One-photon Absorption Research Articles

Near-Infrared Bioimaging Using Two-photon Fluorescent Probes.

Near-infrared (NIR) bioimaging has emerged as a transformative technology in biomedical research. Among many fluorescent probes that are suitable for NIR imaging studies, two-photon absorption (TPA) ones represent a particularly promising category, because TPA fluorescent probes can overcome the inherent limitations of one-photon absorption (OPA) counterparts. By leveraging the unique properties of two-photon absorption, TPA fluorescent probes achieve superior tissue penetration, significantly reduced photodamage, and enhanced spatial resolution. This perspective article delves into the fundamental principles, design strategies, and representative TPA probes for various imaging applications. In particular, a number of molecular fluorescent probes, ranging from organic, inorganic, and COF/MOF-based systems are highlighted to showcase the vast scope of possible TPA probe design and application scenarios. In addition, the employment of stimulated TPA probes that are responsive to different external factors, including pH, redox species, enzymes, and hypoxia, is also discussed. In the end, the future perspectives for the continuous advancement of TPA fluorescent probes in the NIR bioimaging field are presented. For instance, it is essential to transition from cellular to in vivo imaging studies to obtain more physiologically relevant insights. Additionally, the development of "dual-function" TPA probes for both disease diagnosis and therapeutic treatment is particularly promising.

Electronic Structures and Spectra of Donor-Acceptor Conjugated Oligomers.

Narrow band gap donor-acceptor conjugated polymers present excellent paradigms in photonics and optoelectronics due to their chemical tunability, correlated electronic structures, and tunable open-shell electronic configurations. However, rational design for enhancing the properties of these molecular systems remains challenging. In this study, we employed density functional theory (DFT) calculations to investigate prototypical narrow band gap donor-acceptor conjugated oligomers, consisting of alternating cyclopentadithiophene (CPDT) donors paired with benzothiadiazole (BT), benzoselenadiazole (BSe), benzobisthiadiazole (BBT), and thiadiazoloquinoxaline (TQ) acceptors. Analyses of structures, singlet-triplet gaps, and absorption spectra of oligomers with up to ten repeat units have shown that when incorporating the BT, BSe, and TQ acceptors, the backbone curvature resulted in spiral structures that were energetically favored over their linear counterparts, causing differences in the calculated circular dichroism spectra. Oligomers with BBT-based acceptors preferred, however, a linear geometry, consistent with an open-shell electronic structure. Calculated singlet-triplet splittings demonstrated the importance of long chains and specific structures for consistency with the experiment, while effects of the solvent were also quantified. Based on the predicted low-energy conformations, one-photon absorption spectra for the considered oligomers have shown that using the Tamm-Dancoff approximation within time-dependent DFT for the large systems offers good agreement with the first absorption maxima in measured experimental spectra, thus validating the method for large donor-acceptor oligomers. Natural transition orbital analyses provided insights into the excited-state characteristics. Two-photon absorption maxima were accurately predicted, but the cross-sections were overestimated or underestimated, as dependent on the level of theory employed, to be addressed in future work.

Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene.

Organic dyes-based photothermal agents (OPTAs) have received increasing attention as alternative to inorganic materials due to their higher biocompatibility and extensive diversification. Maximizing nonradiative deexcitation channels is crucial to improve the photothermal conversion efficiency (PCE) of OPTAs. This is typically achieved through individual molecular design or collective enhancement using supramolecular strategies. Furthermore, photothermal therapy (PTT) generally relies on linear one-photon absorption of the light source by the OPTA, with less consideration given to nonlinear two-photon absorption (2PA) strategies, despite their potential benefits. Here, a synergistic strategy, which combines intramolecular and intermolecular quenching, is employed to maximize the photothermal efficiency of diphenylamino-substituted distyryl dicyanobenzene (DSB), an outstanding two-photon-absorbing chromophore. One to three DSB units have been introduced on the conic p-tert-butyl-calix[4]arene (CX), serving as a preorganizing platform to allow aggregate formation and promote intramolecular quenching within the multichromophoric systems. Importantly, the multichromophoric molecules had very high two-photon absorption capabilities with cross sections (δ2PA) reaching maximal values of 3290 GM at 810 nm. Experimental data accompanied by large-scale molecular dynamics simulations and time-dependent density functional theory calculations shed light onto the interaction mechanism in those multiple DSB-appended CX compounds to rationalize their optical properties. Then, the formulation with Pluronic F127 amphiphile yields water-dispersible nanoprecipitates (Nps), in which the PCE is further maximized and the photobleaching is reduced due to the combination of intra- and intermolecular quenching. The high two-photon absorption in the near-infrared (NIR) window associated with the high PCE of these nanosized OPTAs could serve as a basis to future in vivo 2P-PTT applications.

