Mikhail V. Bondar

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.

Nonlinear absorption in a series of Donor–π–Acceptor cyanines with different conjugation lengths

A detailed experimental and theoretical study of the linear and nonlinear absorption of a series of asymmetrical D–π–A cyanine dyes with the same trimethylindolin donor (D) and diethylamino-coumarin-dioxaborine acceptor (A) terminal groups and different conjugation lengths, is presented. Strong solvatochromic behavior affecting the fluorescence quantum yields, lifetimes, and the linear and nonlinear absorption properties is observed due to the presence of permanent ground state dipole moments. Detailed experimental studies of lifetime dynamics are performed by direct time-correlated single photon counting and pump–probe techniques. We find that an increase in π-conjugation in the investigated series of dyes leads to an enhancement of the excited-state absorption and two-photon absorption (2PA) cross-sections (δ2PA). The 2PA spectra for all of the investigated dyes consist of two well-separated bands. The first band occurs at two-photon excitation into the vibrational levels and not into the absorption peak of the main transition, S0 → S1, which is more typical of that observed for symmetrical cyanines. The position of the second 2PA band for all the molecules remains unchanged in solvents of different polarity contrary to the large solvatochromic shift of the S0 → S1 band, resulting in a large intermediate state resonance enhancement and, therefore, a larger 2PA in acetronitrile (δ2PA ≈ 10000 GM) compared to toluene (δ2PA ≈ 4700 GM).

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.

Photophysics of dimethylamino-substituted polymethine dye in polymeric media

Abstract The photophysical mechanism responsible for the large Stokes shift in dimethylamino-substituted polymethine dye (DMA-PD) has been studied by steady-state and time-resolved fluorescence spectroscopy. Experiments in low-temperature ethanol glass and in polymer polyurethane acrylate (PUA) in glassy and elastic states allow us to conclude that the emission occurs mainly from two excited states with lifetimes of 2–3 and 6–7 ns, respectively. The excited-state dynamics includes fast subnanosecond rotation of the DMA group around the carbon–dimethylamino bond of the dye molecule and stabilization of this form due to environmental rearrangement. The large Stokes shift remains for this dye in solid media. This process is dynamic even in polymeric media, where it occurs in the subnanosecond–nanosecond time scale. The major spectral shift can be described as an intramolecular excited-state reaction affecting either or both emissive species. We suggest a model in which the twisting of the small dimethylamino group modulates the effective length of the polymethine chromophore, producing the spectral shift. In addition, time-resolved fluorescence anisotropy decays show a slow rotational motion (about 40 ns) of the dye inside the polymer microcavities. These interesting features of the spectral behavior of the PUA matrix doped by DMA-PD may find various applications.

Simple solid state polymeric dye laser for scientific research and biomedical applications

Simple dye laser with the solid state polymeric active medium pumped by the second harmonic of Nd:YAG laser (532 nm) is presented. Active medium is made of dye-doped polyurethane acrylate (PUA) prepared by radical photopolymerization. Laser dyes in PUA are characterized by some peculiarities in spectral and photochemical properties which are important for lasing. To attain high laser efficiency, the hemispherical resonator with total length of 12 cm was used. In this case laser efficiency exceeds 30% with tuning range of 545-750 nm and spectral linewidth 0.4 nm. For removing the thermo-optical distortions in active polymeric medium and increasing its operational lifetime the original scanning system of active element has been constructed. It provides a possibility to realize the repetition rate up to 25 Hz (at peak intensity up to 80 MW/cm2) and to obtain the operational lifetime more than one million pulses for the proposed dyes. The elaborated laser has the certain advantages as compared to solid state Ti:Sapphire laser in relation to the peak power output and optical damage of active medium.