INTER AND INTRAMOLECULAR INTERACTIONS OF PORPHYRINS WITH 5,5′-DITHIOBIS(2-NITROBENZOIC ACID)

Photochernisrry and Photobiology Vol. 56, No. 2, pp. 145-156, 1992 Printed in Great Britain. All rights resewed

0031-8655/92 SO5.00+0.00 Copyright 0 1992 Pergamon Press Ltd

INTER AND INTRAMOLECULAR INTERACTIONS OF

ACID) PORPHYRINS WITH 5,5’-DITHIOBIS(2-NITROBENZOIC

FRANCIS D’SOUZA and V. KRISHNAN*~ Department of Inorganic and Physical Chemistry, Indian Institute of Science,

Bangalore 560 012, India

(Received 2 July 1991; accepted 2 January 1992)

Abstract-meso-Tetraphenylporphyrin and its metal [zinc(II) and copper(II)] derivatives form both inter and intramolecular complexes with 5,5‘-dithiobis(2-nitrobenzoic acid) (DTNB). The nature of interaction is predominantly charge transfer (CT) in origin, with the porphyrin functioning as a II- donor and DTNB as an acceptor. Among the covalently linked intramolecular systems, the magnitude of CT interaction varies with the position (of one of the aryl groups of the porphyrin) to which DTNB is attached as orrho > mefa > para. Steady-state and time-resolved fluorescence studies revealed electron transfer to be the dominant pathway for the fluorescence quenching in these systems. Steady- state photolysis experiments probed using EPR and optical absorption studies have shown that electron transfer (from the excited singlet state of the porphyrin) to DTNB results in the formation of thiyl radical and production of free thiolate anion. It is found that the products of electrochemical reduction of covalently linked porphyrin-DTNB systems are different from those observed for the photochemical studies.

INTRODUCTION

Photophysical studies of porphyrins bearing electron acceptors have received considerable attention in recent years owing t o their importance in solar energy conversion and storage process. Nitro- aromatic compounds have been widely used as electron acceptors in the complexation study of porphyrins (Gouterman and Stevenson, 1967; Hill et al., 1969; Fulton and La Mar, 1976; Grigg and Grim- shaw, 1978; Chandrashekar and Krishnan, 1981, 1982). Maiya and Krishnan (1985) demonstrated photoreduction of trinitrobenzene moiety in the intramolecularly linked porphyrin-trinitrobenzene systems. Among the nitroaromatic compounds, 5, 5’-dithiobis-(2-nitrobenzoic acid) (DTNB)$ is a well known reagent for the determination of sulfhydryl groups in proteins since the reduction of DTNB yields a colored thiolate anion (Ellman, 1959). This property has been exploited by Seely (1972, 1989) to elucidate energy and electron transfer reactions in chlorophyll-DTNB systems supported on a poly- mer matrix. More recently, Seely and Rehms (1991) have studied in detail the photochemical and spectral properties of a particulate model system con-

*To whom correspondence should be addressed. tIn association with Jawaharlal Nehru Center for

Advanced Scientific Research, Indian Institute of Sci- ence, Bangalore, India.

$Abbreviariom: CT, charge transfer; CTC, center-to- center; CuTPP, 5,10,15,20-tetraphenylporphinato- copper(I1); DMF, N,N-dimethylformamide; DTNB, 5, 5’dithiobis(2-nitrobenzoic acid); ET, electron transfer; H,TPP, 5,10,15,20-tetraphenylporphyrin; TLC, thin layer chromatography; ZnTPP, 5,10,15,20-tetraphenylporphinatozinc(I1).

sisting of chlorophyll adsorbed to a suspension of polyethylene in tetradecane. It has been demonstrated that chlorophyll sensitized reduction of DTNB proceeds from the singlet excited state of chlorophyll in the presence of a sacrificial donor, hydrazobenzene. Here, we report a novel approach towards the design of porphyrin-DTNB systems and describe structural and photophysical properties of these entities.

Efficient photoinduced charge-separation in molecular systems is often achieved by the rapid removal of cation and anion radicals from the reaction site to avoid charge-recombination. Elegant molecular systems comprising triads and tetrads with a variation in the redox potentials of the donor and acceptors have been designed to achieve long- lived charge-separation (Wasielewski er al., 1985; Gust et al . , 1986, 1988, 1991). Here, the primary photoinduced charge-separation results in rapid migration of charges via the strategically positioned donors and acceptors leading to the charge-separ- ated species. Alternatively, one could explore the property of the acceptor to obtain the rapid separation of the radicals. 5,5’-dithiobis(2-nitrobenzoic acid) is endowed with two carboxylate groups which provide convenient sites of covalent attachment to the porphyrins to enable the synthesis of intramolecular donor-acceptor systems. In the present study meso-tetraphenylporphyrin and its metal [zinc(II) and copper(II)] derivatives were chosen as donors. The functionalization of one of the meso aryl groups of tetraphenylporphyrin at the (0, m or p) position and subsequent linking of DTNB leads to porphyrin-DTNB systems wherein the DTNB is oriented at different positions with respect to the

145

146

(i)

