Nonlinear Optical Chromophores Research Articles

High-throughput virtual screening of organic second-order nonlinear optical chromophores within the donor-π-bridge-acceptor framework.

In view of the theoretical importance and huge application potential of second-order nonlinear optical (NLO) materials, it is of great significance to conduct high-throughput virtual screening (HTVS) on a compound library to find candidate NLO chromophores. Under the donor-π-bridge-acceptor structural framework, a virtual compound library (size = 27 090) was constructed by enumeration of structural fragments. The kernel property adopted for optimization is the static first hyperpolarizability (β0). By combining machine learning and quantum chemical calculations, we have performed an HTVS procedure to sieve NLO chromophores out, and the response mechanism of the selected optimal NLO chromophores was examined. We have found: (a) The multi-layer perceptron/extended connectivity fingerprint combination with 20% selection ratio gives the highest prediction accuracy for the studied systems. (b) The two optimal donors are bis(4-diphenylaminophenyl)aminyl and bis(4-tert-butylphenyl)aminyl; the optimal π-bridges are composed of two thiophenyl, selenophenyl or furanyl units; and the two optimal acceptors are tri-s-triazinyl and 2,3-dicyanopyrazinyl. (c) The no. 1 candidate molecule can exhibit a calculated β0 equal to 8.55 × 104 a.u. (d) The difference in NLO responses of the optimal 16 molecules comes from the synergistic interaction of ES1, Δμ and f, by employing the two-level model. In addition, the sizable Δμ and f allow the studied optimal molecules to obtain a large NLO response in the meantime keeping a not-too-low excitation energy (retaining good optical transparency in the restricted range of the visible spectrum region). (e) With further modification on the acceptor, the designed DPA-π-TRZ-A' (A' = CN or NO2, π = oligo-thiophenyl or selenophenyl) systems can exhibit a rather large NLO response (maximum β0 = 3.17 × 105 a.u.), hence should have considerable potential as second-order NLO chromophores. With the above observations, we expect to provide some insight for the research community into the HTVS of organic second-order NLO chromophores.

Study on the structure–performance relationship of nonlinear optical chromophores with different donors, bridges, and acceptors

Three series of high performance nonlinear optical chromophores based on triphenylamine or N,N-bis-(4-methoxyphenyl)arylamine as donors and three kinds of acceptors were synthesized and investigated.

First hyperpolarizability of the di-8-ANEPPS and DR1 nonlinear optical chromophores in solution. An experimental and multi-scale theoretical chemistry study

The solvent effects on the linear and second-order nonlinear optical properties of an aminonaphtylethenylpyridinium (ANEP) dye are investigated by combining experimental and theoretical chemistry methods. On the one hand, deep near infrared (NIR) hyper-Rayleigh scattering (HRS) measurements (1840–1950 nm) are performed on solutions of di-8-ANEPPS in deuterated chloroform, dimethylformamide, and dimethylsulfoxide to determine their first hyperpolarizablity (βHRS). For the first time, these HRS experiments are carried out in the picosecond regime in the deep NIR with very moderate (≤3 mW) average input power, providing a good signal-to-noise ratio and avoiding solvent thermal effects. Moreover, the frequency dispersion of βHRS is investigated for Disperse Red 1 (DR1), a dye commonly used as HRS external reference. On the other hand, these are compared with computational chemistry results obtained by using a sequential molecular dynamics (MD) then quantum mechanics (QM) approach. The MD method allows accounting for the dynamical nature of the molecular structures. Then, the QM part is based on TDDFT/M06-2X/6-311+G* calculations using solvation models ranging from continuum to discrete ones. Measurements report a decrease of the βHRS of di-8-ANEPPS in more polar solvents and these effects are reproduced by the different solvation models. For di-8-ANEPPS and DR1, comparisons show that the use of a hybrid solvation model, combining the description of the solvent molecules around the probe by point charges with a continuum model, already achieves quasi quantitative agreement with experiment. These results are further improved by using a polarizable embedding that includes the atomic polarizabilities in the solvent description.

