Very large infrared two-photon absorption cross section of asymmetric zinc porphyrin aggregates: Role of intermolecular interaction and donor-acceptor strengths

Very large two-photon absorption (TPA) cross sections at the infrared region have been revealed for J-aggregates of asymmetric zinc porphyrin using quantum-chemical calculation. The TPA properties are evaluated for monomer and aggregates of a series of push-pull porphyrins, whose syntheses are known in the literature. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor. We also present a quantum-chemical analysis on porphyrin aggregates to understand the role of intermolecular interactions and the relationship between structural and collective nonlinear optical properties. It has been observed that the TPA properties change tremendously as monomers undergo J-aggregation and the magnitudes of TPA cross sections are highly dependent on the nature of aggregates. The importance of our results with respect to the design of photonic and photodynamic therapy materials has been discussed.     

Theoretical study of two-photon absorption in donor-acceptor chromophores tetraalkylammonium halide/carbon tetrabromide

Two-photon absorption properties of a series of donor-acceptor chromophores of tetraalkylammonium halide/carbon tetrabromide ([NR4h.CBr4], h = Cl, Br, I; R = Me, Et, Pr) complexes are investigated in terms of the calculated results by the time-dependent density functional theory (TDDFT) technique combined with the sum-over-states (SOS) method. The modeling two-photon absorption spectra show that these charge-transfer complexes have large two-photon absorption (TPA) cross sections and the [NEt4I.CBr4] has the largest TPA cross section delta with the value of 5.0 x 10(-45) cm4 s photon(-1). The maximum values of delta increase with increasing separations between the donor/acceptor in the order Cl...Br < Br...Br < I...Br for [NEt4h.CBr4] complexes; however, the TPA cross sections delta vary slightly as the size of the alkyl group increases from methyl to propyl for the bromide as a donor, and the maximum wavelength of the TPA peak lambdamax indicates a bathochromic shift. The charge transfers from the halide anion to the carbon tetrabromide make a significant contribution to the excited states, and the donor-acceptor charge transfer plays an important role in the TPA activity, whereas changes in size of alkyl group do not make a substantial contribution to TPA.     

Aggregates of quadrupolar dyes: giant two-photon absorption from biexciton states

A model for aggregates of quadrupolar (DAD or ADA) molecules is presented that relaxes the dipolar approximation for intermolecular electrostatic interactions. New effects, including the appearance of bound biexcitons in clusters of nonpolar molecules, are predicted with interesting and unforeseen consequences on the material properties. Specifically, we show that the large two-photon absorption cross-section, typical of quadrupolar chromophores, can be further amplified by orders of magnitude as a result of aggregation.     

Enhancement of two-photon absorption cross-section and singlet-oxygen generation in porphyrins upon beta-functionalization with donor-acceptor substituents

The microwave-enhanced synthesis, comparative singlet oxygen sensitization efficiency, and nonlinear optical characterization of a new beta-functionalized porphyrin and its copper complex are described. We show that the introduction of a donor-acceptor push-pull conjugated fragment in the beta position strongly perturbs the porphyrin electronic structure leading to a remarkable one- and two-photon NIR absorption enhancement.     

Theoretical insight into linear optical and two-photon absorption properties for a series of N-arylpyrrole-based dyes

N-arylpyrrole-based dyes possessing excellent opto-electronic properties are promising candidates for two-photon fluorescence labeling materials. The systematic investigation of novel N-arylpyrrole derivatives is of great importance for both fine-tuning electronic spectra and designing two-photon absorption (2PA) materials. We thoroughly studied influences of the π-conjugated center and N-substituted pyrrole moieties on the linear optical and 2PA properties. Our results show that introducing N-arylpyrrole produces bathochromic-shifts of the absorption and emission bands and an enhancement of the 2PA cross section (δ(max)), demonstrating that the electron-rich pyrrole moieties can efficiently increase intramolecular charge transfer. Substitution of fluorenyl with benzothiadiazole produces the lower energy gap, higher emission lifetime, red-shift of 2PA spectra and larger δ(max). The absorption and emission bands are highly dependent on the nature of the active building blocks. The aromatic rings attached to pyrrole can modulate the absorption peaks in the high energy and thus subtly modify the electronic properties.     

A TD-DFT study of the absorption spectra of fast dye salts

The UV/visible spectra of a series of diazonium fast dyes have been evaluated by using a time-dependent density functional theory approach explicitly taking into account bulk solvation effects. Using the PBE0 functional with the 6-311G(2d,2p) atomic basis set, the agreement between theory and experiment is excellent for these cationic species. The effects on the spectra of chemical substitution are analysed.     