Optoelectronic Properties of Triply Twisted Möbius Carbon Nanobelt and the Design of Its Isomeric Nanomaterials

The triply twisted Möbius carbon nanobelt (TMCNB) possesses an extremely distinctive geometric and electronic structure and therefore is anticipated to demonstrate outstanding optical properties. In this paper, through integrating quantum chemical calculations and wave function analysis approaches, in-depth studies are conducted on the one-photon absorption (OPA) and two-photon absorption (TPA) characteristics, aromaticity, and circular dichroism of the TMCNB. Inspired by this structure, we further deform it to construct a novel structure, MCNB2, and verify the stability of this structure, thereby confirming its rationality. Since variations in structure will result in dissimilar optical properties, we also undertake theoretical analyses on the absorption properties and circular dichroism of MCNB2. The outcomes of this study offer a significant theoretical foundation for the design and construction of chiral optoelectronic materials.

Electron–photon–phonon interactions in Dirac semimetals: Magneto-optical absorption and mobility analysis

We study the magneto-optical properties of Dirac semimetal (DSM) slabs with particular emphasis on Cd3As2 through electron–photon–phonon interactions, focusing on the magneto-optical absorption coefficient (MOAC) and full-width at half maximum (FWHM). Studying the Landau level (LL) energy of DSMs in the (xy) plane and the z-direction revealed a unique deviation from the square root dependence on the magnetic field, distinguishing DSMs from other semiconductors. At high magnetic fields, the electron–hole symmetry in the LL spectrum is broken, indicating a topological phase in DSMs. For undoped DSMs, MOAC is driven by interband transitions, with peaks from one-photon absorption being smaller and positioned to the left of two-photon ones. Increasing the magnetic field increases peak values. FWHM for one- and two-photon processes increases with the magnetic field and follows a T dependence on temperature. In doped DSMs, both intraband and interband transitions occur, with new interband peaks emerging at higher temperatures near the Fermi energy. Increased electron density shifts the peak position slightly toward higher energy. Peaks from optical phonon emission are consistently higher and located to the right of those from optical phonon absorption, indicating a stronger emission process. The FWHM data allow for the estimation of electron mobilities, and using a reasonable broadening parameter, our predicted mobility values agree with experimental results.

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.

Multiphoton Ionization/Dissociation of Molecular Sulfur S2 in the UV Region.

The multiphoton ionization/dissociation dynamics of molecular sulfur (S2) in the ultraviolet range of 205-300 nm is studied using velocity map ion imaging (VMI). In this one-color experiment, molecular sulfur (S2) is generated in a pulsed discharge and then photodissociated by UV radiation. At the three-photon level, superexcited states are accessed via two different resonant states: the B3Σu- (v' = 8-11) valence states at the one-photon level and a Rydberg state at the two-photon level. Among the decay processes of these superexcited states, dissociation to electronically excited S atoms is dominant as compared to autoionization to ionic states S2+ (X2Πg) at wavelengths λ < 288 nm. The anisotropy parameter extracted from these images reflects the parallel character of these electronic transitions. In contrast, autoionization is found to be particularly efficient at S(1D) and S(1S) detection wavelengths around 288 nm. Information obtained from the kinetic energy distributions of S atoms has revealed the existence of vibrationally excited S2+ (X2Πg (v+ > 11)) that dissociates to ionic products following one-photon absorption. This work also reveals many interesting features of S2 photodynamics compared to those of electronically analogous O2.

