +, TNB.

(C,) of the quencher

in

3.2. intramolecular systems The absorption and fluorescence spectra of the intramolecular D-A complexes were measured in all solvents used in this study. Covalent attachment of the nitroaromatic acceptor unit results in a slight red shift in the absorption band maxima of the porphyrin chromophores and a decrease in their molar extinction coefficients. Similarly, the fluorescence emission spectral maxima of these intramolecular complexes are also red shifted by about 2-3 nm in comparison with the spectral maxima of the unlinked porphyrins (i.e H2TPP or ZnTPP). Interestingly, the extent of the shifts in the absorption and fluorescence spectral maxima and the changes in the apparent molar extinction coefficients of these intramolecular porphyrin-nitroaromatic systems depend on the position (i.e. ortho, meta or para) at which the acceptor unit is linked to the phenyl group of the donor porphyrin moiety [ll, 121. The fluorescence decay curves for ZnTPP-based D-A complexes in CHC13 are shown in Figs. 4 and 5. For these systems and the H,TPP-based systems, similar profiles were obtained in all solvents used in this study. The results from the analysis of these fluorescence decay profiles are collected in Tables 2-5. A single-exponential decay function,f(t) =A exp( -t/7), is found to be adequate ‘The free energy change accompanying the excited state (singlet) electron transfer (AGO) can be expressed as A@=Em-Emd-EMfAG,

TABLE 1. Intermolecular roform at 25 “C” System

H,TPP-DNB HJJ?P-TNB ZnTPP-DNB ZnTPP-TNB

fluorescence quenching

results in chlo-

109 k, (Mm’ SK’)

k, /kd”

AG”’ W)

3.2 4.5 9.1 14.3

0.25 0.34 0.70 1.09

0.11 0.03 -0.36 -0.44

‘Error limits: k,, + 15%; AG”, f 0.07 eV. bk,,=SRT/3000s is the diffusion limit. ‘The AG” values were calculated as described

in Section 3.1.

where

where E”” and Ered are the oxidation and reduction potentials of the donor and acceptor respectively (measured in CHICII, 0.1 M tetra-n-butyl ammonium perchlorate (TBAP)), ro (7.25 A) and rA (3.5 A) are the effective radii of the donor cation and acceptor anion respectively, and sand l arc the dielectricconstants of the reference solvent (C&Cl,) and the other solvent (CHCIs) respectively. R,, is taken to be the sum of r, and r, for these intermolecular cases. ‘?The redox potentials were either measured in our laboratories or obtained from refs. 24 and 25.

b

4.2

8.4

TIME

12.6

/ns

Fig. 4. Fluorescence decay cures for ZnTPP (a), ZnTPP-p-DNB (b), ZnTPPm-DNB (c) and ZnTPP-o-DNB (d) systems in acetonitrile (see Table 4 for decay parameters).

meta positions are best described by a singleexponential decay function in each solvent (see Tables 2 and 3). The lifetimes are either equal to or less than the lifetime of the non-linked fluorophore (i.e. H,TPP). A biexponential equation was found to be necessary to fit the decay of H,TPP-o-DNB (see Table 2) and H,TPP-o-TNB (see Table 3). The decay curves of ZnTPP-o,m,p-DNB and ZnTPP-o,mg-TNB fit a two-exponential function in all solvents, except for ZnTPP-p,m-DNB in CHCI,, which can be fitted adequately by a threeexponential decay equation. For example, the fluorescence decay of ZnTPP-p-DNB in CHCl, can be fitted adequately (x2= 1.29) for the following three lifetimes (and amplitudes): 0.17 ns (O.SO), 0.53 ns (0.19) and 2.20 ns (0.01). It should be noted that, in these two cases, one of the lifetimes is longer than that of the free fluorophore, the relative amplitude of this component is very small (less than 1%) and the lifetime of the shortest lived component is similar to that obtained in an analysis involving a biexponential fit.