FRANCIS D’SOIJZA and V. KRISHNAN

NHz

(ii) NHLOH

HN \

M = 2H M = Zn(ll) M = CU(U) ortho : H2Pp ~n DTNB ZnPp DTNB CUPQ vr DTNB meta : H 2 P m ~ DTNB ZnPm* DTNB CuPmm DTNB para : HzPp DTNB ZnPe m DTNB CuPe DTNB MP - DTNB

NO2

Figure 1. Synthetic route for the DTNB appended porphyrins.

porphyrin core. Photoexcitation of porphyrin- DTNB systems would lead to the singlet excited state of the porphyrin which is a good electron donor. The subsequent reduction of DTNB would proceed in several interesting pathways:

MP + R-S-S-R 3 MP* + R-S-S-R

MP - R-S-S-R 3 MP* - R-S-S-R

MP* + R-S-S-R MP? + R-S- + RS‘ (2)

MP* R-S-S-R j MP?- R-S- + R-S’(3) MP* - R-S-S-R MP? - R-S‘ + R-S- (4)

MP? + RS’ - products (5 )

R-S’ + R-S’ R-S-S-R (6) where P = porphyrin, R = C6H4(N02) (COOH), M = 2H, Zn(1I) and - = covalent linkage.

In intermolecular systems the photoreduced DTNB could result in the formation of a thiolate anoin and its radical analog [reaction (2)], while in the intramolecular systems reactions (3) and (4) could occur. We examine these aspects in the present study.

MATERIALS AND METHODS

The materials employed in this study were of analytical grade. Solvents were of spectral grade and distilled before use. The spectral measurements have been carried out under a blanket of argon and exposure to air is avoided.

Figure 1 presents the synthesis of intramolecularly linked porphyrin-DTNB systems. The starting material, 5(o, m, or p)aminophenyl-10,15,20-triphenylporphyrin, for the synthesis of DTNB appended porphyrin was prepared by condensing pyrrole (4 mM), benzaldehyde (3 mM) and o, m orp-nitrobenzaldehyde (1 mM) in propi- onic acid and subsequent reduction of the nitro derivative to the amino derivative using HCl and stannous chloride (Sun et al., 1986). The amino derivatives were purified on a silica gel column using CH2Cl2 as eluent. DTNB was

procured from Sigma Chemicals, St. Louis, MO and recrystallized in acetic acid. The 5,5’-dithiobis(2-nitrobenzoic acid chloride) was prepared according to the method of Collman and Groh (1982). The DTNB (1 g) was treated with freshlj distilled thionyl chloride (30 mL) and a cata- lytic amount of N,N-dimethylformamide (DMF) (0.3 mL). The reaction mixture was refluxed under nitrogen for 45 min until the solution was a clear orange-yellow in color. The residual liquid was then removed under vac- uum. The orange-yellow solid thus obtained was immedi- ately used for further reactions.

Synthesis of S,S’-dirhiobis(2-nitrobenzoic acid) appended porphyrim. To a solution of the free-base porphyrin, 5(0, m or p)aminophenyl-l0,15,20-triphenylporphyrin (0.02 mM) in SO mL of CH2C12 containing 1 mL of pyridine, the acid chloride of DTNB (0.03 mM) in 15 mL of CH2C12 was added dropwise for a period of 15 min. The reaction mixture was refluxed for 30 min and evaporated to dryness. The contents were dissolved in 50 mL of CH,C12 and washed first with 10% NH,OH and then with 10% brine solution, and dried over anhydrous Na,SO,. The compound was purified on a basic alumina column using CH2CI, as eluent. The second fraction obtained on increasing the polarity of the eluent (3% CH30H) was found to contain the desired compound. Thin layer chromatography (TLC) of the synthesized porphyrins revealed the presence of only one component. Anal. Calcd. for C,,H,,N7S20,: C, 69.03; H, 3.69; N, 9.71; Found, C, 70.48; H, 3.72; N , 9.79. The elemental analysis of the meta and para DTNB substituted derivatives yielded similar results. ‘H NMR in CDCI, (6 in ppm, nature of the multiplet, number of protons, position of the protons): (i) H,P*DTNB: 8.82 (m, 8H, pyrrole-H), 8.20 (m, 6H, triphenyl ortho-H), 7.75 (m, 9H, triphenyl mera and para-H), 8.51 (m, 6H, DTNB- H), 7.35 (m, 4H, sub. phenyl-H), 3.72 (s, lH, arnide-H), -2.72 (s, 2H, imino-H). (ii) H,PmDTNB: 8.83 (m, 8H. pyrrole-H), 8.19 (m. 6H, triphenyl orrho-H), 7.76 (m, 9H, triphenyl meta and para-H), 7.39 (m, 4H, sub. phenyl-H), 7.82 (m, 6H, DTNB-H), 3.04 (s, lH, amide-H), -2.77 (s, 2H, imino-H), (iii) H,PpDTNB: 8.84 (m. 8H, pyrrole-H), 8.41 (m, 6H, triphenyl orrho-H), 7.75 (m, 9H, triphenyl meta and para-H). 7.36, 8.10 (d,d, 4H, sub. phenyi-H), 7.89 (m. 6H, DTNB-H), 3.19 (s, lH, amide-H), -2.76 (s, 2H, imino-H).