Quantum Size-Driven Spectral Variations in Pillar[n]arene Systems: A Density Functional Theory and Wave Function Assessment.

This study explores the quantum size effects on the optical properties of pillar[n]arene (n = 5, 6, 7, 8) utilizing density functional theory (DFT) and wave function analysis. The mechanisms of electron transitions in one-photon absorption (OPA) and two-photon absorption (TPA) spectra are investigated, alongside the calculation of electron circular dichroism (ECD) for these systems. Transition Density Matrix (TDM) and electron-hole pair density maps are employed to study the electron excitation characteristics, unveiling a notable size dependency. Analysis of the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM) reveals the electromagnetic interaction mechanism within pillar[n]arene. Raman spectra computations further elucidate vibrational modes, while interactions with external environments are studied using electrostatic potential (ESP) analysis, and electron delocalization is assessed under an external magnetic field, providing insights into the magnetically induced current phenomena within these supramolecular structures. The thermal stability of pillar[n]arene was investigated by ab initio molecular dynamics (AIMD).

Ion-core switching in Rydberg series of XeKr

We investigate XeKr in high Rydberg states by mass-resolved optical-optical double resonance excitation spectroscopy. We excite XeKr via two-photon absorption to the v* = 0 and v* = 2 vibrational levels of an intermediate electronically excited state of Xe*Kr correlated to Xe*6p[5/2]2 and excite Xe*Kr further via one-photon absorption to the high Rydberg states of Xe**Kr in the energy range of 93200–97400 cm−1. The potential energy curves and the quantum defects of the d-Rydberg and s-Rydberg series of Xe**Kr, converging the A 2Π3/2 state of XeKr+ are determined. From the kinetic energy release in the predissociation process, we conclude that the ion-core switching occurs by the interaction between the bound electronic states of Xe**Kr converging to the A 2Π3/2 state of XeKr+ and the repulsive electronic states of low-lying electronic states of XeKr**. Based on numerical simulations, we interpret the dependence of the yield of the ion-core switching on the excitation energy in terms of the population transfer from the bound electronic states of the high Rydberg states Xe**Kr, converging to the A 2Π3/2 state of XeKr+, to those converging to the electronic ground X2 Σ 1 / 2 + state of XeKr + .

Tuning the linear optical properties and two-photon absorption on the cinnamylideneacetophenone derivatives: The effect of methoxy group by a experimental and theoretical point of view

The present work studied the effect of the methoxy group position in two cinnamylideneacetophenone (CA) derivatives on both one-photon (1PA) and two-photon absorption (2PA) processes. Our outcomes exhibit a dependence on the methoxy group position spectrally and in magnitude for both. A decrease in the lower energy 1PA band of 0.23eV was observed when the methoxy group was moved to the more conjugated part of the cinnamaldehyde molecule. The opposite was also observed for the next higher absorption band. The Quantum Chemical Calculations (QCCs), besides a good agreement between the experimental and simulated 1PA spectra, except for the magnitude of the high energy absorption band, showed that the effect behind the observed experimental red-shift of the lower absorption band is caused by the higher charge transfer when the methoxy group is bounded to the longer conjugated part of the cinnamaldehyde derivative. For experimental 2PA spectra, the higher energy band is strongly affected by the presence of the methoxy group, pointing out an increase of about 56 % in the values of the two-photon absorption cross-section (2PACS). Furthermore, the phenomenological Sum-Over-Essential States (SOS) approach and QCCs were performed for further clarification in the 2PACS experimental results. Hence, our findings demonstrate that changing the methoxy position on opposite sides of the CA may open up a novel insight for designing new organic molecules, particularly when seeking to enhance the values of 2PACS significantly of CA derivatives.

