Fluorescence emission anisotropy coupled to an electrochemical system: Study of exciton dynamics in conjugated polymers (original) (raw)
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Excited-state relaxation in π-conjugated polymers
Physical Review B, 2002
We study ultrafast relaxation processes of odd-(B u) and even-parity (A g) exciton states in poly͑p-phenylene vinylene͒ derivatives. The B u states are studied using a regular two-beam pump-and-probe spectroscopy, which can monitor vibronic relaxation and exciton diffusion. In order to observe the A g states, a three-beam femtosecond transient spectroscopy is developed, in which two different excitation pulses successively generate odd-parity (1B u) excitons at 2.2 eV and then reexcite them to higher A g states. We are able to distinguish two different classes of A g states: one class (mA g) experiences ultrafast internal conversion back to the lowest singlet exciton, whereas the other class (kA g) in violation of the Vavilov-Kasha's rule undergoes a different relaxation pathway. The excitons subsequently dissociate into long-lived polaron pairs, which results in emission quenching with the action spectrum similar to that of the intrinsic photoconductivity. We conclude that the A g states above 3.3 eV (kA g) are charge-transfer states, that mediate carrier photogeneration.
Electronic Delocalization, Vibrational Dynamics, and Energy Transfer in Organic Chromophores
The Journal of Physical Chemistry Letters, 2017
The efficiency of materials developed for solar energy and technological applications depends on the interplay between molecular architecture and light-induced electronic energy redistribution. The spatial localization of electronic excitations is very sensitive to molecular distortions. Vibrational nuclear motions can couple to electronic dynamics driving changes in localization. The electronic energy transfer among multiple chromophores arises from several distinct mechanisms that can give rise to experimentally measured signals. Atomistic simulations of coupled electron-vibrational dynamics can help uncover the nuclear motions directing energy flow. Through careful analysis of excited state wave function evolution and a useful fragmenting of multichromophore systems, through-bond transport and exciton hopping (through-space) mechanisms can be distinguished. Such insights are crucial in the interpretation of fluorescence anisotropy measurements and can aid materials design. This Perspective highlights the interconnected vibrational and electronic motions at the foundation of nonadiabatic dynamics where nuclear motions, including torsional rotations and bond vibrations, drive electronic transitions.
Unraveling Excitonic Effects for the First Hyperpolarizabilities of Chromophore Aggregates
The Journal of Physical Chemistry C, 2019
Excitonic interactions often significantly affect the optoelectronic properties of molecular materials. However, their role in determining the nonlinear optical response of organic electro-optic materials remains poorly understood. In this paper, we explore the effects of excitonic interactions on the first hyperpolarizability for aggregates of donor− acceptor chromophores. We show that calculations of the first hyperpolarizabilty of chromophore aggregates based on a two-state model agree well with the more rigorous coupled perturbed Hartree−Fock method. We then use both time-dependent density functional theory calculations and the molecular exciton approximation to parametrize the two-state model. Use of the molecular exciton approximation to the two-state model (i) is appropriate for disordered aggregates (unlike band theory), (ii) is computationally efficient enough for calculating the first hyperpolarizability of materials that consist of thousands of interacting chromophores, and (iii) allows the unraveling of the effects of both excitonic interactions and electrostatic polarization of the chromophore electron density by its environment on the first hyperpolarizability of molecular materials. We find that use of the molecular exciton approximation to the two-state model does not introduce significant additional errors compared to those introduced by applying the two-state model alone. We determine that the absolute change to the first hyperpolarizability of chromophore aggregates due to excitonic interactions increases with the size of the aggregate. For all sizes of disordered aggregates of chromophores considered in this paper, the inclusion of excitonic interactions on average decreases the magnitude of the first hyperpolarizability by 12−14% compared to the case of non-interacting chromophores. Finally, we present a method for analytically calculating the first hyperpolarizability of a one-dimensional periodic array of chromophores within the molecular exciton approximation to the two-state model. This technique can be used to include an approximate correction for excitonic effects when simulating the electro-optic response of disordered and ordered organic materials.
Journal of Photochemistry and Photobiology A: Chemistry, 2008
This paper describes the optical properties of an electroluminescent poly(phenylene vinylene-cofluorenylene vinylene) (BPPPV-PF)-based -conjugated polymer using absorption, photoluminescence (PL), time-resolved photoluminescence (TRPL), continuous wave (CW) and transient-photoinduced absorption (PA) spectroscopic techniques. The TRPL decay spectra of BPPPV-PF in chloroform and film exhibit single exponential decay with PL lifetimes of 0.8 ns and 0.2 ns, respectively. The CW-PA spectrum exhibits a single well-defined band centred at 1.5 eV, which is assigned to the T-T* transition of the triplet excitons. The monomolecular lifetime ( ) of the triplet excitons was estimated from the intensity dependence of CW-PA and found to be ∼1.2 ms at 80 K. The temperature dependence of the CW-PA signal at 1.5 eV was studied for a temperature range from 80 K to 298 K. It was observed that the PA signal at 1.5 eV was persistent to relatively high temperatures, which may be due to the bulky side chain of the polymer and morphology of the film. The relaxation process of this triplet excitation was studied by measuring the transient decay of the PA signal for various temperatures. The transient-PA signal shows monomolecular recombination process at all recorded temperatures and the temperature dependence of monomolecular lifetime ( ) was studied. It was found that the monomolecular lifetime ( ) is dependent on temperature but independent of the laser excitation intensity. The obtained results are analyzed based on the molecular structure of the PPV-PF copolymer.
