Quantum Chemistry Study on Internal Conversion of Diphenyldibenzofulvene in Solid Phase

Theoretical Insights into the Photophysical Properties of 4CzIPN Doped in Different Hosts: A Multiscale Study

As a typical thermally activated delayed fluorescence (TADF) emitter with green emission, 4CzIPN has attracted much attention recently. Most studies indicated that 4CzIPN doped in different hosts presented different performances; thus, the hosts should have an obvious influence on its photophysical properties. Herein, the influence of four kinds of hosts, including m-CzPym, m-CzTrz, p-CzPym, and p-CzTrz, on the photophysical properties of 4CzIPN is investigated. Molecular dynamics simulations were performed to simulate the host-guest conformations, and the photophysical properties were studied using the combined quantum mechanics/molecular mechanics method coupled with the thermal-vibration correlation function method. It is found that 4CzIPN in doped films has larger transition dipole moments and spin-orbital coupling constants compared to that in nondoped films. Faster radiative decay, intersystem crossing rates, and higher fluorescence efficiency could be obtained in doped films. Our work helps to better understand the photophysical properties of 4CzIPN in doped films and may favor the design of new hosts.

Influence of plasmons on the luminescence properties of solvatochromic merocyanine dyes with different solvatochromism

The effect of localized surface plasmon resonance (LSPR) of a system consisting of a highly dipolar merocyanine dye and a silver nanoparticle (NP) was studied experimentally and theoretically. A theoretical model for estimating the fluorescence quantum yield (φfl) using quantum chemical calculations of intramolecular and intermolecular electronic transition rate constants was developed. Calculations show that the main deactivation channels of the lowest excited singlet state of the studied merocyanines are internal conversion (kIC(S1 → S0)) and fluorescence (kr(S1 → S0)). The intersystem-crossing transition has a low probability due to the large energy difference between the singlet and triplet levels. In the presence of plasmonic NPs, the fluorescence quantum yield is increased by a factor of two according to both experiment and computations. The calculated values of φfl, when considering changes in kr(S1 → S0) and the energy-transfer rate constant (ktransfer) from the dye to the NP was also twice as large at distances of 6-8 nm between the NP and the dye molecule. We also found that the LSPR effect can be increased or decreased depending on the value of the dielectric constant (εm) of the environment.

Mechanisms of fluorescence quenching in prototypical aggregation-induced emission systems: excited state dynamics with TD-DFTB

A recent implementation of time-dependent tight-binding density functional theory is employed in excited state molecular dynamics for the investigation of the fluorescence quenching mechanism in 3 prototypical aggregation-induced emission systems.

Aggregation-caused quenching versus crystallization induced emission in thiazolo[5,4-b]thieno[3,2-e]pyridine (TTP) derivatives: theoretical insights

We report a QM (TD-DFT) and QM/QM' (ONIOM) study of the modulation of emission in a series of thiazolo[5,4,b]thieno[3,2-e]pyridine (TTP) derivatives [Huang et al., J. Mater. Chem. C, 2017, 14, 3456]. By computing the excitation energy transfer couplings and the Huang-Rhys (HR) factors, we rationalize the aggregation-caused quenching (ACQ) observed for the parent molecule and the crystallization-induced emission (CIE) observed for the derivatives presenting intra-molecular H-bonding. We also show that the CIE strategy relying on the rigidification of the arch-bridge-like stator should be considered with caution since it can promote the energy dissipation through vibrational motions.

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

A method for calculating the rate constants for internal-conversion (k_IC) and intersystem-crossing (k_ISC) processes within the adiabatic and Franck–Condon (FC) approximations is proposed.