4. Discussion

76’6.K TIME/ns Fig. 5. Fluorescence decay curves for ZnTPP (a), ZnTPP-p-TNB (b), ZnTPP-m-TNB (c) and ZnTPP+-TNB (d) systems in acetonitrile (see Table 5 for decay parameters).

to describe the decay of those systems for which values of only_4 and 7 are given. For others, where values of A,, AZ, 71 and 72 are given, a twoexponential decay equation, f(t) =A, exp( - t/ 7,) +A, exp( -t/&!, is used since the fit for a oneexponential decay equation is clearly inadequate. The fluorescence decays of H,TPP complexes linked with either TNB or DNB at the para or

The intramolecular D-A complexes studied in this work are not linked by rigid spacer units. Flexibility in the distance and orientation of the acceptor with respect to the porphyrin plane permits the possibility of numerous conformations in which the D-A distances and relative orientations vary. The excited state reaction rate coefficients for these conformations may be different. It is possible to visualize the existence of conformations in which the D-A distances and relative orientations are favourable for efficient quenching and such conformations can be termed “closed” conformations. Other conformations in which the D-A distances and D-A orientations are not favourable for efficient quenching can be termed “extended” conformations. Although, for discussion purposes, these two forms are viewed as two limiting cases (i.e. “closed and “extended” conformations), in reality they can exist as two structural families each of which is an ensemble of several physical conformations. An intramolecular D-A system predominantly existing in either the “extended” form or the “closed” form can give rise to a fluorescence decay which can be fitted to a one-exponential function. It is expected that, in the former case, the lifetime will be almost equal to that of the free fluorophore and, in the latter case, the lifetime will be shorter than that of the free fluorophore. However, it

144

E.G.

Maiya et aL / Time-m&ed

fluorescence

of porphyti-nifroarene

TABLE 2. Intramolecular fluorescence quenching results of H,TPP-DNB or two-exponential fits (reduced x2 values are given in parentheses) System

HzTPP H,TPP-p-DNB H*TPPm-DNB HzTFP-o-DNB

compkxes

systems: fractional amplitudes and lifetimes (ns) of one-

Solvent Toluene

CHCI,

CH,OH

CH,CN

PC

l.o”, 9.44b (1.21) 1.0, 9.50 (1.05) 1.0, 9.52 (1.14) 0.75, 9.43 0.25, 0.35 (0.99)

1.0, 8.00 (1.04) 1.0, 6.63 (1.17) 1.0, 6.16 (1.38) 0.08, 7.24 0.92, 1.10 (1.36)

1.0, 9.01 (1.05) 1.0, 8.55 (1.40) 1.0, 8.20 (1.35) 0.05, 8.38 0.95, 0.36 (1.72)

1.0, 9.20 (1.15) 1.0, 8.93 (1.19) 1.0, 8.01 (1.06) 0.09, 8.01 0.91, 2.10 (2.29)

1.0, 10.9 (1.39) 1.0, 10.33 (1.24) 1.0, 10.31 (1.12) 0.09, 9.76 0.91, 2.95 (1.53)

PAmplitude. bLifetime. cFitting the data to a three-exponential decay equation did not improve the quality of the fit as suggested by high x2 values. TABLE 3. Intramolecular fluorescence quenching results of H,TPP-TNB and two-exponential fits (reduced ,$ values are given in parentheses) System

H;TPP HJPP-p-TNB H;rPP-m-TNB H;TPP-o-TNB

systems: fractional amplitudes and lifetimes (ns) of one-

Solvent Toluene

CHCI,

CHaOH

CH,CN

PC

LO’, 9.44b (1.21) 1.0, 9.68 (1.10) 1.0, 9.31 (1.33) 0.75, 9.67 0.25, 0.91 (1.53)

1.0, X.00 (1.04) 1.0, 7.95 (1.30) 1.0, 7.99 (1.83) 0.78, 8.18 0.22, 0.70 (0.92)

1.0, 9.01 (1.05) 1.0, 8.74 (1.42) 1.0, 8.68 (1.18) 0.77, 9.30 0.23, 0.54 (1.09)

1.0, 9.20 (1.15) 1.0, 9.13 (1.01) 1.0, 8.91 (1.65) 0.79, 9.38 0.21, 0.52 (0.85)

1.0, 10.9 (1.39) 1.0, 10.39 (1.15) 1.0, 10.55 (1.28) 0.65, 11.25 0.35, 2.34 (1.57)

‘Amplitude. %ifetime. In these cases, a two- or three-exponential decay a better fit, but the amplitude of one of the components was insignificant _ gave I (less than 0.03) to warrant application of such an analysis.

