Spectral Characteristics of the Various Prototropic Species of 2-(4‘-N,N-Dimethylaminophenyl)pyrido[3,4-d]imidazole

Abstract

Spectral characteristics of 2-(4'-N,N-dimethylaminophenyl)pyrido[3,4-d]imidazole (DMAPPI) have been studied in two different solvents and H(0)/pH/H(-) range of -10 to +16. Combining the results observed in the absorption, fluorescence, and fluorescence excitation spectra, it is found that (i) only one kind of monoanion (deprotonation of >N-H group) and one kind of trication, (ii) two kinds of monocations (MC1 and MC3) in the ground state and (MC2 and MC3) in the excited singlet state, and (iii) three kinds of dications (DC1, DC2, and DC3) are observed. Semiempirical quantum mechanical calculations (AM1) have been carried out on all kinds of ionic species and their resonating structures. The spectral characteristics have been assigned to various prototropic species combining the experimental and theoretical results.

Dual Fluorescence of 2-(4′-N,N-dimethylaminophenyl)pyrido [3,4-d]imidazole in a TritonX-100/n-Hexanol/Water Reverse Micelle in Cyclohexane: Effect of pH and Trifluoroacetic Acid

“…MC1 and MC3 are present in equilibrium in the S 0 state and the equilibrium between MC2 and MC3 is not established in the S 1 state, as both have different excited state lifetimes (1.2 and <0.2 ns are the excited state lifetimes of MC2 and MC3 in TX-100 normal micelles, respectively (23)). From our earlier study (25) and the present one, it is very difficult to ascertain the presence of different kinds of MCs of DMAPPI in the S 0 state from the absorption spectra. Thus, we recorded the fluorescence excitation spectra of the MCs of DMAPPI in TFA/TX-100/n-hexanol/water in cyclohexane at different w 0 values.…”

“…The excitation band at 376 nm can be assigned to the MC3, as the agreement with the absorption band maximum in the reverse micelle and the results observed in an aqueous medium is quite good. The band at ∼350 nm is different from that observed in aqueous or methanol mediums (25). It is well established (35)(36)(37)(38) that the protonation at the =N-atom of the imidazole ring of the benzimidazoles leads to a red shift in the absorption and/or fluorescence excitation spectra of the neutral species and the red shift observed is smaller than that noticed in the protonation of the =N-atom of the pyridine ring.…”

“…Three possible monocations (MCs) can be obtained by protonating three different basic centers as shown in Scheme 1, i.e., MC1 by protonating the -NMe 2 group and MC2 and MC3 by protonating the =N-atoms of the imidazole and pyridine rings, respectively. AM1 semiempirical quantum mechanical calculations carried out on these MCs (25) have shown that MC2 is the least polar and thus may be stable in nonpolar solvents, whereas MC1 is the most polar and probable ionic species in polar media. Results have shown that MC1 and MC3 were observed in methanol and water in the S 0 state, whereas MC2 and MC3 were present in the S 1 state.…”

“…We have also studied the effect of the surfactant and cosurfacant concentration, pH, and trifluoroacetic acid (TFA). The results thus obtained have been compared with those obtained from the AM1 calculations (20,25).…”

“…(i) MC1 is formed by the protonating -NMe 2 group, (ii) MC2 by the protonating =N-atom of the imidazole ring, and (ii) MC3 by the protonating =N-atom of the pyridine ring (Scheme 1). Spectral characteristics of the MCs of DMAPPI in aqueous and methanol solutions as solvents have been studied in detail (25). In an aqueous medium, MC1 (fluorescence excitation spectral band maximum λ protonates the imidazole =N-atom to form MC2 in the S 1 state only).…”