Yuriy L. Slominsky

Molecular structure—two-photon absorption property relations in polymethine dyes

We performed a comprehensive experimental investigation of two-photon absorption (2PA) spectra of a series of 12 symmetrical and asymmetrical cationic polymethine dyes, including complete one-and two-photon excitation anisotropy measurements. Quantum-chemical calculations were performed with the goal of understanding the nature of 2PA bands and of uncovering structure-property relations. We found that there are 2PA bands in the spectral region between the first absorption band and that for twice its energy. A weakly allowed 2PA band within the short-wavelength shoulder of the first absorption band was observed owing to the effects of vibrational and charge distribution symmetry breaking. The nature of the strongest 2PA band is connected to the electron transition from the molecular orbital localized at the benzene rings of the terminal groups to the lowest unoccupied molecular orbital (LUMO). Structure-property relations revealed that the 2PA cross section tends to be enhanced by either an increase in the length of the polymethine chromophore or an increase in the donor strengths in the terminal groups.

Experimental and theoretical approaches to understanding two-photon absorption spectra in polymethine and squaraine molecules

We performed a detailed experimental investigation and quantum-chemical analysis of two-photon absorption (2PA) spectra of a series of symmetrical cationic polymethines and neutral squaraines having similar structures. Degenerate 2PA spectra of these molecules are taken by using two-photon fluorescence spectroscopy and the Z-scan technique. All measurements are made with 150 fs laser pulses of 1 kHz repetition rate in the tuning range of 520-2100 nm(0.6-2.4 eV). Comparing 2PA spectra of polymethines and squaraines, we find that we can access considerably larger 2PA cross sections (≥8600×10−50 cm4s/photon) in the squaraines owing to narrower linear absorption spectra and an increase in the density of unoccupied molecular orbitals introduced by the squaraine acceptor group in the conjugated chain.

POLYMETHINE AND SQUARYLIUM MOLECULES WITH LARGE EXCITED-STATE ABSORPTION

Abstract We study nonlinear absorption in a series of ten polymethine dyes and two squarylium dyes using Z-scan, pump-probe and optical limiting experiments. Both picosecond and nanosecond characterization were performed at 532 nm, while picosecond measurements were performed using an optical parametric oscillator (OPO) from 440 to 650 nm. The photophysical parameters of these dyes including cross sections and excited-state lifetimes are presented both in solution in ethanol and in an elastopolymeric material, polyurethane acrylate (PUA). We determine that the dominant nonlinearity in all these dyes is large excited-state absorption (ESA), i.e. reverse saturable absorption. For several of the dyes we measure a relatively large ground-state absorption cross section, σ 01 , which effectively populates an excited state that possesses an extremely large ESA cross section, σ 12 . The ratios of σ 12 / σ 01 are the largest we know of, up to 200 at 532 nm, and lead to very low thresholds for optical limiting. However, the lifetimes of the excited state are of the order of 1 ns in ethanol, which is increased to up to 3 ns in PUA. This lifetime is less than optimum for sensor protection applications for Q-switched inputs, and intersystem crossing times for these molecules are extremely long, so that triplet states are not populated. These parameters show a significant improvement over those of the first set of this class of dyes studied and indicate that further improvement of the photophysical parameters may be possible. From these measurements, correlations between molecular structure and nonlinear properties are made. We propose a five-level, all-singlet state model, which includes reorientation processes in the first excited state. This includes a trans – cis conformational change that leads to the formation of a new state with a new molecular configuration which is also absorbing but can undergo a light-induced degradation at high inputs.

Excited-state absorption dynamics in polymethine dyes detected by polarization-resolved pump–probe measurements

Polarization-resolved excitation-probe measurements are performed for a new series of polymethine dyes in several solvents and a polyurethane acrylate elastopolymer. We describe our experimental studies and give an analysis of the nature of the rotational motions of excited molecules and orientation of the excited-state transitions relative to transitions from the ground state.