The zinc(I1) and copper(I1) derivatives of these synthesized porphyrins were prepared using metal acetates as metal carriers according to the procedure described in the literature (Dorough el al., 1951). The DTNB appended metalloporphyrins were purified on a basic alumina

Porphyrin-DTNB molecular systems 147

column using CH,CI2/CH,OH solvent mixture as eluent. The metal derivatives gave satisfactory elemental analysis corresponding to the indicated composition.

The spectrometers employed in this study are the same as described in our earlier work (D'Souza and Krishnan, 1990a). EPR measurements were carried out on a Varian E-109 spectrometer. Diphenylpicrylhydrazide was used as the 'g' marker. The electrochemical generation of radicals in the EPR cavity was performed using a Varian E-256 electrolytic cell. A platinum-iridium mesh was used as an anode and a platinum wire as a cathode. Ag/AgCI was used as a reference electrode. Electrolysis of the solutions was performed using this cell with a Bank Electronic (Germany) model MP-81 potentiostat. The potential drift during electrolysis was less than f 10 mV. Electrochemi- cal studies were performed on a BAS-1WA electrochemical working station. A three electrode assembly consisting of a glassy carbon working electrode (BAS MF-2012), a platinum wire auxiliary electrode and a Ag/AgC1 reference electrode was employed. Porphyrin solutions (1 mM) in acetonitrile containing 100 mM tetrabutylammonium perchlorate as the supporting electrolyte was employed for redox potential measurements. The surface of the working electrode was cleaned using fine alumina powder prior to each run to avoid adsorption of the sulfur bearing compounds on the electrode surface. The details of the thin layer spectroelectrochemical cell are given elsewhere (D'Souza and Krishnan, 1990b).

Fluorescence lifetime of these derivatives were mea- sured by using a picosecond laser excitation single photon counting method. The samples were excited by 4 ps laser pulses at a repetition rate of 800 kHz. Fluorescence decay data were collected at either 42 pskhannel (for ZnTPP based systems) or 160 pslchannel (for H,TPP based systems) resolution on a multichannel analyzer. The excitation wavelength ranged between 560-580 nm corresponding to the Q bands of the porphyrins. The fluorescence was collected at 610 nm for ZnTPP based systems and at 645 nm for H,TPP based systems. The concentration of the porphyrins in CH$N was maintained around 1 p M . The details of the experimental setup are given elsewhere (Periasamy er al., 1988). Analysis of the fluorescence decay to fit a single or multiexponential function was carried out by the non-linear least-squares method using iterative reconvolution and Marquardt procedure for the optimization of parameters. The quality of the fits was judged by the reduced x2 and inspection of the residuals and the auto-correlation functions (Demas, 1983; OConnor and Phillips, 1984).

Light source for the study of irradiation experiments observed through EPR was a 150 W Varian E-MAC xenon lamp coupled to a regulated power supply. The UV radi- ation was cut off by using glass filters so that only h > 350 nm falls on the sample. Steady-state irradiation experiments were carried out using a halogen lamp (150 W). The light from the source is channeled through a three compartment filter cell consisting of copper(I1)ammonium sulfate and focused on the sample through a lens. The test solutions were purged with argon throughout the irradiation time.

RESULTS

The reaction between the 5,5'-dithiobis(2-nitrobenzoic acid chloride) and 5(0, m, or p)aminophenyl-10,15,20-triphenyl-porphyrin results in the formation of amide linked porphyrin-DTNB systems. We found that among the two acid chloride groups one readily formed the amide linkage with the amino substituted porphyrin while the other acid chloride was hydrolyzed during the working

0.0 I I I

350 400 450 500 550 600 650 h h m ) -

Figure 2. Electronic absorption spectrum of ZnTPP containing varying concentration (0,O. 1,0.2,0.4 and 0.6 mM) of DTNB in CH,CN at 298 K. The spectrum is expanded

5 times (Y-axis) in the region 500-650 nm for clarity.

up procedure to iorm carboxylic acid. Elemental analysis of the porphyrin-DTNB systems corre- sponds to the composition, indicating the presence of a carboxylic acid group in these systems. It has not been possible to locate the proton resonance of the free carboxylic acid group in the porphyrin-DTNB systems. The proton resonance of the 0, m , orp substituted aryl groups of the porphyrin in the porphyrin-DTNB systems are characteristic and provide further structural details.

Intermolecular complexation

The optical absorption method has been used to study the molecular complexation properties of H2TPP and ZnTPP with DTNB. It is seen that increasing addition of DTNB to CHJCN solution of the porphyrins results in a decrease in absorbance of Soret and visible bands accompanied by the appearance of several isosbestic points (Fig. 2). This suggests the formation of molecular complex between the porphyrin and DTNB in solution. The binding constant (K,) has been evaluated from the Nash plots (Fig. 3), (Nash, 1960). The linearity of the plots indicates the existence of 1:l (porphyrin:DTNB) stoichiometry for the molecular complexes. The linear least-squares analysis of these plots yield K, values of 30 and 36 M-' dm3 for the free-base porphyrin (H2TPP) and its zinc(I1) derivatives (ZnTPP) respectively. The intermolecular complexation behavior of the porphyrins with DTNB was followed by the singlet emission spectral method. It is found that the emission intensity of the Q(0,O) and Q(1,O) bands of the porphyrin (H2TPP and ZnTPP) in acetonitrile are quenched

148 FRANCIS D'SOIJZA and V. KRISHNAN

dO1 (do- d 1

Figure 3. Nash plots for the interaction of DTNB with (a) HzTPP and (b) ZnTPP in CH,CN at 298 K. Here, C, represents the concentration of DTNB employed, and d and d denote the absorbance values of the porphyrin in absence and presence of DTNB. The changes in the absorbance values at 417 and 419 nm are monitored for the complexation of the free-base porphyrin and its zinc(I1)

derivatives respectively.

