Pyromellitic Acid-Based Cocrystals: A Mechanistic Study for the Origin of Semiconductivity via H-Coupled Charge Transfer

Organic cocrystals consisting of pyromellitic acid-4-nitro phenylenediamine (1) and pyromellitic acid-ethylene diamine (2) are reported. Cocrystals 1 and 2 were fully characterized by single-crystal X-ray diffraction, NMR, and IR spectroscopies. Conductivity measurements indicated that cocrystals 1-2 had semiconducting behavior in the voltage range studied (−10 to +10 V). To investigate the origin of semiconductivity, the newly synthesized cocrystals 1-2 and our previous pyromellitic acid-orthophenylenediamine cocrystal (3) were analyzed for temperature-dependent Raman spectroscopy and EPR spectroscopy. The variable-temperature Raman spectra showed altering intensity of the spectra as a function of temperature as evidence of proton transfer, whereas the EPR spectra showed a signal corresponding to single electron transfer in all three cocrystals 1-3. DFT calculations performed with the crystal structures of 1-3 with transferred protons from the pyromellitic acid to the respective amines as well as the optimized structures with original untransformed states showed a consistent decrease of the HOMO-LUMO gap in favor of the crystal structures for all three cocrystals. Development of ground (NH₃⁺···^-OOC) and excited electronic states (NH₂···HOOC) based on the proton transfer coordinate along the N··H··O hydrogen bonding results in the spectral red shift in favor of crystal geometry, which is observed via TDDFT calculations for cocrystals 1-3. The theoretical calculation of the band gap and hopping rate further supported the formation of closely spaced ground and excited electronic states to be the origin of semiconductivity for all three cocrystals 1-3.