Correlation of vibrational intensity with fluorescence lifetimes in π conjugated polymers
Polymer, 2008
A series of novel pi (p) conjugated polymers, originating from the archetypical Polyphenylene vinylene, in which the phenyl units are successively replaced by the larger naphthyl and anthryl acene units, were previously found to have a well-defined relationship between their relative fluorescence yields and their vibrational characteristics, as determined by Raman spectroscopy. In this study the Strickler-Berg equation is used to probe the influence of continual substitution of higher order acene units into the conjugated backbone in terms of the variation of the radiative and non-radiative rates. The deconvolution of the radiative and non-radiative rates enables the correlation of the reduction of the Raman intensity and concomitant increase in the fluorescence yield with the reduction of the non-radiative rate. This confirms that the reduction of the non-radiative rate is the dominant process introduced by the vibrational confinement originating from systematic substitution of higher order acene units into the polymer backbone.
The Journal of Physical Chemistry A, 2009
This paper provides an overview of recent research activities concerning the quantum-mechanical description of structures and properties of electronically excited chromophores in solution. The focus of the paper is on a specific approach to include solvent effects, namely the polarizable continuum model (PCM). Such a method represents an efficient strategy if coupled to proper quantum-mechanical descriptions such as the time-dependent density functional theory (TDDFT). As a result, the description of molecules in the condensed phase can be extended to excited states still maintaining the computational efficiency and the physical reliability of the ground-state calculations. The most important theoretical and computational aspects of the coupling between PCM and TDDFT are presented and discussed together with an example of application to the study of the low-lying electronic excited states of push-pull chromophores in different solvents.
Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers
Physical Review B, 1997
We have measured the quadratic electroabsorption ͑EA͒ spectrum of a variety of soluble luminescent and nonluminescent-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly͑phenylene-vinylene͒, poly͑phenylene ethylene͒, and polythiophene; the nonluminescent polymers include poly͑diethynyl silane͒ and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton (1B u) and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states (A g), the strongest of which (mA g) is located at an energy about 1.3 times that of the 1B u exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent A g state (kA g) at an energy of about 1.6 times that of the 1B u. We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im͓ 3 (Ϫ;,0,0)͔, using a summation over states model dominated by the ground state, the 1B u exciton, two strongly coupled A g states ͑mA g and kA g ͒, and their most strongly coupled vibrations, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results in a distribution of excited state energies, was also incorporated into the model. The decomposition of the EA spectra due to the conjugation length distribution was then used to calculate the 1B u exciton polarizability (⌬p) using first derivative analysis. For the longest conjugation lengths in our films, we found ⌬p to be of order 10 4 (Å) 3 in luminescent polymers and 10 3 Å 3 in nonluminescent polymers, respectively, in good agreement with recent subnanosecond transient photoconductivity measurements. We also found that the Huang-Rhys parameter of the 1B u exciton varies between 0.25 and 0.9, being in general smaller for the luminescent polymers. The consequent exciton relaxation energies were calculated to be of order 100 meV. ͓S0163-1829͑97͒03948-9͔
Journal of Physical Chemistry B, 2008
Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support π-electron conjugation across ∼80°curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.
A General control mechanism of energy flow in the excited state of polyenic biochromophores
Faraday Discussions, 2011
Quantum dynamics in photobiology is a highly controversial subject of modern research. In particular, the role of low-frequency vibrational coherence of biochromophores has been intensely discussed. Coherent control of polyenic chromophores, like carotenoids and retinoids, has been showing that the manipulation of such low frequency coherences may play a crucial role in the evolution of excited population and therefore in the efficiency of photosynthesis. However, no precise control mechanism has been derived. In order to clarify this open question, we combined quantum dynamical modelling with a sensitive experimental technique, namely Pump-Degenerate Four Wave Mixing (Pump-DFWM). In this work we investigate in detail the internal conversion channel of b-carotene, an important polyenic chromophore, under multipulse excitation and focus on the role of the non-adiabatic coupling between excited-state potentials and the internal energy loss. Our control mechanism is based on the interference between wavepackets in the excited state, which leads to a transient evolution of the vibrational population dependent on the relative phase between excitation sub-pulses. Such a transient evolution can affect the branching ratio between competing channels in the excited state. Therefore, our results are able to rationalize pulse shapes found in a whole class of coherent control experiments involving polyenic biochromophores, like in light harvesting complexes and in bacteriorhodopsin.