Structural, optical characteristics of 3-(p-N, N-dimethyl aminophenyl) 5-phenyl-1H-pyrazole: tuning band gaps with acid solutions

1H-pyrazole has a unique chemistry that can be dramatically affected by acid and base solutions. In this work, the photophysical characteristics of 3-(p-N, N dimethyl aminophenyl)-5-phenyl-1H-pyrazole dye (DAPPP) were investigated. The absorption spectra of DAPPP showed two bands around 321 and 391 nm. Regardless of the optical density, the shape of the spectra does not change with increasing the concentration, indicating that there is no dimer formed for all concentrations used. The fluorescence spectra showed only one band at ~ 469 nm. The intensities and the peak positions exhibited a strong association with the concentration of DAPPP in the solution. Furthermore, the pH variation of the DAPPP solutions has a dramatic effect on the spectral properties. The band gap of DAPPP in different solvents and pH solutions were studied. The studies have shown that the energy gap is significantly affected by pH solutions. For the higher pH solutions (pH 13), the band gaps values were 2.9 eV; the band gap has decreased significantly to 2.36 eV when the pH was reduced to 0.6. This significant reduction in the energy gap could be attributed to increasing electrons cloud in the pyrazole ring through the re-hybridization of pyridine-like nitrogen to be pyrrole-like nitrogen by protonation. This approach can be used in order to have lower energy gaps in other organic compounds. For example, the organic compounds used in solar cells as P-type. Furthermore, Stokes’ shift and quantum yield of fluorescence were calculated and discussed.