Transient excited singlet state absorption in POPOP and dimethyl POPOP

Transient excited singlet state absorption (ESSA) has been measured in POPOP solution in ethanol and dimethyl POPOP solution in toluene at room temperature in the region 4580–6880 A using a nitrogen laser and nitrogen laser pumped dye laser. Extensive absorption with several submaxima and shoulders, which represent the vibrational structure, has been observed in both molecules in the region covered in the present study. Energy level schemes of the two molecules have been obtained with the help of the ground state absorption and fluorescence spectra recorded for the purpose. The observed structure in the ESSA has been tentatively interpreted to be due to transitions between the different vibrational levels of the lowest excited state S1 and two other upper singlet electronic states S3 and S4 or Sn. The occurrence of transitions from the higher vibrational levels of S1 in addition to those from its lowest vibrational level could be understood on the basis of the fact that the pump and the probe beams are made to overlap for a period of ∼ 6–7 ns during which they interact simultaneously with the dye molecules. A plausible explanation for the relative variation in the strength of different transitions from S0 as well as the observed difference (or otherwise) in the strength of these transitions relative to those of S1 in each molecule is given on the basis of parity/symmetry selection rules. The experimental results obtained and the transition assignments made are being reported for the first time.