Two-photon Absorption Response Research Articles

Effect of reverse conjugated structure on charge transfer and photoabsorption characteristics of multi-branched molecules

The present work investigates effect of conjugated structure on two-photon absorption (TPA) and charge transfer characteristics of multi-branched molecules by DTF/TD-DFT and visualization methods. Results show that, compared with V-shaped and T-series counterparts, Y-shaped and R-series molecules exhibit great enhancement on TPA response. Strengthened cooperative effect among branches has been obtained by reversing conjugated structure or changing conjugated structure from V-shape to Y-shape, which has been approved by the TDM and CDD maps in the second step transition of TPA process. Our results indicate that constructing typical structures with suitable charge transfer directions is of great significance for improving performance.

Ultrafast response of diketo-pyrrolo-pyrrole dyes with large two-photon absorption enhancement as well as AIE property

To study the effect of terminal groups substituent on the two-photon absorption (TPA) properties of [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes with diketo-pyrrolo-pyrrole (DPP) as central electron acceptor group, linear/transient absorption spectroscopy, one/two-photon fluorescence spectra, open-aperture [Formula: see text]-scan, and excited states quantum chemical calculations have been performed to systematically study the intermediate product electron acceptor J0 as well as three [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes using benzene, triphenylamine and 2,3-methoxy-triphenylamine as terminal electron donors, named as J1, J2, and J3, respectively. The intermediate product electron acceptor itself exhibits a relatively large TPA response, which indicates the DPP group has a large conjugated system. J2, with strong electron-donating ability groups triphenylamine as terminal groups, shows a large TPA cross-section of 534[Formula: see text]GM, which is approximately 10 times larger than that of J1 with weak electron pushing/pulling abilities groups benzene as terminal groups (51[Formula: see text]GM). The result indicates that the electron-donating abilities of terminal groups do play a key role in TPA response. Furthermore, methoxy substituent on the terminal triphenylamine groups can further effectively increase the TPA response, J3exhibits a 15-fold enhancement compared with that of J1. Transient absorption spectra and quantum chemical calculation results are well accorded with the experimental results. Terminal substituents with strong electron donor groups are beneficial for the formation of intramolecular charge transfer (ICT) process. Besides, methoxy substituent in the terminal of triphenylamine groups can further enhance the degree of ICT and induce AIE characteristics in the molecule.

Effect of terminal substituents on intramolecular charge transfer in one- and two-photon absorption of zinc phthalocyanine derivatives

Zinc phthalocyanine derivatives have unique structure and photophysical properties, which make them a remarkable potential in the fields of optoelectronics as well as biomedicine. In order to reveal the effect of the terminal substituents on the electron transition mechanisms, one-photon absorption (OPA) and two-photon absorption (TPA) processes of three zinc phthalocyanines modified with different terminal substituents (PcZn-0, PcZn-1 and PcZn-2) have been analyzed by using density-functional theory (DFT) in combination with the sum-over-states (SOS) model, the visualization of the transition density matrix (TDM) as well as charge density difference (CDD) calculations. Red-shifted OPA absorption peaks with enhanced intensities, have been observed in molecules PcZn-1, PcZn-2 with long terminal substituents. However, for TPA process, a more effective charge transfer over almost the entire molecule has been observed in PcZn-1 molecule with 4-methoxybenonic acid as terminal substituents. When H-atoms in the hydroxyl groups of four terminal 4-Methoxybenzoic acid groups were substituted by dispersed red-one chains, a limited range of intramolecular charge transfer and localized excitation inside the dispersed red-one chain were observed, leading to a localized excitation absorption peak and expanding the wavelength region of TPA response. Furthermore, terminal substituents with conjugated branches exhibit a stronger cooperative effect between the terminal chains and shows larger TPA response. On the contrary, excessive dihedral angles between different groups can weaken the conjugation degree of molecules and affect intramolecular charge transfer as well as TPA response.

