Characterization of the indacene S0/S1 conical intersection: An MMVB and CASSCF study

Abstract

A conical intersection between the So and SI states of s-indacene has been located with the molecular mechanics with valence bond (MMVB) and complete active space SCF (CASSCF) methods. In both cases the intersection minimum is close in energy and geometry to the overall minimum on the S1 surface, and can be reached directly along the coordinate which leads from the Franck-Condon geometry to the minimum on S1. The existence of such an accessible SI/So surface crossing leading to efficient non-radiative decay can explain the short S, excited state lifetime and lack of fluorescence for s-indacene. Based on a comparison with X-ray structures for a derivative of s-indacene, we argue that the CASSCF ordering of So and SI states is the correct one, whereas MMVB inverts the two states. However, because the energy differences involved are a few kcal mol-', a definitive prediction will probably require a multireference correlated level of theory with gradient optimization.

Unusually short excited state lifetimes of indenofluorene and fluorenofluorene derivatives result from a conical intersection

“…Another plausible mechanism for the lack of emission is internal conversion through a conical intersection between the ground state, S 0 , and the singlet first excited state, S 1 . Such a mechanism is the cause for efficient internal conversion in 4, a substructure of 2 [21].…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Internal conversion is the dominant relaxation process in these cases. The S 1 → S 0 internal conversion has been further characterized by the occurrence of a S 1 /S 0 conical intersection close in energy and geometry to the S 1 minimum [219]. Due to this an efficient non radiative decay channel is promoted thus leading to a short S 1 lifetime and lack of S 1 → S 0 fluorescence.…”

CASSCF calculations for photoinduced processes in large molecules: Choosing when to use the RASSCF, ONIOM and MMVB approximations

“…(As discussed in Section 3.3.1, MMVB cannot properly describe ionic excited states, and pyracylene has several). Nonetheless, MMVB has proved to be a very useful tool for generating initial geometries for CASSCF geometry optimizations 20,63,106,113 , and an extensive CASSCF study of pyracylene only becomes feasible for us when using MMVB geometries as an initial guess. Although we will discuss some of the MMVB work on pyracylene here, we will focus on the CASSCF and RASSCF calculations.…”

“…indacene 63 , c.f. [60][61][62] ) and an active space that cannot represent the chosen states in an unbiased way over the required range of geometries.…”

“…[57][58][59] An alternative approach would be to truncate the active space, although the results of this for a π system can be unpredictable, due to instabilities [60][61][62] . For example, trying to represent a 12-electron π system with only 6 active π orbitals led to symmetry breaking for indacene 63 .…”