Resonance hyper-Raman excitation profiles of a donor-acceptor substituted distyrylbenzene: one-photon and two-photon states

Resonance Raman and resonance hyper-Raman spectra of the "push-pull" conjugated molecule 1-(4'-dihexylaminostyryl)-4-(4"-nitrostyryl)benzene in acetone have been measured at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra), resonant with the first two bands in the linear absorption spectrum. The theory of resonance hyper-Raman scattering intensities is developed and simplified using assumptions appropriate for intramolecular charge-transfer transitions of large molecules in solution. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles, all in absolute intensity units, are quantitatively simulated to probe the structures and the one- and two-photon transition strengths of the two lowest-energy allowed electronic transitions. All four spectroscopic observables are reasonably well reproduced with a single set of excited-state parameters. The two lowest-energy, one-photon allowed electronic transitions have fairly comparable one-photon and two-photon transition strengths, but the higher-energy transition is largely localized on the nitrophenyl group while the lower-energy transition is more delocalized.     

Effective two-photon absorption cross section of heteroaromatic quadrupolar dyes: dependence on measurement technique and laser pulse characteristics

The linear and nonlinear optical properties of the heteroaromatic push-pull-push two-photon absorbing dye N-methyl-2,5-bis[1-(N-methylpyrid-4-yl)ethen-2-yl]-pyrrole ditriflate (PEPEP) are reported. The determination of the two-photon absorption (TPA) cross-section spectrum has been performed with different techniques: femtosecond TPA-white light continuum probe experiments, two-photon-induced fluorescence, and open aperture Z-scan measurements using both nanosecond and femtosecond laser pulses. The measured TPA cross sections and their wavelength dispersion show a marked dependence on the parameters of the laser pulses and on the measurement technique employed. These properties are discussed in terms of the different microscopic mechanisms that can contribute to the multiphoton absorption processes, with different weight depending on the measurement conditions and on the photophysical parameters of the dye.     

Two-photon absorption in quadrupolar pi-conjugated molecules: influence of the nature of the conjugated bridge and the donor-acceptor separation

Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.     

TDDFT study of one- and two-photon absorption properties: donor-pi-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.     

Resonance hyper-Raman excitation profiles and two-photon states of a donor-acceptor substituted polyene

Resonance Raman and resonance hyper-Raman spectra and excitation profiles have been measured for a "push-pull" donor-acceptor substituted conjugated polyene bearing a julolidine donor group and a nitrophenyl acceptor group, in acetone at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra). These wavelengths span the strong visible to near-UV linear absorption spectrum, which appears to involve at least three different electronic transitions. The relative intensities of different vibrational bands vary considerably across the excitation spectrum, with the hyper-Raman spectra showing greater variation than the linear Raman. A previously derived theory of resonance hyper-Raman intensities is modified to include contributions from purely vibrational levels of the ground electronic state as intermediate states in the two-photon absorption process. These contributions are found to have only a slight effect on the hyper-Rayleigh intensities and profiles, but they significantly influence some of the hyper-Raman profiles. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles are quantitatively simulated under the assumption that three excited electronic states contribute to the one- and two-photon absorption in this region. The transition centered near 400 nm is largely localized on the nitrophenyl group, while the transitions near 475 and 355 nm are more delocalized.     

Two-photon absorption strength: a new tool for the quantification of two-photon absorption

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDOS and HF+CIS3-21G) two-photon absorption properties of the 2,2'-bi(9,9-dihexylfluorene) molecule are compared.     

Investigation of the structures and absorption spectra for some hemicyanine dyes with pyridine nucleus by TD-DFT/PCM approach

Density-functional theory calculations of the Rayleigh optical activities of small isolated polyglycine molecules are reported. Fully extended β-sheet-like conformations of polypeptides of glycine, (Gly)_n (with n=1–5) are considered. After geometry optimizations, dipole moments and dipole polarizabilities (both the mean and the anisotropic components) are calculated using the B3LYP and B3P86 functionals in three basis sets. The polarizabilities are used to analyze the Rayleigh scattering activities and depolarization ratios. The convergence of the average dipole polarizability per monomer is analyzed. The differences in activity and depolarization for Rayleigh scattered radiation between the extended β-sheet-like and the folded -helix-like forms of tetraglycine are analyzed and found to be relevant, suggesting its possible use in experimental characterization.     

New cyano-substituted organic dyes containing different electrophilic groups: aggregation-induced emission and large two-photon absorption cross section

Three aggregation-induced emission active dyes (3a–c) were synthesized and their one- and two-photon absorption properties have been investigated. They were all found to be weakly fluorescent in THF solution, while they exhibited dramatic fluorescence enhancement in water/THF mixtures. The solid fluorescence of 3a–c was recorded and their fluorescence quantum efficiency (Φ_F) values were determined to be 8.0%, 8.1%, and 16.4%, respectively. Moreover, the two-photon absorption (2PA) cross-sections (σ) of 3a–c were measured and 3a showed the highest value of 702 GM. The excellent aggregation-induced emission and 2PA properties provide a promising alternative for biophotonic materials.     

