Solid-state [2+2] photodimerization and photopolymerization of α,ω-diarylpolyene monomers: effective utilization of noncovalent intermolecular interactions in crystals

[2+2] Photocycloaddition of olefins is a very useful reaction in synthetic organic chemistry to obtain cyclobutane-containing molecules, which are almost inaccessible by other methods. The reaction, when performed in the crystalline state, occurs more efficiently and selectively than in homogeneous solution due to tight and regular molecular arrangement in the crystal state. Despite numerous examples for the solid-state [2+2] photodimerization of monoenes, however, it is still a challenge to prepare not only dimers but also higher oligomers and polymers from conjugated polyenes, which have multiple reactive double bonds in a molecule. In our recent studies of the solid-state photoreactions of α,ω-diarylpolyenes, noncovalent intermolecular interactions in crystals were effectively utilized to prealign molecules in stacking arrangements, suitable for the [2+2] reaction. With appropriate ring-substituents, [2+2] photodimerization and photopolymerization of the polyenes took place, although the degree of polymerization was relatively low. This review will describe the details of these reactions.

Single-crystal-to-single-crystal [2 + 2] photodimerizations: from discovery to design

The purpose of this review is to address fundamentals and applications of single-crystal-to-single-crystal (SCSC) reactions, based on the [2 + 2] photodimerization. An overview of SCSC [2 + 2] photodimerizations, which comprise single- and multi-component solids, will be given. We reveal that materials that exhibit SCSC reactivity can be considered to have been achieved via either discovery or design. We suggest that SCSC [2 + 2] photodimerizations, if generally achieved, could provide new avenues to control bulk physical properties of solid-state materials.

Intramolecular electron transfer in the photochemistry of some nitrophenyldihydropyridines

4-Phenyl-1,4-dihydropyridine-3,5-dicarboxylates contain two pi chromophores separated by an sp3 carbon. The lowest singlet is localized on the dihydropyridine moiety (1PyH2-Ph) and emits a blue fluorescence (with close to unitary efficiency in glass at 77 K). In 3-nitrophenyl derivatives (PyH2-PhNO2, some of which are photolabile drugs) the fluorescence is completely quenched. Reasonably, this is due to intramolecular electron transfer between the close-lying donor and acceptor moieties to give the charge-separated species (PyH2*+-PhNO2*-). In EPA glass at 77 K, back-electron transfer gives the dihydropyridine-localized triplet (3PyH2-PhNO2), which emits a yellow phosphorescence. In solution, deprotonation from the radical cation on the dihydropyridine moiety initiates rearomatization, finally giving Py-PhNO2 with low quantum yield (5 x 10(-4) to 5 x 10(-3), increasing up to 0.013 by irradiation at 254 nm, where direct excitation of the nitrophenyl chromophore contributes). In the presence of triethylamine, the reaction changes to neat reduction of the nitro group. When a tethered alkylamino group is present, oxidative degradation of that moiety occurs, again via an electron-transfer intramolecular process. This has been found with the drug nicardipine, where photodegration is more efficient (phi 0.02 to 0.1). Donor-acceptor dyads of this type, easily available through the Hantzsch synthesis, may be useful for building new photoinduced electron-transfer systems.

Solid state photochemistry of 1,4-dihydropyridines

Photochemical behavior of some 1,4-dihydropyridines has been investigated in the solid state. Whereas upon irradiation of 1,4-dihydropyridines in solution phase, their photo-oxidation and formation of pyridine derivatives have been observed, irradiation of these compounds in the solid state decreases their light sensitivity. In many cases photo-oxidation has been observed only in very low yields.