would be too simplistic to expect a porphyrin-acceptor complex in an “extended” form to have a fluorescence lifetime exactly identical with that of the free fluorophore. This is because the “electronic structure” of the porphyrin would be altered by the linkage and also by the presence of the acceptor in the vicinity of the fluorophore. A simple model for the flexibly linked D-A systems is that linked molecules coexist in “extended” and “closed” forms and interconversion between them is slow compared with the nanosecond lifetime of the excited state. This model predicts that the fluorescence decay will follow a multiexponential function and the longest lifetime of this function will be closest to the lifetime of the free fluorophore. However, it should be noted that when the rates of conformational change are

rapid (between the “extended” and “closed” forms and also between themselves), it is possible to describe the fluorescence decay by a single- or muitiexponential function depending on the details of the dynamics. In such cases, the lifetime in a one-exponential fit and the longest lifetime in a multiexponential fit may be shorter than that of the free fluorophore. In the present study, it is observed that, in general, the experimental decay data can be fitted to a sum of not more than two (or three, in a very few cases) exponential decays, and that in no case is the longest lifetime obtained in a multiexponential fit considerably less than that of the free fluorophore. Therefore it has been assumed that, in a given D-A complex, all “extended” forms have the same fluorescence lifetime (close to that of the free fluorophore) and that the “closed” forms can be classified into one group

E.G. Ma&

et al. / lhe-resolved

fluorescence ofpotphyrin+ai@oarene complexes

TABLE 4. Intramolecular fluorescence quenching results of ZnTPP-DNB or two-exponential fits (reduced ,$ values are given in parentheses) System

ZnTPP ZnTPP-p-DNB

2nTPP-wDNB

ZnTPP-o-DNB

systems: fractional

and lifetimes (ns) of one-

Solvent Toluene

CHCI,

CH,OH

CH&N

PC

1.w, 2.OOb (1.00) 0.66, 1.73 0.34, 1.00 (1.W 0.45, 1.60 0.55, 0.87 (1.00) 0.05, 1.82 0.95, 0.27 (1.31)

1.0, 1.62 (1.36) 0.03, 1.51 0.97, 0.30 (0.90) 0.02, 1.77 0.98, 0.21 (1.21) 0.20, 1.67 0.80, 0.11 (1.43)

1.0, 1.92 (1.23) 0.05, 2.25 0.95, 0.29 (1.05) 0.06, 2.19 0.94, 0.27 (1.05) 0.14, 1.97 0.86, 0.09 (1.10)

1.0, 1.87

1.0, 2.07 (1.34) 0.02, 2.10 0.98, 0.35 (1.26) 0.03, 2.14 0.97, 0.40 (0.98) 0.19, 2.02 0.81, 0.15 (1.86)

“Amplitude. bLifetime. 7n these cases, a three-exponential fit gave a better ,$ value, but the amplitude than 0.01) to warrant such an analysis.

TABLE 5. Intramolecular fluorescence quenching results of ZnTPP-TNB or hvo-exponential fits (reduced ,$ values are given in parentheses) System

amplitudes

14s

(1.X) 0.01, 1.82 0.99, 0.44 (1.16) 0.01, 2.51 0.99, 0.40 (1.02) 0.15, 1.93 0.85, 0.10 (1.54)

of one of the components

systems: fractional

amplitudes

was insignificant

and lifetimes

(less

(ns) of one-

Solvent Toluene

CHCI,

CH,OH

CH,CN

PC

ZnTPP

1 .O’, 2.00b

ZnTPP-p-TNB

(1.W 0.71, 1.80 0.29, 1.08

1.0, 1.62 (1.36) 0.76, 1.70 0.24, 1.01 (1.05) 0.84, 1.61 0.16, 1.09 (0.9 1) 0.60, 2.10 0.40, 1.31 (1.58)

1.0, 1.92 (1.23) 0.70, 1.73 0.30, 0.66 (1.01) 0.77, 1.82 0.23, 1.16 (1.14) 0.50, 2.35 0.50, 1.67 (0.99)

1.0, 1.87 (1.24) 0.65, 1.80 0.35, 0.86 (1.04) 0.74, 1.86 0.26, 0.88 (1.33) 0.70, 2.19 0.30, 1.35 (1.11)

1.0, 2.07 (1.34) 0.71, 1.98 0.29, 0.30 (1.73) 0.75, 1.90 0.25. 0.96 (0.94) 0.70, 2.40 0.30, 1.50 (0.98)