Synthesis and two-photon absorption property of novel salen complexes incorporated with two pendant azo dyes

New salen compounds have been developed to possess two pendant azo dye chromophores. The two-photon absorption properties have been observed which result from the chromophores. The additive property has been found to exist as a result of no detrimental dipole-dipole interaction between chromophores.     

Effects of conjugation length, electron donor and acceptor strengths on two-photon absorption cross sections of asymmetric zinc-porphyrin derivatives

Exceptionally large two-photon absorption cross sections at the infrared region have been revealed by time-dependent density functional theory calculations for asymmetric charge-transfer conjugated zinc-porphyrin derivatives. The largest two-photon cross section is found to be more than one order of magnitude larger than for the conventional two-photon active organic molecules. The calculations show that the formation of strong charge-transfer states depends on the length of the conjugation bridge between the zinc-porphyrin core and the electron donor/acceptor. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor.     

Two novel terpyridine-based chromophores with donor-acceptor structural model containing modified triphenylamine moiety: Synthesis, crystal structures and two-photon absorption properties

Two novel terpyridine-based chromophores with D-A (D = donor, A = acceptor) structural model containing modified triphenylamine moiety (L1 and L2 ) have been conveniently synthesized via formylation and reduction in satisfactory yields, and fully characterized. The single crystals of them were obtained and determined by X-ray diffraction analysis. The relationships between structure and photophysical properties of the two chromophores were investigated both experimentally and theoretically. The measured maximum TPA cross-sections per molecular weight (δmax /MW) of the chromophores are 0.63 GM/(g mol) (L1) and 0.72 GM/(g mol) (L2), respectively, in DMF as a high polar solvent. The results indicate that the value of δmax/MW could be well tuned by the intramolecular charge transfer (ICT), which could be realized by introducing additional elecron-donor/acceptor groups.     

Theoretical study of two-photon absorption properties of a series of ferrocene-based chromophores

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.     

Calculation of two-photon absorption strengths with the approximate coupled cluster singles and doubles model CC2 using the resolution-of-identity approximation

An implementation of two-photon absorption matrix elements using the approximate second-order coupled-cluster singles and doubles model CC2 is presented. In this implementation we use the resolution-of-the-identity approximation for the two-electron repulsion integrals to reduce the computational cost. To avoid storage of large arrays we introduce in addition a numerical Laplace transformation of orbital energy denominators for the response of the doubles amplitudes. The error due to the numerical Laplace transformation is found to be negligible. Using this new implementation, we performed a series of benchmark calculations on substituted benzene and azobenzene derivatives to get reference values for TD-DFT results. We show that results obtained with the Coulomb-attenuated B3LYP functional are in reasonable agreement with the coupled-cluster results, whereas other density functionals which do not have a long-range correction give considerably less accurate results. Applications to the AF240 dye molecule and a weakly bound molecular tweezer complex demonstrate that this new RI-CC2 implementation allows for the first time to compute two-photon absorption cross sections with a correlated wave function method for molecules with more than 70 atoms and to apply this method for benchmarking TD-DFT calculations on molecules which are of particular relevance for experimental studies of two-photon absorption.     

Understanding on absorption and fluorescence electronic transitions of carbazole-based conducting polymers: TD-DFT approaches

The electronic excitation transitions of carbazole-based oligomers, (Cz-co-Cz) _N , (Cz-co-Fl) _N and (Cz-co-Th) _N (N = 2–4) were investigated using density functional theory (DFT) and time-dependent (TD) DFT methods. Our results show that the calculated ground state geometries favor a more aromatic, planer structure, while the electronically excited geometries favor a quinoidic type structure. Absorption and fluorescence energies have been obtained from TD-B3LYP/SVP calculations performed on the S₁ optimized geometries and are in excellent agreement with experimental data. The experimental fluorescence excitation energies for (Cz-co-Cz)₄, (Cz-co-Fl)₄ and (Cz-co-Th)₄ (2.76, 2.63, and 2.25 eV, respectively) correspond closely with the predicted S₁ transitions (2.84, 3.91 and 2.43 eV, respectively). We also report the predicted radiative lifetimes 0.52, 0.47, and 0.99 ns for (Cz-co-Cz) _N , (Cz-co-Fl) _N and (Cz-co-Th) _N , discuss the origin of the small stoke shift of the carbazole based oligomers and the magnitude of bathochromic shifts. We conclude by discussing the benefits of theoretical calculations, which can provide critical structural and electronic understanding of excitation–relaxation phenomena that can be exploited in design of novel optical materials.