Mechanistic Insights into the Solvatochromism and Surfactant-Mediated Aggregation of Tetracationic [H2TMPyP]4+: A Combined TD-DFT and Experimental Approach.

The electronic and optical properties of tetracationic free-base porphyrin, 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin ([H2TMPyP]4+) were investigated through a systematic combination of spectrophotometric measurements and Density Functional Theory (DFT/TD-DFT) calculations. We explored the influence of solvent polarity, dielectric constant, and surfactant-mediated environments on the porphyrin's Soret and Q-bands. Experimental results reveal significant solvatochromic shifts and intensity variations in various organic solvents, such as DMSO, DMF, ethanol, and methanol, compared to water, which are primarily attributed to differential solvation and modification of the transition dipole moment in less polar media. To validate these observations, eight DFT functionals were benchmarked, identifying ωB97XD/6-31+G(d,p) as the most accurate for predicting the electronic transitions of this cationic system. The theoretical analysis reveals that the solvent dielectric constant significantly influences the torsional angles of the peripheral pyridinium rings, which in turn modulate the macrocycle's conjugation. Furthermore, the interaction with various surfactants such as SDS, CTAB, and Triton X-100 suggests a concentration-dependent mechanistic transition: electrostatic neutralization leads to the formation of H-aggregates in the premicellar region, while micellar encapsulation above the CMC restores monomeric behavior. These insights into the structural-spectral correlations of [H2TMPyP]4+ provide a fundamental basis for its application in advanced biosensing and photodynamic molecular probes.