Resonance Raman scattering spectra of Co(II)- and Cu-5,10,15,20-tetrakis [4-(n-methylpyridyl)]porphyrin in the excited $dd$-state and the mechanisms of its deactivation in solution and complexes with DNA

A study of the resonance Raman scattering (RRS) spectra of Co(II)- and Cu-5,10,15,20-tetrakis [4-(N-methylpyridyl)]porphyrin (Co$^{\mathrm{II}}$TMpyP4, CuTMpyP4) in various solvents and in complex with DNA was carried out. Additional lines were found in the RRS spectra of Co$^{\mathrm{II}}$TMpyP4 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 Co$^{\mathrm{II}}$TMpyP4 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 Co$^{\mathrm{II}}$- 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 $d(z^{2})\to d(x^{2}-y^{2})$ transition, the lifetime of which increases with the formation of complexes of Co$^{\mathrm{II}}$TMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in the rate constant of the internal conversion $k_{\mathrm{ic}}$ due to an increase in the energy gap $\Delta E$ between the ground and dd states of Co$^{\mathrm{II}}$TMpyP4. In the case of Co$^{\mathrm{II}}$ and CuTMpyP4 binding to DNA, the decrease in $k_{\mathrm{ic}}$ 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 $\Delta E$.