<p>Relative energies of the ground state isomers of 1,4-diphenyl-1,3-butadiene (DPB) are determined from the temperature dependence of equilibrium isomer compositions obtained with the use of diphenyl diselenide as catalyst. Temperature and concentration effects on photostationary states and isomerization quantum yields with biacetyl or fluorenone as triplet sensitizers with or without the presence of O<sub>2</sub>, lead to significant modification of the proposed DPB triplet potential energy surface. Quantum yields for <em>ct</em>-DPB formation from <em>tt</em>-DPB increase with [<em>tt</em>-DPB] revealing a quantum chain process in the <em>tt</em> → <em>ct</em> direction, as had been observed for the <em>ct</em> → <em>tt</em> direction, and suggesting an energy minimum at the <sup>3</sup><em>ct</em>* geometry. They confirm the presence of planar and twisted isomeric triplets in equilibrium (<em>K</em>), with energy transfer from planar or quasi-planar geometries (quantum chain events from <em>tt</em> and <em>ct</em> triplets) and unimolecular decay (<em>k</em><sub>d</sub>) from twisted geometries. Starting from <em>cc</em>-DPB, <em>ϕ</em><sub><em>cc</em>→<em>tt</em></sub> increases with increasing [<em>cc</em>-DPB] whereas <em>ϕ</em><sub><em>cc</em>→<em>ct</em></sub> is relatively insensitive to concentration changes. The concentration and temperature dependencies of the decay rate constants of DPB triplets incyclohexane are consistent with the mechanism deduced from the photoisomerization quantum yields. The experimental Δ<em>H</em> between <sup>3</sup><em>tt</em>-DPB* and <sup>3</sup><em>tp</em>-DPB*, 2.7 kcal mol<sup>−1</sup>, is compared with the calculated energy difference [DFT with B3LYP/6-31+G(d,p) basis set]. Use of the calculated Δ<em>S</em> = 4.04 eu between the two triplets gives <em>k</em><sub>d</sub> = (2.4–6.4) × 10<sup>7</sup> s<sup>−1</sup>, close to 1.70 × 10<sup>7</sup> s<sup>−1</sup>, the value for twisted stilbene triplet decay. Experimental and calculated relative energies of DPB isomers on the ground and triplet state surfaces agree and theory is relied upon to deduce structural characteristics of the equilibrated conformers in the DPB triplet state.</p>
J. Saltiel, Dmitrenko, O., Dr. Zeena S. Pillai, Klima, R., Wang, S., Wharton, J. T., Huang, Z. - N., de Burgt, van, and Arranz, J., “Triplet and Ground State Potential Energy Surfaces of 1,4-Diphenyl-1,3-butadiene: Theory and Experiment”, Photochem. Photobiol. Sci., 2008.