Exploring the two-photon absorption response in short π-chain organic compounds: Hydrazones and pyrazolines derivatives

We compared the experimental two-photon absorption spectra of two hydrazone derivatives of chalcone (HDC-1 and HDC-2) and two pyrazoline derivatives of dibenzylideneacetone and chalcone (PD-1 and PD-2). The compounds were synthesized by the Claisen-Schmidt reaction and characterized using NMR, FTIR, and UV–Vis. Using femtosecond pulses, the Z-scan technique recorded two-photon absorption cross-section (σ2PA) spectra of the investigated compounds, which presented values of σ2PA, ranging from ∼12 GM to ∼50 GM. Such σ2PA difference is generally associated with the conjugated chain length and the different functional and peripherical groups used. For example, increasing the π-chain and peripherals donor groups p-OCH3 from HDC-1 to HCD-2 led to a change in the σ2PA magnitude (σHDC−12PA∼35GM, σHDC−22PA∼50GM). The modification on the pyrazoline derivatives, i.e., increasing the conjugated chain and adding p-OCH3 in both phenyls in the PD-2 compared to compound PD-1 doubled the σ2PA magnitude (σPD−12PA∼12GM, σPD−22PA∼22GM). According to the obtained results and considering the investigated compounds' short π-chain, we could claim that these classes of organic molecules promise nonlinear optical materials for optical limiting due to optical transparency ranging from 450 nm for pyrazolines to 550 nm for hydrazone derivatives.

Study of two-photon absorption and excited-state dynamics of coumarin derivatives: the effect of monomeric and dimeric structures.

Intramolecular charge transfer (ICT) and π-electron delocalization are two key factors affecting the nonlinear optical absorption of organic molecules. To clarify the different influences of ICT and π-electron delocalization on two-photon absorption (TPA) and excited-state absorption (ESA), monomeric coumarin C1 and dimeric coumarin C2 are synthesized and studied. Transient absorption spectroscopy (TAS) analysis of these coumarin derivatives in solvents of varying polarities describes the polarity-dependent excited-state dynamics and reveals the ESA signals of the charge transfer state (CTS) and local excited state (LES) with different spectral features. Femtosecond broadband Z-scan experiments indicate that dimeric coumarin C2 has a more significant TPA response than monomeric coumarin C1 in the near-infrared region. Natural transition orbital (NTO) analysis further theoretically characterizes the electron transition feature induced by TPA. Our results reveal that the TPA of these coumarin derivatives can be significantly enhanced by expanding π-electron delocalization, but their ESA is mainly dominated by ICT performance. This study indicates that coumarin derivatives will exhibit extremely broad application prospects in the field of ultrafast optical limiting (OL) through reasonable molecular design.

Crystallography, Charge Transfer, and Two-Photon Absorption Relations in Molecular Cocrystals for Two-Photon Excited Fluorescence Imaging.

Two-photon excited fluorescence imaging requires high-performance two-photon absorption (TPA) active materials, which are commonly intramolecular charge transfer systems prepared by traditional chemical synthesis. However, this typically needs harsh conditions and new methods are becoming crucial. In this work, based on a collaborative intermolecular charge transfer (inter-CT) strategy, three centimeter-sized organic TPA cocrystals are successfully obtained. All three cocrystals exhibit a mixed stacking arrangement, which can effectively generate inter-CT between the donor and acceptor. The ground and excited state characterizations compare their inter-CT ability: 1,2-BTC > 2D-BTC > 1D-BTC. Transient absorption spectroscopy detects TCNB•-, indicating that the TPA mechanism arises from molecular polarization caused by inter-CT. Meanwhile, 1,2-BTC exhibits the highest excited-state absorption and the longest excited-state lifetime, suggesting a stronger TPA response. First-principles calculations also confirm the presence of inter-CT interactions, and the significant parameter Δµ which can assess the TPA capability indicates that inter-CT enhances the TPA response. Besides, cocrystals also demonstrate excellent water solubility and two-photon excited fluorescence imaging capabilities. This research not only provides an effective method for synthesizing TPA crystal materials and elucidates the connection between inter-CT ability and TPA property but also successfully applies them in the fields of multi-photon fluorescence bioimaging.

Giant two-photon absorption response from a silicon quasi-BIC metasurface

We propose an amorphous silicon (a-Si) metasurface based on a quasi-bound state in the continuum (q-BIC) with a theoretical Q-factor up to 930, which consists of Π-like nanostructures with symmetrical defects. The influences of slot offset parameter and the size of added symmetric defect on the near- and far-field characteristics of the metasurface have been analyzed. Due to local near-field enhancement by the tailored high-Q quasi-BIC resonance, our designed metasurface exhibits significantly enhanced two-photon absorption (TPA) response in the experiment, i.e., the TPA coefficient β0 = 173.2 cm/MW (3–4 orders of magnitude larger than a-Si film) and the TPA saturation intensity Isat = 93.0 MW/cm2. Our work offers a promising alternative scheme to improve nonlinear optical effects at the nanoscale.

