Synthesis and photophysical properties of 2,6-dicyano-p-phenylenediamine

The photophysical and electrochemical properties of p-phenylenediamine (PPD) are strongly affected by the addition of cyano groups to the aromatic ring. In 2,3,5,6-tetracyano-p-phenylenediamine (TCPPD) the photophysics is governed mostly by the solvent basicity (β) whereas in 2,6-dicyano-N,N,N′,N′-tetramethyl-p-phenylenediamine (DCTMPPD) by the solvent polarity/polarizability (π*). In order to study the interactions of cyano-substituted PPDs with the solvent molecules in more detail as well as to clarify the role and origin of hydrogen bonding differences for TCPPD and DCTMPPD, another cyano substituted PPD, 2,6-dicyano-p-phenylenediamine (DCPPD) has been synthesized. The photophysical properties have been measured in a wide range of solvents. The fluorescence lifetimes (from 14 ns to 20 ns) and quantum yields (from 0.7 to 0.85) are not very sensitive to the environment. The solvatochromism is analyzed by a linear solvation energy relationship (LSER) using parameters developed by Kamlet, Taft and co-workers. It has been found that both absorption and emission of DCPPD depend on specific as well as non-specific interactions of the solute with the solvent molecules. The ground and excited state pKa values for DCPPD have also been determined.