Two-photon Absorption Spectra Research Articles

Two-Photon Absorption-Molecular Structure Investigation Using a Porphycene Chromophore with Potential in Photodynamic Therapy

Porphycenes have been shown to exhibit many advantageous properties when it comes to the application of two-photon absorption (TPA), a technique with potential use in the area of photodynamic therapy (PDT). A computational study of structure-reactivity relationships in the one- and two-photon absorption spectra of a series of 2,7,12,17-substituted porphycenes has been carried out using linear and quadratic density functional response theory. The focus has been on determining the effect on the spectra of electron donating and withdrawing substituents, the outcome of extending the conjugation lengths to these substituents, and the consequence of formation of metallo-porphycene complexes. In particular, we have looked at the use of TPA in order to improve the penetration depth of the therapeutic light dose, in terms of the position of the absorption maximum with respect to the optical window of tissue penetration, as well as the effect on the TPA cross section. The extent of conjugation was shown to be particularly crucial for increasing the TPA cross section, for both the electron withdrawing and donating substituents, while the inclusion of a metal in the center of the macrocycle was shown to benefit the absorption wavelength in terms of tissue penetration considerations.

Two-photon absorption spectrum of silver nanoparticles

We have investigated the spectral dispersion of real and imaginary parts of third-order nonlinear optical susceptibility of silver nanospherical particles in the aqueous solution using Z-scan and antiresonant ring interferometric nonlinear spectroscopic (ARINS) techniques by femtosecond pulses from the modelocked Ti:Sapphire laser over the spectral range of 720–820nm. The observed characteristic of imaginary part shows a peak at ∼766nm which coincides with the peak of spectral dispersion of real part induced thermally. The observed peak in the dispersion of nonlinear absorption coefficient at ∼766nm is attributed to the two-photon absorption. This two-photon resonance peak is attributed to l=2 plasmon (quadrupolar mode) arising from the phase retardation effects in the spherical nanoparticles present in our polydispersed sample with their diameters (80±20nm) beyond the quasi-static limit.

Quantum-mechanical calculations of the intensity distribution in spectra of adenine tautomers: II. Two-photon absorption spectra

We have performed quantum-mechanical calculations of the two-photon absorption spectra of three tautomeric forms of adenine in the gas phase and of four forms in the aqueous solution. Based on a comparison of the results of these calculations with experimental data, the occurrence of three structures, Ade-N9,N1H+, Ade-N9H (am), and Ade-N7H(am), in aqueous solution of adenine has been confirmed.

Temperature Effect on the Two-Photon Absorption Spectrum of All-trans-β-carotene

In this report, we investigate the influence of temperature on the two-photon absorption (2PA) spectrum of all-trans-β-carotene using the femtosecond white-light-continuum Z-scan technique. We observed that the 2PA cross-section decreases quadratically with the temperature. Such effect was modeled using a three-energy-level diagram within the sum-over-essential states approach, assuming temperature dependencies to the transition dipole moment and refractive index of the solvent. The results show that the transition dipole moments from ground to excited state and between the excited states, which governed the two-photon matrix element, have distinct behaviors with the temperature. The first one presents a quadratic dependence, while the second exhibits a linear dependence. Such effects were attributed mainly to the trans→cis thermal interconversion process, which decreases the effective conjugation length, contributing to diminishing the transition dipole moments and, consequently, the 2PA cross-section.

