Two-Photon Absorption and Two-Photon Circular Dichroism of a Hexahelicene Derivative with a Terminal Donor–Phenyl–Acceptor Motif

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2018-04, Vol.122 (13), p.3365-3373
• Main Authors: Vesga, Yuly, Diaz, Carlos, Crassous, Jeanne, Hernandez, Florencio E
• Format: Article
• Language: English
• Subjects: Chemical Sciences; or physical chemistry; Theoretical and
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.8b00347

Herein, we report on the theoretical–experimental analysis of the two photon absorption and circular dichroism spectra of 1-(2-pyridyl)-4-methoxy-carbo[6]­helicene derivative (P6). The primary outcomes of our investigation on this particular helicene derivative with a donor–acceptor motif on one end led to two important conclusions: (1) the lengthening of the π-electron delocalization within the helical core of P6 predominantly increases the contribution of the magnetic dipole transition moment to the two-photon circular dichroism (TPCD) signal; and (2) the electric quadrupole transition moment contribution to the TPCD signal is enhanced by the intramolecular charge transfer (ICT) produced by the donor–acceptor combination on one end of the molecule. To corroborate our results, we performed a comparative theoretical analysis of the effect of the energy gap and ICT on TPCD on a series of P6-like helicenes with different donor–acceptor combinations. Two-photon absorption and TPCD spectra were obtained using the double L-scan technique over a broad spectral range (400–900 nm) using 90 fs pulses at a low repetition rate (2–50 Hz) produced by an amplified femtosecond system. The theoretical simulations were calculated using time dependent density functional theory at the CAM-B3LYP/6-311++G­(d,p) level of theory.