Excited State Decay Mechanisms Research Articles

Photoluminescence properties of polystyrene-hosted fluorophore thin films

We report on a photo-luminescence study of four different fluorophores: Coumarin 6, 2,5-Diphenyloxazole (PPO), 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) and Para-terpehnyl (PTP), doped in a polystyrene-based thin film. All of the samples are prepared by spin coating from a non-polar polymer solution at various concentrations. Their emission spectra and transient properties are characterized by photoluminescence measurements. Red-shifts in the emission spectra are observed for all four types of fluorophores as their concentration increases. We explain this phenomenon based on concentration dependence of solvatochromic effects and the results show good agreement with existing literature. We also show that the singlet-singlet annihilation processes are possibly a prevalent mechanism in the high concentration regime that affects the steady state and transient emission characteristics of the fluors. With the exception of PTP, photoluminescence quenching occurs as the fluorophore concentration in the polymer is increased. Rate equations for excited state decay mechanisms are analysed by considering different radiative and non-radiative energy transfer mechanisms. The results show consistency with our experimental observations. PTP shows the best photoluminescence results as an efficient fluor in the thin film, whereas PPO shows the strongest concentration dependent quenching and an anomalous lifetime distribution.

Excited-state deactivation of 5-vinyluracil: Effects of π-π conjugation and intramolecular hydrogen bond C H⋯O C

The excited-state decay mechanisms of 5-vinyluracil (5VU) are explored by using a combination of experimental and theoretical methods. The effects of vinyl substitutent, including the π-π conjugation effect and intramolecular hydrogen bond, are discussed. 5VU comprises the conformers 5VUA and 5VUB at ground state, with and without an intramolecular hydrogen bond CH⋯OC, respectively. The steady-state absorption and fluorescence spectra of 5VU are significantly red-shifted with respect to that of uracil. The time-resolved experimental results show that the excited-state decays of 5VU can be described by using three time constants: 2.26 ps, 13.45 ps and 4.66 ns. The decay pathways of 5VU obtained with the linearly interpolated internal coordinate method, predict that the two shorter lifetimes of 2.26 ps and 13.45 ps are attributed to 5VUB, and that the longest lifetime of 4.66 ns is attributed to 5VUA.

Gas-Phase Femtosecond Particle Spectroscopy: A Bottom-Up Approach to Nucleotide Dynamics.

We summarize how gas-phase ultrafast charged-particle spectroscopy has been used to provide an understanding of the photophysics of DNA building blocks. We focus on adenine and discuss how, following UV excitation, specific interactions determine the fates of its excited states. The dynamics can be probed using a systematic bottom-up approach that provides control over these interactions and that allows ever-larger complexes to be studied. Starting from a chromophore in adenine, the excited state decay mechanisms of adenine and chemically substituted or clustered adenine are considered and then extended to adenosine mono-, di-, and trinucleotides. We show that the gas-phase approach can offer exquisite insight into the dynamics observed in aqueous solution, but we also highlight stark differences. An outlook is provided that discusses some of the most promising developments in this bottom-up approach.

Photophysical Properties of Self-Assembled Multinuclear Platinum Metallacycles with Different Conformational Geometries

In this work, spectroscopic techniques and quantum chemistry calculations were used to investigate the photophysical properties of various multinuclear platinum complexes with different conformational geometries. This suite of complexes includes a Pt-pyridyl square, a Pt-carboxylate triangle, and a mixed Pt-pyridyl-carboxylate rectangle, as well as two mononuclear Pt model complexes. Studying the individual molecular precursors in the context of larger assemblies is important to provide a complete understanding of the factors governing the observed photophysical properties of a given system. The absorption and emission bands of the parent linear dipyridyl donor (ligand 1) are largely preserved in the [4 + 4] square and the multicomponent [4 + 2 + 2] rectangle (3 and 4, respectively), with significant red shifts. The [3 + 3] Pt-carboxylate triangle containing p-phthalic acid is nonemissive. Phosphorescence and nanosecond transient spectroscopy on 3 and 4 reveal that the introduction of platinum atoms enhances spin-orbital coupling, thereby increasing the rate of intersystem crossing. This phenomenon is consistent with the low fluorescence quantum yields and short fluorescence lifetimes of 3 and 4. Moreover, the electronic structures for the ground state and low-lying excited states of these compounds were studied using quantum chemistry calculations. The fluorescent states of the platinum complexes are local excited states of ligand-centered π-π* transition features, whereas the nonfluorescent states are intramolecular charge-transfer states. These low-lying intramolecular charge-transfer states are responsible for the nonemissive nature of small molecules 1 and 2 and triangle 5. As the interactions between these components determine the properties of their corresponding assemblies, we establish novel excited-state decay mechanisms which dictate the observed spectra.

Quantum dynamics study of fulvene double bond photoisomerization: The role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.

