Bimetallic transition metal complexes have gained increasing attention because of their versatile functions in solar energy conversion and photonics applications arising from intermetal electronic coupling. In bimetallic platinum (Pt) complexes, electronic communication between the Pt-centered and ligand-centered moieties has been shown to be critical for defining their excited-state dynamic trajectories undergoing either localized ligand-centered (LC)/metal-to-ligand charge-transfer (MLCT) transitions or delocalized metal−metal-to-ligand charge-transfer (MMLCT) transitions. The branching of the excited-state intersystem crossing (ISC) trajectories is modulated through structural factors that alter the relative energies of the different states. In this study, we investigated the correlation of the structural factors influencing the excited-state trajectories. With the use of femtosecond broad-band transient absorption (fs-BBTA) spectroscopy, ultrafast dynamics in the excited state of two select Pt(II) dimers have been mapped out using their coherent vibrational wavepacket signatures in the corresponding transient absorption spectra. To examine how the ligand moieties of the Pt(II) dimers influence excited-state dynamics and the coherent vibrational wavepacket behavior, we carried out comparative studies on two pyrazolate-bridged Pt(II) dimers of the general formula [Pt( t Bu 2 Pz)(N^C)] 2 [ t Bu 2 Pz is 3,5-di-tert-butylpyrazole; N^C is 7,8benzoquinoline (bzq, 1) or 1-phenylisoquinoline (piq, 2)]. We found that photoexcitation into the low-energy absorption bands of 1 and 2, respectively, induces the formation of 1 MMLCT states from which ultrafast ISC proceeds, resulting in stimulated emission quenching and decoherence of the vibrational wavepacket motions. The results obtained in this study suggest that both the energetics and the structural rigidity of the aromatic cyclometalating ligands in 1 and 2 can significantly influence the dynamics along the excited-state trajectory characterized by dephasing of the coherent oscillations. The collective results provide direct evidence of how ligand structure alters electronic dynamics along excited-state trajectories associated with ISC processes, providing insight into using ligand design to steer photochemical processes.
Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1−Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH 2 Cl 2 , τ = 2.5−4.3 μs. This series of molecules also feature high PL quantum efficiencies (Φ PL = 5.3−12% in CH 2 Cl 2 ). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a 1 MLCT excited state lying slightly above the 3 MLCT state, 1050−1490 cm −1 . Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH 3 CN. Combined PL and electrochemical data provided evidence that Cu1−Cu4 are highly potent photoreductants (E ox * = −1.73 to −1.62 V vs Fc +/0 in CH 3 CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that longrange electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1−Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 10 8 M −1 s −1 . The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.
Dinuclear d8 Pt(II) complexes, where two mononuclear square planar Pt(II) units are bridged in an “A-frame” geometry, possess photophysical properties characterised by either metal-to-ligand- (MLCT) or metal-metal-ligand-to-ligand charge transfer (MMLCT)...
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