ZnTPP-m-TNB

ZnTPP-o-TNB

(1.W 0.69, 1.81 0.31, 1.18 (1.02) 0.80, 2.33 0.20, 1.20 (1.08)

“Amplitude. bLifctime. ‘A three-exponential

analysis did not improve the ,$ values to give a better

(or two groups in a few cases) with respect to the fluorescence lifetime. In the majority of studies on intramolecular D-A complexes with flexible linkages [l-lo], the fluorescence decay data have generally been interpreted using a model that predicts multiexponential behaviour, usually two or three. The shortest lifetime is associated with the “closed” form and the longest lifetime with the “extended” form, Siemiarczuk et al. 1271 have observed that, in a few flexibly linked porphyrin-quinone systems, two or three exponentials are inadequate. A distribution of lifetimes was suggested to be the

fit.

appropriate form, but no analysis was carried out. Electron transfer rates k,, were calculated from the shortest decay time of the two- or threeexponential fits. The k,, value thus obtained is the maximum experimental value, which is useful for comparisons between different related systems or the same system in different solvents. We have adopted a similar approach in this paper. As far as the quenching mechanisms are concerned, the AG” values (Table 6) for intramolecular electron transfer reactions in the acceptor-linked porphyrins predict a most favourable electron transfer from the singlet excited states of zinc(I1)

TABLE 6. Corrected free energy changes (AGO) for PET reactions of isomeric D-A porphyrins” Compound

H,TPP-p-DNB H2TPP-m-DNB H,TPP-o-DNB H,TPPq-TNB H,TPP-m-TNB H,TPP+TNB ZnTPP-p-DNB ZnTPP-m-DNB ZnTPP-o-DNB ZnTPP-p-TNB ZnTPP-m-TNB ZnTPP-a-TNB

AG” (eV) Toluene (2.38)b

CHCI, (4.80)b

C&OH (32.70)b

CH,CN (37.50)b

PC (69.0)b

0.82 0.68 0.60 0.68 0.66 0.52 0.36 0.29 0.15 0.18 0.11 0.02

0.40 0.32 0.29 0.31 0.33 0.24 - 0.06 - 0.07 - 0.16 -0.19 - 0.22 - 0.26

0.05 0.01 0.03 0.00 0.07 0.02 0.41 0.38 0.42 0.50 0.49 0.48

0.04 0.01 0.03 - 0.01 0.06 0.01 - 0.42 -0.39 - 0.42 -0.51 -0.49 -0.49

0.02 - 0.02 0.01 - 0.03 - 0.06 - 0.01 - 0.45 - 0.41 - 0.44 - 0.53 - 0.51 - 0.51

-

‘AG’ values were estimated as described in Section 3.1 using the electrochemical data obtained in CH$N (0.1 M TBAP) for these intramolecular complexes. RDn values were obtained from molecular mechanics calculations (see footnote, this page). bDielectric constant.

derivatives linked to nitroaromatic acceptors+. Electron transfer from the triplet states of either the free-base or the zinc(I1) porphyrin is found to be less favourable by about 0.48 eV in each case [lo, 111. Thus a favourable electron transfer in the zinc(H) porphyrins and a less favourable electron transfer in the free-base porphyrins, together with other quenching mechanisms such as enhanced intersystem crossing, energy transfer, electron transfer, etc., may form possible pathways for the singlet state quenching of these systems (see below). With this background, we now examine the details of the fluorescence decay data of porphyrin-nitroarene systems and identify their fluorescence quenching mechanisms. 4.1. Free-base porphyrins An examination of the fluorescence decay results (see Tables 2 and 3) of para- and meta-linked H,TPP systems (Le. H,TPP-p,m-DNB and H,TPP*,m-TNB) indicates that the profiles are adequately fitted by a one-exponential decay equation except for H2TPP-m-TNB in CHCl, and CH,CN (x2> 1.4). The lifetimes of these D-A complexes are less than or equal to the lifetime of the unlinked fluorophore (i.e. H,TPP) in each ‘Incorporation of solvent correction terms for low polarity solvents is rather risky and hence the AG” values listed in Table 6 for solvents having low dielectric constants (toluene and CHCI,) should be regarded only as rough estimates.

