Transient absorption spectroscopy is used to study the excited-state dynamics of Co(3)(dpa)(4)(NCS)(2), where dpa is the ligand di(2-pyridyl)amido. The pi pi*, charge-transfer, and d-d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low-lying metal-centered d-d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand-centered pi pi* and the charge-transfer states to this metal-centered d-d state within 100 fs. The excited molecule then crosses to a second d-d state within the ligand-field manifold, with a time coefficient of 0.6-1.4 ps. Because the ground-state bleaching band recovers with a time coefficient of 10-23 ps, we propose that an excited molecule crosses from the low-lying d-d state either directly within the same spin system or with spin crossing via the state (2)B to the ground state (2)A(2) (symmetry group C(4)). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.

We studied the excited-state dynamics of trans-4-(N-arylamino)stilbenes with aryl = phenyl (p1H), 4-methoxyphenyl (p1OM), or 4-cyanophenyl (p1CN) in solvents of varied polarity and viscosity by using femtosecond transient absorption and time-correlated single photon counting techniques. In nonpolar solvents the decay is triexponential, in which the rapid component corresponds to vibrational cooling combined with solvation, the intermediate temporal component 41-120 ps to trans-cis isomerization, and the long one ∼1 ns to fluorescence decay of the S state. The S state has a delocalized geometry and charge-transfer characteristics, corresponding to a planar intramolecular charge transfer (PICT) state. In polar solvents, an excited-state absorption band appears near 520 and 480 nm for p1OM and p1CN, respectively but not for p1H. This band has a rise lifetime of 4.3/7.5, 16.3/9.4, and 29.5/16 ps for p1CN/p1OM in acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively and matches the decay of the 600 nm PICT band. This band is thus assigned to the absorption of a singlet twisted intramolecular charge transfer state (TICT). The conversion rate decreases as the solvent viscosity is increased and is consistent with a large structural variation amplitude. Theoretical calculations using density functional theory (DFT), method PEB0, were employed to obtain the optimized structures and energies of those states. The PICT state possesses delocalized π electrons along the molecule. The TICT for p1CN is formed by twisting about the aminostilbene-benzonitrile C-N bond by ∼90°, but it is about the stilbene-aniline C-N bond for p1OM. We observed faster conversion rates for p1CN in alcoholic solvents, in which the lifetimes for both the PICT and TICT states are shortened to 20-99 ps and 120-660 ps, respectively, as a result of solvent-solute H-bonding interactions. In p1OM, the TICT state has an elongated C[double bond, length as m-dash]C bond in the stilbene moiety, which might facilitate the trans-cis isomerization reaction and thus account for the relatively short lifetime of 58-420 ps in polar solvents.

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The excited-state dynamics of Ni(3)(dpa)(4)X(2), in which dpa is the ligand di(2-pyridyl)amido and X = NCS or Cl, are investigated by transient absorption spectroscopy. The pi pi* and dd states are excited upon irradiation at wavelengths of 330 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution. The transient species also show weak absorption. It is proposed that a low-lying metal-centered dd state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal a rapid conversion from a ligand-centered pi pi* state to a metal-centered dd state in 0.1-0.4 ps. Vibrational cooling occurs with a time coefficient of 3.0-15.9 ps. From the spectral shift observed in the transient spectra relative to the steady-state spectra, the dd state is assigned as B(1)/B(2)(Ni(t)). This dd state eventually converts to the electronic ground state, in about 100 ps for the isothiocyanate complex and 200 ps for the chloride. In this trimetal string complex, relaxation to the ground electronic surface after excitation is therefore rapid.

Herein, the excited-state dynamics of an extended metal atom chain complex, Cr(dpa)Cl (dpa = dipyridylamide), in tetrahydrofuran solution were investigated using femtosecond transient absorption spectroscopy. Upon excitation at a wavelength of 330 nm, two distinct excited-state absorption species with varied dynamics were identified and assigned to the symmetric (s-) and unsymmetric (u-) Cr(dpa)Cl. The major species is s-Cr(dpa)Cl that undergoes rapid conversion at less than 100 fs from the ligand-centred π-π* state, which is the initially accessed state, to the metal-centred d-d state and then vibrational cooling accompanying the structural relaxation at a time constant ∼2.2 ps. Most of the s-form is recovered to the ground state at ∼200 ps. For u-Cr(dpa)Cl, a similar rapid conversion to d-d states is observed, and the geometric/vibrational relaxation is ∼0.8 ps. The second recovery of the ground state with approximately equal amplitude is observed at a time constant of ∼5 ns. This might be because many d-d states exist and about half of them inefficiently couple with the ground state surface.