Elsevier

Chemical Physics Letters

Volume 679, 1 July 2017, Pages 60-65
Chemical Physics Letters

Research paper
Theoretical study on photoexcitation dynamics of a bis-diimine Cu(I) complex in solutions

https://doi.org/10.1016/j.cplett.2017.04.082Get rights and content

Highlights

  • Excited-state MD simulations of a Cu(I) complex in solutions were carried out.

  • The calculated dynamics in solutions agree well with the experimental ones.

  • The photoexcitation dynamics is largely affected by solvent.

Abstract

We investigate the photoexcitation dynamics of [Cu(phen)2] + (phen = 1,10-phenanthroline) in solutions by using the molecular mechanics with Shepard interpolation method, which enables us to generate potential energy surfaces of the Cu complex efficiently. The calculated flattening rates and coherent vibration motions are in good agreement with the experimental results. We find that the photoexcitation dynamics in the gas phase is much different from those in solutions, which indicates the importance of the solvent effects on the photoexcitation dynamics.

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 θ 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.

References (35)

  • M.W. Mara et al.

    Interplays of excited state structures and dynamics in copper(I) diimine complexes: Implications and perspectives

    Coord. Chem. Rev.

    (2015)
  • D.V. Scaltrito et al.

    MLCT excited states of cuprous bis-phenanthroline coordination compounds

    Coord. Chem. Rev.

    (2000)
  • D.R. McMillin et al.

    Exciplex quenching of photo-excited copper complexes

    Coord. Chem. Rev.

    (1985)
  • N. Armaroli

    Photoactive mono- and polynuclear Cu(I)-phenanthrolines. A viable alternative to Ru(II)-polypyridines?

    Chem. Soc. Rev.

    (2001)
  • O. Sato

    Optically switchable molecular solids: Photoinduced spin-crossover, photochromism, and photoinduced magnetization

    Acc. Chem. Res.

    (2003)
  • Z.A. Siddique et al.

    Structure-dependent photophysical properties of singlet and triplet metal-to-ligand charge transfer states in copper(I) bis(diimine) compounds

    Inorg. Chem.

    (2003)
  • M.Z. Zgierski

    Cu(i)-2,9-dimethyl-1,10-phenanthroline: Density functional study of the structure, vibrational force-field, and excited electronic states

    J. Chem. Phys.

    (2003)
  • L.X. Chen et al.

    MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations

    J. Am. Chem. Soc.

    (2003)
  • G.B. Shaw et al.

    Ultrafast structural rearrangements in the MLCT excited state for copper(I) bis-phenanthrolines in solution

    J. Am. Chem. Soc.

    (2007)
  • M. Iwamura et al.

    Real-time observation of the photoinduced structural change of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) by femtosecond fluorescence spectroscopy: A realistic potential curve of the Jahn-Teller distortion

    J. Am. Chem. Soc.

    (2007)
  • J.V. Lockard et al.

    Influence of ligand substitution on excited state structural dynamics in Cu(I) bisphenanthroline complexes

    J. Phys. Chem. B

    (2010)
  • M. Iwamura et al.

    Coherent nuclear dynamics in ultrafast photoinduced structural change of bis(diimine)copper(I) complex

    J. Am. Chem. Soc.

    (2011)
  • G. Capano et al.

    A quantum dynamics study of the ultrafast relaxation in a prototypical Cu(I)-phenanthroline

    J. Phys. Chem. A

    (2014)
  • M. Iwamura et al.

    Substituent effect on the photoinduced structural change of Cu(I) complexes observed by femtosecond emission spectroscopy

    Phys. Chem. Chem. Phys.

    (2014)
  • L. Hua et al.

    The substituent effect on the MLCT excited state dynamics of Cu(I) complexes studied by femtosecond time-resolved absorption and observation of coherent nuclear wavepacket motion

    Phys. Chem. Chem. Phys.

    (2015)
  • M. Iwamura et al.

    Ultrafast excited-state dynamics of copper(I) complexes

    Acc. Chem. Res.

    (2015)
  • G. Capano et al.

    Theoretical rationalization of the emission properties of prototypical Cu(I)-phenanthroline complexes

    J. Phys. Chem. A

    (2015)
  • Cited by (13)

    • New luminescent copper(I) complexes with extended π-conjugation

      2018, Polyhedron
      Citation Excerpt :

      This deactivation is due to the strong stabilization of the lowest excited states during flattening favouring non radiative decay by virtue of the gap law. Additionally, the distortion of the complex frees space above the copper(II) ion allowing the latter to be attacked by any kind of nucleophile to reach the stable five-coordinate square planar pyramidal geometry; solvent molecules play an important role in this process [6,19,20]. A collection of copper(I)-bis(diimine) luminescent complexes, both homoleptic ([Cu(L)2]+) and heteroleptic ([Cu(L)(L′)]+) [21,22] exhibiting interesting properties in their excited states have thus been isolated and studied in various domains (dye sensitized solar cells [23–31], photo-induced water splitting [32], organic photochemistry [32–37], supramolecular photo-induced electron transfer) [38–46].

    • Heteroleptic diimine–diphosphine Cu(I) complexes as an alternative towards noble-metal based photosensitizers: Design strategies, photophysical properties and perspective applications

      2018, Coordination Chemistry Reviews
      Citation Excerpt :

      However, if the substituents become too bulky ligand–ligand repulsion might destabilize the electronic ground state in [Cu(N^N)2]+ and [Cu(N^N)(PPh3)2]+ systems and lead to ligand dissociation in solution [48,52,53,62,64,67]. The luminescence of the [Cu(N^N)2]+ complexes was temperature-dependent and was explained with a long-term holding two-state model of thermal equilibration between emissive 1MLCT and 3MLCT states [66,90], in which the photoluminescence from the 1MLCT state is nowadays confirmed to be mainly thermally activated delayed fluorescence (TADF, as discussed in more detail in the Sections 3.3 and 3.4.2) [17,35,51,91,92]. Several factors account for the above-mentioned correlation between the bulkiness of ligand and the luminescence properties: the radiative rates of (TAD) fluorescence and phosphorescence, their respective contributions to the overall emission, and the quantum yield of emission.

    View all citing articles on Scopus
    View full text