TPA Spectra Research Articles

Fully Polarizable QM/Fluctuating Charge Approach to Two-Photon Absorption of Aqueous Solutions.

We present the extension of the quantum/classical polarizable fluctuating charge model to the calculation of single residues of quadratic response functions, as required for the computational modeling of two-photon absorption cross sections. By virtue of a variational formulation of the quantum/classical polarizable coupling, we are able to exploit an atomic orbital-based quasienergy formalism to derive the additional coupling terms in the response equations. Our formalism can be extended to the calculation of arbitrary order response functions and their residues. The approach has been applied to the challenging problem of one- and two-photon spectra of rhodamine 6G (R6G) in aqueous solution. Solvent effects on one- and two-photon spectra of R6G in aqueous solution have been analyzed by considering three different approaches, from a continuum (QM/PCM) to two QM/MM models (nonpolarizable QM/TIP3P and polarizable QM/FQ). Both QM/TIP3P and QM/FQ simulated OPA and TPA spectra show that the inclusion of discrete water solvent molecules is essential to increase the agreement between theory and experiment. QM/FQ has been shown to give the best agreement with experiments.

One- and two-photon absorptions of the Cn and Cn−1 Si fullerenes in gas phase and solution

We systematically investigated the one- and two-photon absorption (OPA and TPA) spectra of the C n and C n−1Si fullerenes (n = 20, 30, 40, 50, and 60) in gas phase and solution using the sophisticated time-dependent density functional theory based response approach. The experimental geometry and OPA spectrum of C60 were used to justify the selection of level of theory for geometry optimizations and spectral calculations, respectively. These two kinds of spectra are dramatically enhanced or magnified in the solution compared to the gas phase. This enhancement is much larger in TPA than OPA and can be understood by an increase in transition dipole moment. The TPA spectra are more appropriate than the OPA spectra for distinguishing the isomers of the silicon doped fullerenes. A solvent dependence of the OPA and TPA spectra is also discussed.

Generalized time‐dependent approaches to vibrationally resolved electronic and Raman spectra: Theory and applications

In this review, we summarize the computationally efficient time‐dependent approaches to calculating the vibronic spectra including one‐photon/two‐photon absorption (OPA/TPA), emission, circular dichroism (CD), and resonance Raman/hyper‐Raman scattering (RRS/RHRS) spectra with inclusion of the mode‐mixing, Frank–Condon (FC) and Herzberg–Teller (HT) effects. At first, a unified dependency of spectral differential cross sections (SDCSs) on a generalized linear polarizability is derived in view of the relationship between SDCSs and the linear or nonlinear polarizabilities. Then, the Green's function technique is adopted to calculate the polarizability to avoid the tedious summation over the large number of vibrational states. To demonstrate the computational accuracy and efficiency, we apply the generalized time‐dependent approaches to calculate the OPA(E)/TPA, RRS, and RHRS spectra of a series of radicals and organic molecules. The computational accuracy is demonstrated by a close comparison between the simulated spectra and the available experimental data. In the spectral calculations, we apply our recently developed analytical energy‐derivative approaches for the excited states within the framework of time‐dependent density functional theory (TDDFT) to calculate the electronic‐structure parameters which enter the expressions of SDCSs, such as the excited‐state geometries, energy gradient, harmonic vibrational frequencies, the geometrical derivatives of transition dipole moments, and so forth. It is found that the integrated approach which combines the time‐dependent approach to SDCSs with the analytical energy‐derivative approaches for TDDFT excited‐state structure parameters breaks the bottlenecks of vibronic spectroscopy calculation down. © 2014 Wiley Periodicals, Inc.

Role of non-Condon vibronic coupling and conformation change on two-photon absorption spectra of green fluorescent protein

Two-photon absorption spectra of green fluorescent proteins (GFPs) often show a blue-shift band compared to their conventional one-photon absorption spectra, which is an intriguing feature that has not been well understood. We present here a systematic study on one- and two-photon spectra of GFP chromophore by means of the density functional response theory and complete active space self-consistent field (CASSCF) methods. It shows that the popular density functional fails to provide correct vibrational progression for the spectra. The non-Condon vibronic coupling, through the localised intrinsic vibrational modes of the chromophore, is responsible for the blue-shift in the TPA spectra. The cis to trans isomerisation can be identified in high-resolution TPA spectra. Our calculations demonstrate that the high level ab initio multiconfigurational CASSCF method, rather than the conventional density functional theory is required for investigating the essential excited-state properties of the GFP chromophore.

