The photochemistry and photophysics of all-trans-1,4-diindanylidenyl-2-butene, a rigid analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene

all-trans-1,4-Diindanylidenyl-2-butene (ttt-stiff-5-DPH), a torsionally constrained analogue of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH), was synthesized and studied in order to evaluate the role of phenyl-vinyl torsional motions in the photophysical and photochemical responses of the DPH chromophore. Spectroscopic and photoisomerization measurements reveal that the behavior of the rigid DPH analogue is very similar to that of the parent DPH. This similarity is obtained despite the fact that the alkyl substitution from the five-membered rings selectively lowers the energy of the 1 (1)B(u)* state, leading to inversion of the order of the 1 (1)B(u)* and 2 (1)A(g)* energy levels in hydrocarbon solvents. In stiff-5-DPH, as in DPH, an increase in solvent polarity enhances terminal over central bond photoisomerization. Analyses of fluorescence and photoisomerization quantum yields show that, as in DPH, the torsional relaxation channel on the singlet excited state manifold is inefficient, falling far short of accounting for all radiationless decay. Significant (approximately 50 and 80% of all singlet decay in Bz and AN, respectively), photochemically unproductive, radiationless decay channels exist in both molecules. Competing one bond photoisomerizations give the two major photoproducts: tct-stiff-5-DPH and ctt-stiff-5-DPH. They were isolated in pure form and were spectroscopically characterized. Biacetyl-sensitization was used to study the behavior of the stiff-5-DPH triplet state. As in the parent DPH, stiff-5-DPH triplets undergo relatively efficient concentration dependent geometric photoisomerization.

Synthesis and multitopic complex formation of a photochromic bis(crown ether) based on benzobis(thiazole)

2,7-Bis[2-(6,7,9,10,12,13,15,16-octahydro-5,8,11,14,17-pentaoxabenzocyclopentadecen-2-yl)vinyl]-benzo[1,2-d;3,4-d']bisthiazole, 2, with crown ether styryl moieties substituted on a heterocyclic core, was synthesized and its complex forming ability with several metal cations was evaluated in acetonitrile by absorption and fluorescence spectroscopy. The results are compared to those for the analogous ligand possessing a single crown ether styryl moiety. Selective binding of the metal cation at the heterocyclic core of both ligands was observed only for Hg2+. Alkali and alkaline earth cations bind selectively at the crown ether moieties. Stability constants and pure spectra of defined stoichiometry were determined with the use of HYPERQUAD, a least-squares fitting program, and the results were validated in one case by subjecting the titration spectral matrix to singular value decomposition with self-modeling (SVD-SM). The multitopic ligand 2 forms relatively strong 2:2 stoichiometric complexes with K+, among the alkali metal cations, and Ba2+, among the alkaline earth metal cations, and is a promising selective optical sensor for these ions.

Competitive radiative and reactive relaxation channels in the excited state decay of some thio-analogues of EE-distyrylbenzene

The photophysical and photochemical properties of some thio-analogues of symmetrically substituted EE-1,4-distyrylbenzene (linear conjugation) and EE-1,3-distyrylbenzene (crossed conjugation), where thiophene rings replace the side benzene rings or the central benzene ring, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that found for the corresponding hydrocarbons. The photobehaviour markedly depends on the type of conjugation and on the position (2' or 3') of the ethenic bridge with respect to the sulfur atom. The main effect of the heteroatom is an increase in the photoisomerization yield due to a decrease of the torsional barrier in S(1) and to an increase in the S(1)--> T(1) intersystem crossing, which opens the way to isomerization in the triplet manifold. Conformational equilibria, due to restricted rotation around the quasi-single bonds with the ethenic carbons, have also been investigated by selective photoexcitation for the compounds with side 3'-thienyl groups.