Photochemirrry and Photobiology Vol. 39, No. 3, pp. 281 - 285,1984 Printed in Great Britain. All rights reserved

003 1-8655/84 SO3.OO t 0.00 Copyright 0 1984 Pergamon Press Ltd

PHOTOSENSITIZATION VIA CHARGE TRANSFER OR REVERSIBLE ELECTRON TRANSFER. OXIRANE

ISOMERIZATION AND SULFUR DIOXIDE EXTRUSION

P.K. DAS*, A.J. MULLERf, G.W. GRIFFIN+, I.R. GOULDS, C.-H. TUN@, and N.J. TURROSB

*Radiation Laboratory, University of Notre Dame, South Bend, IN 46556. USA. tuniversity of New Orleans-Lakefront, New Orleans, LA 70148, USA and $Chemistry Department, Columbia

University New York, NY 10027, USA

(Received 1 June 1983; accepted 23 September 1983)

Abstract-N,N,N',N'-Tetramethylbenzidine (NTMB) photosensitizes the cis-iram isomerization of stilbene oxiranes (SO) and the extrusion of SO2 from dibenzyl sulfone (DBS). In acetonitrile solution it is found that in the absence of SO or DBS, singlet NTMB undergoes three processes: intersystem crossing to triplet NTMB ($Isc = 0.63, klsc = 6.3 X lo7 s-I), fluorescence ($f = 0.30, kf = 3 X 107s-'), and formation of a cation by electron ejection (hi(," = 0.09). Both singlet and triplet sensitization are observed. A charge transfer or reversible electron transfer mechanism is proposed to explain the results.

INTRODUCTION

Photosensitized electron transfer reactions, although known for over a decade (Koizumi era/., 1978), have attracted ever increasing attention of organic photo- chemists in recent years. A range of reactions such as isomerizations, rearrangements, and cleavages have been initiated via sensitized electron transfer. A simplified mechanism of photosensitized electron transfer which reflects current views, is presented in the Scheme. In general, a charge transfer stabilized

Scheme 1

complex (exciplex) or radical ion pair are considered to be key intermediates (Rehm and Weller, 1970). In non-polar solvents, exciplexes are the favored species (Gordon and Ware, 1975) in polar solvents, free ions are often observed (Masuhara and Mataga, 1981). The reactions are easily understood in terms of simple molecular orbital theory which predicts that excited states are both good electron donors and acceptors (Turro, 1978). A relationship between the free energy change associated with the electron transfer process and the oxidation and reduction potentials of the donor and acceptor, and the excited state energy has been demonstrated by Weller (1970). and is shown in Eq. 1.

~~ _______~

$To whom correspondence should be addressed. ((Abbreviations: DBS, Dibenzyl Sulfone; DPE, Dipheny-

lethane; ISC, Intersystem crossing; NaC104, Sodium per- shlorate; NTMB, N,N,N',N'-tetramethylbemidine; SO, Stilbene oxiranes.

AG (kcal mol-') = 23.06 x [,!?(DID+) -E(A/A-) -e,;/ac] - AE,,, (kcal mol-') (1)

Clearly, donors and acceptors which are more likely to undergo these reactions will have low oxidation and high reduction potentials, respectively. Typical electron donors which may react via paths a or b in the Scheme are aromatic amines. In the course of our studies, we have investigated the tertiary aromatic amine, N,N,N',N'-tetramethylbenzidine (NTMB)II as a possible photosensitizer in charge transfer or reversible electron transfer reactions, namely the sensitized reactions of oxiranes and sulfones. In addition, the photophysical parameters of NTMB have been investigated.

NTMB

MATERIALS AND METHODS

Photocleavage of DBS. Typically, 2 mC of a solution of DBS (Parish, two times recrystallized from EtOH) (0.01 M) and NTMB (Aldrich) (0.005 M) in acetonitrile was freezelthaw degassed 5 times and irradiated at 313 nm (Turro and Wriede, 1970) using a 400 W XelHg Hanovia Light source for 10 min. Analysis was by gas-liquid chromatography.

The fluorescence quantum yield was determined in acetonitrile solvent relative to quinine bisulfate in 1N sulfuric acid. on a Perkin-Elmer MPFJL spectrofluorimeter. Spectra were corrected for instrument response and solvent effects (Berlman, 1971).

