Photophysics of Auramine-O: electronic structure calculations and nonadiabatic dynamics simulations

Abstract

Diphenylmethane dyes are very useful photoinduced molecular rotors; however, their photophysical mechanisms are still elusive until now. In this work, we adopted combined static electronic structure calculations (MS-CASPT2//CASSCF) and trajectory-based surface-hopping dynamics simulations (OM2/MRCI) to study the S1 excited-state relaxation mechanism of a representative diphenylmethane dye Auramine-O. On the basis of the optimized S1 minima and the computed emission bands, we have for the first time assigned experimentally proposed three transient states (i.e. S1-LE, S1-I1 or S1-I2, and S1-II). Mechanistically, upon irradiation to the S1 state, the system first relaxes to the locally excited S1 minimum (S1-LE). Starting from this point, there exist two kinds of relaxation paths to S1-II. In the sequential path, the system first evolves into S1-I1 or S1-I2 and then runs into S1-II; in the concerted one, the system, bypassing S1-I1 and S1-I2, directly runs into S1-II. In addition, the system can decay to the S0 state in the vicinity of three S1/S0 conical intersections i.e. S1S0-I1, S1S0-I2, and S1S0-II. In the S1 dynamic simulations, 54% trajectories decay to the S0 state via S1S0-II; the remaining trajectories are de-excited to the S0 state via S1S0-I1 (11%) and S1S0-I2 (35%). Our present theoretical investigation does not support the experimentally proposed S1 excited-state hypothesis that the intramolecular rotation of the two dimethyl groups around the C-N bond is responsible for the rapid decay of the emission band at about 500 nm; instead, it should be heavily interrelated with the rotation of the two dimethylanilino groups. Finally, this work provides important mechanistic insights into similar diphenylmethane dyes.

Semiempirical Quantum-Chemical Orthogonalization-Corrected Methods: Benchmarks of Electronically Excited States

“…The OM2/MRCI method has been used extensively for the optimization of minimum-energy conical intersections (MECIs) in small- and medium-sized organic molecules. − ,,,,,,,− ,,,,,− ,,− These studies have confirmed on a case-by-case basis through comparisons with the results from high-level ab initio methods (e.g., CASPT2 and MRCI) that OM2/MRCI can often be applied successfully to find the relevant MECIs and to compute reasonable structures and energies for them.…”

“…In recent years, the OMx/MRCI methods have been used for the calculation of vertical excitation energies of organic molecules, ,− for exploring the reaction paths of photochemical processes, ,,,,,,,,,− for optimizing excited-state equilibrium geometries and conical intersections (see refs − , , , , , , , − , , , , , − , − ) and for performing photoinduced nonadiabatic dynamics simulations (see refs , − , , , − , − , − …”

Altered relaxation dynamics of excited state reactions by confinement in reverse micelles probed by ultrafast fluorescence up-conversion

“…For example, it is proposed that in some circumstances the ring torsion may be accompanied by changes in the orientation of the dimethylamino group (although recent calculations favour ring torsion as the major pathway). [135][136][137][138] Because of these complications it is best to compare AO dynamics measured in similar media. For consistency and to avoid effects associated with the specific probe we will focus on AO in this section.…”

Photochromic Mechanism of a Bridged Diarylethene: Combined Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

“…The empirical decoherence correction (0.1 in our simulations) proposed by Granucci et al was employed . Further technical details were given in previous publications. − …”