Effect of the diradical character on static polarizabilities and two-photon absorption cross-sections: A closer look with spin-flip equation-of-motion coupled-cluster singles and doubles method
We present static polarizabilities and 2PA cross-sections for the low-lying electronic states of prototypical diradicals such as benzynes and analogues of m-xylylene and p-quinodimethane computed with the spin-flip equation-of-motion coupled-cluster singles and doubles (EOM-SF-CCSD) method. The static polarizabilities were calculated as analytic second derivatives of the EOM energies and the 2PA cross-sections were calculated using the expectation-value approach. We explain the trends in the nonlinear responses of the SF target states by constructing few-states models based on truncated sum-over-states expressions for these nonlinear properties. By using a Huckel-type treatment of the frontier molecular orbitals that host the two unpaired electrons, we rationalize the trends in the dipole interactions between the SF target states relevant in the few-states models. We demonstrate the correlation between the nonlinear responses of these electronic states in these diradicals and the diradical character.