An extensive investigation of structural, spectral, optical, electrical, and nonlinear optical properties of a novel crystal (S) 4-(5-bromo-2-acyloxyphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid ethyl ester

The organic nonlinear optical chromophore crystal (S) 4-(5-Bromo-2-acyloxyphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimidine-5-carboxylic acid ethyl ester, abbreviated as BTHPM, was synthesized using Tetrahydro Furan (THF) as a solvent and crystals were grown using a slow solvent evaporation process. One pot multicomponent reaction of 5-Bromo-2-acyl salicylaldehyde with ethyl acetoacetate and urea using (−) Binol borane as a chiral lewis acid. The crystal structure and the lattice inter perfection of the material were determined by SXRD (Single crystal X-ray diffraction) analysis. The functional groups of the title compound were characterized by Fourier Transform Infrared (FTIR) spectral studies. The direct optical band gap was determined to be 5.9 eV, allowing the transparency down to 342 nm. The bond formation, splitting pattern and the atom environment was evidenced in 1H Nuclear Magnetic Resonance (1H NMR), and 13C Nuclear Magnetic Resonance (13C NMR) studies, using Deuterated chloroform (CDCl3) as reference. The analysis of photoconductivity has confirmed the crystal-positive photoconductivity. To investigate the third-order Nonlinear Optical (NLO) behaviour of BTHPM crystal, the nonlinear absorption coefficient, nonlinear refractive index, and third-order nonlinear susceptibility were measured using a Z-scan experiment. The BTHPM crystal dielectric properties were investigated at a range of frequencies.

Comparative study on twelve kinds of electron donors for organic functional materials

The development and comparison of different kinds of donors showed very important role in the field of organic functional materials. A series of nonlinear optical chromophores 1–12 were developed based on twelve different types of electron donors, with the same isophorone based bridge and TCF acceptor. The donors studied include Ethyl methylaniline, tetrahydroquinoline, Ethoxykowloon aniline, triphenylamine, thiophen-2-amine derivatives, bis(alkyl)aniline derivatives and poly heteroatom aniline. Thermal stability analysis, optical properties, Density functional theoretical calculation (DFT) and electro-optical coefficient test of these chromophores was systematically studied to display the effect of different donors on the properties of Chromophores. The results show that the introduction of one or more heteroatoms can effectively improve the electron donating ability of the donor, so as to improve the first hyperpolarizability of the chromophore. The doped film containing the 25 wt% chromophore 1–12 showed electro-optic coefficients from 54 to 163 p.m./V, respectively.

Nonlinear optical materials based on fluorinated polyurethane-imides and their application in waveguide devices

Two monomers, a second-order nonlinear optical azo chromophore C containing a tricyanofuran electron acceptor and a dihydroxyethyl nitrogen electron donor, and a bisphenol AF-type diether dianhydride (BPAFDA), were designed and synthesized. Fluorinated polyurethaneimide (PUI) electro-optic (EO) waveguide materials were prepared using the synthesized monomers polymerized with 4,4′-diphenylmethane diisocyanate (MDI). The structures of the synthesized chromophore C, BPAFDA, and polymers of PUI were characterized by 1HNMR and FTIR, and the thermal properties of the polymers were characterized by DSC and TGA. The prepared PUI exhibited good film-forming properties with glass transition temperatures (Tg) between 160–169°C and over 300°C at 5% thermal weight loss in a nitrogen atmosphere. The experimental results showed that the fluorinated PUIs possessed an EO coefficient of 56–60 p.m./V at 1550 nm and the optical propagation loss of the polymer waveguide was between 1.3–1.4 dB/cm at 1550 nm. Using PUI as the core material of the waveguides, EO modulators with Mach-Zehnder (MZ) structure were designed and prepared, showing good EO modulation performance at 1550 nm.