The nature of excited-state absorption in polymethine and squarylium molecules

Subpicosecond transient absorption measurements were performed for several polymethine and squarylium dyes in ethanol solution and a polymeric host over the spectral range 400-1500 nm. A variety of nonlinear effects including saturable absorption, reverse saturable absorption, and gain were observed and analyzed. We observe strong excited-state absorption (ESA) in all dyes in the range 450-600 nm. We also report the first prediction and observation of additional ESA bands in the near-infrared range. The predictions were based on quantum chemical calculations and the ESA experiments were performed with femtosecond pump-continuum probe techniques. For polymethine dye 2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene) ethylidene]-2-phenyl-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-phenylindolium perchorate, an additional ESA band was detected near 1250 nm, and for squarylium dye 1,3-Bis-[(1,3-dihydro-1-butyl-3,3-dimetyl-2H-benzo[e]indol-2-ylidene)methyl]squ araine, two additional ESA bands were found around 870- and 1380-nm, respectively. To further study the nature of these transitions, the steady-state excitation anisotropy was also studied and compared with predictions. The relationship between ESA spectra of organic dyes and their molecular structure is discussed.

Femtosecond-to-nanosecond nonlinear spectroscopy of polymethine molecules

The linear and nonlinear optical properties of a series of polymethine molecules are investigated to study the effects of molecular structure and the host environment on overall nonlinear absorption performance. The linear characterization includes measuring the solvatochromic shifts between absorption and fluorescence peaks and studying the excited-state orientational diffusion kinetics. The nonlinear characterization involves measuring the excited-state absorption spectra with a femtosecond white-light-continuum pump-probe technique and performing Z scans and nonlinear transmission measurements from the picosecond to the nanosecond time regimes. The results of these experiments allow us to develop an energy-level structure for the polymethines, which accurately predicts nonlinear absorption properties from the picosecond to the nanosecond time regimes. From this model we are able to identify the key molecular parameters for improved nonlinear absorption.

Picosecond absorption anisotropy of polymethine and squarylium dyes in liquid and polymeric media

Abstract Time-resolved excitation-probe polarization measurements are performed for polymethine and squarylium dyes in ethanol and an elastopolymer of polyurethane acrylate (PUA). These molecules exhibit strong excited-state absorption in the visible, which results in reverse saturable absorption (RSA). In pump–probe experiments, we observe a strong angular dependence of the RSA decay kinetics upon variation of the angle between pump and probe polarizations. The difference in absorption anisotropy kinetics in ethanol and PUA is detected and analyzed. Anisotropy decay curves in ethanol follow a single exponential decay leading to complete depolarization of the excited state. We also observe complete depolarization in PUA, in which case the anisotropy decay follows a double exponential behavior. Possible rotations in the PUA polymeric matrix are connected with the existence of local microcavities of free volume. We believe that the fast decay component is connected with the rotation of molecular fragments and the slower decay component is connected with the rotation of entire molecules in local microcavities, which is possible because of the elasticity of the polymeric material.

Femtosecond to nanosecond characterization of the excited-state properties of polymethine molecules

Saturation limits excited-state absorption (ESA) in many organic molecules. We studied polymethines using continuum spectroscopy to determine ESA spectra, two-color pump-probe anisotropy for transition dipole moment orientations, picosecond and nanosecond z-scans and nonlinear transmission measurements.

Comparison of Linear and Nonlinear Absorption in Three Series of Similar Cyanine Dyes: D-?-D, A-?-A and D-?-A

We report the linear and nonlinear spectra of three series of cyanine dyes (donor-?-acceptor, acceptor-?-acceptor, and donor-?-donor) including effects of conjugation length and terminal groups. Two-photon-absorption up to 16,000 GM is observed in acceptor-?-acceptor structures.

Excited state absorption and femtosecond lifetime dynamics in a new series of near IR dyes

Large excited-state-absorption (ESA) spectra and lifetime dynamics for a new series of NIR cyanine-like dyes are compared to similar visible dyes. Strongly red-shifted (200-300 nm) and increased ESA are observed and explained through quantum-chemical calculations.