Ultrafast restricted intramolecular rotation in molecules with aggregation induced emission

In this work, the ultrafast intramolecular rotation behavior of 1,1,2,3,4,5-hexaphenylsilole has been investigated in several solutions with different viscosities using femtosecond transient absorption spectroscopy combined with density functional theory and time-dependent density functional theory calculations. It is demonstrated that the nonradiative process, which competes with radiative decay, involves two main stages, namely the restricted intramolecular rotation and internal conversion processes. The intramolecular rotation depends on viscosity and presents a significant restriction. The restricted rotational rate is determined to be dozens of picoseconds. The following nonradiative process is strongly dominated by intramolecular rotation. The nonradiative decay rate will decrease with the increase in viscosity, leading to a rise in the radiative probability and photoluminous yield. These results have borne out the mechanism of ultrafast restricted intramolecular rotation of aggregation induced emission and provided a detailed photophysical picture of nonradiative processes.

Vibronic coupling and ultrafast relaxation dynamics in the first five excited singlet electronic states of bithiophene

The vibronic structure and nuclear dynamics in the first five excited singlet electronic states of bithiophene (2T) are investigated here. Specifically, considerations are given to comprehend the first two structureless and broad electronic absorption bands and the role of nonadiabatic coupling in the excited state relaxation mechanism of 2T in the gas phase. Associated potential energy surfaces (PESs) are established by constructing a model vibronic coupling Hamiltonian using 18 vibrational degrees of freedom and extensive ab initio electronic structure calculations. The topographies of these PESs are critically examined, and multiple conical intersections are established. The nuclear dynamics calculations are performed by propagating wave packets on the coupled electronic manifold. The present theoretical results are in good agreement with the experimental observations. It is found that strong nonadiabatic coupling between the S1-S4 and S1-S5 states along totally symmetric modes is predominantly responsible for the structureless and broad first absorption band, and overlapping S2, S3, S4, and S5 states form the second absorption band. Photorelaxation from the highly excited S5 to the lowest S1 state takes place through a cascade of diabatic population transfers among the S1-S4-S5 electronic manifold within the first ∼100 fs. Totally symmetric C=C stretching, C-S stretching, C-H wagging, ring puckering, and inter-ring bending modes collectively drive such relaxation dynamics.

Triplets with a Twist: Ultrafast Intersystem Crossing in a Series of Electron Acceptor Materials Driven by Conformational Disorder

Control over the populations of singlet and triplet excitons is key to organic semiconductor technologies. In different contexts, triplets can represent an energy loss pathway that must be managed (i.e., solar cells, light-emitting diodes, and lasers) or provide avenues to improve energy conversion (i.e., photon upconversion and multiplication systems). A key consideration in the interplay of singlet and triplet exciton populations in these systems is the rate of intersystem crossing (ISC). In this work, we design, measure, and model a series of new electron acceptor molecules and analyze them using a combination of ultrafast transient absorption and ultrafast broadband photoluminescence spectroscopies. We demonstrate that intramolecular triplet formation occurs within several hundred picoseconds in solution and is accelerated considerably in the solid state. Importantly, ISC occurs with sufficient rapidity to compete with charge formation in modern organic solar cells, implicating triplets in intrinsic exciton loss channels in addition to charge recombination. Density functional theory calculations reveal that ISC occurs in triplet excited states characterized by local deviations from orbital π-symmetry associated with rotationally flexible thiophene rings. In disordered films, structural distortions, therefore, result in significant increases in spin-orbit coupling, enabling rapid ISC. We demonstrate the generality of this proposal in an oligothiophene model system where ISC is symmetry-forbidden and show that conformational disorder introduced by the formation of a solvent glass accelerates ISC, outweighing the lower temperature and increased viscosity. This proposal sheds light on the factors responsible for facile ISC and provides a simple framework for molecular control over spin states.