Octupolar Merocyanine Dyes: A New Class of Nonlinear Optical Chromophores

A set of new octupolar merocyanine chromophores was designed and synthesized. These compounds were prepared from the reaction of 1,3,5-triformyl-2,4,6-trihydroxybenzene with heterocyclic nucleophiles. Octupolar dyes were formed exclusively in their open-dye form. The one- and two-photon-absorption spectra of the dyes consist of two bands: The long-wavelength band in the two-photon absorption spectrum (a few hundreds GM above 1000 nm) matches well with the intense, long-wavelength-absorption band that is located in the visible region in the linear spectrum. Interestingly, an additional, much-more-intense TPA band in the NIR region is observed at higher energy, which corresponds to a weakly allowed one-photon electronic transition. Changing the peripheral heterocyclic moieties allows tuning of the optical properties to approach the cyanine limit (i.e., polymethine state), thus resulting in a red-shift of the low-energy one-photon-absorption band as well as to the rise of an intense two-photon-absorption band in the NIR region. To the best of our knowledge, this is the first synthesis and TPA characterization of octupolar merocyanine chromophores with typical low-bond-length alternation.

Computational design of improved two‐photon active caging compounds based on nitrodibenzofuran

Nitrodibenzofuran (NDBF) has recently been established as photolabile protecting group and efficiently used as two-photon active cage. In this work, a computational approach is exploited to rationally design improved two-photon active caging groups based on this NDBF chromophore. For this objective, first the two-photon absorption (TPA) properties of NDBF are investigated in detail and a suitable theoretical approach for the reliable simulation of TPA spectra of this class of compounds is identified. Then, virtual chemical modifications are performed by introduction of substituents at the chromophore and replacement of the central furan ring by pyrolle, thiophene, and borrole heterocycles. Subsequently, the TPA properties of the resulting compounds are computed, and the influences of the chemical modifications on TPA properties investigated in detail. The most promising candidates with largely increased two-photon uncaging efficiencies are dimethylamino-substituted derivatives of NDBF, nitrodibenzopyrrol, and nitrodibenzothiophene.

Two-Photon Absorption Spectrum of a Single Crystal Cyanine-like Dye.

The two-photon absorption (2PA) spectrum of an organic single crystal is reported. The crystal is grown by self-nucleation of a subsaturated hot solution of acetonitrile, and is composed of an asymmetrical donor-π-acceptor cyanine-like dye molecule. To our knowledge, this is the first report of the 2PA spectrum of single crystals made from a cyanine-like dye. The linear and nonlinear properties of the single crystalline material are investigated and compared with the molecular properties of a toluene solution of its monomeric form. The maximum polarization-dependent 2PA coefficient of the single crystal is 52 ± 9 cm/GW, which is more than twice as large as that for the inorganic semiconductor CdTe with a similar absorption edge. The optical properties, linear and nonlinear, are strongly dependent upon incident polarization due to anisotropic molecular packing. X-ray diffraction analysis shows π-stacking dimers formation in the crystal, similar to H-aggregates. Quantum chemical calculations demonstrate that this dimerization leads to the splitting of the energy bands and the appearance of new red-shifted 2PA bands when compared to the solution of monomers. This trend is opposite to the blue shift in the linear absorption spectra upon H-aggregation.

Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents

Joint experimental and theoretical work is presented on two quadrupolar D-π-A-π-D chromophores characterized by the same bulky donor (D) group and two different central cores. The first chromophore, a newly synthesized species with a malononitrile-based acceptor (A) group, has a V-shaped structure that makes its absorption spectrum very broad, covering most of the visible region. The second chromophore has a squaraine-based core and therefore a linear structure, as also evinced from its absorption spectra. Both chromophores show an anomalous red shift of the absorption band upon increasing solvent polarity, a feature that is ascribed to the large, bulky structure of the molecules. For these molecules, the basic description of polar solvation in terms of a uniform reaction field fails. Indeed, a simple extension of the model to account for two independent reaction fields associated with the two molecular arms quantitatively reproduces the observed linear absorption and fluorescence as well as fluorescence anisotropy spectra, fully rationalizing their nontrivial dependence on solvent polarity. The model derived from the analysis of linear spectra is adopted to predict nonlinear spectra and specifically hyper-Rayleigh scattering and two-photon absorption spectra. In polar solvents, the V-shaped chromophore is predicted to have a large HRS response in a wide spectral region (approximately 600-1300 nm). Anomalously large and largely solvent-dependent HRS responses for the linear chromophores are ascribed to symmetry lowering induced by polar solvation and amplified in this bulky system by the presence of two reaction fields.