Visible-light optical limiting of vanadia–polyvinylpyrrolidone nanofibers

In this work, vanadium pentoxide (V2O5) nanoparticles-filled electrospun polyvinylpyrrolidone (PVP) nanofibers were investigated systematically at various nanofiller weight percentages (8 and 10 wt%) and input intensities to reveal the effective optical limiting feature in the visible spectrum. XRD analysis demonstrated the purity of the produced V2O5 nanoparticles. According to SEM findings, V2O5 nanoparticles were effectively integrated into the PVP nanofibers. Two distinct absorption bands were observed at around 400 and 217 nm. These bands were related to PVP and V2O5 nanoparticles in linear absorption measurements, respectively. Moreover, an increased Urbach energy value was obtained with an increase in V2O5 nanofiller content within PVP. Open-aperture Z-scan measurements were taken at 532 nm considering the band gap energy of the V2O5 nanofillers in PVP composite nanofibers. In 8 wt% V2O5 nanofilled PVP nanofibers, one-photon absorption (OPA) was the main nonlinear absorption (NA) mechanism, and the defect states of the V2O5 nanoparticles had no contribution to NA. On the other hand, sequential two-photon absorption was the main NA mechanism, and the defect states of the nanoparticles caused more efficient NA behavior in 10 wt% V2O5 nanofilled PVP nanofibers. The effective optical limiting behavior was obtained for this composite nanofiber with lower limiting threshold as 1.49 × 10–5 J/cm2. The V2O5 nanofilled PVP nanofibers presented strong potential optical limiters in the visible wavelength region. This was attributed to their high linear transmittance at low input intensities and their robust NA behavior at higher input intensities.

Physical insight into optical spectroscopy of chiral covalent organic pillars as molecular nanotubes

In this paper, we theoretically investigate chiral covalent organic pillars synthesized by connecting rim-desymmetrized macrocycle, stimulated by recent experimental synthesis [Nat. Synth. 2023, 2, 395–402]. Transition densities reveal physical mechanism of spectral intensity in one-photon absorption (OPA) and two-photon absorption (TPA), and solvent effect strongly influence the spectral profile and intensity in TPA, but little influence the spectrum in OPA. The intra-unit and inter-unit charge transfer are visualized in five phenylenediamine linkers. The chirality of covalent organic pillars is revealed by electronic circular dichroism (ECD) spectroscopy, and interpreted by the visualized transition electric and magnetic dipole moments. Molecular vibrational information is demonstrated by normal and resonance Raman spectra and vibrational modes. The visualization of static and dynamic (hyper) polarization can reveal the physical mechanisms of polarization dependent normal and resonance Raman scattering. Our results provide deeper understanding on the physical mechanism of chiral covalent organic pillars as molecular nanotubes.

High-directivity far-field radiation of quantum dot-based single-photon emitter coupled to polymeric circular waveguide resonant grating

Solid-state single-photon emitters (SPEs) commonly encounter the limitation of quasi-omnidirectional radiation patterns, which poses challenges in utilizing their emission with conventional optical instruments. In this study, we demonstrate the tailoring of the far-field radiation patterns of SPEs based on colloidal quantum dots (QDs), both theoretically and experimentally, by employing a polymer-based dielectric antenna. We introduce a simple and cost-effective technique, namely low one-photon absorption direct laser writing, to achieve precise coupling of a QD into an all-polymer circular waveguide resonance grating. By optimizing the geometry parameters of the structure using 3D finite-difference time-domain simulations, resonance at the emission wavelength of QDs is achieved in the direction perpendicular to the substrate, resulting in photon streams with remarkably high directivity on both sides of the grating. Theoretical calculations predict beam divergence values below 2°, while experimental measurements using back focal plane imaging yield divergence angles of approximately 8°. Our study contributes to the evaluation of concentric circular grating structures employing low refractive index polymer materials, thereby expanding the possibilities for their application.

Optical and morphological properties of DCM thin films co-doped of Znq2 by PVD: Theoretical and experimental investigations