The Impact of Aggregation of Quaterthiophenes on the Excited State Dynamics

Oligothiophenes and their aggregates play a dominant role in optoelectronic and light-harvesting applications. Here, we controlled the degree of aggregation of 2,2':5',2″:5'',2‴-quaterthiophene (QTH) to shed light on the impact of the aggregation on the excited state dynamics. QTH aggregation realized the control over the Intersystem Crossing (ISC) rates and, in turn, the formation of triplet excited states via the simple addition of water to QTH solutions in THF. From global target analysis, the time scale was 345.5 ps for ISC for QTHs in THF, but it was 2.33 ns in the case of QTH solutions featuring 70% water. Notably, the excitonic coupling between closely packed QTHs occurred predominantly in the aggregates formed in the presence of large water concentrations. Relaxation dynamics of the resulting QTH-aggregates differed substantially from QTH solutions at lower water content. For example, QTH-aggregates lacked any triplet excited states, and the unusual emission occurs from lower excitonic states from these predominantly H-aggregates.

Chlorophyll-Based Near-Infrared Fluorescent Nanocomposites: Preparation and Optical Properties

Near-infrared (NIR) fluorescence has attracted much attention in biomedical fields because it offers deep tissue penetration and high spatial resolution. Herein, a method is developed for the preparation of NIR fluorescent nanocomposites (NCs) by encapsulating natural chlorophyll (Chl) into the micelles of octylamine-modified poly(acrylic acid) (OPA). Both femtosecond transient absorption spectra and isothermal titration calorimetry thermogram reveal that the micelles of OPA provide a hydrophobic environment for the improved fluorescence efficiency. Hence the resulted Chl NCs possess unique properties such as ultrasmall size, outstanding photostability, good biocompatibility, and superbright NIR fluorescence emission. In vivo imaging of sentinel lymph node is achieved in nude mice, demonstrating the potential of Chl NCs in biomedical applications. This work provides a new strategy for the preparation of highly biocompatible NIR fluorescence labeling nanocomposites.

Excitation wavelength dependent ICT character and ISC efficiency in a photocleavage agent of 1-aminoanthraquinone

Anthraquinone derivatives have been widely used as photocleavage agents and dyes. Here, photoinduced excited state intramolecular charge transfer (ICT) dynamics of 1-aminoanthraquinone in ethanol are studied by femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical (QC) calculations. Four decay associated difference spectra and corresponding lifetime components were obtained by singular value decomposition and global fitting analysis from the fs-TA spectra. The QC calculations prove that the S₁ state has obvious ICT character. Planar ICT (PICT) and twisted ICT (TICT) reaction coordinates are observed. On the PICT coordinate, vibrational relaxation (VR) and intersystem crossing (ISC) processes are observed. The ISC efficiency is dependent on excitation wavelength, which elucidates that T'₂ state participates in the ISC process on a higher-level than the S'₁ state. On the TICT coordinate, the TICT process is dependent on excitation wavelength, which elucidates a direct experimental evidence of an energy barrier in excited state TICT potential energy.

Quantum Mechanics/Molecular Mechanics Density Functional Theory Simulations of the Optical Properties Fingerprinting the Ligand-Binding of Pentameric Formyl Thiophene Acetic Acid in Amyloid-β(1-42)

The binding pocket proposed by König [ Chem. Commun. 2018 , 54 , 3030 - 3033 ] for the biomarker pentameric formyl thiophene acetic acid (p-FTAA) in the fibrillar structure of amyloid-β(1-42) has been put to the test by the comparison of theoretical and experimental optical absorption and fluorescence spectra obtained in a water environment and inside the protein scaffold. The optical absorption/emission properties of this luminescent conjugated oligothiophene were studied by means of classical force field molecular dynamics simulations to account for the sampling of configuration space in conjunction with polarizable embedding time-dependent density functional theory calculations of spectra. The nuclear motions of residues in the β-sheet were found to be modest, and the time dependence of embedding parameters was shown to be negligible so that a time-independent parameter set could be derived and used for all 300 snapshots considered in the spectrum averaging. In regard to linear absorption spectra, the calculated red shift due to protein binding for the dominant S₁ ← S₀ transition in p-FTAA was found to be equal to 23 nm (0.17 eV), which is in excellent agreement with the corresponding experimental result of 18 nm and taken as corroborating evidence for having correctly identified the binding pocket of p-FTAA in the amyloid. The underlying mechanisms for the calculated red shift were disentangled, and it is shown that some 20 nm (0.15 eV) of the total 23 nm (0.17 eV) is associated with increased planarity of p-FTAA in the binding pocket, whereas a mere 3 nm (0.02 eV) is associated with changes in the environment. In regard to emission spectra, we demonstrate that intersystem crossing from the excited S₁ state to the triplet manifold of states is a less likely event for p-FTAA in the binding pocket as compared to in the aqueous solution, and we thereby partly explain the much higher quantum yield of fluorescence for the more rigid p-FTAA in the binding pocket. Two-photon absorption in p-FTAA is shown to predominantly occur to an overall symmetric excited state and be more than twice as strong for the biomarker in the binding pocket as compared to in water. The corresponding red shift, on the other hand, is very small. Earlier experimental two-photon fluorescence imaging using p-FTAA is shown not to target the dominant two-photon state, and ways to reach a higher image quality (lower signal-to-noise ratio) are proposed in terms of tuning the laser wavelength toward the region of 600 nm or the synthesis of asymmetric ligands with S₁ states that are both one- and two-photon allowed.

