Abstract
The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S2(La, CT) and S1(Lb, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S2 to S1. Polar solvent effects are not found to have a significant influence on this situation. Applying Cs symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.
Highlights
The investigation of intramolecular charge transfer (ICT) in donor−acceptor systems such as substituted benzenes reveals many interesting processes occurring in electronically excited states
In view of the still existing different interpretations and structural models for the explanation of the dual fluorescence of DMABN, the purpose of the present study is to perform a detailed survey of the excited-state mechanism of ICT using multireference methods[51] which can simultaneously describe the energy surfaces of the local excitation (LE) and CT states and their nonadiabatic crossing regions and are capable of full optimization of excited-state geometries and conical intersections
To assess the accuracy of the different multireference approaches used, the two low-lying vertical excitation energies of DMABN were computed at complete active space selfconsistent field (CASSCF), MRCIS, and MRCISD levels of theory using the Atomic natural orbital (ANO)-DZ and ANO-TZ basis sets and compared to the experimental gas phase data (Table 1)
Summary
The investigation of intramolecular charge transfer (ICT) in donor−acceptor systems such as substituted benzenes reveals many interesting processes occurring in electronically excited states. The position and intensity of the red-shifted band show a marked dependence on solvent polarity. This property was interpreted as an indication for a large dipole moment of the emitting state and for its charge-transfer character. In the gas phase, the emission spectrum of DMABN consists of a single local excitation (LE) fluorescence band S1(1Lb), and evidence for a LE → ICT process in DMABN was not found.[2,3] The structural and electronic nature of the red-shifted emission band of DMABN has been explained by means of several excited-state ICT mechanisms
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