Structural Variations and Multiple Charge Transfer Transitions between Chloranil and Carbazole Derivatives

Asymmetric charge transfer (CT) spectra from combinations of carbazole derivatives and chloranil are shown to consist of two bands originating from the highest (HOMO) and second highest (HOMO 2) energy occupied molecular orbitals of carbazole. Symmetry arguments are used to indicate tha t of two possible (parallel plane) alignments of donor and acceptor, t h a t which is totally symmetric gives rise to only the lower energy C T transition whereas the unsymmetrical alignment which is energetically preferred permits t he two observable C T transitions. Low molecular weight model carbazoles all show the asymmetric C?' bands which have been resolved into two Gaussian components by means of a computer assisted analysis. Significantly, poly(N-vinylcarbazole) (PVCA) gives rise to C T hands much less asymmetric t han corresponding model systems and it is concluded tha t steric interactions in PVCA greatly reduce the possibility for interaction of t he carbazole units with chloranil in the 1:l asymmetric arrangement preferred by the model compounds. As expected from the theoretical argument, poly(N-ethyl-3-vinylcarbazole) and poly(N-ethyl-2-vinylcarbazole), both of which are derived from unsymmetrically substi tuted carbazoles, give with chloranil highly asymmetric C T spectra which are very similar t o those of t he appropriate model compounds. Carbazole and its ring and N-alkylated derivatives like other aromatic amines exhibit long wavelength charge transfer (CT) transitions with acceptors because of relatively high energies for the highest occupied molecular orbitals.' This property is manifest in low ionization potentials,' low oxidation potentials (-1.2 V (SCE)),2 and a very high propensity to oxidative ~ o u p l i n g . ~ Observation of CT transitions when organic electron donors and acceptors are allowed to interact is now a commonplace p h e n ~ m e n o n , ~ it being frequently concluded that these transitions arise from so-called CT complexes without proper regard for the magnitude and nature of the binding forces in such complexes. Carbazoles, no less than other good organic electron donor molecules, readily participate in this type of intermolecular association with electron acceptors, the required close approach of donor and acceptor being favored by the planarity of the carbazole ring system. Particular interest in the formation and properties of CT complexes of carbazole derivatives arise from (mainly two) quite different types of study. Hoeglj was the first to show that poly(N-vinylcarbazole) (PVCA) was a useful organic photoconductor and, more importantly, light absorption and photoconductivity were improved when PVCA was mixed with a variety of organic acceptor molecules. Several years later Ellinger6 snowed that many organic electron acceptors, including chloranil, were useful initiators for the polymerization of monomeric N-vinylcarbazole (NVCA). Related observations were published independently a t about the same time by Scott, Miller, and Labes.i These early disclosures stimulated research studies of the formation and photoelectrical properties of CT complexes of PVCA, leading to a commercial process for electrophotography based on compositions formed from PVCA and 2,4,7-trinitrofluorenone (TNF). In complete contrast, and despite extensive studies by several groups of workers,1° the reactions of NVCA with organic electron acceptors have not yet resulted in developments significant in other than a purely mechanistic sense; a critical survey of the scope and value of such studies has been given recently by Hyde and Ledwith.ll Reactions of NVCA and chloranil have been extensively investigated. Originally it was claimed by Ellinger6 that chloranil was a useful initiator for cationic polymerization of NVCA in toluene, but subsequently other workers12 found that purified chloranil was inactive in this particular system. Rather, it was shown that the strong protonic acid, 1,4-dihydroxy-2,3,5,6-tetrachlorobenzene (the dihydro reduction product of chloranil), thought to be an impurity in chloranil, was the true initiator. In more polar solvents the situation is even more c~nfused . '~ Another original observation by Ellinger6 was that mixtures of NVCA apd chlorani1 in acetone gave rise to formation of the cyclodimer of NVCA when exposed to uv light or strong sunlight. This result has been amply confirmed by subsequent studies14J5 and it is now known that cyclodimerization of NVCA oc834 Marsh e t al. Macromolecules curs by a novel chain reactionI6 in which the photoexcited chloranil (or other suitable oxidant) undergoes an electron transfer reaction with NVCA to produce NVCA cation radicals, which then function as chain carriers. There have been extensive studies of the effects of solvent and other variables on the chloranil sensitized cyclodimerization and polymerization of NVCA.17 For interpretations of photoelectrical properties of complexes of PVCA and of photoinduced and thermal reactions of NVCA with acceptors such as chloranil, the energies and types of CT transitions play a crucial role. I t is surprising therefore that the very obvious asymmetry of CT spectra of carbazole complexes has not been the subject of serious comment. The two highest energy filled molecular orbitals of carbazoles are separated by only approximately 0.5 eV1 and it is a reasonable assumption (vide infra) that the asymmetric nature of CT spectra of carbazole derivatives arises from CT transitions involving the highest and second highest energy occupied molecular orbitals of the carbazole moiety with a common lowest energy empty molecular orbital of the acceptor molecule. Multiple CT transitions, with similar origins, are not uncommon for aromatic donor molecules, with the best known example involving naphthalene and te t ra~yanoethylene .~J~ For CT spectra which are asymmetric it is particularly misleading to treat the experimentally observed vmax as a measure of the energy of the CT transition and some kind of band analysis is necessary before making correlations. Nevertheless the values of v, (or more usually corresponding values of A, , ) have been used in several studies of CT spectra of carbazole derivative^,'^ including PVCA,20-z4 resulting in some erroneous conclusions. I t is the purpose of the present paper to clarify the nature and origins of CT spectra of carbazoles with chloranil and to indicate how conformational preferences (especially in the case of PVCA) affect the relative intensities of available multiple CT transitions producing in consequence a dramatic effect on the wavelength sensitivities of such combinations. Since this work was completed two important publications have appeared relating to multiple CT transitions. The first25 deals with relative intensities of CT transitions in phenylpentamethyldisilane-tetracyanoethylene complexes and is notable for a proper band analysis by a computer assisted resolution of experimental curves into skewed Gaussian components. A second publication by Okamoto et aLZ6 gives information as to the thermodynamics constants of CT complex formation of carbazoles, including PVCA, with a series of acceptor molecules and makes clear the different shapes of CT spectra for common symmetrical acceptor molecules with low molecular weight carbazoles and PVCA, respectively. The more general results reported by Okamoto et aLZ6 are entirely complementary to the specific data reported herein for chloranil containing systems and provide strong support for a theoretical explanation of the origins and intensities of CT transitions of carbazole derivatives. Experimental Section Materials. Poly(N-vinylcarbazole) (PVCA) obtained from BASF (Luvicon) was dissolved in tetrahydrofuran and precipitated with methanol seven times. T h e PVCA was redissolved in benzene and freeze dried. T h e molecular weight was determined by membrane osmometry and intrinsic viscosity, whereas the molecular weight distribution was determined by gelqermeation chromatosraDhv. The aolvmer samDle used had an M , = 2.53 X lo5 and an-MWD = 5.6i. " PolviN-ethvl-2-vinvlcarbazole) (P2VCA) was DreDared anioni. . cally by conventional high-vacuum techniques. Syntheses of the monomer27 and polymerz8 have been described previously. Molec0 4 O 6 n 4