In
this investigation, we report the first hyperpolarizabilities
and two-photon absorption cross sections of a large series of 12 push–pull
cationic chromophores. All of these dyes show a dipolar acceptor+–π–donor structure, where the nature of
the donor and acceptor units and π-bridge was synthetically
tuned to allow insightful comparisons among the molecules. The hyperpolarizability
was obtained through a solvatochromic method, by exploiting the rare
negative solvatochromism exhibited by the investigated compounds.
The two-photon absorption cross sections were determined through two-photon
excited fluorescence measurements by means of a tunable nanosecond
laser system for sample excitation. The nonlinear optical properties
were discussed relatively to the photoinduced intramolecular charge
transfer occurring in these donor–acceptor systems, investigated
by femtosecond transient absorption experiments. We found a strong
increase in hyperpolarizability upon increasing the molecular conjugation.
Unexpectedly, the hyperpolarizability is almost unaffected by an increase
in donor/acceptor strength and intramolecular charge transfer degree.
Differently, the two-photon absorption cross sections of these dyes
are enhanced by an increase in both molecular conjugation and intramolecular
charge transfer efficiency. Several recent literature works have reported
at the same time scattered information about the hyperpolarizability
and two-photon absorption of small organic molecules. Our investigation
is, to the best of our knowledge, the first attempt to uncover detailed
structure–property relationships for these two nonlinear optical
properties. Our results represent a promising route to achieve large
hyperpolarizability and two-photon absorption in push–pull
dyes and may drive the design of new efficient nonlinear optical materials.