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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]

Four sandwich phthalocyanines with different rare earth metals (La, Y, Yb and Sc) are synthesized to investigate the influence of aggregation on nonlinear optical limiting effect. Hyperchem fitting results show that the distance between the phthalocyanine rings of the four phthalocyanine compounds is 2.81, 2.69, 2.56 and 2.38 Å for LaPc (Tetra-β-(4-nitro) lanthanum phthalocyanine), YPc (Tetra-β-(4-nitro) yttrium phthalocyanine), YbPc (Tetra-β-(4-nitro) ytterbium phthalocyanine) and ScPc (Tetra-β-(4-nitro) scandium phthalocyanine), respectively. As the radius of rare earth ions decreases, the interlayer spacing between phthalocyanine rings decreases, which greatly increases the degree of aggregation between phthalocyanine rings. As a result, the photophysical and nonlinear absorption coefficients of different rare-earth phthalocyanines also show regular changes. The nonlinear absorption coefficients of the four rare-earth phthalocyanines were LaPc (β = 1.86ⅹ10−10 m/W), YPc (β = 9.78ⅹ10−11 m/W), YbPc (β = 6.63ⅹ10−11 m/W), ScPc (β = 7.43ⅹ10−11 m/W), respectively, indicating that the degree of aggregation between phthalocyanine rings gradually increases as the radius of rare earth metals changes from La, Y, Yb, to Sc, which inhibits the reverse saturable absorption process of the triplet states, resulting in a gradual decrease of the nonlinear absorption coefficient. The experimental results fully reveal the effect of the aggregation effect between phthalocyanine rings on photophysical and nonlinear optical properties, and would provide a theoretical basis for the design of high-performance nonlinear optical materials. Image 1 • Sandwich phthalocyanines with different metals (La, Y, Yb, Sc) are designed to investigate the effect of aggregation on nonlinear optical effect. • Results show that the distance between the phthalocyanine rings are 2.81, 2.69, 2.56 and 2.38 Å for LaPc, YPc, YbPc and ScPc, respectively. • As the metal ion radius decreases, the aggregation effect between phthalocyanine rings increases, which results in worse NOL behaviour. • The influence of aggregation on structural, photophysical and NOL properties was obtained by adjusting the distance between phthalocyanine rings. [ABSTRACT FROM AUTHOR]