We report on two-photon fluorescence excitation (TPE) action
cross sections for five widely used molecular
fluorophores. Measurements were performed by use of ultrashort
(∼100-fs) Ti:sapphire pulsed excitation over the range 690–960
nm. TPE spectra were obtained by comparison with a fluorescein
calibration standard. Large cross sections were found for the
cyanine reagent Cy 3 (∼140 GM) and for Rhodamine 6G (∼150
GM), both at 700 nm [1 GM = 10-50
(cm4 s)/photon]. Several fluorophores show
interesting and desirable blue shifts with respect to twice the
one-photon absorption wavelength. Fluorophore fluorescence
intensities showed no significant departure (±4%) from quadratic
illumination power dependence, indicating genuine two-photon
processes. Implications of these measurements for two-photon
laser-scanning microscopy are discussed.

Two-photon fluorescence excitation cross sections of biomolecular probes from 690 to 960 nm

Fluorescence and photoinduced proton transfer in the protolytic forms of fluorescein: Experimental and computational study

Solvent effects on the one- and two-photon absorption (1PA and 2PA) of disperse orange 3 (DO3) in dimethyl sulfoxide (DMSO) are studied using a discrete polarizable embedding (PE) response theory. The scheme comprises a quantum region containing the chromophore and an atomically granulated classical region for the solvent accounting for full interactions within and between the two regions. Either classical molecular dynamics (MD) or hybrid Car–Parrinello (CP) quantum/classical (QM/MM) molecular dynamics simulations are employed to describe the solvation of DO3 in DMSO, allowing for an analysis of the effect of the intermolecular short-range repulsion, long-range attraction, and electrostatic interactions on the conformational changes of the chromophore and also the effect of the solute–solvent polarization. PE linear response calculations are performed to verify the character, solvatochromic shift, and overlap of the two lowest energy transitions responsible for the linear absorption spectrum of DO3 in DMSO in the visible spectral region. Results of the PE linear and quadratic response calculations, performed using uncorrelated solute–solvent configurations sampled from either the classical or hybrid CP QM/MM MD simulations, are used to estimate the width of the line shape function of the two electronic lowest energy excited states, which allow a prediction of the 2PA cross-sections without the use of empirical parameters. Appropriate exchange-correlation functionals have been employed in order to describe the charge-transfer process following the electronic transitions of the chromophore in solution.

The Role of Molecular Conformation and Polarizable Embedding for One- and Two-Photon Absorption of Disperse Orange 3 in Solution B

This review presents the theoretical background concerning simplified quantum chemistry (sQC) methods to compute non‐linear optical (NLO) properties and their applications to large systems. To evaluate any NLO responses such as hyperpolarizabilities or two‐photon absorption (2PA), one should evidently perform first a ground state calculation and compute its response. Because of this, methods used to compute ground states of large systems are outlined, especially the xTB (extended tight‐binding) scheme. An overview on approaches to compute excited state and response properties is given, emphasizing the simplified time‐dependent density functional theory (sTD‐DFT). The formalism of the eXact integral sTD‐DFT (XsTD‐DFT) method is also introduced. For the first hyperpolarizability, 2PA, excited state absorption, and second hyperpolarizability, a brief historical review is given on early‐stage semi‐empirical method applications to systems that were considered large at the time. Then, we showcase recent applications with sQC methods, especially the sTD‐DFT scheme to large challenging systems such as fluorescent proteins or fluorescent organic nanoparticles as well as dynamic structural effects on flexible tryptophan‐rich peptides and gramicidin A. Thanks to the sTD‐DFT‐xTB scheme, all‐atom quantum chemistry methodologies are now possible for the computation of the first hyperpolarizability and 2PA of systems up to 5000 atoms. This review concludes by summing‐up current and future method developments in the sQC framework as well as forthcoming applications on large systems.This article is categorized under:
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Structure and Mechanism > Molecular Structures
Electronic Structure Theory > Density Functional Theory
Electronic Structure Theory > Semiempirical Electronic Structure Methods


Simplified quantum chemistry methods to evaluate non‐linear optical properties of large systems

The solvation and the solvatochromic behavior of 5-(dimethylamino)-5′-nitro-2,2′-bithiophene 1, the basis of a π* scale of solvent polarities, was investigated theoretically in toluene, dichloromethane, methanol and formamide with a Monte Carlo and quantum mechanics (QM/MM) iterative approach. The calculated transition energies of the solvatochromic band of 1, obtained as averages of statistically uncorrelated configurations, including the solute and explicit solvent molecules of the first solvation layer, besides showing good agreement with the experimental transitions, reproduced very well the positive solvatochromism of this probe in various solvents.

A Monte Carlo–quantum mechanics study of a solvatochromic π* probe

One- and two-photon absorption (1PA and 2PA) of fluorescein dianion (FSD) in water were studied using a combined and sequential Quantum Mechanics/Molecular Dynamics methodology. Different sets of 250 statistically relevant (uncorrelated) configurations composed by the solute and several solvent molecules were sampled from the classical simulation. On these configurations, the electronic properties were calculated a posteriori using the Zerner's intermediate neglect of differential overlap (ZINDO) method. The linear and nonlinear absorption of FSD in water were calculated using discrete and explicit solvent models. In the largest case, the relevant configurations are composed by FSD and 47 explicit water molecules embedded in the electrostatic field of all remaining water molecules. Both INDO/CIS and INDO/CISD calculations were performed to study the absorption processes of FSD and the Sum-Over-States (SOS) model was used to describe the 2PA process. A semi-classical method for spectrum simulations was employed to simulate the 1PA and 2PA cross-section spectra of FSD in water. For comparison purposes, in the case of the 2PA process two approaches, the "full expression" and "resonant expression" methods, were employed to simulate the nonlinear spectrum. The last method assumes resonant conditions and on the computation point of view it represents an interesting option to study the 2PA process. The INDO/CI calculations give a satisfactory description of the 1PA spectrum of FSD and properly describe the unusual blue-shift of its first π→π(*) transition in water. In the case of 2PA, the introduction of doubly excited configuration interactions (INDO/CISD) has proven to be essential for an appropriate description of the process at the higher energy spectral region. It was observed that the solvent effects do not drastically change the cross-sections of both processes. The simulated 2PA cross-section spectrum provided by the "full expression" method presents a better definition of the bands which appear along the experimental spectrum than the one provided by the "resonant expression" method. However, both approaches provide similar description for the effect of the solvent environment on the 2PA process of FSD in water.

One- and two-photon absorption of °uorescein dianion in water: a study using S-QM/MM methodology and ZINDO method.

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One- and two-photon absorption of °uorescein dianion in water: a study using S-QM/MM methodology and ZINDO method.