The intersystem crossing yield was determined in acetonitrile solvent, using 313 nm excitation light, using the piperylene isomerization method (Lamola and Hammond, 1965) relative to benzo- phenone as a sensitizer of known intersystem crossing yield.

N T M B fluorescence yield.

NTMB intersystem crossing yield.

28 1

282 P. K. DAS et al.

At the concentration of piperylene (0.02 M) employed, negligible quenching of NTMB fluorescence occurred. We conclude that isomerization of the piperylene is only a triplet process.

Absolute quantum yields for reaction were determined using NTMB (0.005 M), 313 nm excitation light. The benzophenone/ benzhydrol system was used for actinometry (Hammond et a l . , 1961), and irradiations were performed using a merry-go-round apparatus. AU solutions were freezehhaw degassed.

Laser phorolysb. The pulsed laser photolysis equip- ment at Columbia uses a Lambda Physik EMG 101 (308 nm, 15 ns) laser, and has previously been described (Turro eral., 1980). The system (Encinas and Scaiano, 1979) at Notre Dame uses a Molectron UV-400 nitrogen laser (336 nm, 8 ns) .

Preparation and irradiation of oxiranes. The oxiranes were synthesized (Muller et al., 1982) by reductive conden- sation of the corresponding methyl arylglyoxates or benzoyl cyanides induced by hexamethylophosphorous triamide. Irradiations were conducted in N,-purged acetonitrile solutions, contained in serum-capped, 15 X 2 cm, Pyrex test tubes, employing a Rayonet RPR-100 chamber reactor (Southern New England Ultraviolet Company) equipped with 16 8-W F8T5 broad emission lamps (290.420 nm, centered at 350 nm).

Reduction potential of NTMB. This value was mea- sured in acetonitrile (0.2 M NaCIO,) vs S.C.E. by Dr. G. Kavarnos. CYTO Medical Laboratories, Inc., Norwich, Connecticut.

Quantum yields for DBS photocleavage.

RESULTS

Photochemistry

Steady state photolysis (A > 300 nm) of deaerated solutions of the oxiranes lc, 2c, and 3 in acetonitrile in the presence of NTMB, such that only the latter absorbs light, results in cis-trans-isomerization (Eq. 2a), with the photostationary composition in favor of the trans isomer. Our results are given in Table 1.

Table 1. Photostationary states for sensitized oxirane isomerizations

Quencher cb:trans

lc 1:8.5 2c 1:9.5 3 1:3.0

l a RI=R2=CN, R2=R3=pMe0-C6H4 l b Rl=R2=CN, R2=R3=pMeO-C&14

2a R1=R3=C02Me, R2=R4=pMe0-ChH4 2b R1=R3=CO2Me, R2=R4=pMe-C6H4 2c R1=R3=C02Me, R2=R4=C&

IC RI=R2=CN, R ~ = R ~ = C ~ H S

3 RI=R3=H, R ~ = R ~ = P C N - C , ~ H ~

Irradiation of dibenzylsulfone (DBS), 4, and NTMB in deaerated acetonitrile (A > 300 nm) gives diphenylethane (DPE) and loss of sulfur dioxide (Eq. 2b). The yield of DPE is 100% (based upon consumed DBS). In both cases, NTMB was re- covered quantitatively.

PhCH2S02CH2Ph &jjj PhCH2CH2Ph + SO;?

(2b) 4

Photophysical properties of NTMB

At room temperature in acetonitrile, NTMB fluoresces strongly with A,,, (corrected) at 404 nm. The lifetime of the 'NTMB in acetonitrile deter- mined by single photon counting (TJ is 11 ns. The absolute quantum yield of fluorescence is 0.30 f 0.01, and the absolute quantum yield for intersystem crossing to lowest triplet state is 0.63 f 0.08. From kf , +f, and +ac the constants kf and klsc were deter- mined to be 3.0 x lo7 s-', and 6.3 x lo7 s-', respectively. Since the sum +f + +Isc = 0.93 f 0.08 is close to unity, we deduce that fluorescence and intersystem crossing dominate the decay of 'NTMB.

