Multiphoton Absorption in Expanded Porphyrins

We have calculated the multiphoton absorption cross-sections for three expanded porphyrin derivatives using the sum-over-states-involved tensor approach in combination with the strongly correlated multireference determinant single- and double-configuration interaction method. The calculated results showed that the two- and three-photon energies corresponding to the first peak of the multiphoton absorption spectra showed a decrease (red-shifted) with the number of inserted thiophene groups, whereas the cross sections showed a remarkable increase, particularly for three-photon absorption cross-section. However, the larger twist of the molecular plane for the expanded molecule resulted in an obvious drop in the increasing trend for three-photon absorption cross-section.     

扩展卟啉分子的多光子吸收特性

发展关联电子体系的多参考组态相互作用方法, 应用态求和的张量方法, 计算研究了三种扩展卟啉分子的多光子吸收特性. 计算结果表明, 通过中间插入噻吩杂环基团, 扩展卟啉分子的双光子和三光子吸收峰发生较大红移, 对应的吸收截面得到显著的提高, 并且三光子吸收截面的增加更为明显; 但是由于卟啉环扩大导致分子平面发生扭曲, 三光子吸收截面的增大趋势明显减弱.     

The correction vector method for three-photon absorption: the effects of pi conjugation in extended rylenebis(dicarboximide)s

A correction vector method within the multireference determinant single and double configuration interaction approximation coupled with the semiempirical intermediate neglect of differential overlap Hamiltonian has been developed for the computation of single and multiphoton absorption spectra of conjugated molecules. We study the effect of pi conjugation on these properties in the extended rylenebis(dicarboximide)s. The one-, two-, and three-photon absorption cross sections of the lowest-lying excited states show a power law dependence on the conjugation length, with exponents of about 1.3, 2.6, and 5.6, respectively. The maximum value of the three-photon absorption cross section in these molecules is calculated to be 1.06x10(-78) cm6 s2photon2 for photon energy at 0.57 eV.     

One-, two-, and three-photon absorption induced fluorescence of a novel chromophore in chloroform solution

One-, two-, and three-photon absorption induced fluorescence intensities of a novel nonlinear optical chromophore have been measured by using a tunable femtosecond pulsed laser as the excitation. Four resonance peaks are observed as the excitation wavelength is tuned from 600 to 2000 nm. These peaks correspond to the one-, two- and three-photon fluorescence resonance. Except for intensity difference, the lifetime and the fluorescence spectrum are found to be the same for the one-, two-, or three-photon resonance, hence suggesting that the same excited energy level is involved in emitting the fluorescence intensity. A three-level model is developed to account for the incident excitation laser intensity dependence of the one-photon and multiphoton fluorescence intensity. The model allows the multiphoton absorption cross sections to be extracted; it can also account for the deviation observed in the linear, square, and cubic intensity dependence of the one-, two-, and three-photon fluorescence intensity, respectively. To determine the absorption cross sections, the present method does not require the fluorescence quantum efficiency data, needed in the low intensity technique.     

Effects of donor/acceptor strengths on the multiphoton absorption: an EOM-CCSD correction vector study

We have developed a correction method (CV) to calculate the single- and multiphoton absorption (MPA) spectra of organic pi-conjugated systems within the equation of motion coupled-cluster method with single and double excitations (EOM-CCSD). The effects of donor/acceptor strengths on the multiphoton absorption in a series of symmetrically substituted stilbene derivatives have been reinvestigated at both the ab initio and the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian levels. Both ab initio and INDO calculations show that the electron-donating or electron-withdrawing substituents lead to enhancements of two- and three-photon absorption cross sections, more pronounced for two-photon absorption than for three-photon absorption. The ab initio calculations usually produce larger excitation energies than the semiempirical, which lead to lower MPA cross sections.     

Two-photon absorption of metal-organic DNA-probes

We report remarkable multiphoton absorption properties of DNA intercalating ruthenium complexes: (1) [Ru(phen)(2)dppz](2+); (2) [(11,11'-bidppz)(phen)(4)Ru(2)](4+); (3) [11,11'-bipb(phen)(4)Ru(2)](4+). Two-photon spectra in the range from 460 to 1100 nm were measured using the Z-scan technique. In particular, complex 2 was found to exhibit very strong two- and three-photon absorption properties, which could be an effect of symmetric charge transfer from the ends towards the middle of the conjugated dimeric orbital system. We propose that these molecules could provide a new generation of DNA binding nonlinear chromophores for wide applications in biology and material science. The combination of a large two-photon cross section and strong luminescence quantum yields for the molecules when intercalated makes the compounds uniquely bright and photo-stable probes for two-photon luminescence imaging and also promising as enhanced photosensitizers in two-photon sensitizing applications.     