DTNB (mM) - Figure 4. Stern-Volmer plots of (a) H,TPP-DTNB and (b) ZnTPP-DTNB in CH,CN at 298 K. Here, I,, and I represent the fluorescence intensity of the porphyrins in the absence and presence of DTNB. The wavelengths of excitation employed for ZnTPP and HzTPP are 546 and

513 nm respectively.

on increasing addition of DTNB. The Stern-Volmer plots (Fig. 4) revealed a marginal upward curvature at concentrations of quencher greater than 6 mM. This indicates the presence of more than one quenching process, e.g. complex formation, diffusive quenching followed by ET. The Stern-Volmer constants (Ksv) of quenching for the different porphyrins have been obtained from the linear part of the plot using least-squares analysis. The K,, values of quenching of the fluorescence of H2TPP and ZnTPP were found to be 61 and 20 M-I respectively. It is possible to obtain rate coefficients for bimolecular quenching (k,) from the K,, values and the lifetime of the excited singlet state of the porphyrins. Time-resolved fluorescence study of HzTPP and ZnTPP in CH3CN exhibits a single exponential decay with lifetime at 8.80 ns ( x 2 = 1.13) and 1.90 ns ( x 2 = 1.19) respectively. The k, values for HzTPP and ZnTPP were calculated and found to be 7.1 x loy M-' s-l and 10.5 x loy M-' s-l respectively. It is of interest to compare these values with those obtained from the lifetime quenching data. It is found that on addition of DTNB (1 mM), H2TPP and ZnTPP (1 p M ) exhibits monoexponential decays with excited singlet lifetimes of 8.50 ( x 2 = 1.18) and 1.78 ns (x2 = 1.21) respectively. It may be noted, however, that on addition of DTNB (> 5 mM) to H2TPP and ZnTPP a biexponential decay is observed similar to that observed for intramolecular systems (vide infra).

The k, values calculated from the lifetime data are found to be 4.02 x 10' M-I s-I and 4.35 x loy M - L s-I respectively for H2TPP and ZnTPP. The higher magnitude of k, values obtained from steady-state fluorescence measurements reveal ground-state complexation in these systems. The rate of diffusion controlled reaction ( k d ) has been calculated using Debye-Smouluchowski's equation. According to this, the kd is given as

where N is the Avegadro's number, k is the Boltz- man constant, q is the viscosity of the solvent in cp, and rA and rB are the radii of the reactants. The radius of the porphyrin was taken from the known crystal structure (Silvers and Tulinsky, 1967) and the radius of the DTNB molecule was estimated using CPK models. A comparison of the calculated kd value (1.8 x loLD M-I s-l) with the experimen- tally observed k, values indicate that the major part of the quenching proceeds through diffusion.

Intramolecular complexation

The synthesized DTNB appended porphyrins reveal many interesting optical properties. The optical absorption spectra of two representative porphyrin derivatives, H2Po-DTNB and ZnPm-DTNB are shown in Fig. 5 and the data


0.2 +

I W

Y

'./..

0 I I 300 350

1.0- 0.i

500 600 O

Figure 5. Optical absorption spectra of a representative free-base porphyrin, H,Po-DTNB (-) and its zinc(I1) derivative, ZnPo-DTNB (- - -) in CH,CN at 298 K. The concentration of porphyrins

employed is 0.01 mM.

are summarized in Table 1. DTNB in acetonitrile exhibits an absorption band at 312 nm. In covalently linked porphyrin systems, this band was found to be blue shifted and occurs in the region 270 nm, where the UV bands of porphyrin appear (Gouterman, 1978). It is seen that the Soret and visible bands of the porphyrin are shifted to the red region with a decrease in intensity (E values) relative to the unappended porphyrins. It is interesting to note that the magnitude of the shift and decrease in intensity of the absorption bands vary with the nature of the substitution of the porphyrin entity as ortho > meta > para. The fluorescence spectra of the intramolecularly linked porphyrins exhibit nor- mal emission bands corresponding to the Q(0,O) and Q(1,O) transitions (Fig. 6). The emission profiles of the covalently linked systems remain unaltered on changing the wavelength of excitation from the Soret to the Q band of the porphyrins. Interestingly, the relative fluorescence quantum yields are found to be dependent on the nature of the substitution and decrease as ortho < rneta < para for both free- base and zinc(I1) porphyrin-DTNB derivatives (Fig. 6). These results of the optical absorption and emission studies indicate the presence of intramolecular interactions in these systems.