Pulsed laser photolysis of NTMB in deaerated acetonitrile leads to absorptions due to NTMB cation (NTMB') and the triplet state of NTMB (3NTMB) (Alkaitis and Gratzel, 1976). The spectra of these two species overlap extensively in the spectral region 35G500 nm; however, they can be separated by employing 2,5-dimethyl-2,4-hexadiene as a specific triplet quencher (kqT - 10' M-I s - ' ) . At concentrations of the diene in the order of 0.05 M, the lifetime of 3NTMB is decreased sufficiently (-20 ns) to allow monitoring of the NTMB' absorbance at times close to the laser pulse. The total yield of NTMB' was determined to be -0.09 for solutions containing 0.05 M diene. Thus, in the absence of added quenchers, +f + is^ + +ion - 1.0. Analysis of the cation absorption as a function of laser intensity reveals that it is formed via monophotonic excitation of NTMB to 'NTMB, followed by spontaneous ejection of an electron.

In the absence of solvated quenchers, the cation absorption is essentially constant over the time scale of the triplet decay. Thus, subtraction of the long-lived absorptions from those at the end of the pulse yields the absorption spectra of both species (Fig. 1). It is clear that absorptions at wavelengths greater than 500 nm are exclusively due to the triplet. The lifetime of the triplet state, monitored at 510 nm (73) is 5 ks.

Photosensitization via charge transfer 283

0.18 .

0.10 - f 0 0

0.05 -

0.00 -

400 460 820 500 M

A-

Figure 1. Absorption spectrum of 'NTMB* (- ) and NTMB+ (- - -) in acetonitrile.

Quenching studies

The oxiranes and DBS quench 'NTMB as deter- mined by steady state fluorescence quenching. Plots of Iof/If vs. quencher concentration were linear (e.g., Fig. 2) as were plots of T O ~ / T ~ vs quencher concentra- tion (Fig. 3). The slope of the plot and T~ gave the quenching constants summarized in Table 2. The decay of the NTMB triplets (monitored at 510 nm) produced by laser photolysis is found to be quenched by both the oxiranes and DBS (Table 2). Plots of T ~ - ' vs quencher concentration were linear and from the slopes the quenching constants were evaluated.

I . . . . . . . . 0.002 0006 001 0.014 [ i S ] ( M )

Figure 3. Plot of 7',hf vs [DBS] in acetonitrile.

Table 2. Rate constants for quenchingof the excited states of NTMB.

Quencher ks x lo-' kT x lo-' ( e moI-'s-I) ( e mo1-I s-I)

la 21.8 lb 19.8 lc 23.7 20 2.4 2b 8.9 2c 12.8 4 10.0

28 79

180

- co.05

2.0

Quantum yields for product formation

The absolute quantum yield for DPE formation has been determined as a function of [DBS] (dots in Fig. 4). The plot leads to a curve, rather than a straight line, indicating a complex quenching mech- anism.

30 / - : CALCULATED CURVE

10 /' : EXPERIMENTAL POINTS

Figure 2. Plot of fo,4ff vs [DBS] in acetonitrile. Figure 4. Plot of +& vs [DBSI-' in acetonitrile.

284 P. K. DAS el al.

DISCUSSION

Kinetic scheme

The oxiranes studied and DBS are efficient quenchers of the excited states of NTMB. A triplet energy transfer mechanism for quenching can easily be ruled out as triplet sensitizers having triplet energies in the range 264 kJ/mol (63 kcal mol-'), near that for NTMB, are ineffective in causing either the isomerization or cleavage reactions. A reaction mechanism involving attachment of the photo- ionized free electron is ruled out as the data clearly demonstrate the involvement of the excited states of NTMB. No evidence for ground state charge- transfer interactions between the sensitizer and any of the substrates could be found from examination of the UV spectra of mixtures of these compounds. The reduction potential for DBS (4) has been determined to be -0.23 V. Since the oxidation potential for NTMB is known (Zweig et al., 1967), the free energy changes for quenching of the singlet and triplet states of NTMB can be calculated for this quencher using the Weller equation (Eq. 1). Thevaluesof AG which are calculated for singlet and triplet quenching are -280 kJ/mol (-67 kcal mol-') and -197 kJ/mol (-47 kcal mol-I), respectively. The oxiranes, 1, which possess cyan0 groups are found to quench sensitizer singlets at a rate close to that for diffusion controlled processes (2 x 10'" M-' s-I). The oxiranes, 2, which are poorer electron acceptors than 1, are less effective quenchers. Thus, the value of k, appears to decrease as the electron donating ability of the phenyl substituents increase. Similar trends are noted in the quenching of triplet sensitizers by oxiranes. These observations are completely consis- tent with previous proposals and conclusions (Albini and Arnold, 1978). We will, therefore, restrict our attention to certain aspects of the electron transfer mechanism.

