Theoretical Study of the Intramolecular Localization and Migration of a Triplet Exciton in the N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) Molecule

Abstract

The intramolecular localization and migration of a triplet exciton in the N,N′-di­(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) molecule is studied by the XMCQDPT/CASSCF method. Energy profiles corresponding to linear interpolations between different localized states show that the barriers between minima are comparable with the vibrational energies of the soft modes corresponding to the transition of one structure to another. The triplet exciton lifetimes and characteristic migration times are estimated. It is shown that the intramolecular migration is 3 orders of magnitude faster than the main triplet decay process in α-NPD, namely, nonradiative deactivation. Therefore, at room temperature, the excitons can freely migrate over the molecule and become effectively delocalized during their lifetime.

In-Depth Ab Initio Study of Thermally Activated Delayed Fluorescence in 4,5-Di(9H-carbazol-9-yl)-phthalonitrile

“…Vibronically assisted 3 LE(PN) → 1 ICT RISC with k RISC = 0.738 s −1 (τ RISC = 1.35 s) is even slower than phosphorescence. However, crossing between 3 LE(PN) and 3 ICT may facilitate the participation of 3 ICT in the RISC process: a conservative estimate based on the Arrhenius equation shows that the 3 LE(PN) → 3 ICT transition may take place on a picosecond time scale, 15,27 because the frequency of the 3 LE(PN) → 3 ICT reorganization mode is at least ∼10−100 cm −1 , and the barrier height is at most ∼0.1 eV.…”

“…The geometry of the S 1 state was optimized by time-dependent (TD) DFT, while the geometry of the triplet states were optimized the Tamm−Dancoff approximation (TDA) as being more suitable for triplets. 27,28 The TDA-DFT calculations were performed by the ORCA program. 29,30 The state energies for each of the found geometries were calculated using the state-averaged CASSCF with 16 electrons on 12 orbitals (SA(10)-CASSCF (16,12) (see, e.g., ref 31 and the refs therein)) with averaging over 10 states, both singlets and triplets, to obtain the energies of S 0 and all other target states simultaneously.…”

“…The geometries of the S 0 , S 1 , and triplet states of 2CzPN were optimized by BHHLYP/6-31G­(d,p) using the Firefly package partially based on the GAMESS-US code . Numerous benchmarks (see, e.g., refs and ) show that, while the energy characteristics, such as excitation energies and S 1 –T n energy differences, are extremely sensitive to the amount of the HF exchange in the functional, the state geometries are much less sensitive and fairly well agree with those optimized by Complete Active Space Self-Consistent Field (CASSCF). The geometry of the S 1 state was optimized by time-dependent (TD) DFT, while the geometry of the triplet states were optimized the Tamm–Dancoff approximation (TDA) as being more suitable for triplets. , The TDA-DFT calculations were performed by the ORCA program. , …”

Theoretical Study of Charge-Transfer Exciplexes in Organic Photovoltaics

“…Since we are dealing with charge-transfer states, the results are extremely sensitive to the choice of the density functional. Numerous benchmarks , show that while the energy characteristics, such as excitation energies and S 1 –T n energy differences, strongly depend on the amount of the HF exchange in the functional, the state geometries are much less sensitive and fairly well agree with those optimized by CASSCF . An extensive work was performed in ref to fine-tune global hybrid or range-separated functionals for the proper reproduction of excited states of different (local and charge transfer) nature.…”

Use of effective fragment potentials for simulation of excited states in an inhomogeneous environment

“…It is known that charge carriers and excitons in large molecules of organic semiconductors are localized on the individual fragments of the molecule, but can easily migrate from one fragment to the other under the influence of small perturbations, such as inhomogeneous electrostatic fields induced by the neighborhood of the active molecule in a condensed (crystal or amorphous) phase. These intra‐ and intermolecular migrations govern the charge and exciton mobility in organic semiconductors.…”