Rational design and investigation of nonlinear optical response properties of pyrrolopyrrole aza-BODIPY-based novel push-pull chromophores

Abstract

Intramolecular charge transfer (ICT)-based chromophores are highly sought after for designing near-infrared (NIR) absorbing and emitting dyes as well as for designing materials for nonlinear optical (NLO) applications. The properties of these 'push-pull' molecules can easily be modified by varying the electronic donor (D) and acceptor (A) groups as well as the π-conjugation linker. This study presents a methodical approach and employs quantum chemical analysis to explore the relationship between the structural features, electro-optical properties, and the NLO characteristics of molecules with D-π-A framework. The one- and two-photon absorption (2PA), linear polarizability (α), and first hyperpolarizability (β) of some novel chromophores, consisting of a dimeric aza-Boron Dipyrromethene (aza-BODIPY) analogue, called, pyrrolopyrrole aza-BODIPY (PPAB), serving as the acceptor, have been investigated. The electronic donors used in this study are triphenylamine (TPA) and diphenylamine (DPA), and they are conjugated to the acceptor via thienyl or phenylene π-linkers. Additionally, the Hyper-Rayleigh Scattering (β_HRS), which enables direct estimation of the second-order NLO properties, is calculated for the studied chromophores with 1064 nm excitation in acetonitrile. The β value shows a significant increase with increasing solvent polarity, indicating that the ICT plays a crucial role in shaping the NLO response of the studied molecules. The enhancement of the 2PA cross-section of the investigated molecules can also be achieved by modulating the combinations of donors and linkers. The results of our study indicate that the novel D-π-A molecules designed in this work demonstrate considerably higher hyperpolarizability values than the standard p-nitroaniline, making them promising candidates for future NLO applications.

Fig. 1. The 2-D representation of the acceptor, linker, and donor components.

Fig. 2. Design strategy of novel PPAB chromophores for NLO applications: upper panel shows the optimized geometries of D–π–A–π–D series of molecules (1–3), used earlier for designing NIR-emitting dyes [ref. 27]. As NLO response of these molecules are quite low, presumably due to their centrosymmetric nature, using the same donor and acceptor groups we have designed D–π–A chromophores PPAB (1–3), whose optimized geometries are shown in the lower panel.

Fig. 3. The HOMO and LUMO images with the energies of the D–π–A molecules, PPAB (1–3) in acetonitrile solvent.

Fig. 4. The electrostatic potential (ESP) of D–π–A molecules. The potential distribution in the ESP map increase in the order red < yellow < green < blue. The range of ESP map is ranging from −0.07 a.u. to 0.07 a.u.