State-of-the-art
femtosecond spectroscopies and quantum-chemical
methods were used to investigate the excited-state dynamics of D−π–A+ (C1) and D−π–A+–π–D (C2) methylpyridinium (acceptor
unit, A) derivatives bearing dibutylamino groups as strong electron
donors (D) and bithiophenes as highly effective π-rich spacers.
The absorption spectra of C1 and C2 are
broad and shifted to the red side of the visible spectral range. A
significant negative solvatochromism was observed for the absorption
bands of the investigated salts with increasing solvent polarity that
was rationalized in terms of the change in electron density upon excitation.
The absorption spectra of C2 are red-shifted with respect
to those of C1, whereas the emission bands of the two
compounds overlap, suggesting a localization of the excitation on
just one branch of the quadrupolar compound, which becomes the fluorescent
portion. This is in agreement with our quantum-mechanical calculations,
which predict that the symmetry of C2 is broken in the
relaxed S1 geometry. Excited-state symmetry breaking was
observed in all of the investigated solvents regardless of their polarity.
Femtosecond transient absorption and fluorescence up-conversion measurements
revealed that the excited-state dynamics of C1 is essentially
dominated by solvent relaxation, whereas in the case of C2, two distinct excited singlet states were detected in polar solvents,
where an intramolecular charge-transfer (ICT) state is efficiently
produced. The main photoinduced decay pathway of both compounds was
found to be internal conversion in all of the investigated media.
High two-photon-absorption cross sections of 500 and 1400 GM for C1 and C2, respectively, were obtained by means
of femtosecond-resolved two-photon excited fluorescence measurements,
thus demonstrating the enhancement in the nonlinear optical properties
of the quadrupolar compound over its dipolar counterpart, in agreement
with the more efficient ICT observed in the case of C2.