The electrochemical redox behavior of these donor-acceptor systems have been studied to arrive at the energy of the charge-transfer states. The cyclic voltammograms of a representative H2Pm-DTNB derivative are presented in Fig. 7. Similar voltammograms were obtained for the free- base porphyrin-DTNB and their zinc(I1) and copper(I1) derivatives. The data are summarized in Table 2. We found that all the compounds exhibit two one-electron quasi-reversible oxidation and two one-electron quasireversible reduction peaks corre-

sponding to the porphyrin ring oxidation and reductions, respectively (Felton, 1978; Kadish, 1986). Interestingly, an irreversible peak - -0.8 V

Table 1. Optical absorption data of porphyrin-DTNB systems in CH3CN at 298 K'

DTNBt Compound Q, Qz Q3 Q4 B, UV band

HzTPP 646 592 549 513 417 - (3.52) (3.74) (3.72) (4.16) (5.76)

(3.20) (3.49) (3.62) (4.06) (5.37)

(3.32) (3.50) (3.65) (4.10) (5.41)

(3.38) (3.53) (3.69) (3.92) (5.48)

(4.30) (5.82)

(3.82) (5.22) (4.62)

(3.92) (5.38) (4.59)

(3.93) (5.48) (4.52)

H~Po-DTNB 647 591 550 516 420 274 (4.52)

(4.58)

(4.68)

H2Pm-DTNB 648 590 551 515 419 276

HzPp-DTNB 647 592 553 517 420 278

ZnTPP 596 546 419 -

ZnPo-DTNB 598 550 421 267

ZnPm-DTNB 597 548 420 270

ZnPp-DTNB 598 549 420 276

CuTPP 538 411 - (4.37) (5.73)

CuPo-DTNB 540 413 275

CuPm-DTNB 539 412 279

CuPp-DTNB 539 412 273

(4.12) (5.38) (4.55)

(4.18) (5.47) (4.62)

(4.25) (5.55) (4.67)

*A in nm the value in parentheses refers to log c. +The free DTNB in CH3CN exhibits an absorption band at

312 nm (log c = 4.70).

FRANCIS D'SOUZA and V. KRISHNAN

1 I 635 665

A (nrn) - Figure 6. Fluorescence spectra of (i) ZnTPP, (ii) ZnPp- DTNB, (iii) ZnPm-DTNB and (iv) ZnPo-DTNB in CH,CN at 298 K. The concentration of porphyrins

employed is 1 pm.

is observed in all the covalently linked compounds and this has been ascribed to the one-electron reduction of the disulfide group of the appended DTNB moiety. Evidence for the reduction of the disulfide group leading to the formation of thiyl radical and thiolate anion is presented in a later section. It may be noted that using the platinum working electrode results in decrement of the peak current and shifts in the potentials during repetitive runs due to the strong adsorption of sulfur bearing compounds on the metal surface. It can be seen that the ring oxidation and reduction potentials of the free-base and metalloporphyrins in the linked systems show a marginal change relative to the potentials observed for the respective unappended derivatives.

Time-resolved fluorescence studies on these covalently linked molecules reveal interesting results. The fluorescence decay profiles of representative free-base and its zinc(I1) derivatives are shown in Fig. 8 along with the result of fittings. The decay profiles obtained can best be fitted to a biexponential function as revealed by their reduced x 2 values. Treatment of the data for either a monoexponential or triexponential fit leads to a large x2 value (x2 > 1.2) and non-linear nature of the residuals, indicating that biexponential decay function describes the decay adequately. In a few cases, the amplitude values of the short component are less than 5 % ; however, these values are real owing

Figure 7. Cyclic voltammograms of H,Pm-DTNB (1 mM) in CH,CN containing 100 mM tetrabutylammonium perchlorate at 298 K. Scan rate 100 mVls. The scan of potentials in cathodic (a) and anodic (b) region are shown. The scan of the potential range 0.0 to -1.0 V is

given separately in the inset.

to the best fit obtained with low x2 values and the linear form of the residuals. The two different lifetimes and their relative amplitudes are summarized in Table 3. In all these compounds, the weighted average lifetime of the species are found to be smaller than the corresponding unsubstituted porphyrins (ZnTPP and H2TPP). It is seen that the lifetime of the long component of the porphyrin-DTNB derivatives is almost coincident with the lifetime observed for their respective unappended porphyrins. It is reasonable to assume that the short lifetime component could represent the electron transfer, and the rate of electron transfer (k,,) in such a system is given by

k,, - (7,' - 7,') (8)

where 7, is the lifetime of the short component of the linked derivative and 7, is the lifetime of the free donor (H2TPP and ZnTPP). The calculated values of k,, (Table 3) reveal that the k,, values are higher for zinc(I1) derivatives than for the free-base porphyrin-DTNB systems. It may be mentioned here that the k,, values do not significantly change with varying concentration of the porphyrin deriva-


Table 2. Electrochemical data on the DTNB appended porphyrins and its metal derivatives in CH3CN at 298 K*t

