Research paperTheoretical study on photoexcitation dynamics of a bis-diimine Cu(I) complex in solutions
Graphical abstract
Introduction
Bis-diimine copper(I) complexes have been attracting much interest as inexpensive optical materials [1], [2], [3], [4]. Therefore, the fundamental properties of the Cu(I) complexes have been extensively studied both experimentally and theoretically [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. In the ground state, the two ligands of the Cu(I) complex are perpendicular to each other. Upon the metal-to-ligand charge transfer (MLCT) excitation, structural change to square-planer structure is induced by the pseudo-Jahn-Teller effect (Fig. 1). Time-resolved spectroscopic techniques revealed that the timescales of this flattening motion upon the MLCT excitation are several hundred femtoseconds in solutions, and strongly affected by substituents of ligands. For example, the flattening timescales of [Cu(phen)2]+ (phen = 1,10-phenanthroline) and [Cu(dmphen)2]+ (dmphen = 2,9-dimethyl-1,10-phenanthroline) upon the S0 → S1 excitation were found to be ∼200 fs and ∼800 fs in dichloromethane, respectively [12], [15], [16]. The coherent vibrational motions associated with the photoreaction were also observed by the pump-probe spectroscopy.
These results indicate a complicated character of multidimensional potential energy surfaces (PESs) of the Cu(I) complex. Furthermore, several previous studies showed that the photoexcitation dynamics of the Cu(I) complex in solution is also affected by surrounding solvent molecules [1], [2], [5], [8], [10], [11], [15]. For example, the flattening rates in acetonitrile are faster than those in dichloromethane [10], [15]. In addition, it was demonstrated that vibrational coherence associated with an excited-state reaction in solution is much different from that in the gas phase [20]. Therefore, the multidimensional PESs of the Cu(I) complex including solvent molecules should be considered to understand the complicated photoexcitation dynamics.
In this letter, we theoretically investigate the photoexcitation dynamics of a simple bis-diimine copper(I) complex with no substituents, [Cu(phen)2]+, in solutions. Although there are many theoretical studies on the excited-state properties of bis-diimine copper(I) complexes [7], [13], [17], [19], direct molecular dynamics (MD) simulations of the photoreactions in the presence of solvent molecules have not been performed, to the best of our knowledge. We thus carry out nonequilibrium excited-state MD simulations in two solvents, dichloromethane and acetonitrile. Although such simulations require high computational cost in electronic structure calculations, we drastically reduce the computational cost by employing the molecular mechanics with Shepard interpolation correction (MMSIC) method [21]. The MMSIC method enables us to generate an accurate semiglobal PES of a molecule in condensed phases efficiently and run a large number of MD trajectories. To clarify the solvent effects, we also carry out nonequilibrium excited-state MD simulations in the gas phase. From these nonequilibrium excited-state MD simulations, the solvent effects on the photoexcitation dynamics such as the flattening rates and coherent vibrational motions are investigated.
Section snippets
Method
Since details of the MMSIC method are described elsewhere [21], we only describe the method briefly here. The MMSIC method is based on the combined quantum mechanical and molecular mechanical (QM/MM) method. The most time-consuming terms of QM/MM potential energy calculation are those dependent on the QM electronic wave function, Ψ, which is generally expressed as the sum of QM electronic energy and QM-MM electrostatic (ES) interaction energy. When we adopt a site-site representation of the
Results and discussion
We first calculated the S0 and S1 potential energy profiles of [Cu(phen)2]+ in the gas phase along the dihedral angle between two ligands with the DFT and TDDFT method (Fig. 1). In the S0 ground state, the high-symmetry D2d structure with the dihedral angle of 90° is the most stable, consistent with the previous DFT calculations for bis-diimine Cu(I) complexes [6], [7], [12], [13], [19]. On the other hand, in the S1 excited state, the optimized geometry with the dihedral angle of 90° is a C2v
Conclusions
In this Letter, we investigated the photoexcitation dynamics of [Cu(phen)2]+ complex in CH2Cl2 and CH3CN solutions through nonequilibrium excited-state MD simulations using the MMSIC method. The calculated flattening rates and coherent vibration modes in solutions were in good agreement with the experimental results, and much different from the calculated results in the gas phase. The present results reveal that the photoexcitation dynamics is largely affected by surrounding solvent molecules.
Acknowledgement
The authors are grateful to Dr. Takeuchi and Dr. Tahara for valuable discussion. The computations were partly performed at the Research Center for Computational Science, Okazaki. This work was supported by MEXT/JSPS KAKENHI (Grant-in-Aid for Scientific Research) Grant Numbers 26810008, 16H00778, 16KT0165, and 17K05757.
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