Quantum chemical studies of three-photon absorption of some stilbenoid chromophores

Three-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to the third-order response function and its residues. The results obtained by using different functionals are compared with experimental data for similar systems obtained from the literature. With a Coulomb attenuated, asymptotically corrected functional, the excitation energy to the first resonance state is much improved. Comparison with experiment indicates that this is the case for the three-photon cross section as well. In particular, the overestimation of the cross sections and underestimation of excitation energies offered by the density functional theory using common density functionals is corrected for. It is argued that a reliable theory for three-photon absorption in charge transfer and other chromophore systems thereby has been obtained. Further elaboration of the theory and its experimental comparison call for explicit inclusion of solvent polarization and pulse propagation effects.     

Highly efficient multiphoton-absorbing quadrupolar oligomers for frequency upconversion

Two series of quadrupolar diphenylamino-endcapped oligofluorenes, PhN-OF(n)-NPh (n=2-5) and PhN-OF(n)-TAZ-OF(n)-NPh (n=1-4), which have an electron-withdrawing 1,2,4-triazole (TAZ) moiety as central core, with D-π-A-π-D structural motif (D=donor, A=acceptor), have been synthesized by palladium-catalyzed Suzuki cross-coupling of 9,9-dibutyl-7-(diphenylamino)-2-fluorenylboronic acid and the corresponding (1,2,4-triazole-based) aryl halide as key step. On pumping with infrared femtosecond lasers, these oligomers showed very strong multiphoton-excited blue photoluminescence. These D-π-D and D-π-A-π-D quadrupolar oligofluorenes exhibit superior three-photon absorption properties compared to the respective D-π-A counterparts with a highest three-photon absorption cross-section (σ(3)) of up to 2.72×10(-77) cm(6) s(2) . Despite the comparable linear and multiphoton absorption properties of the two types of quadrupolar oligomers PhN-OF(n)-NPh and PhN-OF(n)-TAZ-OF(n)-NPh, only the former exhibit remarkably intense and highly efficient multiphoton-excited frequency-upconverted deep blue lasing, which gives rise to record high lasing efficiency of 0.097% and very narrow of full width at half-maximum of the lasing spectra. Our findings suggest that quadrupolar-type molecules/oligomers are superior for multiphoton excited frequency upconverted lasing to their dipolar counterparts and also provide important guidelines to design highly efficient three-photon absorption molecules for photoluminescence and lasing applications.     

Exceptionally strong multiphoton-excited blue photoluminescence and lasing from ladder-type oligo(p-phenylene)s

We report the synthesis and investigation of multiphoton absorption properties of a novel series of diphenylamino-end-capped ladder-type oligo(p-phenylene)s which exhibit greatly enhanced and efficient multiphoton (from two- to five-photon) upconverted blue photoluminescence with which the record-high intrinsic three-photon absorption cross-section of 4.56 × 10(-76) cm(6) s(2) in the femtosecond regime has been obtained. Exceptionally efficient two- to five-photon-excited lasing in the blue region has also been demonstrated in which the highest two-photon-excited lasing efficiency of 0.34% has been achieved.     

Three-photon absorption cross-section enhancement in two symmetrical fluorene-based molecules

In this article, we report the pure three-photon absorption effects of two novel symmetrical fluorene-based molecules with 2D-pi-2D [9,9-diethylhexyl-2,7-bis-(N,N-diphenylamine)fluorene] and 2D-D-pi-D-2D [5,5'-(9,9-bis-(2-ethylhexyl)-9H-fluorene-2-yl)bis(N,N-diphenylthiophen-2-amine)] structural motifs. The obtained three-photon absorption cross sections, (4.74 +/- 0.01) x 10(-76) and (6.77 +/- 0.02) x 10(-76) cm(6) s(2) for the 2D-pi-2D and 2D-D-pi-D-2D archetypes, respectively, are rather high. The geometries and electronic excitations of the two molecules were systematically studied by the PM3 and ZINDO/S methods. The influence of the different molecular structures on the three-photon absorption cross section is discussed micromechanically. The experimental and theoretical results demonstrate that the transition dipole moment between the ground and final states is the most definitive factor. A new fitting method that is more accurate than those reported previously was used to obtain the 3PA coefficient.     

A molecular theory for two-photon and three-photon fluorescence polarization

In the analysis of molecular structure and local order in heterogeneous samples, multiphoton excitation of fluorescence affords chemically specific information and high-resolution imaging. This report presents the results of an investigation that secures a detailed theoretical representation of the fluorescence polarization produced by one-, two-, and three-photon excitations, with orientational averaging procedures being deployed to deliver the fully disordered limits. The equations determining multiphoton fluorescence response prove to be expressible in a relatively simple, generic form, and graphs exhibit the functional form of the multiphoton fluorescence polarization. Amongst other features, the results lead to the identification of a condition under which the fluorescence produced through the concerted absorption of any number of photons becomes completely unpolarized. It is also shown that the angular variation of fluorescence intensities is reliable indicator of orientational disorder.     