Nonlinear optical properties and transient absorption in Hibiscus Sabdariffa dye solution probed with femtosecond pulses

Highly conjugated organic molecules are attractive chromophores for nonlinear optics due to their significant third-order nonlinear response near the absorption bands located in the broad spectral range from 200 to 900 nm. In this work, we report on the wavelength-dependence of one- and two-photon absorption processes in Hibiscus sabdariffa organic dye excited by femtosecond (fs) laser pulses centered at 1030 nm and 515 nm. Z-scan and transient absorption (TA) techniques have been employed to investigate the absorption and relaxation processes in the target dye. The dye is found to have a significant negative nonlinear absorption coefficient (β) of value ∼ −2.7 × 10−11 cm/W at 515 nm probing laser pulses, while it has a positive value equal to +2.1 × 10−13 cm/W when 1030 nm laser pulses were employed. The TA decay measurements using 40 fs laser pulses, centered at 515 nm wavelength, reveal that the dye exhibits a short relaxation time of ∼3 ps attributed to the involvement of the excited state proton transfer (ESPT) process in the Hibiscus dye solution. Moreover, our time-integrated fluorescence measurements revealed that no fluorescence emission could be detected. The application of ultrashort pulses of ∼40 fs duration enabled the tracking of ESPT in the Hibiscus dye, which was not detected in previous studies that employed laser pulses of picoseconds and nanoseconds duration.

Systematic Study on Nonlinear Optical Chromophores with Improved Electro-Optic Activity by Introducing 3,5-Bis(trifluoromethyl)benzene Derivative Isolation Groups into the Bridge

A series of novel chromophores A, B, C, and D, based on the julolidinyl donor and the tricyanofuran (TCF) and CF3-tricyanofuran (CF3-Ph-TCF) acceptors, have been synthesized and systematically investigated. The 3,5-bis(trifluoromethyl)benzene derivative isolation group was introduced into the bridge in the chromophores C and D. These nonlinear optical chromophores showed good thermal stability, and their decomposition temperatures were all above 220 °C. Density functional theory (DFT) was used to calculate the energy gaps and first-order hyperpolarizability (β). The macroscopic electro-optic (EO) activity was measured using a simple reflection method. The highest EO coefficient of poled films containing 35 wt% of chromophore D doped in amorphous polycarbonate afforded values of 54 pm/V at 1310 nm. The results indicate that the 3,5-bis(trifluoromethyl)benzene isolation group can suppress the dipole-dipole interaction of chromophores. The moderate r33 value, good thermal stability, and good yield of chromophores suggest their potential use in the nonlinear optical area.

Size economy and first hyperpolarizability: synthesis and nonlinear optical behavior of ferrocene-based donor-acceptor chromophores lacking π-linkers.

The question of size economy in the design of chromophores for nonlinear optics is addressed in this investigation. We have synthesized directly linked donor-acceptor dyads, which lack a π-conjugated linker, the presence of which is usually considered obligatory in materials designed for nonlinear optics. Correlating linear optical data, electrochemical data, computational data and hyper Rayleigh scattering (HRS) data on ferrocene (Fc) based dyads, we demonstrate that the first hyperpolarizability of such size economical chromophores is significantly better compared to that of Fc based, traditional, larger, donor-π-acceptor chromophores. Arguably, a larger π-conjugated linker decreases the electronic communication between the donor and the acceptor and weakens the intramolecular charge transfer in such chromophores.

1,8‐Diarylnaphthalenes: Synthesis, Properties, and Applications

Abstract1,8‐Diarylnaphthalenes are a fascinating example of strained organic molecules that have found many practical applications due to unusual parallel face‐to‐face arrangement of two peri‐aryl rings in close proximity almost perpendicular to the central naphthalene backbone. 1,8‐Diarylnaphthalenes show notable potential as photoluminescent and chiral sensors, stereodynamic switches, nonlinear optic chromophores, blue‐transparent frequency doublers, materials for creating high‐performance blue and green OLEDs, chiral ligands and so on. Molecules of this type have also been used as convenient models for studying π‐π, dipole‐π and cation‐π interactions. 1,8‐Diarylnaphthalenes are also interesting as objects to study atropisomeric transformations: if the peri‐aryl rings have ortho‐ or meta‐substituents, then the anti‐conformer becomes C2‐symmetrical (hence chiral), and the syn‐isomer upon symmetrical substitution is achiral. In this review, data on synthetic methods, applications, atropisomerism and structural characteristics of 1,8‐diaryl‐ and 1,8‐dihetarylnaphthalenes are summarized.

