Coherent Nuclear Dynamics in Ultrafast Photoinduced Structural Change of Bis(diimine)copper(I) Complex

Abstract

The photoinduced structural change of a prototype metal complex, [Cu(dmphen)(2)](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), was studied by ultrafast spectroscopy with time resolution as high as 30 fs. Time-resolved absorption measured with direct S(1) excitation clearly showed spectral changes attributable to the D(2d) (perpendicular) → D(2) (flattened) structural change occurring in the metal-to-ligand charge transfer singlet excited state ((1)MLCT) and the subsequent S(1) → T(1) intersystem crossing. It was confirmed that the two processes occur with time constants of ~0.8 ps (structural change) and ~10 ps (intersystem crossing), and their time scales are clearly well-separated. A distinct oscillation of the transient absorption signal was observed in the femtosecond region, which arises from the coherent nuclear motion of the perpendicular S(1) state that was directly generated by photoexcitation. This demonstrated that the perpendicular S(1) state has a well-defined vibrational structure and can vibrate within its subpicosecond lifetime. In other words, the S(1) state stays undistorted in a short period, and the coherent nuclear motion is maintained in this state. Time-dependent density functional theory (TDDFT) calculations gave consistent results, indicating a very flat feature and even a local minimum at the perpendicular structure on the S(1) potential energy surface. The vibrational assignments of the S(1) nuclear wavepacket motion were made on the basis of the TDDFT calculation. It was concluded that photoexcitation induces a(1) vibrations containing the Cu-ligand bond length change and a b(1) vibration attributed to the ligand-twisting motion that has the same symmetry as the flattening distortion. Ultrafast spectroscopy and complementary quantum chemical calculation provided an overall picture and new understanding of the photoinduced structural change of the prototypical metal complex.

Signaling-State Formation Mechanism of a BLUF Protein PapB from the Purple BacteriumRhodopseudomonas palustrisStudied by Femtosecond Time-Resolved Absorption Spectroscopy

“…52 Briefly, the light source was a Ti:sapphire oscillator/regenerative amplifier system (MIRA-900F/Legend Elite, Coherent) that generates 1.1 mJ pulses at 800 nm with a duration of ∼80 fs at 1 kHz. 52 About 80% of the 800 nm output was used to pump an optical parametric amplifier (TOPAS, Light Conversion) to generate a 450 nm pump pulse. This pulse was attenuated down to 240 nJ and focused onto the sample for photoexcitation (the beam diameter was ∼150 μm at the sample position).…”

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

“…[1][2][3][4] As a prototype of such Cu(I) complexes, [Cu(dmphen) 2 ] + (dmphen = 2,9-dimethyl-1,10-phenanthroline) has been studied extensively. [5][6][7][8][9][10][11][12][13][14][15] This complex has a D 2d structure in the electronic ground (S 0 ) state, where two phenanthroline ligands are attached to the central copper ion perpendicular to each other. 16,17 As the copper ion is formally oxidized with the MLCT transition, the conformation of this complex changes from the initial tetrahedral-like D 2d structure to the square-planar structure that is preferred for the Cu(II) complexes.…”

“…11 Quantum chemical calculations also supported such a flattening distortion in the excited state. [10][11][12]14,18 Because the structural change in the metal complexes is usually a very fast process, the flattening distortion can compete with other fast processes such as internal conversion (IC) and/or intersystem crossing (ISC). To obtain proper understanding about the dynamics and mechanism of photochemical/ physical processes of [Cu(dmphen) 2 ] + , various time-resolved methods have been applied, including picosecond timecorrelated single photon counting, 10 femtosecond fluorescence up-conversion, 13,19,20 and femtosecond time-resolved absorption spectroscopies.…”

“…To obtain proper understanding about the dynamics and mechanism of photochemical/ physical processes of [Cu(dmphen) 2 ] + , various time-resolved methods have been applied, including picosecond timecorrelated single photon counting, 10 femtosecond fluorescence up-conversion, 13,19,20 and femtosecond time-resolved absorption spectroscopies. 11,14,[20][21][22][23] Although there was initial controversy for the interpretation of the transient data, 10,11 we now have a consensus about the dynamics occurring on the femto to picosecond time scale. 13,14 In particular, our femtosecond time-resolved emission study in solution clarified that the 1 MLCT (S 1 ) state having the perpendicular structure is generated with photoexcitation, and that it stays undistorted for a short time (B0.7 ps) before undergoing a flattening distortion in the S 1 state.…”

“…11,14,[20][21][22][23] Although there was initial controversy for the interpretation of the transient data, 10,11 we now have a consensus about the dynamics occurring on the femto to picosecond time scale. 13,14 In particular, our femtosecond time-resolved emission study in solution clarified that the 1 MLCT (S 1 ) state having the perpendicular structure is generated with photoexcitation, and that it stays undistorted for a short time (B0.7 ps) before undergoing a flattening distortion in the S 1 state. 13 The ISC process occurs from the 'flattened' S 1 state to the T 1 state ( 3 MLCT state) with a time constant of B7 ps.…”