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 investigation of the two-photon absorption properties of 3,6-bis(4-vinylpyridinium) carbazole derivatives—new biological fluorescent probes

The present study focuses on a series of carbazole derivatives, which are being investigated as potential two-photon fluorescent probes (TFP) for DNA detection and two-photon bioimaging. The geometric structures, electronic structures, and the one-photon (OPA) and two-photon (TPA) absorption properties of 3,6-bis(1-methyl-4-vinylpyridinium) carbazole (BMVC) derivatives, as well as their dications and diiodized derivatives, were studied using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) method. The results showed that the TPA spectra of the diiodized BMVC derivatives and their dications are all found in the near-infrared region (NIR). At the same time, the diiodized BMVC derivatives presented larger TPA cross-sections than the neutral BMVC derivatives and the dications. These theoretically derived values were also in good agreement with their corresponding experimentally observed values, and they indicated that the diiodized BMVC may be the form of this TFP that combines with DNA. The diiodized BMVC derivatives and the dications have the potential to be excellent TPA materials, especially when used as TFPs in nucleic acid imaging applications.

Influence of Excitonic Interactions on the Transient Absorption and Two-Photon Absorption Spectra of Porphyrin J-Aggregates in the NIR Region

The transient absorption (TA) and two-photon (TPA) absorption spectra of a water-soluble porphyrin, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H4TPPS2-) diacid, in the monomeric and J-aggregated form are reported. TA spectra, measured with ultrafast laser pulses, were excited at 403 nm and probed at wavelengths varying in the 440−1100 nm range. While the TA spectrum of the J-aggregate form has been already investigated, but only in the 450−750 nm region (Kano, H.; Kobayashi, T. J. Chem. Phys. 2002, 116, 184), the TA spectrum of the monomer has never been investigated previously. The TPA spectrum, also measured with ultrafast laser pulses in the wavelength range of 380−440 nm, shows an increase of the TPA cross-section per single molecule by a factor of 30 upon aggregation. Even more intriguing is the appearance of a two-photon allowed transition around 410 nm in the TPA and TA spectra of the J-aggregate form, not present either in the TPA or in the TA spectra of the monomer. The physical nature of the two-photon allowed state giving rise to such a band is thoroughly discussed, in relation to the peculiar excitonic properties of molecular aggregates, and it is tentatively attributed to a bi-exciton state.

Effects of conjugation in length and dimension on two-photon properties of fluorene-based chromophores

We report calculated two-photon (TPA) absorption spectra based upon results obtained from quadratic response time-dependent density functional theory for fluorene-based donor-π-acceptor molecules. Coulomb attenuated functionals with a long-range exchange contribution are applied to predict TPA excitation energies and cross-sections. Observed spectra are explained, and the effects of conjugation and multibranching on the TPA spectra are discussed.

Role of Donor−Acceptor Strengths and Separation on the Two-Photon Absorption Response of Cytotoxic Dyes: A TD-DFT Study

Time-dependent density functional theory (TD-DFT) is applied to model one-photon (OPA) and two-photon (TPA) absorption spectra in a series of conjugated cytotoxic dyes. Good agreement with available experimental data is found for calculated excitation energies and cross sections. Calculations show that both OPA and TPA spectra in the molecules studied are typically dominated by two strong peaks corresponding to different electronic states. We find that donor-acceptor strengths and conjugated bridge length have a strong impact on the cross-section magnitudes of low- and high-frequency TPA maxima, respectively. These trends are analyzed in terms of the natural transition orbitals of the corresponding electronic states. Observed structure-property relationships may have useful implications on design of organic conjugated chromophores with tunable two-photon absorption properties for photodynamic therapy applications.

TDDFT Study of One- and Two-Photon Absorption Properties: Donor−π−Acceptor Chromophores

We report a comprehensive time-dependent density functional theory (TDDFT) study of one-photon and two-photon absorption (OPA and TPA, respectively) spectra for donor-pi-acceptor molecules. The calculated excitation energies were generally shown to be in good agreement with experiment, particularly when compared to results from measurements carried out in a nonpolar solvent, although the oscillator strengths were overestimated in some cases. Calculated TPA cross sections applying the two-state approximation were shown to be highly dependent on the form of the line-shape function used. Although a good agreement with experimental TPA spectra was generally observed, the wide range in the experimentally measured values and lack of systematic experimental data on solvent effects limited a detailed comparison as yet.

