2PA Properties in Branched Alkene and Alkyne Chromophores
A R T I C L E S
individual arms.30 The interest in multiply branched systems
for 2PA also stems from the fact that relatively large 2PA cross
sections may primarily be realized from low-lying excited states.
Consequently, considerable research has been carried out toward
the design and synthesis of multiply branched chromophores.
Several configurations, such as donor-π-acceptor, acceptor-
π-donor, and chromophores with different branching centers,
π-bridging units, and tri- and tetra-branched structures, have
been synthesized and studied.29-50
molecules have been synthesized with varying donor-acceptor
strength, substituents, and conjugation length.30-33
Though larger donor-acceptor strength is important for
higher 2PA cross section, the nature and length of the π-linker
do play a decisive roles in increasing the 2PA cross section
and nonlinear optical (NLO) properties. It has been shown in
dipolar chromophores that richer π-electron systems improve
the intramolecular charge-transfer character and thereby enhance
the NLO properties.51 Several π-bridging chromophores have
been considered for improving the 2PA cross section. Among
them, alkene π-bridging is most widely studied, and it has been
suggested that, in many of the linear systems, alkene π-bridging
is better than alkyne π-bridging.52,53 However, for several
reasons pertaining to intra-arm electronic coupling, charge
delocalization, and charge transfer, the consequences of π-bridg-
ing on NLO and 2PA properties in branched systems may be
different from those in the linear quadrupolar molecules.54-56
It has been theoretically predicted from ab initio calculations
that the 2PA cross sections of alkyne π-bridge-containing
quadrupolar molecules are not significantly different from those
of their alkene π-bridged analogues.56,57 However, a systematic
experimental study of 2PA in branched trimers with different
π-bridging and the underlying mechanism has not been reported.
To understand the effect of π-bridging, acceptor strength, and
conjugation length on 2PA cross sections of multiply branched
chromophores, we have synthesized new triphenylamine-core
branched molecules with pyridine as acceptor (molecules A-D,
Figure 1). Two-photon absorption cross sections of these
molecules have been measured by two-photon excited fluores-
cence and non-degenerate pump-probe techniques. Compari-
sons of 2PA cross sections were also made with N,N,N-tris[4-
{2-(4-{5-[4-(tert-butyl)phenyl]-1,3,4-oxadiazol-2-yl}phenyl)-1-
ethenyl}phenyl]amine (PRL-701) and tris[4-(3′,5′-di-tert-
butyldistyrylbenzenyl)phenyl]amine (N(DSB)3). Here, we have
varied the nature of π-bridging from alkenes (A and C) to
alkynes (B and D), conjugation length within individual branches
(A to B, C to D), and acceptor strength at the end of each branch
(B, PRL-701, and N(DSB)3). Steady-state and time-resolved
spectroscopic techniques are employed to probe the mechanism
of 2PA. We have correlated the observed 2PA cross sections
with the extent of intramolecular charge transfer probed by
femtosecond transient absorption spectroscopy. The results of
2PA measurements have shown that alkene π-bridged chro-
mophores have superior 2PA properties over the alkyne
π-bridged counterparts at low energy, and we were able to
successfully correlate the observed trend with the results of
transient absorption spectroscopy. Further, an increase in
acceptor group strength as well as an increase in conjugation
length within a branch was found to increase 2PA cross sections.
Mechanistic investigations aimed at understanding the factors
that influence the magnitude of the 2PA cross section in multiply
branched systems suggest that it is a complex interplay between
intra-arm coupling, electronic delocalization, and extent of
intramolecular charge transfer.37-44 Beljonne et al.29 have
suggested that, within the exciton picture, the 2PA cross section
in octupolar molecules should scale by a factor of 3 in
comparison to the dipolar counterparts by virtue of increased
charge transfer among the three arms. Cho et al.50 have shown
theoretically that, in branched molecules, the 2PA cross section
increases as the strength of the donor-acceptor interaction
increases. Among several branching centers investigated in
octupolar systems, triphenylamine systems were found to show
better enhancement of 2PA cross sections with increased
branching.29-32 Prasad and co-workers30 have shown a 7-fold
enhancement of 2PA cross section for trimer (PRL series) over
monomer in a triphenylamine-core branched molecule. Macak
et al.49 suggested that electronic coupling is nominal between
the arms and vibronic coupling between the arms is significant
in enhancing the 2PA of the PRL series of dyes. Mechanistic
investigations on the same series of dye molecules suggested
that effective electronic delocalization44 and increased intramo-
lecular charge transfer are possible reasons behind the enhanced
2PA cross section. Also, several triphenylamine-core branched
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