The impact of the pi-electron conjugation length on the three-photon absorption cross section of fluorene derivatives

The three-photon absorption cross sections of three different fluorene derivatives, with extended pi-electron conjugation lengths was experimentally measured and compared with shorter pi-electron conjugation length analogs. The effect of the conjugation length on the three-photon absorption cross section sigma(3) (') of this family of molecules has been elucidated. It is demonstrated that sigma(3) (') of the asymmetric compound D-pi-pi-pi-A is 6.6 times larger than its shorter configuration D-pi-A, while for the symmetric compounds D-pi-pi-pi-D and A-pi-pi-pi-A a two-fold enhancement was found relative to their shorter conjugation length analogs. Measurements of the three-photon excitation of these compounds in THF solution (10(-3)M) were accomplished with a tunable optical pulse generation pumped by a 25 ps Nd-YAG laser.     

Spectroscopic Properties and Three-photon Absorption Induced Optical Limiting of Series of Novel Nonlinear Chromophores

Three novel nonlinear chromophores with symmetric D-π-D molecular structure and extended conjugated length were synthesized. Solvatochromism analysis shows great symmetric intramolecular charge transfer occurring in chromophores by the enhancement in the dipole moment between the ground and excited states. The properties of optical power limiting induced by three-photon absorption (3PA) are demonstrated. Large 3PA coefficients and the corresponding molecular cross sections as high as 10-74 cm6s2 were obtained for nanosecond laser pulses at 1.06 μm from nonlinear transmission measurements.     

High-order multiphoton fluorescence of organic molecules in solution by intense femtosecond laser pulses

The five and possibly seven-photon fluorescence was observed for organic molecules in solution for the first time. A high-intensity laser enabled us to measure the properties of the high and any-order processes, and the emission could be directly visualized by the eye. These results showed that the common two-photon microscope could be upgraded to the higher order multiphoton microscope by choosing suitable excitation wavelengths. The multiphoton absorption cross sections differed by a factor of 10(33) as the order of the multiphoton process increased.     

Mono- and bichromatic electron dynamics: LiH, a test case

This paper describes an electron dynamics method where the time dependence of an external oscillating electric field is the perturbing part of the Hamiltonian. Application of the electric field induces charge movement inside the molecule and electronic transitions between the molecular orbitals. The test system is the neutral LiH molecule. The method is applied to wave functions calculated using the B3LYP (hybrid) density functional, with the STO-3G and the 6-31+G basis sets. The molecule undergoes full population inversion between the HOMO and the LUMO when the electric field is in resonance with the HOMO-LUMO energy gap. The magnitude of the electric field directly affects the rate at which electronic transitions occur and the rate at which charges move between lithium and hydrogen atoms. The method is used to model both monochromatic and bichromatic multiphoton effects in LiH. Monochromatic one-, two- and three-photon transitions occur between the HOMO, LUMO and two other virtual orbitals. There is evidence of both [1+2] direct and [1+1+1] stepwise multiphoton transitions. Bichromatically, two "laser" pulses are applied at different frequencies. Electronic transitions can be fine-tuned to occur via pre-specified pathways of virtual molecular orbitals.     

One-color reversible control of photochromic reactions in a diarylethene derivative: three-photon cyclization and two-photon cycloreversion by a near-infrared femtosecond laser pulse at 1.28 μm

One-color control of colorization/decolorization reactions of diarylethene molecules was attained by using nonresonant high-order multiphoton absorption processes with a near-infrared (NIR) femtosecond laser pulse at 1.28 μm with 35 fs full width at half-maximum (fwhm). The intensity of a rather weak laser pulse (<1 nJ/pulse) can induce the simultaneous three-photon absorption leading to the colorization, while much weaker intensity induces two-photon absorption resulting in the decolorization. The spatial patterning concomitant with higher-order multiphoton absorption processes was also demonstrated.     

Modular fluorescent benzobis(imidazolium) salts: syntheses, photophysical analyses, and applications

A series of benzobis(imidazolium) (BBI) salts has been prepared and studied as a new class of versatile fluorescent materials. Using a high yielding, modular synthetic strategy, BBI salts with a range of functionality poised for investigating fundamental and applications-oriented characteristics, including emission wavelength tunability, solvatochromism, red-edge excitation, chemical stability, multiphoton excitation, and protein conjugation, were prepared in overall yields of 40-97%. Through structural variation, the BBIs exhibited lambda(em) ranging between 329 and 561 nm while displaying phi(f)s up to 0.91. In addition, the emission characteristics of these salts were found to exhibit strong solvent dependencies with Stokes shifts ranging from 4570 to 13 793 cm(-1), depending on the nature of the BBI core. Although red-edge effects for BBI salts with Br and BF4 counterions were found to be similar, unique characteristics were displayed by an analogue with MeSO4 anions. The stability of an amphiphilic BBI was quantified in aqueous solutions of varying pH, and >85% of the emission intensity was retained after 2 h at pH 3-9. Through multiphoton excitation experiments in aqueous solutions, a BBI salt was found to exhibit three-photon fluorescence action cross sections similar to serotonin. The application of BBI salts as fluorescent protein tags was demonstrated by conjugating bovine serum albumin to a maleimide-functionalized derivative.     

Nature of the aqueous hydroxide ion probed by X-ray absorption spectroscopy

X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately "hyper-coordinated" solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.