Templating Polymer/Chromophore Crystallization in a Gyroid Matrix

Blending the small molecule nonlinear optical chromophore, 2-chloro-4-nitroanaline (CNA), with a semicrystalline block copolymer, polystyrene-poly(ethylene oxide) (PS-PEO), drives the formation of a bicontinuous cubic microstructure, known as gyroid (GYR), demonstrating Ia3̅d cubic space group symmetry. The morphology transition is due to the selective partitioning of CNA into the PEO domains, which happens to be the majority phase while PS forms the GYR network. Furthermore, PEO and CNA cocrystallize together, forming a single crystalline phase that exhibits a different crystal structure from the starting PEO and CNA materials. At room temperature, PEO crystals and PEO/CNA cocrystals coexist and display different melting temperatures, Tm,PEO = 45 °C and Tm,PEO/CNA = 78 °C, respectively. Interestingly, although the PEO domain is the matrix, the GYR network templates the crystallization of the PEO/CNA cocrystal by precisely controlling the cocrystal long period. Furthermore, circular dichroism (CD) measurements indicate that the crystallized sample contains a chiral component when the PEO/CNA cocrystal is present, and upon melting of the PEO/CNA cocrystal (Tm,PEO/CNA = 78 °C), the CD signal vanishes. The results shown here indicate that adding a small molecule to semicrystalline block copolymer in which one block will form a cocrystal with the small molecule opens the possibility of controlling hierarchical structure and material functionality.

Organic nanomedicine containing nonlinear optical chromophores for ultrastable photo-to-heat converting theranostics in the near-infrared window

Organic molecules that are active in the near-infrared (NIR) bio-window for photo-to-heat energy-converting theranostics have attracted much attention in cancer therapy. However, current organic NIR-absorbing molecules are still suffering from the inefficient non-radiative decay or serious photobleaching under light irradiation. Herein, we design a convenient strategy to load a new kind of NIR-absorbing nonlinear optical (NLO) chromophore FTC-3f with the biocompatible polymer, methoxy poly(ethylene glycol)-poly(lactideco-glycolide) (mPEG-PLGA). The resulted FTC-3f/PLGA nanoparticles showed excellent stability, high photothermal conversion efficiency (∼51.11%) and good photoacoustic performance. Moreover, in vitro anticancer experiments demonstrated that the FTC-3f/PLGA nanoparticles possess a high PTT (photothermal therapy) efficiency under NIR laser irradiation. This work thus provides an insight into the exploration of versatile organic NIR-absorbing nanoparticles toward future practical PTT application.

Synthesis of new type of nonlinear optical chromophores: The introduction of a novel aromatic amine donor 1-oxajulolidine to enhance the electro-optical activity of organic second-order nonlinear optical materials

Developing new organic second-order nonlinear optical chromophores with a new electron donor is crucial for enhancing performance of electro-optic (EO) materials. A series of new second-order nonlinear optical chromophores (J1, JS and JO) with new electron donor 1-Oxajulolidine have been designed and synthesized. Ref1, Ref2 and Ref3 with N, N-diethylaniline (DEA), 4-anisole and julolidine as electron donor had also been synthesized for comparison. The result of TGA and DSC tests suggested that the decomposition temperature of the chromophores was higher than 260 °C. As the UV–vis absorption spectra of chromophores in solution, maximum absorption (λmax) of the chromophores J1 with 1-Oxajulolidine group presented red-shift compared to Ref1 and Ref2, but blue-shift to Ref3. Nevertheless, chromophore with 1-Oxajulolidine presented more obvious solvatochromic behavior from dioxane to dichloromethane. Density functional theory had been used to calculate HOMO-LUMO energy gaps and β of these chromophores. The r33 values of guest-host EO polymers Ref1/APC, J1/APC, JS/APC and JO/APC with a loading density of 10 wt% were obtained as 7.5, 9.6, 15.3 and 13.5 pm V−1, respectively. The above result clearly indicated that r33 of J1/APC (9.6 pm V−1) was 27% higher than that of Ref1/APC (7.5 pm V−1). The electro-optic coefficient of JS with the same conjugate bridge and acceptor was higher than that of FTC in the same doping concentration. As revealed by the above results, the introduction of the polycyclic structure of 1-Oxajulolidine, containing two electron-rich atoms (N and O), could efficiently enhance the electro-optical performance. Furthermore, λmax of JS in doped film showed blue-shift compared with FTC which exhibited a promising potential for tackling nonlinearity and transparency trade-off in second-order nonlinear optical (NLO) materials.