solvent. However, the deviations are not significant when we consider that the electronic structure of the D-A complex is different from that of the free fluorophore owing to linkage. Thus, by and large, these cases can be interpreted as examples of conformers whose lifetimes are close to that of the free fluorophore. This situation is especially true for the TNB-linked complexes. A comparison of the results of the meta- and para-substituted H,TPP complexes with those of the ortho analogues (i.e. H,TPP-o-DNB and H,TPP-o-TNB) is interesting. A two-exponential decay analysis was employed to fit the fluorescence profiles of the latter complexes in each solvent and, here, a model involving the coexistence of “closed” and “extended” forms is applicable. A comparison between H,TPP-o-TNB and H,TPP-oDNB reveals that, except for toluene, the short Iifetime component is dominant in DNB complexes, whereas the longer lifetime component is dominant in TNB complexes. This indicates greater efficiency of quenching in DNB complexes. The mechanism of the intramolecular quenching process in these linked systems appears to be complex and is not completely understood at the present time. The results obtained for the intermolecular interactions of HJPP and several nitroaromatic acceptors [20] suggest that a combination of several quenching mechanisms, including those involving electron transfer, exciplex formation, etc., operate. It is expected that, in the intramolecular case, the same mechanisms operate in the fluorescence quenching of the porphyrin donor components. With regard to the participation of the PET mechanism in the quenching of the linked free-base porphyrins, the AG” values for the PET reactions of these systems are more positive than the AG” values for the corresponding ZnTPP-based intramolecular systems (Table 1). Nonetheless, in favourable cases, PET reactions can still occur in the free-base intramolecular systems, albeit with less efficiency than in the ZnTPP-based systems. The data given in Tables 2 and 3 suggest that this is indeed the case. For example, the fluorescence lifetimes of HJPP-oTNB and HJPP-o-DNB are considerably reduced compared with that of H2TPP. However, no radical electron paramagnetic resonance (EPR) signals were observed for any of the investigated H,TPPbased complexes during steady state, in situ irradiation experiments in an EPR cavity [ll, 121. Put together, all of these observations support the proposition that several mechanisms, including PET, contribute to the fluorescence quenching of free-base porphyrins linked to nitroarenes.

B.G. Ma&a et aL I Time-resolved fluorescence

4.2. Zinc (II} povhytis Most of the investigated ZnTPP-based systems give a good fit to a two-exponential equation (Tables 4 and 5). The fluorescence quenching in ZnTPP-DNB (except for ZnTPP-p-DNB in toluene) systems is very effective and this is indicated by shorter lifetimes and larger amplitudes for the “closed” forms (Table 4). On the other hand, for the linked ZnTPP-TNB systems, the amplitude data indicate the dominance of the “extended” forms over the “closed” forms (Table 5). In both cases, however, the lifetimes of the “extended” forms are nearly equal to the lifetime of unlinked ZnTPP. In certain cases, the extended forms have slightly longer lifetimes than that of the free porphyrin and these small differences can be attributed to the differences expected when the porphyrin is linked to an acceptor which can alter the “electronic structure” of the fluorophore. Thus the fluorescence decay of these systems can be considered to fit the model in which “closed” and “extended” forms coexist. As stated earlier, the AGo values (Table 6) suggest that electron transfer from the excited singlet states of the zinc(B) porphyrins to the linked nitroaromatic acceptors provides a major contribution to the observed decrease in the fluorescence lifetimes. Indeed, photoinduced charge separation in these ZnTPP-based systems has been established by on-line irradiation/ESR experiments [ll, 121. Furthermore, both “closed” and “extended” forms having different D-A distances and orientations are observed in these systems and, as indicated by the lifetime data, the electron transfer rate constants (k,J vary depending on the position at which the acceptor unit is linked to the donor. Therefore these complexes are good candidates for testing the distance and orientation dependence of PET reactions.