Compound P + Pf P + P: p: + p2-

H2TPP HzPo-DTNB

HZPp-DTNB H2Pm-DTNB

ZnTPP ZnPo-DTNB ZnPm-DTNB ZnPp-DTNB CUTPP CuPo-DTNB CuPm-DTNB CuPp-DTNB

1.02 1.06 1.04 1.05 0.94 0.99 0.98 0.96 1.02 1.06 1.08 1.06

1.32 1.36 1.34 1.33 1.23 1.27 1.25 1.24 1.26 1.27 1.26 1.22

-1.02 -1.01 -1.00 -1.00 - 1.24 -1.22 -1.23 - 1.24 -1.19 -1.28 -1.31 -1.34

-1.36 -1.34 -1.36 - 1.36 -1.53 - 1.48 -1.50 - 1.52 - 1.65 -1.67 -1.64 -1.60

- -0.78 -0.80 -0.81

-0.79 -0.79 -0.82

-0.81 -0.80 -0.79

-

-

'The reported potentials are with reference to Ag/AgCI electrode: error limits are within

tP , Pt, P2+, P;, P2- correspond to neutral, monocation radical, dication. monoanion

$The reduction of DTNB in these derivatives is found to be irreversible.

2 20 mV.

radical and dianion species of the corresponding porphyrins respectively.

tives (1 to 5 pM). This indicates the absence of inter andor diffusive quenching.

Interesting results were obtained from the EPR

4-

a 2 -

s 1 -

M

0- u r 0.0 4.16 8.32 12.48 16.64 20.80

TIME, NS

Figure 8. Fluorescence decay profiles for (a) H2Pm- DTNB and (b) ZnPm-DTNB obtained from 1 p M solutions in CH3CN at 298 K. The experimental setup is

given in the text.

studies on steady-state irradiation experiments of the covalently linked porphyrin-DTNB systems. We found that irradiation of either the free-base porphyrin-DTNB system or its zinc(I1) derivative in acetonitrile in EPR cavity at 140 K resulted in the formation of weak signals devoid of hyperfine splittings (Fig. 9). These signals have 'g' values at 2.0032 and 2.0015 for ZnPo-DTNB and 2.0017 and 1.9996 for H2Po-DTNB. The 'g' values are not significantly altered in the other meta and para derivatives. The spectral width (A HPp) of these signals was found to be - 26 mT. In order to under- stand the origin of these EPR signals, the following control experiments were performed and the results are presented here. Irradiation of the individual constituents, i.e. porphyrins (free-base and zinc(I1) derivatives), DTNB and solvent CH3CN, in EPR cavity either at room temperature or at low temperature does not generate EPR signals. However, the EPR spectrum of ZnTPF. [generated electrochemically by oxidizing a CH3CN solution of ZnTPP at 0.81 V (vs Ag/AgCI)] exhibits a broad signal at 298 K [Fig. lO(a)]. No apparent hyperfine splitting due to the interaction of the unpaired electron with pyrrolic protons and nitrogens was observed (Fajer

Table 3. Time-resolved fluorescence data for the porphyrin-DTNB systems in CH&N at 298 K'

Compound T$ A, TI A2 x2 k,, x 10-9s-'t

ZnPo-DTNB 0.56 0.09 1.95 0.91 1.09 1.25 ZnPm-DTNB 1.46 0.31 2.03 0.68 1.01 0.15 ZnPp-DTNB 1.35 0.23 1.84 0.76 1.08 0.21 HZPo-DTNB 2.15 0.06 8.95 0.94 1.14 0.35 H,Pm-DTNB 1.62 0.10 9.21 0.89 1.12 0.50 HZPp-DTNB 1.48 0.03 8.80 0.97 1.08 0.56

'Lifetime in nano seconds; Al and A2 are the relative amplitudes of the two components.

tSee text for the calculation of kc,.

152 FRANCIS D’SOUZA and V. KRISHNAN

(a)

Figure 9. Light induced EPR spectra observed from irradiated solutions of (a) H,Po-DTNB and (b) ZnPo-DTNB in CH,CN at 298 K. EPR settings: modulation amplitude, 4 G , microwave power, 2 mW, microwave frequency, 9.05 GHz, time constant, 0.064 s.

et ul., 1973; Fajer and Davis, 1978). The ‘g’ value of the signal was found to be 2.0029 with a spectral width of 22 mT. Interestingly, the EPR spectrum of electrochemically reduced DTNB in CH3CN [produced by electrolyzing DTNB at -0.90 V (vs AglAgCI)] exhibits well resolved signals characteristic of the thiyl radical, RS’, where R = 3-carboxylic-4-nitrothiophenyl radical [Fig. 10(b)]. The spectrum consists of three sets of signals, each of which is a doublet or triplet with equal intensity. The origin of these signals has been analyzed based on the interaction of the unpaired electron of the thiyl radical with one nitro group (aN = 12.8 G) producing the three main signals each of which is further resolved because of interaction with two types of protons, orrho to the nitro group (aHo = 3.2 G) and metu to the nitro group (aHm = 1.2 G). The EPR spectrum of the thiyl radical was simulated and the calculated hyperfine parameters, aN, aHo, and aHm agree well with the exper- imentally observed values. The observed spectrum provides the first experimental demonstration of the formation of the thiyl radical on reduction of DTNB, although the formation has been postulated earlier. The spectral width of thiyl radical signal was found to be 3.20 mT with a ‘g’ value of 2.0082. The ‘g’ value obtained here is close to the value reported for the aliphatic carboxylate bearing thiyl radical by Neta and Fessenden (1971). Digital addition of the EPR spectrum of ZnTPPt and RS’ yielded a signal centered at 3257 G consisting of hyperfine splitting with a large spectral width (A H,, = 3.6 mT). This spectrum does not bear one to one correspondence with that observed on irradiation of the covalently linked porphyrin-DTNB systems. It is likely that the EPR signals observed on irradiation of the covalently linked systems arise from radicals that