Effect of hydrogen bonding on the nonradiative properties of dibenzofuran

Femtosecond time-resolved transient absorption spectroscopy is employed to investigate the ultrafast excited-state dynamics from the S₂ state of dibenzofuran and the hydrogen bonding effect in protic and aprotic solvents. Following the excitation with 266 nm, the initial population on the S₂ state couples to the S₁ state via internal conversion on tens of picoseconds. Afterwards, vibrational energy relaxation (VER) with dozens of picoseconds is determined in all solvents and strongly depended on the polarities of aprotic solvents. And the rising species related with the hydrogen-bonded intermediate S^⁎ state are observed just in ethanol and methanol with the higher Kamlet parameter α. Subsequently, the intersystem crossing from the S₁ state to the triplet manifold is a main deactivation pathway with the decay time of ≫ns. All the relaxation processes exhibit solvent dependence, indicating that hydrogen bonding interaction between solute and solvent enhances VER and significantly influences on the excited state dynamics of dibenzofuran.

Vibrationally resolved emission spectra of luminescent conjugated oligothiophenes from anharmonic calculations

We report on accurate and efficient calculations of vibrationally resolved emission spectra for oligothiophenes from anharmonic vibrational configuration interaction wave-function calculations in reduced vibrational spaces. These reduced spaces are chosen based on the independent mode displaced harmonic oscillator model. Good agreement with experiment is obtained for all-trans oligothiophenes with two to five rings also when employing only a few active modes. Vibrational modes incorporating inter-ring carbon-carbon stretches and a ring breathing mode are found to be the main players in the vibrational progression for the emission from the first excited electronic state for all investigated oligothiophene derivatives. The presented framework is here illustrated for oligothiophenes, but we have made no underlying system-dependent assumptions and believe it to become a valuable tool for the rational design of fluorescence biomarkers.

Ultrafast investigation of photoinduced charge transfer in aminoanthraquinone pharmaceutical product

We investigated the mechanism of intramolecular charge transfer and the following radiationless dynamics of the excited states of 1-aminoanthraquinone using steady state and time-resolved absorption spectroscopy combined with quantum chemical calculations. Following photoexcitation with 460 nm, conformational relaxation via twisting of the amino group, charge transfer and the intersystem crossing (ISC) processes have been established to be the major relaxation pathways responsible for the ultrafast nonradiative of the excited S₁ state. Intramolecular proton transfer, which could be induced by intramolecular hydrogen bonding is inspected and excluded. Time-dependent density functional theory (TDDFT) calculations reveal the change of the dipole moments of the S₀ and S₁ states along the twisted coordinate of the amino group, indicating the mechanism of twisted intra-molecular charge transfer (TICT). The timescale of TICT is measured to be 5 ps due to the conformational relaxation and a barrier on the S₁ potential surface. The ISC from the S₁ state to the triplet manifold is a main deactivation pathway with the decay time of 28 ps. Our results observed here have yield a physically intuitive and complete picture of the photoinduced charge transfer and radiationless dynamics in anthraquinone pharmaceutial products.

Deactivation pathways of thiophene and oligothiophenes: internal conversion versus intersystem crossing

Quantum chemical calculations reveal that excited thiophene decays via a low lying conical intersection seam. In oligothiophenes barriers inhibit this passage while deactivation pathways via intersystem crossing channels open.