16188 | J. Mater. Chem. C, 2020, 8, 16188--16197 This journal is © The Royal Society of Chemistry 2020 Cite this: J. Mater. Chem. C, 2020, 8, 16188 Metal-free carbazole scaffold dyes as potential nonlinear optical phores: molecular engineering† Ravi Ketavath, ‡ a Khevath Praveen Kumar Naik,‡ a Sachin G. Ghugal, ‡ a Naga Krishnakanth Katturi, b T. Swetha, a Venugopal Rao Soma b and Banavoth Murali * a In the light of diverse applications of nonlinear optical (NLO) chromophores in interdisciplinary fields, there is high demand for the quest of cost-effective and facile processing of smart materials. To narrow down such potential sensitizers, a profound physical understanding of the structure–property relation- ship is the need of the hour. The new molecular designs have triggered the development of simple carbazole scaffold dyes with D–p–A configurations, which can act as NLO chromophores. The theoreti- cal and experimental frameworks of these novel molecular-engineered dyes exhibited strong charge- transfer absorption bands in the visible region besides high extinction coefficients and appropriate redox properties, making them promising canditates for NLO devices. Notably, the substitution of p-linkers and the insertion of the azo group on carbazole is critical for the optimal optoelectronic window, as evidenced by the spectral, electrochemical experiments, and density functional theory calculations. The state-of-the-art femtosecond (fs) Z-scan studies unveiled a strong third-order NLO susceptibility of B10 À12 esu. Intriguingly, these dyes also exhibited reversible saturable absorption (RSA) kind of behavior when excited at 800 nm due to two-photon absorption (2PA), further augmenting them as alluring candidates for NLO device applications. The strong 2PA cross-sections [362–1890 GM] obtained for these molecules propitiously augur well for bio-imaging and other photonic applications. Introduction Goppert Meyer theoretically found the possibility for the simul- taneous absorption of two photons by the same molecule, which was experimentally demonstrated in 1961. 1,2 Compared to one-photon absorption, two-photon absorption (2PA) can create excited states by the simultaneous interaction of two photons when focused with a pulsed laser. 3 2PA is a way of accessing a given excited state using photons with half the energy of the corresponding one-photon transition, leading to a wide range of applications, such as optical signal processing, optical switching, and telecommunications. 4–10 Thus, there is a strong demand for efficient 2PA dyes. The primary criteria for a molecule to show two-photon absorption is a large cross- section area for absorption (d). 11 Therefore, several strategies have been developed to enhance the cross-section area and the wavelength of the two-photon absorption peak of p-conjugated organic molecules. To fulfill the above criteria, the molecule should be designed in such a way that the conjugation is increased to increase the charge-transfer distance, the strength of the electron-donating and accepting substituents within the molecule are high so that the energy gap is less, and finally, the degree of non-centrosymmetric distribution leads to enhanced non- linear optical (NLO) properties. 12–16 Keeping this in mind, many researchers are focusing on designing p-conjugated organic dyes. Recently, Cui et al. designed and synthesized series of D–A–D quinacridones, and they observed an enhancement in the nonlinear optical response when the two sides of quina- cridones were connected with the electron donor groups (carbazole). They reported significant 2PA values in the range of 1.23–3 Â 10 À13 mW À1 with 190 fs pulses at 1030 nm wavelength. 17 Ayare et al. investigated the NLO properties of imidazole-fused anthraquinone dyes and found increased ICT upon the addition of carbazole and triphenylamine donor groups, and the 2PA values measured at 532 nm with 5 ns pulses at 10 Hz repetition rate were in the range of B10–12 m W À1 . 18 Huang et al. synthesized large cyano- and triazine-substituted D–p–A–p–D compounds, which demonstrated good TPA cross- section values of 1363 GM, 413 GM and 5782+ GM at 800 nm a Solar Cells and Photonics Research Laboratory, School of Chemistry, University of Hyderabad, Hyderabad 500046, Telangana, India. E-mail: murali.banavoth@uohyd.ac.in; Tel: +91-40-23134828 b Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Hyderabad 500046, Telangana, India † Electronic supplementary information (ESI) available: Details of the charac- terization (NMR, HRMS, LCMS spectra, and EA data), and DFT Calculation of the dyes D1–D5 are described. See DOI: 10.1039/d0tc03866k ‡ R. K., P. K., and S. G. G. have contributed equally to this work. Received 13th August 2020, Accepted 9th October 2020 DOI: 10.1039/d0tc03866k rsc.li/materials-c Journal of Materials Chemistry C PAPER Published on 10 October 2020. Downloaded by University of Hyderabad on 11/27/2020 3:46:33 AM. View Article Online View Journal | View Issue