(a)

Figure 10. EPR spectra of (a) ZnTPP? produced by the electrochemical oxidation of a CH$N solution of ZnTPP (1 m M ) containing 100 mM tetrabutylammonium perchlorate at 0.81 V (vs Ag/AgCI). (b) the thiyl radical produced by reducing a CHICN solution of DTNB (1 mM) containing 100 mM tetra-butylammonium perchlorate at -0.90 V (vs Ag/AgCI). EPR settings: modulation amplitude, 4 G , microwave power, 10 mW, microwave fre-

quency, 9.29 GHz, time constant, 0.064 s.

are stabilized by secondary reactions. It is of interest to probe the EPR active copper(I1)

derivatives of the synthesized porphyrin to study the inter and intramolecular interactions. With this in view the EPR spectra of the copper(I1) derivatives were recorded in acetonitrile at 140 K. Rep- resentative spectra for the CuTPP and CuPo-DTNB are presented in Fig. 11. All the spectra were simulated using the Gaussian quadra- ture method and EPR parameters (g and A) calculated from the spectral features assuming a Spin-Hamiltonian for axial symmetry (Assour, 1965; Manoharan and Rogers, 1969) (Table 4). It is clear from Table 4 that the g,, and gl values for the inter and intramolecular porphyrin-DTNB systems decrease significantly relative to that observed for CuTPP. Moreover, a decrease in the values of AFfand A? and an increase in AYl and A? were observed in the inter and intramolecularly linked system indicating interaction between the porphyrin and DTNB units (Chandrashekar and Krishnan, 1982). The inplane &bonding of the Cu-N bonds in these donor-acceptor molecules has been obtained in terms of the bonding parameter (a’), using the


Figure 11. EPR spectra of (a) CuTPP and (b) CuPo-DTNB in CH,CN at 140 K.

Table 4. EPR parameters of Cu(I1) derivatives of porphyrin-DTNB systems in CH3CN at 140 K

DISCUSSION

Compound g11 gL 104 cm-' Aa2

CuTPP 2.184 2.045 207.0 31.6 14.7 16.1 - CuPo-DTNB 2.173 2.032 198.1 31.5 14.7 16.0 0.045 CuPm-DTNB 2.176 2.038 200.4 31.6 14.8 16.2 0.032 CuPp-DTNB 2.179 2.040 202.2 31.6 14.8 15.9 0.028 CuTPP+DTNB* 2.178 2.038 204.0 31.7 14.8 16.0 0.019

'50-fold excess of DTNB used.

method of Kievelson and Neiman (1961). The difference in the magnitude of the a2 values ( A d ) between CuTPP and the inter and intramolecularly linked copper(I1) porphyrin-DTNB derivatives gives the measure of the covalency factor arising from the charge-transfer interaction. It is found that the AaZ values calculated for the porphyrin-DTNB systems vary with the nature of substitution as ortho > meta > para and the magnitude of these values are relatively higher than those of the intermolecular porphyrin-DTNB (1 50 mMlmM) system. The decrease in Aa2 values for Cu(I1)porphyrin signifies an increase in the u-covalency of the Cu-N bonds in these systems. This is ascribed to the II-electron donation of the Cu(I1)porphyrin to DTNB which increases the Cu-N cr-covalency, thus giving indirect evidence for interaction of the two entities.

The results of the optical absorption and emission and magnetic resonance spectroscopic studies of both the inter and intramolecular systems point out the existence of weak CT interaction between the porphyrin and DTNB moieties. The magnitude of the energy of CT state, Em, can be obtained from electrochemical redox potential measurements. However, the reduction potential of DTNB is found to be irreversible around -0.80 V (vs Ag/AgCl) owing to the cleavage of the disulfide bond resulting in the formation of thiyl radical and thiolate anion. Thus the energy of the CT state, porphy- rint-DTNB;, could not be calculated in these systems, in contrast to other nitroaromatic systems (Maiya and Krishnan, 1985). It is of interest to note that among the covalently linked porphyrin-DTNB systems, the ortho substituted derivatives exhibit dominant CT interaction between the donor and acceptor relative to the meta and para substituted compounds indicating the importance of confor- mational features favorable to such interactions. Molecular model studies have been carried out using a computer programme, CART. For this, the coordinates of HzTPP and its zinc(I1) derivatives obtained from the reported crystal structure (Silvers and Tulinsky, 1967) are taken as the reference frame. The coordinates of DTNB were generated from the known crystal structure of diphenyl disulfide (Lee and Bryant, 1969). It is found that a change in the dihedral angle of the amide bond and the free rotation of the S-S bond allows one of the

154 FRANCIS D’SOUZA and V. KRISHNAN

P A

Figure 12. Schematic representation of two limiting conformers of covalently linked porphyrin systems

(a) “closed” and (b) “extended” conformers.

