The study of the excited states and photophysical characteristics of β-nitro-tetraphenylporphyrin (TPP–NO_2) has been carried out using resonance Raman scattering (RRS) spectroscopy and methods of the density functional theory. The appearance of new lines, the intensity of which depends on the composition of the matrix and excitation wavelength, has been found in the TPP–NO_2 RRS spectra in the low-temperature matrix. The calculation of the vibrational states of TPP–NO_2 allowed the linking of the additional lines with the asymmetric vibrations of the nitro group and valence C–C vibrations of the phenyl ring (Ph1) that was nearest to it. The activation of these modes is related to the specific features of the TPP–NO_2 geometry in the charge transfer (CT) state from Ph1 to the porphyrin macrocycle. It has been concluded on the basis of the analysis of the data of the study of the RRS spectra and the results of calculations that use the СAM-B3LYP and wB97XD functionals that the CT states do not play a significant role in the TPP–NO_2 fluorescence quenching, as previously assumed. The fluorescence quenching owes to strengthening channels of internal and inter-conversion by reducing the energy gaps Δ E ( S _1 – T _1) and Δ E ( S _1 – S _0) as well as increasing the spin-orbit coupling between the S _1 and T _1 states. It has been shown that TPP–NO_2 is characterized by conformational heterogeneity both in the ground and in the excited states, which explains the absence of the monoexponentiality of fluorescence decay kinetics.
A study of the resonance Raman scattering (RRS) spectra of Co(II)- and Cu-5,10,15,20-tetrakis [4-(N-methylpyridyl)]porphyrin (CoIITMpyP4, CuTMpyP4) in various solvents and in complex with DNA was carried out. Additional lines were found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity, as well as for its complex with DNA under nanosecond pulsed excitation. At the same time, under the same excitation conditions, such lines are absent in the spectra of CoIITMpyP4 solutions in pure DMF, dimethyl sulfoxide, water, and alcohols. To interpret the experimental data, we performed calculations of the structure and vibrations for the solvate complexes of CoII - and CuTMpyP4 with water, methanol and formic acid in the ground and in excited states. Based on the data obtained, additional lines in the Raman spectra were assigned to the excited dd-state corresponding to the d(z2) d(x2-y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in the rate constant of the internal conversion kic due to an increase in the energy gap E between the ground and dd states of CoIITMpyP4. In the case of CoII and CuTMpyP4 binding to DNA, the decrease in kic is due to the additional interaction of the extra-liganding water molecule directly or with the participation of several immobilized DNA water molecules with one of its bases. This complicates the conformational rearrangement of the water molecule in the dd-state, which contributes to an increase in E.
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