Electrogenerated chemiluminescence of phenylquinoline-phenothiazine donor-acceptor molecules, dipyrromethene-BF₂ laser dyes and bent-pyrene-containing cyclophanes
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Four aspects of electrogenerated chemiluminescence were studied. First, the effect of molecular geometry on the photophysical, electrochemical and ECL properties of 3,7-[bis[4-phenyl-2-quinolyl]]-10-methylphenolthiazine and 7,7'-bis-(6-hexyl-4 phenylquinolin-2-yl)-10,10'-dimethyl-10H,10H'-[3,3']biphenothiazinyl were investigated. Upon addition of phenylquinoline groups onto the phenothiazine rings, apparent changes in the photophysical and electrochemical properties were observed. The optimized geometry showed that the phenylquinoline moieties were twisted from the phenothiazine rings, thus inhibiting any delocalization of the charge upon electrochemical oxidation or reduction. This geometry allows the production of stable radical ions needed to generate ECL from phenothiazine within the potential window. Second, the effect of chemical structures on the photophysical, electrochemical and ECL properties of four novel pyrenophanes were studied. When compared to pyrene, these compounds displayed a shift in the thermodynamic reduction potentials to more negative values. AM1 calculations also showed that the LUMO energy increased with the degree of nonplanarity. Similar to pyrene, their ECL spectra showed broad “excimer like” emission in addition to the monomer emission. Similar studies were performed on five boron-based laser dyes with different substituents or degree of substitution. High fluorescence quantum yields were obtained for all dyes except PM 597. Moderately intense ECL was observed from PM 567, PM 580, and PM 597, compounds that exhibited reversible electrochemical oxidation and reduction. The third aspect involved the study of polycyclic aromatic hydrocarbons in presence of tri-n-propylamine (TPrA) in MeCN/benzene solutions. ECL was observed from three out of the eight compounds studied. Based on the energetic criterion for ECL, the potential for oxidation of the intermediate free radical was found to be about -1.7 V vs SCE. By studying the fluorescence quenching of several PAHs by TPrA, the standard potential for oxidation of TPrA was estimated as 0.9 V vs SCE. The last aspect involved the study of ECL energy transfer and eventually utilized the self-quenching capability of PM 567 to determine the self-association constant (Ka) for dimeric oligo(aminotriazine) in a mixed solvent system. Analyzing changes observed in the ECL intensity with respect to the concentration of PM 567-labeled oligomers yielded a dissociation constant (Kd) of about 1.4 x 10-3 M (Ka = 7.1 x 102 M-1), a value slightly higher than the literature value.