Charge-transfer excitation of electron donor-acceptor complexes. Direct observation of ion pairs by time-resolved (picosecond) spectroscopy

Abstract

Electron donor-acceptor (EDA) complexes have been observed experimentally3 and have been proposed as intermediates in many types of chemical reactions.4 56Among the earliest and most important examples are the 1:1 EDA complexes of various dienes and dienophiles involved in the Diels-Alder reaction,5,6 Indeed, theoretical studies have delineated the importance of chargetransfer (CT) interactions in thermal [4 + 2] as well as [2 + 2] cycloadditions.7 Experimental support for such formulations is found in the observation of a direct relationship between the second-order rate constant (log k) for the Diels-Alder cycloaddition and the CT transition energy {hvcf) for the EDA complexes of various anthracenes with tetracyanoethylene.8 9"10The EDA complex consisting of either 9-cyanoanthracene (CNA) or indene (IN) with tetracyanoethylene (TCNE) is ideally (1) Bell Laboratories.

Photochemical Perfluoroalkylation with Pyridine N -Oxides: Mechanistic Insights and Performance on a Kilogram Scale

“…82 Although intramolecular donor-acceptor dyads have been used as photosensitizers, 83 literature precedence suggests that the photoexcited state of the intermolecular EDA complexes in question are extremely short lived, to the point where productive intermolecular collisions with the photoexcited state are not likely to occur. Lorance et al 84 have studied the rates of back-electron transfer and fragmentation for a series of reduced N -methoxypyridinium analogs and report impressively fast rates for each of these processes.…”

Electron transfer from anthracenes. Comparison of photoionization, charge-transfer excitation and electrochemical oxidation

“…Thus a slope of only 0.6 in eq. [7] suggests that w actually decreases in proportion to the donor property of the anthracene. Such a trend could arise from increasing steric hindrance at the meso positions attendant upon disubstitution required for better donor properties.…”

“…A more convenient, although less direct, method is based on the charge transfer absorption energy hvCT for electron donor-acceptor or EDA complexes (6). For a series of aromatic hydrocarbons with an electron acceptor (A) in common, the charge transfer excitation of the EDA complex leads to the ion pair (7), i.e., According to the Mulliken theory (8), the value of hvCT for weak complexes is directly proportional to the first vertical I To whom all correspondence should be addressed ionization energy E l (or equivalently the ionization potential Ip) of ArH according to the relationship where EA is the electron affinity of the common acceptor. The term w is to first order the electrostatic energy of the ion pair formed in eq.…”

Charge-transfer excitation: unconventional yet practical means for controlling stereoselectivity in asymmetric photoreactions

“…The photophysical processes of such reactive ECTs have been studied by time-resolved (picosecond) spectroscopies for a number of C-T systems. 18,19 Upon selective photoexcitation at the C-T band, ECT immediately (r0.1 ns) undergoes intra-complex electron transfer to give a singlet RIP. A recent femtosecond stimulated Raman spectroscopic study on the photoinduced charge-separation dynamics of a C-T complex of pyromellitic dianhydride and hexamethylbenezene showed that the structural relaxation is facilitated by solvation dynamics during the charge separation process and that the ECT already possesses the significant charge-separated character and spontaneously relaxes.…”