Synthesis, photophysical and nonlinear optical properties of push-pull tetrazoles

A 2,5-disubstituted tetrazole with p-nitrophenyl and 3-pyridyl units as acceptors (1a), and three push-pull tetrazoles with p-nitrophenyl as an acceptor and phenyl (1b), 2-(dibenzo[b,d]furan-4-yl) (1c), and 4-(N,N-diphenylamino)phenyl (1d) as donor groups, were synthesized by copper-catalyzed aerobic C-N coupling of p-nitrophenyl tetrazole with appropriately substituted aryl boronic acids. The absorption and emission spectra of 1a-c showed minimal dependence on the polarity of the solvent; however, in the case of 1d a blue shift was noted in the longest absorption band (λ ₁) as the polarity increased. The fluorescence intensity of the title compounds was found to be solvent-dependent; however, no apparent correlation to solvent polarity could be established. The absorption and emission characteristics of 1a-d were also influenced by the nature of the substituent as 1d, bearing a strong electron donating 4-(N,N-diphenylamino)phenyl group, displayed a significant red shifted absorption (λ ₁) as well as emission (λ _em) bands compared to other compounds. Time dependent density functional calculations (CAM-B3LYP/6-311++G**) revealed that the longest wavelength band (λ ₁) is associated with an intramolecular charge transfer (ICT) from HOMO/HOMO-1/HOMO-2 → LUMO/LUMO+1 in these molecules. The first hyperpolarizability values, β _HRS, of 1a-d were measured using the solution-based hyper-Rayleigh scattering technique using a femtosecond Ti:Sapphire laser and the highest NLO activity was measured for 1d with the greatest push-pull characteristics. A strong correlation was observed between the calculated hyperpolarizability (β _tot) and experimentally measured values (β _HRS).