ИСТИНА

As is well known, substitution can result in shifting of the frontier levels in conjugated systems. Recently, it has been demonstrated that the magnitude of such shifts in fluorinated stilbenes varies for different pi-orbitals, and the effect can be strong enough to cause reordering of the excited states [1]. Unfortunately, it turns out that the widely used TDDFT methodology can be unreliable even in simple systems like stilbene where the energy of the lowest excited states unevenly varies with the fraction of exact exchange in the exchange-correlation functional [1,2]. Correct description of the manifolds of the lowest excited states thus critically depends on the choice of the functional that can depend on the system in question. In the present work, we consider biphenyl and its fluorinated derivatives as another important model system for investigation of the effects of fluorination on the excited states. The singlet excited states of the biphenyls were studied by means of the XMCQDPT2 quasi-degenerate perturbation theory [3] taken as a reference for the TDDFT calculations with a series of exchange-correlation functionals. We also compare the computational data against the results of the femtosecond and stationary spectroscopy. We report the XMCQDPT2 and TDDFT data for the vertical excitations and the stationary points of the lowest singlet excited states in biphenyls as they change with the fluorination pattern. Unlike in stilbenes, the TDDFT predictions for the relative positions of the excited states depend only moderately on the amount of exact exchange. However, they generally disagree with the XMCQDPT2 results, and the absolute TDDFT values still vary considerably. Yet TDDFT reproduces some important qualitative effects such as spontaneous polarization in the S1 state due to the interaction of a quasi-degenerate pair of excited states of different symmetry. Another important effect consists in an out-of-plane deformation in the perfluorinated biphenyl that additionally enhances polarity of the S1 state. This work was supported by the Russian Science Foundation (RSF-18-13-00337). The calculations were carried out using the equipment of the shared research facilities of HPC computing resources at the Lomonosov Moscow State University. [1] I.N. Ioffe et al., J. Am. Chem. Soc., 2017, 139, 15265-15274. [2] C. Angeli et al., J. Chem. Phys., 2009, 130, art. no. 174307. [3] A.A. Granovsky, J. Chem. Phys., 2011, 134, art. no. 214113.