S1-S2 Conical intersection and ultrafast S2→S1 Internal conversion in pyrazine

Theoretical and experimental determination of the absorption and emission spectra of a prototypical indolenine-based squaraine dye

“…48,49 In such cases a quantitative agreement can only be obtained by using a more sophisticated model including the non-adiabatic coupling between the two electronic states. 5,70 The assignment of the electronic transitions in the high energy region of the absorption spectrum poses several problems. According to TD-DFT results a significant number of transitions fall in that spectral region though only a few of them have a significant oscillator strength.…”

Dynamics of radiationless transitions in large molecular systems: A Franck–Condon-based method accounting for displacements and rotations of all the normal coordinates

“…Indeed, that problem has attracted considerable interest over the past years and therefore experimental and theoretical data are abundant in the literature. [37][38][39][40][41][42][43][44][45][46][47] The absorption spectrum of pyrazine exhibits a well structured band for the S 0 → S 1 (π, π * ) transition and a broad, less structured band for the S 0 → S 2 (n, π * ) one. 37,38 The most significant feature of the S 1 absorption band is the presence of an anharmonic progression of the b 1g mode (ν 10a ); since the molecular symmetry (D 2h ) is retained upon transition to S 1 , 43,46,48 odd transitions of that progression are not allowed in the Franck-Condon approximation (strong frequency variation could be responsible for even transitions), so that other mechanisms must be invoked.…”

“…49 Theoretical computations have confirmed the existence of a conical intersection between the two excited states, 42 moreover, quantum dynamic computations based on the vibronic coupling mechanism were able to reproduce the shape of the S 0 → S 2 absorption band. 41,[50][51][52] In order to apply the procedure outlined in the previous section to the dynamics of the S 2 → S 1 internal conversion, the equilibrium geometries, the harmonic frequencies and the normal modes of the ground state and of the two crossing excited states are necessary. We will adopt diabatic electronic states, which we assume to be harmonic.…”

“…Thus, those parameters, and, as a first approximation, vibrational frequencies and normal modes of vibration have been computed at MCSCF level, see infra for details. All results are summarized in table 1 and table 2. insert table 1 insert table 2 As concerns the coupling between S 2 and S 1 , in D 2h point group Q 10a is the only mode with the right symmetry (b 1g ) for linearly coupling S 2 and S 1 , thus, following previous work 39, 41,50 we have taken a linear expansion over Q 10a :…”

Efficient on-the-fly ab initio semiclassical method for computing time-resolved nonadiabatic electronic spectra with surface hopping or Ehrenfest dynamics

“…The pyrazine molecule exhibits a conical intersection between the first (S 1 ) and second (S 2 ) bright excited states, which influences the dynamics after excitation and the resulting vibronic spectra. Because the pyrazine molecule provides an almost ideal benchmark, its vibronic spectra have been extensively studied both experimentally [37][38][39][40][41][42][43][44][45][46] and theoretically, 6,25,26,31,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] giving us an opportunity not only to assess the accuracy of the MSDR in relation to experiment but also to compare it with exact quantum calculations on simplified models developed by others. In addition, due to efficiency of the MSDR, we were able to go beyond simplified model systems and to compute the absorption spectrum using the ab initio electronic structure computed on the fly in all 24 dimensions.…”