Calculations of nonlinear response properties using the intermediate state representation and the algebraic-diagrammatic construction polarization propagator approach: Two-photon absorption spectra

An earlier proposed approach to molecular response functions based on the intermediate state representation (ISR) of polarization propagator and algebraic-diagrammatic construction (ADC) approximations is for the first time employed for calculations of nonlinear response properties. The two-photon absorption (TPA) spectra are considered. The hierarchy of the first- and second-order ADC∕ISR computational schemes, ADC(1), ADC(2), ADC(2)-x, and ADC(3/2), is tested in applications to H(2)O, HF, and C(2)H(4) (ethylene). The calculated TPA spectra are compared with the results of coupled cluster (CC) models and time-dependent density-functional theory (TDDFT) calculations, using the results of the CC3 model as benchmarks. As a more realistic example, the TPA spectrum of C(8)H(10) (octatetraene) is calculated using the ADC(2)-x and ADC(2) methods. The results are compared with the results of TDDFT method and earlier calculations, as well as to the available experimental data. A prominent feature of octatetraene and other polyene molecules is the existence of low-lying excited states with increased double excitation character. We demonstrate that the two-photon absorption involving such states can be adequately studied using the ADC(2)-x scheme, explicitly accounting for interaction of doubly excited configurations. Observed peaks in the experimental TPA spectrum of octatetraene are assigned based on our calculations.

Nonlinear optical properties for a class of hexa-peri-hexabenzocoronene chromophores: a computational investigation

The systematic investigation of the linear and nonlinear optical properties on such a class of hexa-peri-hexabenzocoronene (HBC) chromophores is of significance for rationally designing two-photon absorption (TPA) materials. The results indicate that increasing the strength of electron-donating or accepting terminal groups leads to bathochromic-shift of the absorption band and enhancement of the TPA cross section (δ(max)). For the molecules with fluorinated methylene and cyano substituents, replacement of a double bond by a triple bond in the conjugated linker produces the increase of δ(max), owing to the lower bond-length alternation and better rigidity of phenylene-ethynylene. In contrast, for the molecules with nitro groups, the similar replacement results in a slight decrease of δ(max) because N-HBC-E-NO2 has excellent planarity architecture and effective electronic coupling. The TPA spectra are red-shifted and the δ(max) values are enhanced as the number of branches increases. Thus, a solvatochromism effect has a positive influence on the TPA response of the nitryl compounds due to larger polarization of the nitro moiety. We have shed light on the linear relationship between the first hyperpolarizability and δ(max). These HBC derivatives hold potential as high-performance nonlinear optical materials.

Effect of alicyclic ring size on the photophysical and photochemical properties of bis(arylidene)cycloalkanone compounds

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4-C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.

Excited state absorption: a key phenomenon for the improvement of biphotonic based optical limiting at telecommunication wavelengths

Spectroscopic properties, two-photon absorption (TPA) and excited state absorption (ESA), of two organic cyanine dyes and of a ruthenium based organometallic cyanine are compared in order to rationalize their similar ns-optical power limiting (OPL) efficiency in the telecommunication wavelength range. The TPA contribution to the ns-OPL behavior is higher for both organic cyanines, while the main process is a TPA-induced ESA in the case of the organometallic system, in which the ruthenium induces a broadening of the NIR-ESA band and resulting in a strong spectral overlap between TPA and ESA spectra.

