Structure-Dependent Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in Copper(I) Bis(diimine) Compounds

Abstract

The photophysical properties of singlet and triplet metal-to-ligand charge transfer (MLCT) states of [Cu(I)(diimine)(2)](+), where diimine is 2,9-dimethyl-1,10-phenanthroline (dmphen), 2,9-dibutyl-1,10-phenanthroline (dbphen), or 6,6'-dimethyl-2,2'-bipyridine (dmbpy), were studied. On 400 nm laser excitation of [Cu(dmphen)(2)](+) in CH(2)Cl(2) solution, prompt (1)MLCT fluorescence with a quantum yield of (2.8 +/- 0.8) x 10(-5) was observed using a picosecond time-correlated single photon counting technique. The quantum yield was dependent on the excitation wavelength, suggesting that relaxation of the Franck-Condon state to the lowest (1)MLCT competes with rapid intersystem crossing (ISC). The fluorescence lifetime of the copper(I) compound was 13-16 ps, unexpectedly long despite a large spin-orbit coupling constant of 3d electrons in copper (829 cm(-1) ). Quantum chemical calculations using a density functional theory revealed that the structure of the lowest (1)MLCT in [Cu(dmphen)(2)](+) (1(1)B(1)) was flattened due to the Jahn-Teller effect in 3d(9) electronic configuration, and the dihedral angle between the two phenanthroline planes (dha) was about 75 degrees with the dha around 90 degrees in the ground state. Intramolecular reorganization energy for the radiative transition of 1(1)B(1) was calculated as 2.1 x 10(3) cm(-1), which is responsible for the large Stokes shift of the fluorescence observed (5.4 x 10(3) cm(-1)). To understand the sluggishness of the intersystem crossing (ISC) of (1)MLCT of the copper(I) compounds, the strength of the spin-orbit interaction between the lowest (1)MLCT (1(1)B(1)) and all (3)MLCT states was calculated. The ISC channels induced by strong spin-orbit interactions (ca. 300 cm(-1)) between the metal-centered HOMO and HOMO - 1 were shown to be energetically unfavorable in the copper(I) compounds because the flattening distortion caused large splitting (6.9 x 10(3) cm(-1)) between these orbitals. The possible ISC is therefore induced by weak spin-orbit interactions (ca. 30 cm(-1)) between ligand-centered molecular orbitals. Further quantum mechanical study on the spin-orbit interaction between the lowest (3)MLCT (1(3)A) and all (1)MLCT states indicated that the phosphorescence borrows intensity from 2(1)B(1). The radiative rate of the phosphorescence was also structure-sensitive. The flattening distortion reduced the transition dipole moment of 2(1)B(1) --> the ground state, and decreased the extent of mixing between 1(3)A and 2(1)B(1), thereby considerably reducing the phosphorescence radiative rate at the MLCT geometry compared to that at the ground state geometry. The theoretical calculation satisfactorily reproduced the radiative rate of ca. 10(3) s(-1) and accounted for the structure-sensitive phosphorescence intensities of copper(I) bis(diimine) compounds recently demonstrated by Felder et al. (Felder, D.; Nierengarten, J. F.; Barigelletti, F.; Ventura, B.; Armaroli, N. J. Am. Chem. Soc. 2001, 123, 6291).

Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

“…Other metal-based systems of Pt(0) and Cu(I) show similar slow (3-15 ps) ISC rates between singlet and triplet MLCT states. [54][55][56] Just as in the case of PtPOP, the lowest singlet and triplet potential curves in these complexes are parallel (identical geometry), and direct SO interaction between them is forbidden. DFT calculations have shown that structural distortions in the singlet MLCT state cause large energy splitting between HOMO and HOMO-1, preventing a very fast ISC induced by strong spin-orbit interactions between these orbitals, which are mainly composed of metal d-orbitals.…”

Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution

“…It is possible that these minor differences signify the strength of the solvent-solute interactions in different solvents as revealed by our previous LITR-XAS results. 31 Whether these inconsistencies carry any significance is unclear, especially given that the position, shape and apparent width of the main band at ~560 nm are nearly identical in the three 34 is due to weakened spin-orbital coupling as the result of the structural rearrangement in the S 1 state, namely the flattening of the tetrahedral geometry of the F-C state via the decrease of φ. 34 Based on the TD-DFT calculations and the fluorescence lifetime measurements by TCSPC, they concluded that the flattening occurred prior to the ISC process rather than vice versa.…”

“…Extensive TD-DFT calculations 31 34 They found that spin-orbit coupling between the F-C MLCT state and the lowest early times after photoexcitation. However, when the dihedral angle between the ligands φ decreases to the MLCT state geometry, the orbital energies for higher triplet states lie above the lowest energy singlet.…”

“…According to the steady-state emission measurements, the emission band is peaked at 750 nm with its blue edge starting from 550 nm. 34 Therefore, our measurements are on the blue side of the steady-state emission band.…”

“…36 Without direct structural measurements with a better time resolution than 100 ps, we tentatively attributed the blue shift and its associated 10-20 ps time constant as a signature of the "flattening" distortion of the phenanthroline moieties based on the TD-DFT results and on the timescales for internal rotations of groups with similar sizes in twisted internal charge transfer (TICT) states. 37 A much slower ISC process has been proposed by Siddique et al 34 based on their time-correlated single photon counting (TCSPC) fluorescence lifetime measurements, which revealed a 13-ps fluorescence lifetime. Their spectral assignments were supported by TD-DFT calculations, revealing that the weakened spin-orbital coupling resulting from the structural distortion from tetrahedral geometry, the "flattening", was attributed to the origin of a remarkably slow ISC process.…”