Photochemistry and Photobiology, 1998, 68(4): 481-486 zyxwvut Salt-induced Charge Separation in Photoinduced Electron Transfer Reactions. The Effect of Ion Size* Viviana N. Grosso, Carlos M. Previtali and Carlos A. Chestat Departamento de Quimica y Fisica, Universidad Nacional de Rio Cuarto, 5800 zyxwv Rio Cuarto, Argentina Received 1 April 1998; accepted 17 July 1998 ABSTRACT The effect of added salts on the efficiency for photoinduced charge separation in two typical electron acceptor (A)/elec- tron donor (D) systems was studied by the technique of laser flash photolysis. We investigate the exciplex-forming pyrenelp-dicyanobeene zyxwvutsrq (Fy/DCB) and pyrenelN,N-dime- thylaniline (Py/DMA) systems in ethyl acetate. The salts se- lected for this study are tetrabutylammonium chloride, tetrahexylammonium chloride, lithium perchlorate, sodium perchlorate, tetrabutylammonium perchlorate, sodium tetraphenylborate and tetrabutylammonium tetraphenyl- borate. In most cases, the salts quench the emission of the exciplexes with rate constants near the ditTusional rate limit in ethyl acetate. An apparent red shift of the fluorescence maximum of the exciplexes with increasing salt concentra- tion is also generally observed. Laser flash photolysis ex- periments showed that in the absence of the salts both zyxwvu A/ D systems yield exclusively the triplet excited state of the polyaromatic. However, in the presence of many of the elec- trolytes studied, induced free radical ion formation zyxwvu is ob- served. The experimental efficiencies for induced charge separation (q) depend on the zyxwvuts A/D system and on the nature of the salt. The measured q values vary between 0 and zyxwvu 0.5. The most striking variation corresponds to the lithium and sodium perchlorates. These salts are almost totally ineffi- cient in quenching the Py/DCB exciplex, while they quench and induce charge separation from the Py/DMA exciplex with a high yield. The effect of the different salts on both exciplexes may be rationalized by using the concept of the softhard character of the interacting ions. INTRODUCTION Photoinduced electron transfer (PET)$ reactions between electron donors (D) and electron acceptors (A) is a topic of *This article is dedicated to the memory of our friend and colleague, Prof. Juan J. Cosa, whose life was an example of integrity and dedication to science. ?To whom correspondence should be addressed at: Departamento de Quimica y Fisica, Universidad Nacional de Rio Cuarto, 5800 Rio Cuarto, Argentina. Fax: 058 zyxwvutsrqp 676233; e-mail: cchesta~exa.unrc.edu.ar $Abbreuiurions: A, electron acceptor; 4-CB, 4-C1-biphenyl; CT, charge transfer; D, electron donor; DCB, p-dicyanobenzene; DMA, N,N-dimethylaniline; PET, photoinduced electron transfer; Py, pyrene; THF, tetrahydrofuran; ZnTPP, zinc tetraphenylpor- phyrin. 0 199X American Society for Photobiology zyxwvutsrqpon 003 1 -8655/98 $S.O0+0.00 current interest from the mechanistic (1) and synthetic point of view (2). High polar media are usually preferred in the synthetic applications of PET reactions because in these media most PET reactions afford free radical ions efficiently. However, it is known that some free radical ions may react quickly with polar solvent molecules (3), suggesting that the utility of PET reactions could be extended if free ions could be generated in low-polarity media. Unfortunately, in nonpolar solvents diffusive separation of radical ions (from contact ratdical ion pair or exciplex intermediates) is prevented by a large Coulombic barrier, making the yield of profitable free radical ions invariably poor. The use of Coulombic effects to favor the separation of chlarged species is an effective approach, as shown by stud- ies of PET reactions within charged D/A reactants (3), mi- cellar media (4) or electrolyte/polyelectrolyte solutions (5- 7). Particularly, the quenching of exciplexes by diverse types of salts has been investigated by several groups (8-12). It is bdieved that electrolytes quench the exciplex emission by electrostatic disruption of its Coulombic binding forces, wlhich may lead to free radical ion formation via an ion- exchange-type mechanism (8,lO). It has also been estab- lished that the quenching mechanism often involves the par- ticipation of a fluorescent intermediate formed by the exci- pl'ex and an undissociated molecule of the salt (9). Recently, we showed the occurrence of an unusually stable fluorescent complex in the quenching of an intramolecular naphthalene/ triethylamine exciplex by tetrabutylammonium perchlorate in tetrahydrofuran (THF) (1 1). However, most of the studies mentioned above were per- formed on one exciplex with a given electrolyte. While the effect of several salts on the exciplex quenching was inves- tigated by other researchers, to our knowledge a comparative study of the effect of changing the type and size of the ions or[ the charge separation quantum efficiency for different ex.cip1exes was not reported in the literature. Here, we report a study of the effect of several salts on th'e quenching and the efficiency of the salt-induced charge separation, in two typical exciplex-forming systems: the py- renelp-dicyanobenzene (Py/DCB) and pyrene/N,N-dimethyl- aniline (Py/DMA) systems in ethyl acetate. In a previous work Kitamura et al. (8) investigated the quenching of the ennission of these two exciplexes by several quaternary zy am- monium salts in chloroform. They found that as the size of the positive ion increases, the quenching efficiency decreas- 481