Spectroscopic and thermodynamic studies of light harvesting perylenediimide derivative - zinc porphyrin complex in aqueous media

Self-assembly of perylene derivative such as N,N'-bis(2(trimethylammonium iodide) ethylene)perylene-3,4,9,10-tetracarboxyldiimide (TAIPDI) can produce one-dimensional form (1D) in an aqueous media. The ability of one-dimensional TAIPDI to form light harvesting complex with water-soluble zinc porphyrin (ZnTPPS₄) via the π-π and electrostatic interactions has been described. Owing to electronic interactions between the π-systems, the complex formation is accompanied by pronounced absorption spectral changes in the UV/Vis absorption bands. The formation constant of the ZnTPPS₄-TAIPDI complex has been determined as 2.60×10⁴M^-1 suggests a moderately stable complex. The steady-state fluorescence measurements exhibited fluorescence quenching of both the singlet TAIPDI and ZnTPPS₄ because of the electron transfer process from the electron-donating ZnTPPS₄ to the electron-accepting TAIPDI. Based on the picosecond time-resolved fluorescence, the rate and quantum yield of the electron transfer were found to be 2.47×10¹⁰s^-1 and 0.99, respectively, indicating fast and efficient electron transfer. The thermodynamic parameters of the complex formation have been determined from the stopped-flow measurements. The interaction between ZnTPPS₄ and TAIPDI occurs in two steps, a fast and reversible step followed by a slow and irreversible one. The activation parameters for the complex formation (ΔH^#=22±5kJmol^-1 and ΔS^#=-123±18JK^-1mol^-1), (ΔH^#=133±4kJmol^-1 and ΔS^#=167±13Jmol^-1K^-1) were determined from variable temperature studies for the "on" and the "off" of the first step and ΔS^#=246±37.89Jmol^-1K^-1 and ΔH^#=130±11kJmol^-1 for the second step. The negative and positive ΔS^# values found for the interaction reactions are consistent with an associative interaction for the first step followed by dissociative mechanism for both the "off" and the second step.