Merostabilization in radical ions, triplets, and biradicals. 6. The excited state behaviour of para-substituted tetraphenylethylenes

Abstract

This paper i~ t1edicatc.d to Prof. G. Ham~notzd on the occ~asion of his 60th birthday WILLIAM J . LEIGH and DONALD R. ARUOLD. Can. J . Chem. 59. 3061 (1981). The half-wave oxidation and reduction potentials. measured by cyclic voltammetry. of tetraphenylethylene and a series of para-substituted (cyano, methyl. and methoxy) derivatives are found to correlate with o+ and 0 -values respectively. The deviations from these correlations that are observed for one of the derivatives (unsymmetrically substituted) are attributed to merostabilization.The nature of the first excited singlet state of these compounds is investigated by spectroscopic and photochemical means. Quantum yields for the direct c i~ -trans photoisomerization demonstrate that this provides the dominant pathway for decay of the singlet state. The n ,n * triplet energies are estimated from a qualitative investigation of the relative efficiencies of triplet-sensitized cis-trans isomerization as a function of sensitizer triplet energy. On a Ctudie la nature du premier etat singulet excite de ces composes par la photochimie et la spectroscopie. Les rendements quantiques de la photoisomerisation directe cis-trans demontrent le mecanisme principal de la desintegration de l'etat singulet. On a evalue les energies du triplet n.x* a partir d'une recherche qualitative sur I'efficacite relative de I'isomerisation c i s -t r a~~s sen~ibilisee par un triplet en fonction de I'energie du triplet sensibilisateur.[Traduit par le journal]

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems

“…Rate constants for energy transfer in all steps of this process were found to be the same within experimental error, and slightly larger than rates previously reported in other systems. This suggests the latter are not fully diffusion controlled, and other factors such as Franck-Condon 55 factors, triplet state energies and steric considerations need to be taken into account. Additionally, we have demonstrated electron transfer in a termolecular system using azulene to transfer the electron hole between an immobile electron acceptor and electron donor.…”

“…There is significantly more reflectance change at longer wavelengths, but the signal to noise ratio in the transient decays was found to be worse in this region. 55 The kinetics of the formation of the β-carotene radical were measured as a function of azulene loading. In all cases, the loadings of anthracene-9-carboxylic acid and β-carotene were kept constant at 1.0 and 0.25 mmol g -1 respectively.…”

“…In the presence of 9-cyanoanthracene, the transient spectrum (Fig. 6) shows an additional broad absorption from the excited triplet state of this 55 species, with absorption bands at 430 and 460 nm 51 , indicating energy transfer has occurred from benzophenone to naphthalene to the lower lying 9-cyanoanthracene triplet state. Direct production of the latter species is negligible 54 .…”

“…In this case we expect that azulene will be the only mobile species on the silica 50 gel surface, and in electron transfer from such a large molecule as β-carotene one might expect that any steric influence to be considerably greater than that observed in the systems referred to above. It should be noted that whilst the β-carotene is relatively large, its size (estimated at 2.8nm using HyperChem) does not 55 preclude access to the porous structure of the silica gel. Adsorption to the surface is via London (dispersive) forces or, in the case of for example anthracene-9-carboxylic acid, the considerably stronger hydrogen bonding interactions.…”

“…In all three cases the bimolecular rate constant for energy 50 transfer to and from naphthalene is the same within experimental error, indicating that the process is dominated by the rapid mobility of naphthalene and its excited triplet state. An advantage of a termolecular system is being able to measure and independently verify the same energy transfer step via different 55 excited states as in the case here with naphthalene triplet state decay and 9-cyanoanthracene formation.…”