Yuriy Stepanenko

Phenylene Vinylene Platinum(II) Acetylides with Prodigious Two-Photon Absorption

The linear and nonlinear optical properties of a series of linear and cross-conjugated platinum(II) acetylide complexes that contain extended p-(phenylene vinylene) chromophores are reported. The complexes exhibit very high femtosecond two-photon absorption (2PA) cross section values (σ(2) up to 10,000 GM), as measured by nonlinear transmission (NLT) and two-photon excited fluorescence (2PEF) methods. The large 2PA cross sections span a broad range of wavelengths, 570-810 nm, and they overlap with the triplet excited state absorption. Spectral coincidence of high cross section 2PA and triplet absorption is a key feature giving rise to efficient dual-mode optical power limiting (OPL).

Proton tunnelling in porphycene seeded in a supersonic jet

Abstract Porphycene, a constitutional isomer of porphyrin, was investigated in a supersonic jet expansion using laser-induced fluorescence and hole burning techniques. The lines recorded in the fluorescence excitation spectrum were split into doublets. No splitting was detected if one or both inner hydrogen atoms were replaced by deuterium. The doublet structure also disappeared upon formation of a complex between porphycene and water or alcohol. These findings indicate that the origin of the doublet structure in bare porphycene is the tunneling splitting due to the exchange of two inner protons between the nitrogen atoms. From the spectral response to deuteration, and to a change in cooling conditions, it is deduced that the tunneling splitting is higher in the ground state than in the first excited singlet state.

Near-infrared two-photon absorption in phthalocyanines: Enhancement of lowest gerade-gerade transition by symmetrical electron-accepting substitution

This paper presents, to the best of our knowledge, the first study of two-photon absorption (2PA) spectra of a number of symmetrically substituted phthalocyanines in the excitation wavelength region from lambda(ex)=800 to 1600 nm. The selected molecules vary by position of substitution (alpha or beta), number of substituent groups (4, 8, or 16), and presence or absence of metal (Zn or Al) in the center. For all phthalocyanines we find a moderately strong (sigma(2) approximately 100-200 GM), pure electronic, gerade-gerade (g-g) 2PA transition, which shows up as a well-resolved relatively narrow peak in the energy region between Q and B bands (lambda(ex)=870-1100 nm). In metallophthalocyanines (MPcs) this lowest g-g transition is followed by the onset of other higher-frequency 2PA transitions. In some metal-free phthalocyanines (H(2)Pcs) we also reveal a second, broader 2PA transition at slightly higher frequency. In both MPcs and H(2)Pcs, we find a strong monotonic increase of integrated strength of the lowest g-g transition as a function of electron-accepting ability of peripheral substituents, expressed as their aggregated Hammett constant. By using few essential states models (three states for MPcs and four states for H(2)Pcs) we demonstrate the primary role of excited-state transition dipole moment in this effect.

Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy

We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis(tributylphosphine)-bis-(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)2(NPh2), NH2, OCH3, t-butyl, CH3, H, F, CF3, CN, benzothiazole, and NO2, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540-810 nm excitation wavelength range by complementary femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S0 → Sn transitions to higher energy states, above the lowest-energy S0 → S1 transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, σ2 ∼ 300 GM (1 GM = 10-50 cm4 s), for R = NPh2. At transition energies overlapping with the lowest-energy S0 → S1 transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, σ2 < 30-100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S0 → S1 transition gradually increases with increasing EW strength, reaching values σ2 = 700 GM for R = NO2, while in the S0 → Sn transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands. Our findings are corroborated by theoretical calculations that show large increase of the permanent electric dipole moment change in the S0 → S1 transition when ligands with strong EW substituents are twisted by 90° relative to the planar chromophore. Our NLT results in the S0 → S1 transition region are quantitatively similar to those obtained from the 2PEF measurement. However, at higher transition energy corresponding to S0 → Sn transition region, the NLT method yields effective multiphoton absorption stronger than the 2PEF measurement in the same systems. Such enhancement is observed in all Pt complexes as well as in all ligand chromophores studied, and we tentatively attribute this effect to nearly saturated excited-state absorption (ESA), which may occur if 2PA from the ground state is immediately followed by strongly allowed 1PA to higher excited states.