Three-branched coumarin derivatives for two-photon uncaging. How does branching influence the efficacy?

In the design of non-linear optical chromophores, the conjugation pathways in multi-branched dyes are known to influence heavily their optical properties. Herein, we investigate this strategy for the design of two-photon (2P) responsive photolabile protecting groups (PPG) by assembling via a triphenylamine core extended coumarinylmethyl derivatives. The experimental study reveals an enhancement of the 2P absorption in the tri-branched compound, but a strikingly different photophysics behaviour resulting in a decrease in bond cleavage efficiency, and aggregation in aqueous acetonitrile. These combined effects results in much poorer 2P uncaging efficiency of the three-branched derivatives. In contrast, the corresponding mono-branched coumarin exhibit very high 2P photochemical efficiency (450 GM).

Design and synthesis of various double donors for nonlinear optical chromophores with enhanced electro-optic activity

Eight nonlinear optical chromophores L1-2, M1-2, N1-2 and O1-2 based on the diethylaniline bis(N,N-diethyl)aniline, bis(N-Ethyl-N-hydroxyethyl)aniline and bis(3-hydroxypiperidin)aniline donor and thiolated isophorone bridge were designed and synthesized. In particular, the bridge sections of the molecules were functionalized with tert-butyltrimethylsilane and tert-butyl(methyl)diphenylsilane isolation groups, respectively. All these eight chromophores exhibited high thermal stabilities with decomposition temperatures above 220 °C. Density functional theory calculations indicated that the chromophores M1-2, N1-2 and O1-2 showed much larger hyperpolarizability than chromophores L1-2 with single donor due to the double donors structure. A large r33 value (198 p.m./V) and poling efficiency (2.03 ± 0.09 nm2/V2) at 1310 nm have been achieved for polymer films containing chromophores N2 which was much larger than the value (104 p.m./V and 1.03 ± 0.05 nm2/V2) of chromophore L2 with single donor. The reasonable design of these double donors can effectively improve the electron donating ability of donors and be applied to advanced organic nonlinear materials.

Dendronized Hyperbranched Polymer: A New Architecture for Second-Order Nonlinear Optics

Organic/polymeric second-order nonlinear optical (NLO) materials, which rely on the poling-induced non-centrosymmetric arrangement of NLO chromophores, have played a very important role in laser technology and optical fiber communication, due to their ultra-fast response speed, excellent machining performance and low dielectric constant. However, the NLO chromophores have the large dipole moments with strong intramolecular charge transfer, which lead to the intermolecular electrostatic interactions to tend to the centrosymmetric arrangement and decrease the poling efficiency. Since the special three-dimensional spatial separation can minimize these strong intermolecular electrostatic interactions during poling process, dendrimers and hyperbranched polymers have been considered as better topology for the next generation of highly efficient NLO materials. In 2013, by the attachment of low generation dendrimers to the hyperbranched backbone, a new dendritic architecture of dendronized hyperbranched polymer (DHP) was proposed for improving the comprehensive performance of NLO materials. Recent results showed many advantages of DHPs in NLO field, such as easy syntheses, large NLO coefficients and high orientation stability, etc. In this review, the latest advancement of DHPs, including the design principle, synthesis, as well as their application as NLO materials is summarized. The new opportunities arising from DHPs are also summarized in the future perspective.

