Spectroscopic studies of hexadecylquinolinium tricyanoquinodimethanide

Hexadecylquinolinium tricyanoquinodimethanide (1), a unimolecular rectifier of electrical current with a large ground-state dipole moment (43 +/- 8 D) , reveals large hypsochromic shifts of the absorption spectrum. Two fluores cent emissions were observed: one in the visible region (quantum yield phi approximate to 0.01, not solvatochromic) and one in the near-infrared spect rum (weakly solvatochromic). Using a prolate spheroidal cavity model and th e absorption maxima measured in eight solvents, the excited-state dipole mo ment of 1 is estimated as 8.7 D. The NMR spectral lines broaden above 330 K and lose the multiplet structure. The core-level N Is XPS spectrum reveals the three expected N valence states. The valence-level XPS spectrum can be correlated with theory. Simultaneous cyclic voltammetry and electron spin resonance of the radical anion of 1 shows that the spin density in the LUMO of 1 is concentrated on the tricyanoquinodimethanide portion of the anion, The molecule is clearly zwitterionic in the ground state, both in LB films and in solution, and is undissociated ("neutral") in its first excited sta te.