aryl groups of DTNB to take up an orientation parallel to the porphyrin plane, leaving the other ring in a perpendicular orientation. It may be mentioned here that for each change in dihedral angle there exists a family of structures and the programme gives the optimally averaged energy confor- mer. The two extreme conformers obtained for ZnPo-DTNB in its ‘closed’ and ‘extended’ orien- tations are shown in Fig. 12. The center-to-center (CTC) distance calculated was found to be - 6.7 A” and - 11.8 A’, respectively, for these two limiting structures. The ‘closed’ orientation would allow maximum CT interaction between the porphyrin ring and DTNB entity. It is known that the rate of photoexcited state electron transfer reaction depends on both the conformation and distal separation between the donor and acceptor (Wasielewski and Neimezyk, 1984; Wasielewski, 1988). However, the rate data obtained from time-resolved fluorescence studies point out no significant change in the magnitude of k,, values among the differently substituted derivatives. The possibility that the k,, values may not be wholly representative of electron transfer is examined. It is found that the magnitude of the lifetimes of the covalently linked compounds remain unchanged with the increase in concentration of the compounds, indicating the absence of intermolecular complexation andor diffusional encounters. Attempts have been made to identify the presence of any short lifetime component up to 50 ps (instrument resolution factor) in these systems which may represent the real k,, values. We are

1 .c

t 5 z W 0

a t Y

0

C

(a’ I

)O

A (nm) - Figure 13. (a) Thin layer optical absorption spectrum of a 1 mM solution of (i) DTNB, (ii) and (iii) represents the spectra of the electrolyzed product after 300 and 600 s respectively in CH,CN. (b) The thin layer optical absorption spectrum of (i) ZnPo-DTNB and (ii) its electrolyzed product after 600 s in CH,CN. Tetrabutyl-ammonium perchlorate (100 mM) was used as a supporting electrolyte and the potential was held at -0.90 V vs Ag/AgCI in both

the cases.

than the observed values. It is likely that the differently substituted porphyrins equilibrate within the time scale of the measurement to a preferred geometry favorable for CT complexation followed by electron transfer reaction. Steady-state photolysis experiments provide evidence for ET reactions.

Attempts have been made to obtain information on the possible channels of the electron transfer reactions. Results of steady-state photolysis experiments and spectroelectrochemical measurements have been found to be useful in this regard. The optical absorption spectrum of the thiophenolate anion (3-carboxylic-4nitrothiophenolate) produced by the electrochemical reduction of DTNB in acetonitrile (at -0.90 V vs Ag/AgCI) in a spectroelectrochemical cell revealed a band at 470nm [Fig. 13(a)]. This band is similar to the thiophenolate anion produced by the photochemical reduction of DTNB using chlorophyllide as a sensitizer in the presence of sacrificial donor, hydrazobenzene

unable to find any component at time scales shorter (Seely, 1972, 1989; Seely and Rehms, 1991). Elec-


1.0

t E

a K

4 W 0

Y

0

0.c 3

Figure 14. (a) The electronic absorption spectrum of an irradiated sample of ZnPo-DTNB in CH3CN at 298 K. (b) The spectra of an irradiated sample of ZnTPP and DTNB in presence of EDTA. (i) and (ii) denote the spectrum before and after 600 s of irradiation in CH,CN respectively. The inset in (b) shows the build-up of the thiolate anion peak during the time course of irradiation

0, 60, 120, 180, 240 and 300 s.

trochemical reduction of the covalently linked ZnPo-DTNB system (at -0.90 V vs Ag/AgCl) in CH3CN revealed an absorption spectrum with bands similar to that observed for a mixture of ZnTPP and RS- (produced electrochemically) [Fig. 13(b)]. The absorption spectrum revealed split Soret band and red shifted visible bands characteristic of a “hyperporphyrin” type absorption pattern indicating the formation of pentacoordinated species (Nappa and Valentine, 1978; D’Souza and Krish- nan, 1990b). Steady-state irradiation of ZnPo-DTNB exhibited two absorption bands at 446 and 750 nm [Fig. l4(a)]. The appearance of the band at 446 nm suggests the presence of thiolate anion and the long wavelength band indicates the existence of ZnTPPt species (Gasyna et al., 1985; Neta et al. , 1986). Interestingly, photolysis of a mixture of ZnTPP and DTNB in the presence of a sacrificial donor, EDTA, resulted in the formation of RS- [Fig. 14(b)]. These results suggest that the electron transfer reaction is the dominant pathway for the fluorescence quenching of porphyrins. In covalently linked molecules, the formation of RS-

clearly indicates that reaction (4) is the major pathway. However, the results obtained on electrochemical reduction of ZnPo-DTNB suggest the formation of a pentacoordinated species. It is likely that electron transfer proceeds in different ways, depending on the mode, either through photoexcited transfer or electrochemical means.

Acknowledgements-The authors are thankful to the Department of Science and Technology, and Department of Non-Conventional Energy Sources, Government of India, New Delhi for financial support. The authors are grateful to the Tata Institute of Fundamental Research, Bombay for extending the picosecond facility.

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INTER AND INTRAMOLECULAR INTERACTIONS OF PORPHYRINS WITH 5,5′-DITHIOBIS(2-NITROBENZOIC ACID)