Photorefractive Response Enhancement in Poly(triarylamine)-Based Polymer Composites by a Second Electron Trap Chromophore.

Photorefractive (PR) performances are affected by the components of the photoconductor, sensitizer, nonlinear optical dye, and plasticizer. A photoconductor with high hole mobility promises a faster response time, whereas it induces higher photoconductivity, which leads to easy dielectric breakdown. Adding a second electron trap is effective in controlling photoconductivity. In this study, the role of a second electron trap 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) was investigated in a PR composite consisting of a photoconductor of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] with a high hole mobility, a nonlinear optical chromophore of piperidinodicyanostyrene, a plasticizer of (2,4,6-trimethylphenyl)diphenylamine, and a sensitizer of [6,6]-phenyl C61 butyric acid-methyl ester. The minimum time response with the maximum optical diffraction efficiency and sensitivity was measured at a 1 wt % content of TmPyPB. These results were consistent with the number of charge carriers trapped per unit volume and per unit time Nc (cm–3 s–1), which is defined as the ratio between the initial trap density Ti (cm–3) and response time τ (s), at a 1 wt % content of TmPyPB. A faster response time of 149 μs, optical diffraction of 24.1% (external diffraction of 4.8%), and a sensitivity of 2746 cm2 J–1 were measured at 50 V μm–1 for the sample with 1 wt % TmPyPB. High loading of 5 wt % TmPyPB led to a large decrease in photoconductivity and effectively suppressed the dielectric breakdown under a stronger electric field, whereas a slower response time with lower diffraction efficiency was observed for optical diffraction.

Influence of acceptor tethering on the performance of nonlinear optical properties for pyrene-based materials with A-π-D-π-D architecture

In this study, we designed a series of pyrene-based donor-π-donor-π-acceptor compounds ( HPTC1-HPTC7 ) by structural tailoring the reference compound ( HPTC ) using acceptor units. Nonlinear optical (NLO) properties, frontier molecular orbitals (FMOs), natural bonding orbital (NBO), transition density matric (TDM) analysis, and absorption spectra of reference and proposed derivatives were calculated at M06/6-31G(d,p) functional. All the designed compounds have smaller energy bandgaps than the HPTC compound. Moreover, the designed compounds exhibited larger global softness values than the reference . The absorption maxima of HPTC2 , HPTC3 , and HPTC7 are blue shifted with respect to HPTC . NBO analysis revealed that prolonged hyper conjugative associations and strong interactions between the donor (π) and acceptor (π*) moieties play a crucial part in their stabilization. The FMO and NBO findings supported the NLO responses of entitled compounds, and consequently, the linear and nonlinear properties of designed derivatives elevate compared to the reference molecule. Promisingly, the NLO response for HPTC7 comprises of highest values of < α > , β total and < γ > as 1.92 × 10 −22 esu, 1.95 × 10 −27 esu, and 4.69 × 10 7 ( a.u ). This NLO behavior shows push–pull NLO chromophores for HPTC7 predicting its role in pursuing NLO materials for optoelectronic applications.

First Order Hyper polarizability and Intramolecular Charge Transfer of N-Ethyl-N-(2-Hydroxyethyl)-4-(4-Nitrophenylazo) Aniline for Photonic Applications

We report a computational methodology for estimating the first order hyperpolarizability ((3) of N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) aniline, a nonlinear optical (NLO) chromophore being used as anazobenzene dye. The planar and non-planar structures of N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) aniline were optimized by B3LYP/6-311++G (D,P) basis set using GAUSSIAN’09W program package. The dipole moment, static polarizability, first order hyperpolarizability were studied using HF/6-31G (D) basis set. The (3 value of the planar and non-planar structure of N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo) aniline is 196 and 124 times higher than that of urea standard, respectively, which qualify both of the novel molecular systems as highly efficient for second order NLO applications and adds evidence to the importance of derealization of ^-electrons in a system for effective second order NLO processes. Experiments were also performed using an Nd-YAG laser of pulses in the range of 5-10 nanoseconds with a tunable peak power of 4.7 W.

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.