Single crystal molecular structure and solution photophysical properties are reported for 1,3-diphenylisobenzofuran (1), of interest its a model compound in studies of singlet fission. For the ground state of I and of its radical cation (1(+center dot)) and anion (1(-center dot)), we report the UV-visible absorption spectra, and for neutral 1, also the magnetic circular dichroism (MCD) and the decomposition of the absorption spectrum into purely polarized components, deduced from fluorescence polarization. These results were used to identify a series of singlet excited states. For the first excited singlet and triplet states of 1, the transient visible absorption spectra, S-1 -> S-x and sensitized T-1 -> T-x and single exponential lifetimes, tau(F) = similar to 5.3 ns and tau(T) = similar to 200 mu s, are reported. The spectra and lifetimes of S-1 -> S-0 fluorescence and sensitized T-1 -> T-x absorption of I were obtained in a series of solvents, as was the fluorescence quantum yield, Phi(F) = 0.95-0.99, No phosphorescence has been detected. The first triplet excitation energy of solid I (11 400 cm(-1)) was obtained by electron energy loss spectroscopy, in agreement with previously reported solution values. The fluorescence excitation spectrum suggests all onset of a nonradiative channel at similar to 37 000 cm(-1). Excitation energies and relative transition intensities are in agreement with those of ab initio (CC2) calculations after all empirical 3000 cm(-1) adjustment of the initial state energy to Correct differentially for a better quality description of the initial relative to the terminal suite of in absorption transition. The interpretation of the MCD spectrum used the semiempirical PPP method, whose results for the S-1 -> S-x spectrum require no empirical adjustment and are otherwise nearly identical with the CC2 results in all respects including the detailed nature of the electronic excitation. The ground state geometry of I Was also Calculated by the MP2, B3LYP, and CAS methods. The calculations provided a prediction of changes of molecular geometry upon excitation or ionization and permitted all interpretation of the spectra in tern-is of molecular orbitals involved. Computations suggest that 1 call exist as two nearly isoenergetic conformers of C-2 or C-x symmetry. Linear dichroism measurements in stretched polyethylene provide evidence for their existence and show that they orient to different degrees, permitting a separation of their spectra in the region Of the purely polarized first absorption band. Their excitation energies are nearly identical, but the Franck-Condon envelopes of their first transition differ to a surprising degree.