Review of NLOphoric azo dyes-Developments in hyperpolarizabilities in last two decades

This review gathers the hyperpolarizabilities of monomeric azo dyes studied by various analytical techniques electric-field-induced second-harmonic generation (EFISHG), hyper-Rayleigh scattering (HRS), Z-scan technique, solvatochromic method, and density functional theory (DFT). Dyes are sub-classified according to their structural characteristics. Absorption-emission maxima, and thermal stabilities of the dyes are also gathered. NLO properties of azo dyes are comparable to styryl type compounds. The combination of azo dyes with extended alkene can improve thermal stability and NLO response. In every section, structures displaying higher NLO response are highlighted. The structural characteristics responsible for the enhanced the hyperpolarizabilities are: incorporation of N, N- dialkyl donor with -NO2 acceptor connected through the heterocyclic ring as a spacer; conjugation extension in the azo dyes with stilbene/styryl type structure; incorporation of strong acceptor 3dicyanomethylene-2,3-dihydrobenzo[b]thiophene-1,1-dioxide in the structure. Moreover, various donoracceptor fragments producing higher hyperpolarizability are mentioned in the 'summary and future aspects section'. This review will help researchers while designing and synthesizing NLOphores.

Automated summary

This review summarizes the hyperpolarizabilities of monomeric azo dyes studied by various analytical techniques, such as EFISHG, HRS, Z-scan technique, solvatochromic method, and DFT. It categorizes dyes based on their structural characteristics and highlights structures that exhibit higher non-linear optical response. Incorporating specific structural features like donor-acceptor pairs and conjugation extension can enhance the hyperpolarizabilities, as discussed in the review.