Covalently linked graphene oxide-transition metal disulfide quantum dots nanocomposites featuring enhanced nonlinear optical absorption

Nanocomposites composed of different nanomaterials are a promising class of optoelectronic materials, owing to their interfacial electronic interactions. Compared to van der Waals forces, covalent bond-based linkages may endow nanocomposites with promoted electronic interactions and reinforced nonlinear optical (NLO) responses. In this study, we constructed covalently linked, mixed-dimensional nanocomposites by combining graphene oxide (GO) with MoS2 and WS2 quantum dots (QDs). These nanocomposites were prepared by employing a bifunctional molecule, 4-mercaptobenzenediazonium tetrafluoroborate, which carries a reactive diazonium group and a thiol group. The reactive diazonium group allowed for the attachment of thiophenol to GO via a radical addition reaction, while the thiol group enabled the subsequent passivation of sulfur vacancies in MoS2 and WS2 QDs. The resulting nanocomposites, denoted as GO-MoS2 and GO-WS2, respectively, exhibited significant fluorescence quenching, indicating effective electron and/or energy transfer from MoS2 or WS2 QDs to GO. Importantly, these covalently linked nanocomposites demonstrated superior two-photon absorption (TPA) responses compared to the individual components (GO, MoS2 and WS2 QDs) as well as the corresponding physical blends, presumably resulting from the efficient electron and/or energy transfer within these nanocomposites. This study not only demonstrates the significant promise of covalently linked GO-MoS2 and GO-WS2 nanocomposites in optical limiting applications, but also opens up new opportunities for the advancement of nanocomposites in ultrafast photonic devices.

One‐Photon and Two‐Photon Absorption Properties of Multi‐Branched Squaraine Dyes Comprised of Triphenylamine Cores and Ethynylene Linkers

AbstractWe developed multi‐branched π‐conjugated systems using squaraine dyes with triphenylamine cores connected by ethynylene linkers. We investigated the influence of interbranch coupling between squaraine branches on their one‐photon (OPA) and two‐photon absorption (TPA) properties. These dyes with triphenylamine components showed a red‐shifted one‐photon absorption (OPA) compared to the precursor squaraine dyes. Among branched dyes, the lack of apparent splitting or shift in the absorption maxima, even as the absorption intensity increased with the number of chromophores, implies the presence of limited exciton coupling between the squaraine branches. The present squaraine dyes with triphenylamine cores exhibited a moderate TPA compared to the precursor squaraine without the triphenylamine core, due to the extended π‐conjugation. Notably, both the 3‐branched and 2‐branched dyes demonstrated additional enhancement in the TPA response, surpassing that of the monochromophoric counterpart. This resulted in achieving a substantial TPA cross section of up to 3905 GM at 830 nm.

Two-Photon Absorption Response of Functionalized BODIPY Dyes in Near-IR Region by Tuning Conjugation Length and Meso-Substituents.

BODIPY dyes substituted by phenol or -COOMe units at the meso-position (C8) with and without a distyryl group including a methoxy moiety at the -C3 and -C5 positions of the BODIPY have been synthesized to analyze the photophysical properties. To clarify the ground-state interaction, absorption and emission features were investigated in the THF environment. Extending the π-conjugation with the methoxy moiety at -C3 and -C5 positions of BODIPY leads to a spectral shifting of the absorption maxima toward red by 120 nm. In addition, attaching the -COOMe unit at the meso-position of the BODIPY structure increases nonradiative molecular relaxation as compared to compounds possessing phenol substituents at the same position. We have investigated the effect of phenol and a -COOMe group and π-extended conjugation length with a methoxy moiety on the properties of two-photon absorption (TPA) and electron transfer dynamics by performing open-aperture (OA) Z-scan and femtosecond transient absorption spectroscopy measurements, respectively. The synthesized BODIPY compounds with the distyryl group including the methoxy unit show TPA character due to the longer conjugation length and therefore intramolecular charge transfer ability. Based on the OA Z-scan experiments upon photoexcitation with 800 nm pulsed laser light, TPA cross-section values were obtained as 74 and 81 GM for the compounds possessing phenol and -COOMe units at the meso-position of BODIPY treated by distyryl group with methoxy moieties, respectively. Additionally, optical and electronic properties were calculated theoretically by using the DFT method.

