Ultrafast Dynamics and Computational Studies on Diaminodicyanoquinodimethanes (DADQs)

Three diaminodicyanoquinodimethanes, 4-(R1R2C)-1-[(NC)2C]-C6H4 (R1,R2 = H2N, 1; R1 = 3,5-Me2-4-OCH4H6N–, R2 = H2N, 2; R1 = 3,5-Me2-4-OCH4H6N–, R2 = 4-Me-C5H9N, 3), were investigated using carbon-13 NMR, steady-state, and ultrafast transient absorption and ultrafast fluorescence spectroscopies to unravel the unusual characteristics of this class of chromophores. Computed (GIAO)B3LYP/6-31G* data for the zwitterions 1–3 using necessary solvation (PCM) models were shown to be in excellent agreement with observed structural and carbon-13 NMR data. The ground-state geometries of 1–3 contain a cationic methine group R1R2C– twisted from the C6H4 ring and an anionic methine group (NC)2C– in plane with the C6H4 ring in solution and solid state. The 13C chemical shifts of the peak corresponding to the methine carbon at the (NC)2C– group of 1–3 are observed at 32.5–34.7 ppm, which are some 55 ppm upfield compared with the 13C chemical shift for the methine carbons in TCNQ, 1,4-[(NC)2C]2-C6H4. The decay of the excited state in diaminodicyanoquinodimethanes is fast and dominated by nonradiative processes on the picosecond time scale, which depends on the viscosity of the medium. The dynamics of the excited-state decay is therefore limited by conformational changes through an intramolecular twisting motion. This twisting motion is hindered by friction, which, in turn, also depends on the functional group size of the system. The dominant nonradiative pathways after excitation are due to twisted excited-state conformers according to TD-DFT computations