'NTMB NTMB + hv (kf) (4)

'NTMB 4 'NTMB (ktsc) (6)

' N T M B A N T M B (k*) (8)

[Q ----NTMB]* ,* products (a,P) (9)

Q + "TMB-)[Q---NTMB]* (kql) (5)

Q + 'NTMB- [Q --- NTMB]' (kq) (7)

Equation 9 represents several steps which eventuate in the formation of products from the species [Q --- NTMB]*. For DBS cleavage, the limiting effi- ciencies of DPE production from quenching of

'NTMB and 'NTMB are defined as a and p. respectively. If we assume that quenching of 'NTMB does not lead to 'NTMB, then according to the above mechanism, we can write an expression for the total quantum yield for DPE production (Eqs. 10-12).

61 = (*) k, [DBS] + k, a

63 = (k, + k:iDBS] ) ( - 9 = ) P k, [DBS] + k,

( 12)

Since all of the rate constants in Eqs. 10-12 are known, we may fit calculated values of +DpE to those determined experimentally, and thus determine values for a and p. The solid line in Fig. 4 represents ~ D P E which has been calculated using values of a = 0.04 and p = 0.38. The date fit the measured +;g well within experimental error.

Mechanism

The electron transfer processes 5 and 7 may result in formation of a solvent caged ion-pair or charge- transfer complex, or free solvated ions (See Scheme). In polar solvents such as acetonitrile, the fomation of free ions is common; however, in this case no evidence for an increased yield of NTMB' when quencher is added, can be found. Thus, we can assume that the reactions occur within a charge- transfer complex, and that the free radical anions of the quenchers need not be formed. The efficiencies of the singlet and triplet sensitization processes (A and B) are considerably less than unity. Back electron transfer within the singlet exciplex or singlet ion pair may lead to ground state singlet species in competition with cleavage. However, back electron transfer from the triplet exciplex or ion pair must lead to a triplet state of a product, if spin is to be conserved. Thus, it is expected that cleavage will be more efficient from the triplet exciplex or triplet ion pair, i.e. p is expected to be larger than a. Experi- mentally, p is ca. 38% and a is ca. 4% in agreement with expectation. In the case of DBS, the equations for DPE formation may be written

6+ 6 - NTMB* + DBS --* [NTMB .... DBS] (13)

6+ 8- [NTMB --- DBS] + NTMB + PhbH2 + h02CH2Ph

(14)

PhCHzSOz + PheH2 + SO1

2 PhCH2 + PhCH2CH2Ph

Photosensitization via charge transfer. 285

The NTMB photosensitized chemistry of the oxirane and sulfone systems investigated in this study are “chemically clean” with respect to product formation. The system is highly reversible in the case of NTMB and sulfones since M NTMB is capable of converting >lo-’ M DBS to product. The conclusions drawn from the present investiga- tion are consistent with those from previous work on related systems (Albini and Arnold, 1978). Our results provide additional information concerning the spin states of the sensitizer and the efficiencies with which these states transform the substrate to product. In the absence of direct evidence for the involve-

ment of ions, the detailed nature of the photosensi- tization remains somewhat ambiguous. Neverthe- less, the concept of electron transfer will serve as a valuable guide for the interpretation of such pro- cesses and for an understanding of structure- reactivity and structure product relations.

Acknowledgements-The authors at Columbia University thank the Air Force Office of Scientific Research and the IBM Corporation for their generous support of this research. The research at Notre Dame was supported by the Office of Basic Energy Sciences, U.S. Department of Energy. This is document No. NDRL 2435 from the Notre Dame Radiation Laboratory. Support for this work at UNO was obtained from the National Science Foundation (Grant CHE-78-0066 15).

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