Light-emitting fluorophores 1-10b have been synthesized. Helical compounds end-capped with different electron-rich and electron-poor aryl moieties are evaluated. Photocyclization of 10a was investigated, and the X-ray crystal structure of 4 obtained. We demonstrate that the optical properties of all of the new compounds, and by extension many conjugated materials, can be tuned over the entire blue range (400-480 nm), below.
The charge-separated state of 8 was observed in the absorption and emission spectra. DFT (B3LYP/ 6-31G*) calculations identified the optimum geometry of 8. The distance between the electron-donating and electron-withdrawing group in 8 is 20 angstrom; through the bond. The helical structure of the T5H bridge causes the distance of donor and acceptor to be as short as 7.06 angstrom. Rapid charge separation and slow charge recombination was detected by femtosecond pump-probe absorption measurements. A through bond electron transfer mechanism is preferred, although the distance through space is much shorter than the thru-bond distance.
The nonlinear optical properties of BDT, T5H, 1, 6, 7, 9 and 11 as well as Rhodamine B have been investigated. Rhodamine B in methanol was used for calibration. π-Centers of BDT and T5H were studied. Compounds 1, 6, 7, 9 and 11 possess either donor-π (D-π) or donor-π-acceptor (D-π-A) structures. Two-photon absorption cross-sections related to the third-order nonlinear susceptibility were measured using Z-scan techniques. Molecular structure and nonlinear optical property relationship was formulated. Compound 9 has a two-photon absorption cross-section of 103 GM when excited at 720nm.