Modeling Infrared Spectral Dynamics upon Symmetry Breaking of a Photo-Excited Quadrupolar Dye

Abstract

A significant number of quadrupolar dyes with a D-π-A-π-D or A-π-D-π-A structure, where D and A are electron donor and acceptor groups, were shown to undergo symmetry breaking (SB) upon optical excitation. During this process, the electronic excitation, originally distributed evenly over the molecule, concentrates on one D-π-A branch, and the molecule becomes dipolar. This process can be monitored by time resolved infra-red (TRIR) spectroscopy and causes significant spectral dynamics. A theoretical model of excited-state SB developed earlier (Ivanov, A. I. J. Phys. Chem. C, 2018, 122, 29165-29172) is extended to account for the temporal changes taking place in the IR spectrum upon SB. This model can reproduce the IR spectral dynamics observed in the −C ≡ C− stretching region with a D-π-A-π-D dye in two polar solvents using a single set of molecular parameters. This approach allows estimating the degree of asymmetry of the excited state in different solvents as well as its change during SB. Additionally, the relative contribution of the different mechanisms responsible for the splitting of the symmetric and antisymmetric −C ≡ C− stretching bands, which are both IR active upon SB, can be determined.

Change of Quadrupole Moment upon Excitation and Symmetry Breaking in Multibranched Donor‐Acceptor Dyes

“…The change of band ratio associated with a 5 ps time constant reflects an increase of the extent of SB, most probably due to diffusive solvent relaxation. As already observed with other dyes, [16,17,38] and in agreement with a recent theoretical modelling of this phenomenon, [42] the initial stage of ES-SB is due to inertial solvent motion that occurs within a few hundreds of fs, [61] and is thus too fast to be resolved here.…”

“…[39] This suggests that the instantaneous asymmetric arrangement of the solvent molecules around the dye, i. e. the asymmetry of the solvent field, and the ensuing solvent relaxation play crucial roles in ES-SB, as confirmed by recent theoretical modelling of this phenomenon. [40][41][42] Depending on the dye, non-dipolar but quadrupolar solvents were found to generate sufficient asymmetry in the local field to induce ES-SB. [19] Moreover, non-specific solute-solvent interactions, such as hydrogen-and halogen-bonding, were shown to significantly enhance the loss of symmetry brought about by dipolar interactions.…”

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor–acceptor branch length

“…Over the last few years, the dynamics of ES-SB as well as the factors that influence this process have been intensively studied both experimentally, and theoretically. [33][34][35][36][37][38] Theoretical investigations by Terenziani and Painelli suggested that ES-SB is triggered by solvent and structural fluctuations. 13,33,34 Experimental studies revealed that the dynamics of ES-SB matches that of solvent motion.…”

Влияние Асимметричного Переноса Зарядана ИК Спектры Возбужденных Квадрупольных Молекул

“…В исследуемой системе нарушение симметрии протекало достаточно медленно и удалось зарегистрировать эволюцию спектров характеристических колебаний. С течением времени нарастало значение параметра асимметрии, что и приводило к наблюдаемым изменениям спектра [19]. Таким образом, экспериментальные данные свидетельствуют о том, что в молекуле с −C≡Nгруппами параметры α и β одного знака, тогда как в молекуле с −C≡C-группами они разного знака.…”

“…Далее была разработана теория, связывающая форму ИК спектров с параметром асимметрии [17]. Эта теория хорошо описывает изменение спектров −C≡N-колебаний в растворителях различной полярности [18] и эволюцию спектров поглощения −C≡C-групп с ростом степени асимметрии [19].…”