In this article, new experimental results of the morphological and optical properties of thin films of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran co-doped by 8-hydroxyquinoline zinc (Znq2) for different concentrations were presented. The physical vapor deposition in a high vacuum was used to create the thin films on glass substrates with a thickness of 100 nm. The morphological properties of the samples were identified using the Atomic Force Microscopy (AFM) apparatus in tapping mode. TEM images show amorphous structures. The phase composition of the samples was assessed using FTIR methods. The Tauc plot approach was used to examine the measured transmittance spectra in order to estimate the energy gap. Analysis of the results showed that the co-doping of DCM thin film by Znq2 influenced the structural and optical properties of the samples. To evaluate the experimental results found, theoretical calculations based on the maximum one-photon absorption (OPA) wavelengths of DCM and Znq2 have been first measured through the UV–Vis spectral technique. Then, the experimental conclusions on values for DCM and Znq2 have been compared with their corresponding simulation data procured from the time-dependent self-consistent-field (TD-SCF) computation utilizing the density functional theory (DFT) at B3LYP/6-311G(d,p) level. Besides, we have explored the first and second frontier molecular orbitals (MOs) for title compounds and their energy gaps via the DFT procedure. The calculations of maximum OPA wavelengths and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) contributions on both molecules have also been crosschecked with the results of already published data in the literature.

Enhanced nonlinear absorption and photoluminescence properties of Zn, Fe, Cu, V and Ni doped MoO3 transition metal oxide thin films

Abstract In this study, the effect of the dopant element on the linear, nonlinear absorption and optical limiting properties of ultrasonically sprayed MoO3 thin films is presented. The linear optical results showed an increase with the density of the defect states in the bandgap with doping, in conjunction with a decrease in bandgap energy and an increase in Urbach energy. Broad photoluminescence emissions are detected in the range of 350 and 600 nm, decreasing in intensity by doping. To reveal the defect states effects’ on the nonlinear absorption (NA) behavior, OA Z-scan data were analyzed with two theoretical models considering only two photon absorption (2PA) (model 1), and one photon absorption (OPA), 2PA and free carrier absorption (model 2). The NA behavior is observed and found to get enhanced by increasing input intensity and doping atoms due to generation of new oxygen vacancies and formation of further defect states. The NA coefficient values of the thin films in model 2 are 100 times higher than that of 2PA coefficient values in model 1. This result revealed the strong effect of defect states on the NA behavior. Among the investigated dopant atoms, Cu resulted in enhanced NA due to the higher density of defect states. While the genuine 2PA is the dominant NA mechanism for V and Fe doped MoO3 thin films, OPA and 2PA are the dominant NA mechanisms for the Ni, Zn and Cu doped MoO3 thin films due to their higher concentration of defect states. Cu-doped MoO3 thin film has a lower optical limiting threshold of 0.026 mJ/cm2 due to its enhanced NA behavior. Considering the results obtained, this study opens the door to the potential of doped MoO3 thin films to be used as optical limiters in the visible wavelength region.

Excitonic one-photon absorption of one- and two-dimensional C60 polymers: Insights from many-body first-principles calculations

By solving the many-body Bethe–Salpeter equation, we explored the excitonic one-photon absorption of one- and two-dimensional C60 polymers. We studied seven polymeric structures including C60-chain, two structures with quasi-tetragonal phases (qTP1 and qTP2), and four structures with quasi-hexagonal phases (qHP1, ML-qHP2, BL-qHP2, and bulk-qHP2). The one-photon excitonic transitions were analyzed in detail relative to the independent particle transitions. Significant red-shifts (0.4 and 0.2 eV for ML-qHP2 and BL-qHP2, respectively) were observed in the excitonic one-photon absorption of low-dimensional polymers compared to bulk polymers. The red-shift can be related to the dimensional and interlayer dependences of screening effect which lead to different binding energies (0.61, 0.44, and 0.27 eV for ML-qHP2, BL-qHP2, and bulk-qHP2, respectively). For two qHPs (i.e., qHP1 and ML-qHP2), different connection modes between C60 molecules lead to different types (direct and indirect for qHP1 and ML-qHP2, respectively) of electronic band gap, and they also have different spectral profiles due to different hole and electron densities of excitons. Finally, the C60-chain formed by [2 + 2] cycloadditions has a direct band gap of 2.09 eV which is desirable for applications in photoelectric devices based on the solar spectrum.

Charge transfer excitons and directional fluorescence of in-plane lateral MoSe2-WSe2 heterostructures.