Two-photon absorption spectroscopy in ZnSe/ZnSSe strained-layer superlattices

We report two-photon absorption photoluminescence excitation (TPA-PLE) measurements in ZnSe/ZnSSe strained-layer superlattices (SLS). Polarization-dependent selection rules are observed in TPA spectra depending on the direction of the exciting field with respect to the growth axis of the SLS. Experimental exciton transition energies have found to be in good agreement with theoretical predictions considering strain and confinement effects. Furthermore by means of comparative non-linear technique the TPA coefficient has been measured at different energies and compared with theoretical predictions including quasi-bidimensional excitonic effects. A satisfactory agreement has been shown for the overall spectral behaviour and in some cases also for the TPA coefficient values.

Polarization dependence of two-photon absorption in ZnSe-ZnSSe strained-layer superlattices

We report two-photon absorption photoluminescence excitation (TPA-PLE) measurements in ZnSe/ZnSSe strained-layer superlattices (SLS). Polarization dependent selection rules are observed in TPA spectra depending on the direction of the exciting field with respect to the growth axis of the SLS. Moreover a comparison between the theoretical predictions for TPA transition rates and experimental results has been attempted.

One- and two-photon spectra of some selected molecules: A comparative study

The complementarity of one- and two-photon spectroscopy has been utilised for throwing light on the following problems of chemical interest: (1) Weak interaction between identical chromophores separated by insulating bridges gives rise to split states of different symmetries. Two-photon spectroscopy (TPA), in conjunction with one-photon absorption (OPA), has been used to identify the states and hence to estimate the magnitude of interaction in bimolecules and trimolecules. From the shifts between the one- and the two-photon spectra, the splittings have been estimated. Calculations confirm that the dominant interaction is the through-bond one. (2) The second type of problem is the identification ofg andu vibrations in molecules. We have initiated studies on three molecules in jet-cooled conditions: 9,10-dihydro-anthracene (DHA). 9,10-dihydro-phenanthrene (DHP) and octa-fluoronaph-thalene (OFN). Only the one-photon fluorescence excitation spectra have so far been obtained by us and the TPA spectra are under investigation. (3) The third class of molecules discussed here are the Ln3+ complexes wheref n⇒ fn transitions are intrinsically two-photon allowed. We have studied two GD3+ single crystals. The CF-splittings, observed clearly in TPA, have been fitted with a parametric model. Some of our observations on the variations of TPA intensity patterns from crystal to crystal, such as circular:linear polarisation ratios, relative intensities of transitions to differentJ-states, do not quite fit in with the Axe-Judd-Downer model. The discrepancies call for a reappraisal of the role of ligand in the TPA process.

Membrane-phorbol ester interactions monitored by circular dichroism

The interaction of phorbol 12,13-dibutyrate (PDBu), 12- O-retinoylphorbol 13-acetate (RPA) and 12- O-tetradecanoylphorbol 13-acetate (TPA) with l-α-phosphatidylserine-containing small unilamellar vesicles or erythrocyte ghosts was monitored by circular dichroism (CD). No change in the CD spectra of PDBu was observed upon binding, while RPA and TPA spectra were slowly affected by the interaction. The changes in RPA and TPA spectra were assigned to the embedding of these molecules in the membrane bilayers. In the presence of 10 8 cells/ml, after one minute incubation, about 2 to 5% of the amount of phorbol ester added is embedded in the membrane. It is suggested that either phorbol esters entering the membrane is not a prerequisite for protein kinase C activation or the amount of phorbol esters necessary to activate protein kinase C is very small.

Direct evidence of incorporation of 12- O-[20- 2H 1]tetradecanoylphorbol-13-acetate into artificial membranes as determined by deuterium magnetic resonance

In order to elucidate the manner of interaction of 12- O-tetradecanoylphorbol-13-acetate (TPA), 2H-NMR spectra of [20- 2H 1]TPA and 31P-NMR spectra were recorded in the presence of multibilayers of dimyristoylphosphatidylcholine (DMPC). Observation of several pairs of quadrupole splittings directly proves the TPA molecules are intercalated into the multibilayers. Further, reduction of the 31P chemical shift anisotropy of DMPC multibilayers by TPA is more pronounced than that of phorbol 12,13-diacetate (PDA) whose activity of tumor promotion is very weak.