A combined quantum mechanics/molecular mechanics study of the one- and two-photon absorption in the green fluorescent protein

We present for the first time a QM/MM study of the one- and two-photon absorption spectra of the GFP chromophore embedded in the full protein environment described by an advanced quantum mechanically derived polarizable force field. The calculations are performed on a crystal structure of the green fluorescent protein (GFP) using the polarizable embedding density functional theory (PE-DFT) scheme. The importance of treating the protein environment explicitly with a polarizable force field and higher-order multipoles is demonstrated, as well as the importance of including water molecules close to the chromophore in the protein barrel. For the most advanced description we achieve good agreement with experimental findings, with a peak at 405 nm for the neutral and a peak at 475 nm for the anionic form of the GFP chromophore. The presence of a dark OPA state, as suggested by other studies to explain the discrepancies between OPA and TPA spectra, is not supported by our calculations.

Theoretical Prediction of One- and Two-Photon Absorption Properties of N-Annulated Quaterrylenes as Near-Infrared Chromophores

Graphene nanoribbons (GNRs) have attracted increasing attention due to high potentiality in nanoelectronics. In the present study, quantum-chemical calculations of structural and nonlinear optical properties have been first carried out for the nanoelectronical materials, a new series of ladder-type N-annulated quaterrylenes and their imide chromophores. The effects of the solvent, terminal groups, the number of N-annulated bridges, and π-conjugated length are discussed in detail. The solvent effect is significant on the one-photon absorption (OPA). Moreover, the OPA and two-photon absorption (TPA) properties of the two series of DI and N-MI molecules show a clear solvent dependence, which is attributed to the carboximide substitution featuring larger polarization. Introducing electron-donating groups and dicarboximides and increasing the conjugated length lead to red-shifts of the OPA, emission, and TPA spectra, lower emission lifetimes, and enhanced TPA cross sections (δ(max)), but further extension of the conjugated framework does not always promote an increase of δ(max). The changing trends of δ(max) can be explained by the transition moment and the intramolecular charge transfer. All N-annulated quaterrylene and their imide derivatives possess small energy gaps, intense near-infrared absorption and emission, and large δ(max), which are important for use as two-photon fluorescent labeling materials.

Extremely nondegenerate two-photon absorption in direct-gap semiconductors [Invited

Two-photon absorption (2PA) spectra with pairs of extremely nondegenerate photons are measured in several direct-gap semiconductors (GaAs, CdTe, ZnO, ZnS and ZnSe) using picosecond or femtosecond pulses. In ZnSe, using photons with a ratio of energies of ~12, we obtain a 270-fold enhancement of 2PA when comparing to the corresponding degenerate 2PA coefficient at the average photon energy (ηω1 + ηω2)/2. This corresponds to a pump photon energy of 8% of the bandgap. 2PA coefficients as large as 1 cm/MW are measured. Thus, by using two widely different wavelengths we are able to access the large 2PA observed previously only in narrow gap semiconductors. We also calculate the corresponding enhancement of nonlinear refraction, consisting of two-photon, AC-Stark and Raman contributions. The net effect is a smaller enhancement, but exhibits very large dispersion within the 2PA regime.

Differences in Two‐Photon and One‐Photon Absorption Profiles Induced by Vibronic Coupling: The Case of Dioxaborine Heterocyclic Dye

A theoretical study of a dioxaborine heterocyclic compound in solution provides a case study for an analysis of the effects induced by the so-called Herzberg-Teller (HT) vibronic coupling on the one-(OPA) and two-photon absorption (TPA) spectra. For TPA, the HT vibronic coupling induces differences in the shapes of the absorption band. The study highlights the importance of vibronic coupling as a potentially important mechanism in absorption spectroscopy, able to explain differences in the OPA/TPA spectra.

