posted on 2016-04-27, 00:00authored byYanli Liu, Javier Cerezo, Giuseppe Mazzeo, Na Lin, Xian Zhao, Giovanna Longhi, Sergio Abbate, Fabrizio Santoro
We present the simulation
of the absorption (ABS), electronic circular
dichroism (ECD), emission (EMI), and circularly polarized luminescence
(CPL) spectra for the weak electronic transition between the ground
(S0) and the lowest excited state (S1) of hexahelicene,
2-methylhexahelicene, 2-bromohexahelicene, and 5-azahexahelicene.
Vibronic contributions have been computed at zero Kelvin and at room
temperature in harmonic approximation including Duschinsky effects
and accounting for both Franck–Condon and Herzberg–Teller
contributions. Our results nicely capture the effects of the different
substituents on the experimental spectra. They also show that HT effects
dominate the shape of ECD and CPL spectra where they even induce changes
of signs; HT effects are also relevant in ABS and EMI, tuning the
relative intensities of the different vibronic bands. HT effects are
the main reason for the differences in the line shapes of ABS and
ECD and of EMI and CPL spectra and for the mirror-symmetry breaking
between ABS and EMI and between ECD and CPL spectra. In order to check
the robustness of our results, given also that few examples of calculations
of vibronic CPL spectra exist, we adopted both adiabatic and vertical
approaches to define the model potential energy surfaces of the (S0) and the (S1) states; moreover we expanded the
electric and magnetic dipole transition moments around both the S0 and S1 equilibrium geometries.