In this article, the linear and nonlinear optical properties of in-plane lateral MoSe2-WSe2 heterostructures are theoretically investigated. The polarization-dependent strongest optical absorption in one-photon absorption occurs in charge transfer excited states, where electrons transfer from WSe2 to MoSe2. This phenomenon is supported by the LUMO (lowest unoccupied molecular orbital) and HUMO (highest occupied molecular orbital) imaging obtained through scanning tunneling microscopy. The charge difference density and transition density matrix are used to interpret the electronic transitions, and these interpretations rely on the concept of transition density. The optical properties of two-photon absorption in its nonlinear optical process are significantly different from the excitation in one-photon absorption, where the strongest optical absorption is contributed from direct transition from the ground state to the final state without going through an intermediate excited state, due to the very large difference of permanent dipole moments between the excited and ground states. Our results also reveal directional fluorescence and physical mechanism of in-plane lateral MoSe2-WSe2 heterostructures. Our work can provide insights into the physical mechanism of the optical properties of in-plane lateral MoSe2-WSe2 heterostructures.

Thermal control over phosphorescence or thermally activated delayed fluorescence in a metal-organic framework.

By integrating a tailor-made donor-acceptor (D-A) ligand in a metal-organic framework (MOF), a material with unprecedented features emerges. The ligand combines a pair of cyano groups as acceptors with four sulfanylphenyls as donors, which expose each a carboxylic acid as coordination sites. Upon treatment with zinc nitrate in a solvothermal synthesis, the MOF is obtained. The new material combines temperature-assisted reverse intersystem crossing (RISC) and intersystem crossing (ISC). As these two mechanisms are active in different temperature windows, thermal switching between their characteristic emission wavelengths is observed for this material. The two mechanisms can be activated by both, one-photon absorption (OPA) and two-photon absorption (TPA) resulting in a large excitement window ranging from ultraviolet (UV) over visible light (VL) to near infrared (NIR). Furthermore, the emission features of the material are pH sensitive, such that its application potential is demonstrated in a first ammonia sensor.

One- and two-photon absorption spectra of organoboron complexes: vibronic and environmental effects.

We synthesized a series of four parent aza-β-ketoiminate organoboron complexes and performed spectroscopic studies using both experimental and computational techniques. We studied how benzannulation influences the vibronic structure of the UV/Vis absorption bands with a focus on the bright lowest-energy π → π* electronic excitation. Theoretical simulations, accounting for inhomogeneous broadening effects using different embedding schemes, allowed gaining in-depth insights into the observed differences in band shapes induced by structural modifications. We observed huge variations in the distributions of vibronic transitions depending on the position of benzannulation. By and large, the harmonic approximation combined with the adiabatic hessian model delivers qualitatively correct band shapes for the one-photon absorption spectra, except in one case. We also assessed the importance of non-Condon effects (accounted for by the linear term in Herzberg-Teller expansion of the dipole moment) for S0 → S1 band shapes. It turned out that non-Condon contributions have no effect on the band shape in one-photon absorption spectra. In contrast, these effects significantly change the Franck-Condon band shapes of the two-photon absorption spectra. For one of the studied organoboron complexes we also performed a preliminary exploration of mechanical anharmonicity, resulting in an increase of the intensity of the 0-0 transition, which improves the agreement with the experimental data compared to the harmonic model.

Theoretical study on photoinduced charge transfer in one-photon and two-photon absorption of branching oligofluorenes

Oligofluorenes have been identified as very promising two-photon absorption (TPA) materials and present great application potential for the fabrication of nonlinear optical devices, but the TPA mechanism and corresponding electron excitation properties have not been studied. Here, the photoinduced charge transfer characteristics of V-shaped and Y-shaped branching oligofluorenes that consist of two and three fluorene units in each branch during one-photon absorption (OPA) and TPA processes are analyzed theoretically using the density functional theory and visualization sum-over-states model. The calculated results show that the OPA intensity and TPA cross-section are significantly enhanced by increasing the branch length or changing the structure from V-shaped to Y-shaped. The long-distance charge transfer only occurs on the second transition of TPA at high excited states. Compared to Y-shaped molecules, V-shaped structures exhibit a stronger cooperative effect among the different branches.