Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications

Fourteen
new dipolar cations have been synthesized, containing
methoxy or tertiary amino electron donor groups attached to helquat
(Hq) acceptors. These Hq derivatives have been characterized as their
TfO– salts by using various techniques including
NMR and electronic absorption spectroscopies. UV–vis spectra
show intense, relatively low energy absorptions with λmax ≈ 400–600 nm, attributable to intramolecular charge-transfer
(ICT) excitations. Single-crystal X-ray structures have been solved
for two of the chromophores, one as its PF6– salt, revealing centrosymmetric packing arrangements (space groups Pbca and P1̅). Molecular quadratic
nonlinear optical (NLO) responses have been determined directly by
using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and indirectly
via Stark (electroabsorption) spectroscopy for the low energy absorption
bands. The obtained static first hyperpolarizabilities β0 range from moderate to large: (9–140) × 10–30 esu from HRS in MeCN and (44–580) ×
10–30 esu from the Stark data in PrCN. The magnitude
of β0 increases upon either extending the π-conjugation
length or replacing a methoxy with a tertiary amino electron donor
substituent. Density functional theory (DFT) and time-dependent DFT
calculations on selected tertiary amino chromophores confirm that
the low energy absorptions have ICT character. Relatively good agreement
between the simulated and experimental UV–vis absorption spectra
is achieved by using the CAM-B3LYP functional with the 6-311G­(d) basis
set. The βtot values predicted by using DFT at the
same level of theory are large ((472–1443) × 10–30 esu in MeCN). Both the theoretical and experimental results show
that para-conjugation between Hq and electron donor
fragments is optimal, and enlarging the Hq unit is inconsequential
with respect to the molecular quadratic NLO response.