How Inter- and Intramolecular Processes Dictate Aggregation-Induced Emission in Crystals Undergoing Excited-State Proton Transfer

Aggregation-induced emission (AIE) offers a route for the development of luminescent technologies with high quantum efficiencies. Excited-state intramolecular proton transfer (ESIPT) coupled to AIE can produce devices with emission across the visible spectrum. We use a combination of theoretical models to determine the factors that mediate fluorescence in molecular crystals undergoing ESIPT. Using two materials based on 2'-hydroxychalcone as exemplar cases, we analyze how inter- and intramolecular processes determine the emissive properties in the crystal environment. This systematic investigation extends the current interpretation of AIE to polar chromophores with multiple decay pathways. We find that population of nonradiative pathways is dictated by the electronic effects of the substituents and the degree of distortion allowed in the crystal environment. Localization of the electron density is crucial to maximize fluorescence via ESIPT. Our conclusions offer design strategies for the development of luminescent molecular crystals.

Tuning the Solid State Emission of the Carbazole and Cyano-Substituted Tetraphenylethylene by Co-Crystallization with Solvents

Solid state emissive materials with high quantum yields and tunable emissions are desirable for various applications. A new TPE derivative (1) with two carbazole moieties and two cyano groups is reported, which shows typical aggregation induced emission behavior. Four crystals 1a, 1b, 1c, and 1d are obtained after crystallization from N,N-dimethylformamid (DMF), trichloromethane (CHCl₃ ), tetrahydrofuran (THF), and dichloromethane (CH₂ Cl₂ ), respectively. Crystal structural analyses reveal that (i) molecules of 1 co-crystallize with DMF, CHCl₃ , THF, and CH₂ Cl₂ in 1a, 1b, 1c, and 1d, respectively, and (ii) conformations of 1 are different within 1a, 1b, 1c, and 1d, and compound 1 within crystal 1a adopts the most twisting conformation. Crystalline solids 1a, 1b, 1c, and 1d exhibit high emission quantum yields up to 0.65, but their emission colors are varied from blue to green. In comparison, the amorphous solid of 1 is yellow-emissive with emission maximum at 542 nm. Moreover, the blue- or green-emissive crystalline solids and the yellow-emissive amorphous solid can be inter-converted by the grinding of crystalline solids and exposure of the amorphous solid to vapors of appropriate solvents. It is also demonstrated that microrods of 1a, 1b, and 1d show typical optical waveguiding behavior.

Aggregation induced blue-shifted emission – the molecular picture from a QM/MM study

Aggregation induced blue-shifted emission is a consequence of the restricted structural relaxation, excited-state planarization and freezing of low-frequency out-of-plane vibrations.

A conical intersection model to explain aggregation induced emission in diphenyl dibenzofulvene

A conical intersection seam is behind the restriction of intramolecular rotation mechanism for aggregation induced emission in diphenyl dibenzofulvene (DPDBF). In solution, the seam is accessed through rotation around the exocyclic fulvene bond, leading to radiationless decay to the ground state. In the solid, the seam cannot be accessed because the torsion is blocked, and DPDBF becomes emissive.

Modularity analysis of tunable solid-state emission based on a twisted conjugated molecule containing 9,9'-bianthracene group

The solid-state luminescence behavior of a compound (BAPN) depends on the molecular aggregation pattern. Modular analysis shows that the two emitting systems within the BAPN molecule are responsible for switching the emission color in various aggregation states.

Quantum chemical insights into the aggregation induced emission phenomena: a QM/MM study for pyrazine derivatives

There have been intensive studies on the newly discovered phenomena called aggregation induced emission (AIE), in contrast to the conventional aggregation quenching. Through combined quantum mechanics and molecular mechanics computations, we have investigated the aggregation effects on the excited state decays, both via radiative and nonradiative routes, for pyrazine derivatives 2,3-dicyano-5,6-diphenylpyrazine (DCDPP) and 2,3-dicyanopyrazino phenanthrene (DCPP) in condensed phase. We show that for DCDPP there appear AIE for all the temperature, because the phenyl ring torsional motions in gas phase can efficiently dissipate the electronic excited state energy, and get hindered in aggregate; while for its "locked"-phenyl counterpart, DCPP, theoretical calculation can only give the normal aggregation quenching. These first-principles based findings are consistent with recent experiment. The primary origin of the exotic AIE phenomena is due to the nonradiative decay effects. This is the first time that AIE is understood based on theoretical chemistry calculations for aggregates, which helps to resolve the present disputes over the mechanism.