Early Events in the Nonadiabatic Relaxation Dynamics of 4-(N,N-Dimethylamino)benzonitrile

Abstract

4-(N,N-Dimethylamino)benzonitrile (DMABN) is the archetypal system for dual fluorescence. Several past studies, both experimental and theoretical, have examined the mechanism of its relaxation in the gas phase following photoexcitation to the S2 state, without converging to a single description. In this contribution, we report first-principles simulations of the early events involved in this process performed using the nonadiabatic trajectory surface hopping (TSH) approach in combination with the ADC(2) electronic structure method. ADC(2) is verified to reproduce the ground- and excited-state structures of DMABN in reasonably close agreement with previous theoretical benchmarks. The TSH simulations predict that internal conversion from the S2 state to the S1 takes place as early as 8.5 fs, on average, after the initial photoexcitation, and with no significant torsion of the dimethylamino group relative to the aromatic ring. As evidenced by supporting EOM-CCSD calculations, the population transfer from S2 to S1 can be attributed to the skeletal deformation modes of the aromatic ring and the stretching of the ring-dimethylamino nitrogen bond. The non- or slightly twisted locally excited structure is the predominant product of the internal conversion, and the twisted intramolecular charge transfer structure is formed through equilibration with the locally excited structure with no change of adiabatic state. These findings point toward a new interpretation of data from previous time-resolved experiments.

Ab Initio Multiple Spawning Photochemical Dynamics of DMABN Using GPUs

“…As expected and in agreement with Ref. 31 , DMABN rapidly and adiabatically visits regions in configuration space close to the S 1 -(T)ICT minimum, i.e., with a twist angle of nearly 90 degrees. However,…”

“…31 , τ 2 could potentially correspond to the time required for the twist angle to reach its equilibrium value after relaxation in S 1 , i.e., a significantly different value from the one observed in the Franck-Condon region. As already mentioned in previous paragraphs, the ultrafast S 2 /S 1 nonadiabatic dynamics produced by AIMS/LR-TDDFT matches the results of recent theoretical studies by Miller et al 31 and Lan et al, 32 as does the smooth and weak population exchange between S 1 and S 2 observed after 125 fs and the weak population of S 3 . The conclusions of the aforementioned studies are that significant DMA twisting is not required for the S 2 /S 1 radiationless transition to take place and that in the early stage of the nonadiabatic dynamics, the pyramidalization coordinate fluctuates around what corresponds to a near-planar geometry.…”

“…or CT. As discussed in Ref. 31 , the initial interpretation of the experimental lifetime is based on the assumption that DMABN requires a twist of the DMA group to reach the S 2 /S 1 crossing region. 29 Fuß et al proposed that an increase in the fragmentation of DMABN should be related to variation of a large amplitude nuclear motion.…”

“…31 branching space does not contain DMA torsion. 110 In addition, the multireference results are consistent with ultrafast S 2 /S 1 decay occurring via mostly planar geometries, as observed in our work and in Ref.…”

“…(C5-C6) reported in recent theoretical work 31 based on TSH dynamics and ADC(2) for the electronic structure. A more extended discussion can be found below.…”