The Photocycle of Channelrhodopsin‐2: Ultrafast Reaction Dynamics and Subsequent Reaction Steps

The photocycle of channelrhodopsin-2 is investigated in a comprehensive study by ultrafast absorption and fluorescence spectroscopy as well as flash photolysis in the visible spectral range. The ultrafast techniques reveal an excited-state decay mechanism analogous to that of the archaeal bacteriorhodopsin and sensory rhodopsin II from Natronomonas pharaonis. After a fast vibrational relaxation of the excited-state population with 150 fs its decay with mainly 400 fs is observed. Hereby, both the initial all-trans retinal ground state and the 13-cis-retinal K photoproduct are populated. The reaction proceeds with a 2.7 ps component assigned to cooling processes. Small spectral shifts are observed on a 200 ps timescale. They are attributed to conformational rearrangements in the retinal binding pocket. The subsequent dynamics progresses with the formation of an M-like intermediate (7 and 120 μs), which decays into red-shifted states within 3 ms. Ground-state recovery including channel closing and reisomerization of the retinal chromophore occurs in a triexponential manner (6 ms, 33 ms, 3.4 s). To learn more about the energy barriers between the different photocycle intermediates, temperature-dependent flash photolysis measurements are performed between 10 and 30°C. The first five time constants decrease with increasing temperature. The calculated thermodynamic parameters indicate that the closing mechanism is controlled by large negative entropy changes. The last time constant is temperature independent, which demonstrates that the photocycle is most likely completed by a series of individual steps recovering the initial structure.

Photoelectron imaging of carbonyl sulfide cluster anions: Isomer coexistence and competition of excited-state decay mechanisms

We investigate the structure and decay of (OCS)n− cluster ions (n=2–4) using photoelectron imaging spectroscopy. The results indicate the coexistence of isomers with OCS− and covalently bound (OCS)2− cluster cores. A several-fold decrease in the relative abundance of the dimer-based species is observed for n=3 and 4 compared to n=2. The OCS−(OCS)n−1 cluster ions undergo direct photodetachment similar to OCS−⋅H2O, while (OCS)2−(OCS)n−2 exhibits both direct electron detachment and cluster decomposition via ionic fragmentation and autodetachment. The autodetachment originates from either the excited states of the parent cluster or internally excited anionic fragments. It is described using a statistical model of thermionic emission, which assumes rapid thermalization of the excitation energy. A decrease in the relative autodetachment yield in the trimer and tetramer cluster ions, compared to the covalent dimer, is attributed to competition with ionic fragmentation.

Spectroscopic and photophysical properties of hexanuclear rhenium(III) chalcogenide clusters.

The electronic, vibrational, and excited-state properties of hexanuclear rhenium(III) chalcogenide clusters based on the [Re(6)(mu(3)-Q)(8)](2+) (Q = S, Se) core have been investigated by spectroscopic and theoretical methods. Ultraviolet or visible excitation of [Re(6)Q(8)](2+) clusters produces luminescence with ranges in maxima of 12 500-15 100 cm(-)(1), emission quantum yields of 1-24%, and emission lifetimes of 2.6-22.4 microseconds. Nonradiative decay rate constants and the luminescence maxima follow the trend predicted by the energy gap law (EGL). Examination of 24 clusters in solution and 14 in the solid phase establish that exocluster ligands engender the observed EGL behavior; clusters with oxygen- or nitrogen-based apical ligands achieve maximal quantum yields and the longest lifetimes. The excited-state decay mechanism was investigated by applying nonradiative decay models to temperature-dependent emission experiments. Solid-state Raman spectra were recorded to identify vibrational contributions to excited-state deactivation; spectral assignments were enabled by normal coordinate analysis afforded from Hartree-Fock and DFT calculations. Excited-state decay is interpreted with a model where normal modes largely centered on the [Re(6)Q(8)](2+) core induce nonradiative relaxation. Hartree-Fock and DFT calculations of the electronic structure of the hexarhenium family of compounds support such a model. These experimental and theoretical studies of [Re(6)Q(8)](2+) luminescence provide a framework for elaborating a variety of luminescence-based applications of the largest series of isoelectronic clusters yet discovered.

Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

Ultrafast time-resolved resonance Raman spectra of carbonmonoxy hemoglobin (Hb), nitroxy Hb, and deoxy Hb are compared to determine excited state decay mechanisms for both ligated and unligated hemes. Transient absorption and Raman data provide evidence for a sequential photophysical relaxation pathway common to both ligated and unligated forms of Hb* (photolyzed heme), in which the excited state 1Q decays sequentially: Q 1 → H b I ∗ → H b II ∗ → Hb Hb ground state. Consistent with the observed kinetics, the lifetimes of these states are <50 fs, ≈300 fs, and ≈3 ps for 1Q, H b I ∗ and H b II ∗ , respectively. The transient absorption data support the hypothesis that the H b I ∗ state results from an ultrafast iron-to-porphyrin ring charge transfer process. The H b II ∗ state arises from porphyrin ring-to-iron back charge transfer to produce a porphyrin ground state configuration a nonequilibrium iron d-orbital population. Equatorial d– π* back-bonding of the heme iron to the porphyrin during the lifetime of the H b II ∗ state accounts for the time-resolved resonance Raman shifts on the ≈3 ps time scale. The proposed photophysical pathway suggests that iron-to-ring charge transfer is the key event in the mechanism of photolysis of diatomic ligands following a porphyrin ring π− π* transition.