Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence in the Pd Complex: Mechanism and Dual Upconversion Channels

Abstract

The Pd complex PdN3N exhibits an unusual dual emission of roomtemperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S 0 , S 1 , T 1 , and T 2 states are involved in the luminescence. Both the S 1 → T 1 and S 1 → T 2 intersystem crossing (ISC) processes are more efficient than the S 1 fluorescence and insensitive to temperature. However, the direct T 1 → S 1 and T 2 -mediated T 1 → T 2 → S 1 reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T 1 phosphorescence and therefore enable TADF. Importantly, the T 1 → S 1 rISC and T 1 phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T 1 phosphorescence is recorded experimentally.

Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials

“…17,26–29 However, now there is consensus in the community that an intermediate state should be involved in the TADF process, which is critical for ISC and rISC, such as the localized triplet excited state ( 3 LE). 6,30–32 Generally speaking, upon photoexcitation, either the localized singlet excited ( 1 LE) state or the 1 CT state is firstly populated, then ISC occurs, and the 3 LE and the charge transfer triplet state ( 3 CT) will be populated. rISC will populate the emissive 1 CT state again.…”

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

“…In this state, the system can radiatively emit fluorescence or nonradiatively hop to the T 1 or S 0 state. The internal conversion process to the S 0 state is very slow due to the large energy gap between S 1 and S 0 because, according to the well-known energy-gap law, this process cannot compete with the ISC process, as done in similar works. ,− The fluorescence emission rate is estimated to be 2.98 × 10 6 s –1 at 300 K, which is much slower than the S 1 -to-T 1 ISC rate of 7.31 × 10 8 s –1 at 300 K. Thus, the forward ISC process is favorable, which populates the T 1 state efficiently. Similarly, in the T 1 state, there are three relaxation channels, i.e., phosphorescence emission, ISC to the S 0 state, and reverse ISC to the S 1 state.…”

“…To date, the reported Cu­(I) complexes have high luminescent quantum efficiencies, and their performance can be comparable to and even surpass the Ir­(III)-based phosphorescence complexes . To rationally design TADF materials, many groups carried out theoretical studies, which provided helpful luminescence insights at the microscopic level. − In 2016, Marian and co-workers investigated a photoluminescent three-coordinate cooper complex using the combined density functional theory and multireference configuration interaction (DFT/MRCI) approach . The calculated ISC and rISC rates are larger than the fluorescence and phosphorescence rates, respectively.…”

Near‐Infrared Room‐Temperature Phosphorescence in Arylselanyl BODIPY‐Doped Materials

“…Considering that the triplet state always has an energy close to that of the singlet state, the ▵E ST value estimated from the emission maxima of the fluorescence and phosphorescence of the MeOSeBOD ‐doped BP system is large (0.347 eV). Therefore, T 3 ‐mediated reverse ISC may occur in this system [33] . In order to strengthen the discussion, we evaluated the reverse ISC (RISC) constant using ORCA program (version 4.2.1) with B3LYP/cc‐PVDZ.…”

“…Therefore, T 3 -mediated reverse ISC may occur in this system. [33] In order to strengthen the discussion, we evaluated the reverse ISC (RISC) constant using ORCA program (version 4.2.1) with B3LYP/cc-PVDZ. Subsequently, the smallest energy gap was calculated to be 0.03 eV for S 1 !T 3 process in the excited state of MeOSeBOD.…”