Thiophene-modified donor-[formula omitted]-acceptor compound with an enhanced two-photon absorption response: An ultrafast dynamics study

Comparative investigations were performed to determine how thiophene group modifications affect the two-photon absorption properties of a donor-π-acceptor compound. Specifically, linear absorption and one/two-photon fluorescence spectra, open-aperture Z-scan, time-resolved spectroscopy and quantum chemical calculations were performed. After thiophene modification, the two-photon absorption cross-section of compound R1’ was determined to be 264 GM, exhibiting a 1.6-fold enhancement compared with that of compound R1. A transient absorption spectra experiment together with quantum chemical calculations were performed to investigate the effect of thiophene modification on the ultrafast response and intramolecular charge transfer properties of the dipolar compounds. The results indicate that the thiophene group modification on the conjugated bridge and terminal ends plays an important role in the two-photon absorption and intramolecular charge transfer properties.

Theoretical Insights on the Sensing Performance for Newly-synthesized Two-photon Fluorescent N2H4 Probes Based on Spirobifluorence.

The development of fluorescent probe for hydrazine (N2H4) detection has attracted much attention due to the important role of N2H4 plays in the fields of medicine, agriculture, biology and environments. In this paper, the optical properties and water solubility of two novel two-photon fluorescent molecular probes (Probe1 and Probe2) before and after the reaction with N2H4 are studied by using the density function theory. The results show that electronic distribution and transition dipole moment of the probes are obviously changed after the reaction with N2H4, thus the optical properties of the molecules are influenced and the detection of N2H4 are realized. In addition, photoinduced electron transfer processes for Probe1 and Probe2 in the presence of N2H4 are theoretically characterized, which explains the experimental observations from the microscopic mechanism. Special attention has been paid on the analysis of the two-photon absorption for the probes with the absence and presence of N2H4 by the response theory method. Both probes with good water solubility show large variation on the two-photon absorption cross section when reacts with N2H4. In particular, the two-photon absorption response of Probe2 is more obvious, so it possesses preferable two-photon fluorescence microscopic imaging ability. More importantly, the receptor effect on the sensing performances of the probes are demonstrated, providing a theoretical reference for the design and synthesis on more efficient two-photon fluorescence N2H4 probes. Our study provides necessary information on the response mechanism of the studied chemosensors and helps to establish the relationship between the structure and optical properties of two-photon fluorescence N2H4 probes.

Modulating the nonlinear absorption response of SnOx thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Microscopic optical nonlinearities and transient carrier dynamics in indium selenide nanosheet.

This work systematically investigates the third-order nonlinear optical (NLO) properties and ultrafast carrier dynamics of layered indium selenide (InSe) obtained by mechanical exfoliation (ME). The two-photon absorption (TPA) effect of layered InSe was tested using micro-Z/I-scan techniques. The results indicate that InSe flakes undergo the TPA response under the excitation of both 520 nm and 1040 nm fs pulses, and that InSe is more likely to achieve TPA saturation under visible light excitation. Furthermore, ultrafast carrier dynamics revealed that InSe flakes in the visible region undergo a transition from photoinduced absorption to photobleaching and exhibit a fast recombination time of ∼0.4-1ps, suggesting a high optical modulation speed as high as ∼1-2.5 THz.

Enhancement of Two-Photon Absorption in Boron-Dipyrromethene (BODIPY) Derivatives

The linear and nonlinear optical properties of two BODIPY derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenylvinyl)phenyl)-boron-diuoride-azadipyrromethene (ZL-22), were comprehensively investigated based on experimental and theoretical studies. It was found that both compounds show a strong two-photon absorption response in the near-infrared regime, and the two-photon-absorption cross-section values of ZL-61 and ZL-22 were determined to be 8321 GM and 1864 GM at 800 nm, respectively. The improvement of the two-photon absorption cross section in ZL-61 was attributed to the enhancement of the donor group, which was confirmed by transient absorption measurements and DFT calculation. Our results indicate that these BODIPY derivatives are a promising candidate for optical limiting and two-photon imaging applications.

Giant two-photon absorption in MXene quantum dots.

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb2C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb2C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb2C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb2C changes to saturable absorption when the incident wavelength is between 400-800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb2C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb2C via morphology or surface termination.

