Singlet fission is a process in which one singlet exciton is converted to two triplets. By using transient absorption and time-resolved emission spectroscopy, recent experimental study (J. Am. Chem. Soc. 2017, 140, 814) investigated how different crystal packing of perylenediimide (PDI) molecules modulates their singlet fission rates and yields. It was observed that the rates vary between 0.33 and 4.3 ns–1. By employing a simple three-state kinetic model and restricted active-space configuration interaction method with double spin-flip, we study the electronic factors (excitation energies and coupling between relevant states) responsible for the variation of singlet fission rates in these PDI derivatives. Our approach reproduces the trends in singlet fission rates and provides explanations for the experimental findings. Our analysis reveals that the electronic energies and the coupling play significant roles in controlling the singlet fission rates. The wave function analysis of the adiabatic electronic states shows that in many model PDI structures, the multiexciton character is spread over several states, in contrast to previously studied systems. This different nature of the multiexciton state poses interesting mechanistic questions. By mapping the relation between the stacking geometries of PDIs and the rates of the singlet multiexciton formation and the binding energies, we suggest favorable PDI structures that should not lead to exciton trapping.

Ultrafast photochemical reactions are initiated by vibronic transitions from the reactant ground state to the excited potential energy surface, directly populating excited-state vibrational modes. The primary photochemical reaction of vision, the isomerization of retinal in the protein rhodopsin, is known to be a vibrationally coherent reaction, but the Franck-Condon factors responsible for initiating the process have been difficult to resolve with conventional time-resolved spectroscopies. Here we employ experimental and theoretical 2D photon echo spectroscopy to directly resolve for the first time the Franck-Condon factors that initiate isomerization on the excited potential energy surface and track the reaction dynamics. The spectral dynamics reveal vibrationally coherent isomerization occurring on the fastest possible time scale, that of a single period of the local torsional reaction coordinate. We successfully model this process as coherent wavepacket motion through a conical intersection on a ∼30 fs time scale, confirming the reaction coordinate as a local torsional coordinate with a frequency of ∼570 cm. As a result of spectral features being spread out along two frequency coordinates, we unambiguously assign reactant and product states following passage through the conical intersection, which reveal the key vibronic transitions that initiate the vibrationally coherent photochemistry of vision.

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In recent years the potential energy surfaces of the penta-2,4-dieniminium cation have been investigated using several electronic structure methods. The resulting pool of geometrical, electronic and energy data, provides a suitable basis for the construction of a topographically correct analytical model of the molecule force field and, therefore, for a better understanding of this class of molecules, which includes the chromophore of visual pigments. In the present contribution, we report the construction of such a model for regions of the force field that drive the photochemical and thermal isomerization of the central double bound of the cation. While previous models included only two modes, it is here shown that the proposed three-mode model, and corresponding set of parameters, are able to reproduced the complex topographical and electronic structure features seen in electronic correlated data obtain at the XMCQDPT2// CASSCF/6-31G* level of theory.