Frequency Upshift in BO2 and CO2+ upon Electronic Excitation: A Twin-State Model Rationalization

The twin-state model, previously shown to provide a simple physical rationalization for the frequency upshift (exaltation) of the Kekulé mode in benzene upon S(0) to S(1) electronic excitation, is extended to the case of the BO(2) radical and the CO(2)(+) radical cation. In the case of BO(2)/CO(2)(+), the ground and excited states are degenerate, yet the model applies to the degenerate two-state system as well. In contrast with a pseudo-Jahn-Teller model, the twin-state one can predict which frequency is exalted and also which pair of electronic states are coupled, thus explaining the specificity of the phenomenon. The frequency exaltation is a spectroscopic manifestation of the resonance between the pair of VB structures describing twin states. In analogy with the case of benzene, it is predicted that the ν(3) asymmetric stretch fundamental will be a dominant peak in the two-photon absorption spectrum of BO(2) and CO(2)(+).

Efficient Photochromic Transformation of a New Fluorenyl Diarylethene: One- and Two-Photon Absorption Spectroscopy

Efficient reversible phototransformation of a new diarylethene-fluorene derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9H-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (1), was demonstrated in organic media under low-intensity laser excitation. Linear photophysical characterization of 1 was performed at room temperature in solvents of different polarity and viscosity. Significantly, close to unity quantum yield for the cyclization reaction of 1 was shown in nonpolar solutions. The lifetimes of the excited states of the open (OF) and closed (CF) forms of 1 were measured by a femtosecond transient absorption technique, and corresponding values of ∼0.7 and ∼0.9 ps were shown in dichloromethane (DCM), respectively. Degenerate two-photon absorption (2PA) spectra of the OF and CF of 1 were obtained over a broad spectral range by the open aperture Z-scan method under 1 kHz femtosecond excitation. The values of 2PA cross sections of the OF in DCM (∼50-70 GM) were found to increase up to 1 order of magnitude (∼600 GM) after cyclization to the CF. The nature of cyclization and cylcoreversion processes were investigated by quantum chemistry with employment of DFT-based methods implemented in the Gaussian'09 program. The potential of 1 for application in optical data storage was shown using poly(methyl methacrylate)-doped films and two-photon fluorescence microscopy readout.

Two-photon STED spectral determination for a new V-shaped organic fluorescent probe with efficient two-photon absorption.

Two-photon stimulated emission depletion (STED) cross sections were determined over a broad spectral range for a novel two-photon absorbing organic molecule, representing the first such report. The synthesis, comprehensive linear photophysical, two-photon absorption (2PA), and stimulated emission properties of a new fluorene-based compound, (E)-2-{3-[2-(7-(diphenylamino)-9,9-diethyl-9H-fluoren-2-yl)vinyl]-5-methyl-4-oxocyclohexa-2,5-dienylidene} malononitrile (1), are presented. Linear spectral parameters, including excitation anisotropy and fluorescence lifetimes, were obtained over a broad range of organic solvents at room temperature. The degenerate two-photon absorption (2PA) spectrum of 1 was determined with a combination of the direct open-aperture Z-scan and relative two-photon-induced fluorescence methods using 1 kHz femtosecond excitation. The maximum value of the 2PA cross section ~1700 GM was observed in the main, long wavelength, one-photon absorption band. One- and two-photon stimulated emission spectra of 1 were obtained over a broad spectral range using a femtosecond pump-probe technique, resulting in relatively high two-photon stimulated emission depletion cross sections (~1200 GM). A potential application of 1 in bioimaging was demonstrated through one- and two-photon fluorescence microscopy images of HCT 116 cells incubated with micelle-encapsulated dye.

Third-order nonlinear spectra and optical limiting of lead oxifluoroborate glasses

We have determined two-photon absorption and nonlinear refraction spectra of the 50BO(1.5) - (50-x)PbF(2) - xPbO glasses (with x = 25, 35, 50 cationic %) at the range of the 470 and 1550 nm. The replacement of fluor atoms by oxygen leads to an increase in the third-order susceptibility, due to the formation of non-bridging oxygens (NBO). The nonlinear index of refraction is one order of magnitude higher than the one for fused silica, and it increases almost twice for the sample with x = 50. This sample has also shown promising features for all-optical switching as well as for optical limiting.