Synthesis, photophysical and nonlinear optical properties of pushpull tetrazoles

A 2,5-disubstituted tetrazole with p -nitrophenyl and 3-pyridyl units as acceptors ( 1a ), and three pushpull 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 CN coupling of p -nitrophenyl tetrazole with appropriately substituted aryl boronic acids. The absorption and emission spectra of 1ac 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 ( 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 1ad 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 ( 1 ) as well as emission ( em ) bands compared to other compounds. Time dependent density functional calculations (CAM-B3LYP/6-311++G**) revealed that the longest wavelength band ( 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, HRS , of 1ad 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 pushpull characteristics. A strong correlation was observed between the calculated hyperpolarizability ( tot ) and experimentally measured values ( HRS ). A 2,5-disubstituted tetrazole with a p -nitrophenyl unit as an acceptor and a 4-( N , N -diphenylamino) phenyl group as a donor exhibits strong pushpull characteristics and displays high NLO activity.