4.3. Dtitance and orientational effects on k, The dependence of the electron transfer rate on the D-A separation distance and orientation has been the subject of several experimental and theoretical studies [1, 281. The distance (r) dependence of the electron transfer rate (ket, s-r) is given by k,, = Y exp( - PrJ

(2)

where y (s-l) depends on the energetics of the reaction including the Franck-Condon factor and p (A-‘) is a function of the overlap integral of the wavefunctions of donor and acceptor. Also, it has been observed that faster electron transfer

of porphyCn_nitroarene compkxe~

147

reactions result when the donor and the acceptor are in a face-to-face orientation 1291. The electron transfer rate for an intramolecular reaction is calculated using the experimentally measured lifetimes according to k,,=T-‘--O-l

(3) where Q is the lifetime of the “free” fluorophore and r is the lifetime of the “closed” conformer (shortest lifetime) to which k,, values are assigned. In view of the fact that the free fluorophore lifetime can be modified by linkage with an acceptor, it is more meaningful to use the longer lifetime obtained in the multiexponential analysis as r,, for the linked complexes. The k,, values thus calculated for the “closed” forms of ZnTPP-based systems are given in Table 7. Molecular mechanics calculations (PCM4 model) of the structures of the D-A complexes suggest that the centre-to-centre (ctc) distances between the porphyrin and the nitroaromatic unit vary as 12.3 f 1.2 8, (para) > 10.2*0.7 %, (meta)> 8.6 &OS w (ortho). Similarly, the edgeto-edge distances between the donor and acceptor are 4.3 + a.7 W (para) >4.03 +O.S A (meta)>2.6tO.l A (ortho). The values are the same for the DNB- and TNB-linked porphyrins. Thus it is now possible to arrive at a distance dependence of PET reactions in these D-A systems. An inspection of Table 7 reveals that, for ZnTPP-DNB systems, k,r values vary as ortho > meta > para. These observations are consistent with eqn. (2). However, results of ZnTPP-TNB systems are not consistent with eqn. (2); the order in which the rate constants vary is opposite to that given above for the DNB-linked systems. We rationalize this difference in the distance dependence of k,, values for the TNB-linked and DNBlinked systems by invoking the orientational dependence of PET reactions. Thus, although the TABLE 7. Rate constants (log&:) transfer reactions of ZSPP-DNB D-A

complex

ZnTPP-p-DNB ZnTPP-m-DNB ZnTPP-o-DNB ZnTPP-p-TNB ZnTPPm-TNB ZnTPP-o-TNB

for the photoinduced electron and ZnTPP-TNB systems’

log k.: Toluene

CHCI,

CH,OH

CH,CN

PC

8.70 8.81 9.51 8.63 8.54 8.52

9.43 9.61 9.93 8.57 8.48 8.15

9.46 9.51 10.03 9.00 8.53 7.90

9.24 9.29 9.98 8.80 8.79 8.34

9.38 9.30 9.79 9.45 8.75 8.28

‘The rate constants were calculated for the “closed” of the D-A complexes using eqn. (3) (see text). b/c,, in reciprocal seconds.

conformers

148

D-A distances vary as ortho < meta < para in the ZnTPP-TNB systems, the D-A orientation may not be conducive for efficient electron transfer. In this regard, it should be noted that molecular mechanics calculations have revealed that the D-A orientations for TNB- and DNB-linked complexes are different+. Intuitively, we ascribe the orientations exhibited by TNB-linked complexes as those which are %nfavourable” for efficient PET. This “unfavourable” D-A orientation is a consequence of steric hindrance between the porphyrin and the NO, groups of the linked TNB unit. The same factor may be responsible for the observed lower amplitude ratios for the “closed” forms of the TNB-linked complexes in comparison with those of the DNB-linked complexes (Ai values in Tables 4 and 5). Indirect evidence for these propositions comes from the ‘H NMR spectra of the complexes. The ‘H NMR spectra of H,TPP-o,m,p-TNB systems show that the porphyrin imino proton resonances are shifted upfield compared with the imino proton resonance position in H2TPP and, more importantly, are split into complex multiplets [ll]. On the other hand, the imino proton resonances of the analogous DNB systems experience only a marginal upfield shift and appear as a singlet as is the case for unlinked HJPP [12]. These results are consistent with the molecular mechanics data described above and can be rationalized only if the orientation of the nitroaromatic unit with respect to the plane of the porphyrin is different for the TNB-linked and DNB-linked systems. Nonetheless, it is difficult to quantify these results in terms of the identification of the most probable D-A orientation, especially in the absence of any knowledge of the accurate energies of the conformers. The rate data given in Table 7 also reveal another interesting aspect, which is related to the relationship between AG” and k,, values and provides further insight into the orientational dependence of the k,, values in the D-A complexes. For the intramolecular ZnTPP systems linked to DNB and TNB units, the AG” values of the former systems are less negative than those of the latter (Table 6). Thus it can be expected that the k,, values for the ZnTPP-TNB systems will be higher than those for the ZnTPP-DNB systems. However, the data given in Table 7 indicate the opposite; by tMolccular mechanics calculations give dihedral angles (#) encompassing C(meso-phenyl)-O-C(O-nitrophenyl)C(nitrophenyl) of 89” (para), 85” (meta) and 92” (ortho) for the TNBlinked compIcxes and 5” (para), 59” (meta) and 88” (ortho) for the DNB-linked complexes. These data pertain to the most favourable ground state conformations.

