A weak band at the tail of the known tolane (diphenylacetylene, DPA) fluorescence spectrum in several solvents is assigned to the forbidden 1(1)A(u) → 1(1)A(g) transition on the basis of its lifetime (∼200 ps) and its fluorescence excitation spectra. The 1(1)A(u) state, generally called the dark state, is not truly dark. We report the temperature (T) dependence of DPA fluorescence quantum yields (ϕ(f)) in methylcyclohexane (MCH) solution and the fluorescence and phosphorescence quantum yields of DPA in glassy MCH at 77 K. Significant differences between fluorescence and phosphorescence excitation spectra reveal that, in addition to the 1(1)B(1u) ← 1(1)A(g) transition, the first DPA absorption band includes a transition to another excited state, most probably the 1(1)B(2u) state, from which intersystem crossing is more efficient. The T dependence of ϕ(f) values in MCH solution is shown to be consistent with the previously reported T dependence of the lifetimes of transient DPA singlet excited state absorptions in the picosecond time scale. Transient absorption decay rate constants in hexane, methylcyclohexane and decalin as a function of T are retreated. Application of the medium enhanced barrier model shows that the medium is fully engaged with the molecular motion that is involved in the activated nonradiative decay path of the 1(1)B(1u) state. In accord with theoretical calculations and experimental observations, that process is assigned to the diabatic internal conversion of the short-lived linear fluorescent π,π* (1(1)B(1u)) state, over a low intrinsic energy barrier, to the longer lived weakly fluorescent trans-bent π,σ* (1(1)A(u)), which is the precursor of the DPA triplet state. Absorption and fluorescence measurements in several solvents show that the 1(1)B(1u)-1(1)A(g) energy gap decreases linearly with increasing medium polarizability. Our results allow a more definitive state order assignment for DPA.

The vibronic structure of the fluorescence spectrum of trans-1,2-di(1-methyl-2-naphthyl)ethene (t-1,1) in methylcyclohexane (MCH) solution at room temperature was expected to become better defined upon cooling of the solution to 77 K. Instead, a broad, λexc-dependent fluorescence spectrum was observed in the glassy medium. Vibronically structured t-1,1 fluorescence spectra were obtained in the MCH glass only upon irradiation at the long-λ onset of the absorption spectrum. The application of singular value decomposition with self-modeling on the fluorescence spectral matrices of t-1,1 allowed their resolution into major and minor pairs of vibronically structured spectra that are assigned to two structural modifications of each of two relative orientations of the 1-methyl-2-naphthyl moieties. The difference between the two structures in each pair lies in the direction of rotation of each naphthyl group away from the plane of the olefinic bond. A complex but different conformer distribution is also responsible for the fluorescence spectra of t-1,1 in 5:5:2 (v/v/v) diethyl ether/isopentane/ethyl alcohol (EPA) glass at 77 K. The conformer distributions are also sensitive to the rate of cooling used in glass formation. Conformer distributions based on predicted small energy differences from gas-phase theoretical calculations are of little value when applied to volume-constraining media. The photophysical and photochemical properties of the analogues of the other two conformers of trans-1,2-di(2-naphthyl)ethene, trans-1-(1-methyl-2-naphthyl)-2-(3-methyl-2-naphthyl)ethene (t-1,3) and trans-1,2-di(3-methyl-2-naphthyl)ethene (t-3,3), were determined in solution. However, it is the calculated geometries and energy differences of the t-1,1 conformers [DFT using B3LYP/6-311+G(d,p)] that are essential guides to the interpretation of the experimental results.

Identical fluorescence lifetimes and spectra of cis- and trans-1,2-di(1-methyl-2-naphthyl)ethene reveal an adiabatic cis → trans photoisomerization pathway that accounts for a significant fraction of observed cis → trans photoisomerization quantum yields. The fluorescence quantum yields of both isomers decrease as the solvent is changed from methylcyclohexane to the more polarizable toluene or to the more polar acetonitrile and there is a corresponding increase in the photoisomerization quantum yield in the trans → cis direction. The pronounced solvent dependence of the contribution of the adiabatic pathway to cis → trans photoisomerization--methylcyclohexane (70%), toluene (45%), acetonitrile (31%)--is consistent with the participation of a zwitterionic twisted intermediate, (1)p*, on the singlet excited state surface which is stabilized as the polarizability and/or polarity of the solvent is increased. Solvent stabilization of (1)p* favours the nonadiabatic photoisomerization pathways of both isomers and diminishes the cis → trans adiabatic pathway.

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cis-1,2-Di(1-methyl-2-naphthyl)ethene, c-1,1, undergoes photoisomerization in methylcyclohexane, isopentane and diethyl ether/isopentane/ethanol glasses at 77 K. On 313 nm excitation the fluorescence of c-1,1 is replaced by fluorescence from t-1,1. Singular value decomposition reveals that the spectral matrices behave as two component systems suggesting conversion of a stable c-1,1 conformer to a stable t-1,1 conformer. However, the fluorescence spectra are λexc dependent. Analysis of global spectral matrices shows that c-1,1 is a mixture of two conformers, each of which gives one of four known t-1,1 conformers. The λexc dependence of the c-1,1 fluorescence spectrum is barely discernible. Structure assignments to the resolved fluorescence spectra are based on the principle of least motion and on calculated geometries, energy differences and spectra of the conformers. The relative shift of the c-1,1 conformer spectra is consistent with the shift of the calculated absorption spectra. The calculated structure of the most stable conformer of c-1,1 agrees well with the X-ray crystal structure. Due to large deviations of the naphthyl groups from the ethenic plane in the conformers of both c- and t-1,1 isomers, minimal motion of these bulky substituents accomplishes cis → trans interconversion by rotation about the central bond.