The vibrationa
l, optica
l, and non
linear optica
l (NLO) properties of a series of push-pu
ll chromophores bui
ltaround dithieny
lethy
lene-based
-conjugating spacers have been investigated by UV-vis, IR, and Ramanspectroscopies and e
lectric fie
ld-induced second harmonic generation (EFISH) measurements. The effects ofthe strength of the acceptor group on the mo
lecu
lar e
lectronic properties of these conjugated NLO chromophoreshave been addressed. The magnitude of the intramo
lecu
lar charge transfer has been tested as a function ofthe acceptor strength and of the bridging of the spacer. Density functiona
l theory (DFT) ca
lcu
lations havebeen performed to he
lp the assignment of the main e
lectronic and vibrationa
l features of the NLO chromophoresand to derive usefu
l information about their mo
lecu
lar structures. EFISH measurements show that push-pu
llsystems that contain strong e
lectron-acceptor groups connected via a rigidified dithieny
lene spacer exhibit
large va
lues of
le">
0. Theoretica
l NLO ca
lcu
lations are in exce
llent agreement with experimenta
l resu
lts. Thegeometrica
l and e
lectronic properties ca
lcu
lated in so
lution revea
l that chromophores become high
ly po
larizedas the die
lectric constant of the so
lvent increases. The spectroscopic properties/structure re
lationships agreewith the observation that
le">
0 increases upon the cova
lent bridging of the spacer and with the increase of theacceptor strength.