and large, the PET reactions of the DNB-linked systems are faster than those of the TNB-linked systems. According to Marcus theory [30], ket should increase as AG” becomes negative and decrease in the so-called “inverted” region. In the present case, the AG” values are only moderately negative such that they do not fall in the inverted region, and thus this cannot explain the faster rates observed for the DNB-linked systems. On the other hand, and as stated earlier, both the edge-to-centre and edge-to-edge distances in the intramolecular complexes remain similar in each isomer irrespective of the identity of the acceptor (i.e. whether TNB or DNB). Thus the differences observed in the rate constants of PET reactions of the D-A complexes seem to suggest that the lower rate coefficient for the ZnTPP-TNB systems is a consequence of an unfavourable D-A orientation.

5. Conclusions On the basis of the thermodynamic data only (i.e. AG” values), PET rates are expected to be higher for the TNB-linked porphyrins than for the DNB-linked analogues. On the other hand, molecular mechanics calculations and ‘H NMR spectral data of the D-A complexes linked via flexible spacer fragments suggest that, in the TNB-linked porphyrins, steric interactions exist between the nitro groups of the TNB unit and the pyrrole and/ or phenyl ring protons of the porphyrin, and that such steric interactions are absent in the DNBlinked porphyrins. This steric hindrance would position the acceptor in an “unfavourable” orientation with respect to the plane of the porphyrin, such that the k,, values in these porphyrins will be smaller than those in the corresponding DNBlinked porphyrins. Thus both orientational and thermodynamic effects operate simultaneously in these D-A systems and the dominance of one effect over the other depends on the type of linked acceptor unit. The time-resolved fluorescence decay data presented here substantiate the above viewpoint. The complex decay kinetics observed indicate the presence of fan&es of conformations that do not equilibrate on the nanosecond time scale. At least two distinct groups of conformers are required to account for the observed time dependence of the fluorescence behaviour in these complexes “closed” and “extended”. In general, the lifetime is shorter and the relative amplitude is greater for the “closed” conformer of DNB-linked por-

B.G. Maiya et al. I i?me-resolved fluorescence

phyrins in contrast with the corresponding conformer of TNB-linked analogues. These observations suggest that steric and orientational factors dominate over the thermodynamic factor for this class of D-A systems in deciding the rates of PET reactions. The origin of this dominance of the steric factor over the thermodynamic factor lies in the structural features of the complexes. A subtle change in position of the attachment of the acceptor to the donor brings about a large change in the D-A distance and, more importantly, in the D-A orientation in these complexes. This contrasts with observations in many other intramolecular D-A systems separated by flexible linkages [l--IO]. The intramolecular porphyrin-nitroarene systems presented here are separated by a short spacer unit (only one oxygen atom) and this accounts for the existence of steric repulsions between the donor and acceptor atoms. In contrast, in the majority of previously reported porphyrin-based D-A systems, the intervening spacer units were not as short as that used in this study (spacers usually have more than one atom in the chain), such that any orientational effect would be “masked” by the facile rotational freedom of the spacer entity. The distance dependence of the k,, vaIues is discernibIe only in the case of DNB-linked zinc(I1) porphyrins. In the case of TNB-linked analogues the orientational effects seem to prohibit PET even though the distance and thermodynamic parameters favour the reaction.

13 14 15

20 21 22 23 24

Acknowledgments

25 26

We are grateful to the Department of Science and Technology, Government of India (New Delhi) for establishing the picosecond fluorescence spectrometer as part of their Intensified Research in High Priority Areas (IRHPA) progranune at the Tata Institute of Fundamental Research (TIFR) and to the Department of Non-Conventional Energy Sources (New Delhi) for financial support of this work.

27

28

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39, Cowlen@ Linked Donor-Accepmr of Photosynthetic Electron and Energy Pergamon, Oxford, Vol. 45, No. 15, 1989.

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