Computational design of ratiometric two-photon fluorescent Zn2+ probes based on quinoline and di-2-picolylamine moieties

To improve two-photon absorption (TPA) response of a newly synthesized probe, a series of ratiometric two-photon fluorescent Zn2+ sensors based on quinoline and DPA moieties have been designed. The one-photon absorption, TPA, and emission properties of the experimental and designed probes before and after coordination with Zn2+ are investigated employing the density functional theory in combination with response functions. The design consists of two levels. In the first level of design, five probes are constructed through using several electron acceptors or donors to increase accepting or donating ability of the fluorophores. It shows that all the designed probes have stronger TPA intensities at longer wavelengths with respect to the experimental probe because of the increased intra-molecular charge transfer. Moreover, it is found that the probe 4 built by adding an acyl unit has the largest TPA cross section among the designed structures due to the form of longer conjugated length and more linear backbone. One dimethylamino terminal attached along the skeleton can improve TPA intensity more efficiently than two side amino groups. Therefore, in the second level of design, a new probe 7 is formed by both an acyl unit and a dimethylamino terminal. It exhibits that the TPA cross sections of probe 7 and its zinc complex increase dramatically. Furthermore, the fluorescence quantum yields of the designed probes 4 and 7 are calculated in a new way, which makes use of the relation between the computed difference of dipole moment and the measured fluorescence quantum yield. The result shows that our design also improves the fluorescence quantum yield considerably. All in all, the designed probes 4 and 7 not only possess enhanced TPA intensities but also have large differences of emission wavelength upon Zn2+ coordination and strong fluorescence intensity, which demonstrates that they are potential ratiometric two-photon fluorescent probes.

Nonlinear optical responses and ultrafast dynamics of three soluble ladder-conjugated molecules: A study on the terminal group effect

To study the structure-property relationship as well as the intramolecular charge transfer properties of typical ladder-conjugated molecules, three ladder-conjugated molecules named LT2, LT3 and BPTF-2 were systematically investigated by UV–vis absorption spectra and fluorescence spectra, the open-aperture Z-scan technique, two-photon excited fluorescence and femtosecond pump-probe experiments. These ladder-conjugated molecules employ the same fluorene group as the π-bridge and the central core, with substituent changes at the terminal position. These molecules exhibit a large two-photon absorption response and two molecules emit intense two-photon fluorescence at a wavelength of 800 nm, which is a commonly used wavelength in two-photon absorption/two-photon fluorescence applications. Our results indicate that the π-bridge and the terminal group play an important role in determining the two-photon absorption response. The dynamic results obtained from degenerated pump-probe experiments revealed that the evolution times of the intramolecular charge transfer state were >1000 ps, >1000 ps, and 40 ps for LT2, LT3 and BDTF-2, respectively. The results match those of the two-photon absorption responses and the fluorescence emission properties of these ladder-conjugated molecules. Our results also indicate that ladder-conjugated molecules have potential applications in two-photon absorption/two-photon fluorescence fields.

Determination of the contributions of degenerate and non-degenerate two-photon absorption response in silicon avalanche photodiode for infrared photon detection

The study of non-linear interactions in semiconductor photo-electronic devices to achieve effective and fast photon identification is an ongoing and important task. We investigated the specific contribution of degenerate and non-degenerate two-photon absorption (D-TPA and ND-TPA) response in silicon avalanche photodiode (Si-APD) for infrared photon detection at room temperature. We experimentally demonstrated that when the two laser pulses overlapped, the average D-TPA quantum detection efficiencies at 1800 nm, and that at 1550 nm were measured as 3.3 × 10−16 counts•pulse/photon2, 4.3 × 10−15 counts•pulse/photon2, respectively. And the ND-TPA quantum detection efficiency of 1800 nm and 1550 nm was measured to be 7.9 × 10−16 counts•pulse/photon2. This study provides a solution for the practical infrared photon detection devices based on TPA effect in Si-APDs.

Two-Photon-Triggered NO Release via a Ruthenium-Nitrosyl Complex with a Star-Shaped Architecture.

We report herein a molecular engineering strategy based on the design of a multipolar ruthenium-nitrosyl (Ru-NO) complex with a three-branched architecture. The three Ru-NO units are introduced at the periphery of a highly π-delocalized truxene core bearing three terpyridine ligands. The two-photon absorption capabilities of the complex were investigated by the Z-scan technique. The strong electronic coupling among the individual arms gives rise to a very strong two-photon absorption response (δ800nm ∼ 1600 GM), which corresponds to a 16-fold enhancement of the capability of a single-arm reference, thereby promoting an efficient light-driven NO release process in aqueous media.