Femtosecond transient fluorescence spectrometer based on parametric amplification

We report an experimental proof-of-principle of a method for recording femtosecond, time-resolved fluorescence spectra in the visible range. The method is based on a noncollinear parametric amplification in a beta barium borate crystal and provides time resolution of the order of 100fs. We demonstrate that with this method, transient fluorescence spectra as wide as 6000cm−1 can be recorded in a single time-delay scan. Fluorescence decay dynamics and transient spectra of Coumarin 6dye dissolved in aniline were measured to test the usefulness of the method.

High-gain multipass noncollinear optical parametric chirped pulse amplifier

We demonstrate a multipass noncollinear optical parametric chirped pulse amplifier seeded by pulses from a femtosecond Ti:sapphire oscillator and pumped by a commercial Q-switched, frequency doubled Nd:yttrium–aluminum–garnet laser. Amplification higher than 106 and pulse energy exceeding 1.7 mJ are achieved with four passes through a single β-barium borate crystal. Good beam quality and high gain, together with broad amplification bandwidth, make it an attractive alternative to Ti:sapphire chirped pulse amplifier systems.

Primary Role of the Chromophore Bond Length Alternation in Reversible Photoconversion of Red Fluorescence Proteins

Rapid photobleaching of fluorescent proteins can limit their use in imaging applications. The underlying kinetics is multi-exponential and strongly depends on the local chromophore environment. The first, reversible, step may be attributed to a rotation around one of the two exocyclic C-C bonds bridging phenol and imidazolinone groups in the chromophore. However it is not clear how the protein environment controls this motion - either by steric hindrances or by modulating the electronic structure of the chromophore through electrostatic interactions. Here we study the first step of the photobleaching kinetics in 13 red fluorescent proteins (RFPs) with different chromophore environment and show that the associated rate strongly correlates with the bond length alternation (BLA) of the two bridge bonds. The sign of the BLA appears to determine which rotation is activated. Our results present experimental evidence for the dominance of electronic effects in the conformational dynamics of the RFP chromophore.

Multi-terawatt chirped pulse optical parametric amplifier with a time-shear power amplification stage

We report on a compact and simple, broadband optical parametric chirped pulse amplifier system that amplifies femtosecond pulses directly from a titanium sapphire oscillator up to 2 TW power. Our system relies on a new technique - time shear - that improves the time overlap between the seed and pump pulses and, thus, improves the efficiency of the power amplification stage pumped with a ns laser. Parametric amplification was achieved in two stages: a multipass, noncollinear geometry preamplifier with a single beta-barium borate crystal, and a power booster stage incorporating three beta-barium borate crystals in the new time-shear design. Both stages were pumped with pulses from a commercial 10 Hz frequency doubled ns Nd:YAG laser. The system delivers 49 mJ pulses with a temporal width of 23 fs and its overall conversion efficiency after pulse compression is 10%.

High-accuracy reference standards for two-photon absorption in the 680-1050 nm wavelength range.

Degenerate two-photon absorption (2PA) of a series of organic fluorophores is measured using femtosecond fluorescence excitation method in the wavelength range, λ2PA = 680-1050 nm, and ~100 MHz pulse repetition rate. The function of relative 2PA spectral shape is obtained with estimated accuracy 5%, and the absolute 2PA cross section is measured at selected wavelengths with the accuracy 8%. Significant improvement of the accuracy is achieved by means of rigorous evaluation of the quadratic dependence of the fluorescence signal on the incident photon flux in the whole wavelength range, by comparing results obtained from two independent experiments, as well as due to meticulous evaluation of critical experimental parameters, including the excitation spatial- and temporal pulse shape, laser power and sample geometry. Application of the reference standards in nonlinear transmittance measurements is discussed.

Laser studies of pyridylindoles in supersonic jets

One- and two-photon spectroscopy has been used to study jet-cooled 2-(2′-pyridyl)indole (PyIn-0) and 3,3′-dimethylene-2-(2′-pyridyl)indole (PyIn-2). The laser-induced fluorescence and resonance-enhanced photoionisation spectra of PyIn-2 are congested due to the flexibility of the ethylene chain. Single vibronic level fluorescence spectra allow identification of equivalent modes in the ground and electronically excited states. Deuteration at nitrogen reveals a 4 cm−1 red shift of the origin. Ab initio calculations were conducted at the TD B3LYP/6-31G(d,p) level of theory. We were able to reproduce the energies of pure electronic transitions both in PyIn-0 and PyIn-2. The properties of pyridylindoles in supersonic jets are compared with those for indole and 2-phenylindole.