Theoretical and experimental studies of aryl-bithiophene based push-pull pi-conjugated heterocyclic systems bearing cyanoacetic or rhodanine-3-acetic acid acceptors for SHG nonlinear optical applications

A series of push-pull aryl-bithiophene based systems 2-3 were designed and synthesized in order to understand how structural modifications influence the electronic, linear and nonlinear optical properties. The push-pull conjugated chromophores 2-3 bear a bithiophene spacer conjugated with a phenyl ring functionalized with N,N-dialkylamino electron-donor groups together with cyanoacetic or rhodanine-3-acetic acid acceptor groups. Theoretical (DFT calculations) and experimental studies were carried out to obtain information on conformation, electronic structure, electron distribution, dipolar moment, and molecular nonlinearity response of the push-pull bithiophene derivatives. This multidisciplinary study revealed that chromophore 2e exhibits the highest value for hyperpolarizability beta (10440 × 10-30 esu) due to the strong electron donating ability of the N,N-diethylamino group, and the ethyne linker that not only lengthens the pi- conjugation path but also grants less distortion to the system.Thanks are due to Fundação para a Ciência e Tecnologia (FCT) for a PhD grant to S. S. M. Fernandes (SFRH/BD/87786/2012) and FEDERCOMPETE for financial support through the CQ/UM (Ref. UID/QUI/ 00686/2013 and UID/QUI/0686/2016). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Reequipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. The pulsed laser system was acquired within the framework of the grant (PTDC/CTM/105597/2008) from the Fundação para a Ciência e Tecnologia (FCT) with funding from FEDERCOMPETE. This work was also supported by the Associated Laboratory for Sustainable Chemistry - Clean Processes and Technologies - LAQV which is financed by Portuguese national funds from FCT/MEC (UID/ QUI/50006/2013) and co-financed by the ERDF under the PT2020 Partnership Agreement (POCI-01-0145-FEDER–007265).