Simulation and Analysis of the Transient Absorption Spectrum of 4-(N,N-Dimethylamino)benzonitrile (DMABN) in Acetonitrile

Abstract

4-( N , N -Dimethylamino)benzonitrile (DMABN) is a well-known model compound for dual fluorescence—in sufficiently polar solvents, it exhibits two distinct fluorescence emission bands. The interpretation of its transient absorption (TA) spectrum in the visible range is the subject of a long-standing controversy. In the present study, we resolve this issue by calculating the TA spectrum on the basis of nonadiabatic molecular dynamics simulations. An unambiguous assignment of spectral signals to specific excited-state structures is achieved by breaking down the calculated spectrum into contributions from twisted and nontwisted molecular geometries. In particular, the much-discussed excited-state absorption band near 1.7 eV (ca. 700 nm) is attributed to the near-planar locally excited (LE) minimum on the S 1 state. On the technical side, our study demonstrates that the second-order approximate coupled cluster singles and doubles (CC2) method can be used successfully to calculate the TA spectra of moderately large organic molecules, provided that the system in question does not approach a crossing between the lowest excited state and the singlet ground state within the time frame of the simulation.

Micro-Solvated DMABN: Excited State Quantum Dynamics and Dual Fluorescence Spectra

“…The decay rates we obtain in our direct dynamics simulations are very similar to those in [ 35 , 36 , 65 ], in which TDDFT was also used for the electronic structure. A very similar decay rate is also observed in ADC2 in the gas phase, water and acetonitrile in [ 33 , 34 , 64 ]. The authors concluded that the solvent environment does not affect the initial internal conversion.…”

“…In a later publication in 2020, Kochman and Durbeej improved the previous setup using QM/MM on DMABN in a water nanodroplet, and observed for the first time the formation of the TICT state at around 600 fs. Very recently, they have applied the CC2 method to compute the transient absorption spectra of DMABN in acetonitrile [ 34 ].…”

Automatized protocol and interface to simulate QM/MM time‐resolved transient absorption at TD‐DFT level with COBRAMM

“…85 However TSH at algebraic diagrammatic construction method showed an almost complete depopulation of S 2 , without back hops to that state. 31 Along our simulation time, we can clearly see the oscillations typical of this behavior in the population profiles, until the S 2 population is 20% of the total one after 100 fs.…”

“…The TA spectrum of DMABN has been studied both experimentally and computationally, 31,74 and helps understanding the ultrafast dynamics of the molecule and the role of the two states involved in the fluorescence of the system. The early spectrum, in the first 100 fs, is composed in the 1-6 eV energy range by two main contributions.…”

“…This approach has been used to successfully reproduce TA spectra using wavefunction methods for the description of the excited states. 9,10,12,31 Recently, Domcke and coworkers reported a TSH compatible protocol within the Condon approximation which accounts for the finite duration and spectral shape of the pump and probe. 9 In this work, we want to introduce a series of tools implemented in the open-source academic software developed in our group, Cobramm is Optimized in Bologna to Run Ab-initio and Molecular-Mechanics calculations (COBRAMM) 32,33 that automatize the whole workflow necessary to simulate a TA spectrum, from building the initial system, through simulating the non-adiabatic dynamics with TSH, until the calculation and convolution of the time-resolved TA spectrum.…”