Impact of different auxochromes hydroxyl and methyl on two photon absorption coefficients and ultrafast dynamics of near-infrared hemicyanine dyes.

There is immense interest in the design of organic nonlinear optical (NLO) chromophores for optical limiting and bioimaging applications. In this work, two near-infrared hemicyanine compounds which are named 2-(2-(3-Hydroxy-8,9-dihydro-7H-pyrido[1,2-a]quinazolin-10-yl)vinyl)-1,1-dimethyl-3-propy l-1H-benzo[e]indol-3-ium iodide (Hemicyanine1) and 1,1-Dimethyl-2-(2-(3-methyl-8,9-dihydro-7Hpyrido[1,2-a]quinazolin-10-yl)vinyl)-3-propyl-1H-benzo[e]indol-3-ium iodide (Hemicyanine2) were synthesized. The difference between Hemicyanine1 and Hemicyanine2 is that one of the auxochrome is hydroxyl and the other is methyl. The third-order nonlinear optical absorption properties of Hemicyanine1 and Hemicyanine2 were investigated using broadband Z-scan measurements at nanosecond time regime. The Z-scan results demonstrate that both chromophores have reverse-saturable absorption (RSA), while the NLO response of Hemicyanine2 is significantly stronger than Hemicyanine1. The corresponding physical mechanism were studied via femtosecond transient absorption (TA) spectroscopy. The results of femtosecond TA measurement demonstrate that the lifetime of excited state of Hemicyanine2 is considerably longer than that of Hemicyanine1. Our study indicated that the auxochrome is critically important in optimizing the NLO response and excited-state lifetime of hemicyanine chromophores for photonic applications. • Two novel near-infrared hemicyanine dyes Hemicyanine1 and Hemicyanine2 which have different auxochromes Hydroxyl and methyl were synthesized to investigate the third-order nonlinear optical properties and ultrafast dynamics. • Both compounds showed reverse saturable absorption and obvious differences under the excitation of nanosecond time regime. • The optical limiting performance and possible damage threshold were investigated on Hemicyanine2. • Femtosecond transient absorption spectra revealed the ultrafast dynamics. • By changing the auxochrome, the lifetimes of excited states can be adjusted to improve the reverse-saturable absorption of Hemicyanine2 , which is significantly enhanced compared to Hemicyanine1. [ABSTRACT FROM AUTHOR]