Knowledge Bank

The electronic structure of polydiacetylene (PDA) crystals has resisted complete explanation. To gain further understanding, detailed analysis of their electronic spectra is desirable. Modulation techniques have proven very useful in the study of optical properties and molecular interactions in the solid state. In general, room temperature modulation spectra correlate with structure normally seen only in low temperature unmodulated spectra. To aid in the assignment of the PDA electronic structure and to answer the question of what enhancement of structure would be seen in low temperature modulation experiments, we have measured the piezomodulated reflection (PMR) spectra at 14K for the PDAs 2,4-hexadiyne-1,6-diol-bis(p-toluene sulfonate) (PTS) and 9, 9'-(2, 4-hexadiyne-1, 6-diyl) bis-9H-carbazole (DCHD). These two PDAS show a strong 0-0 Frenkel exciton transition at 1.97 eV and 1.86 eV, respectively, followed by extensive vibrational side bands. Structure due to intramolecular vibrations seen in the PTS and DCHD PMR spectra have been assigned. The values for the vibrational frequencies determined from the PMR spectra are in good agreement with those reported from Raman $studies1$. Also seen in the PMR spectra are low frequency modes which are associated with lattice modes. Furthermore, assignment of the transverse and longitudinal frequencies, corresponding to the maxima and minima in the PMR spectra, can be made and the stopping bandwidth for each band $determined2$.