Ultrafast photophysics of an orange–red thermally activated delayed fluorescence emitter: the role of external structural restraint

The application of thermally activated delay fluorescence (TADF) emitters in the orange–red regime usually suffers from the fast non-radiative decay of emissive singlet states (kSNR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although kSNR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the kSNR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and kSNR. Spectroscopic and theoretical investigations indicated that S1/S0 ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic–vibrational coupling through the Huang–Rhys factor, while kSNR is largely affected by the reorganization energy of the promoting modes. By restraining S1/S0 ES-SR in doping films, the kSNR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S1/S0 ES-SR, promoting modes and kSNR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.

fs-TA.The fs-TA measurements were performed by using home-built ultrafast pumpprobe spectrometer upon 320 nm optical excitation.Briefly, a commercial 1 kHz Ti:sapphire laser system delivered ~40 fs pulses centered at 800 nm.The ~60 fs excitation pulses (pump = 320 nm) were derived from the second-harmonic output of a collinear optical parametric amplifier.Approximately 0.1-0.3J pulse energy was measured at the sample position.The broadband UV/Vis probe pulses were generated by focusing a small portion of the Ti:sapphire laser fundamental into a linearly moving CaF2 window, resulting in a white-light spectrum between 350 nm and 750 nm.Pump and probe beams were spatially overlapped in a quartz cuvette with 500 m sample thickness or in PS films directly, optical density of sample was controlled to 0.1-0.3 at 320 nm.The polarization of pump and probe beams were set to magic angle of 54.7°.After passing the sample, the probe pulses were dispersed in a grating spectrometer and detected by a linear Si detector array.The measured TA data were evaluated via target analysis with software package Glotaran 2 based on the R-package TIMP. 3 Theoretical calculation.
Electronic structure and wavefunction analysis.All electronic structure calculation of TDBA-Ac and PzTDBA were performed using Gaussian 09 4 and 16 5 software packages.
The geometric structure of investigated emitters was optimized on both ground (S0), singlet (S1) and triplet (T1) excited states at M06-2X/6-311G** level while no imaginary frequency was found by frequency analysis.The vertical (EST) and adiabatic (EST*) energy gaps between singlet (S1) and triplet (T1) excited states were estimated by single point and geometric optimization calculation, respectively.The frontier orbital distribution of investigated emitters were rendered by using the VMD 1.9.3 program. 6The natural transition orbitals (NTOs) and hole-electron analysis of corresponding excited states were performed by using Multiwfn program, 7 which leads to parameters for quantifying charge transfer of corresponding excited states.The Sr index (hole-electron overlap) was calculated as follow: (1 where  hole (r) and  ele (r) denote spatial distribution of hole and electron wavefunction, respectively.
Vibrational analysis.The Huang-Rhys (HR) factor (Sk) and reorganization energy contribution (k) of each vibrational modes were calculated by using MOMAP software on basis of frequency analysis of corresponding states.We further estimated reorganization energy contribution (k) of each vibrational modes between S1 and S0 states with the harmonic oscillator approximation: (2) in which k can be calculated by the corresponding frequency (k) and vibrational displacement (Qk), while Qk can be estimated as a linear combination of internal coordinates, i.e. ( where Dj represents displacement along the internal coordinate j of the equilibrium position in the S1→S0 transition. The HR factor (Sk) of each vibrational mode k can be calculated with its frequency (k) and displacement (Qk). (4) Spin-orbital coupling.The SOC matrix elements between singlet (S1) and triplet (T1) excited states were calculated by using the linear-response LR methods implemented in PySOC program, 8 which has been included in MOMAP software. 9The SOC Hamiltonian can be approximately calculated as (5)   where L and S represent magnetic moment operators resulting from orbital and spin angular momentum with SOC constant (ri).
Structural relaxation.To further quantify S1/S0 structural relaxation, the root of the mean of squared displacement (RMSD) between S0 (xi, yi, zi) and S1 (xi', yi', zi') state was calculated in various solution. ( by summing over all atoms i = 1, ..., N. ISC and RISC rate.The ISC and RISC rate of investigated TADF emitters were calculated by using the thermal vibration correlation function (TVCF) method implemented in MOMAP software, in which geometric displaced, distorted and Duschinsky rotation effects between corresponding electronic states were taken into consideration.Specifically, non-radiative ISC (S1→T1) and RISC (T1→S1) rate can be calculated with SOC term and corresponding TVCF ρISC(t, T) with form of Moreover, ISC/RISC kinetics can be also considered in the framework of Marcus theory, 10 which could be expressed as: where ћ is the reduced Planck constant, kB is the Boltzmann constant, T is the temperature,  is the reorganization energy, and EA is the activation energy to reach the crossing seam.In the case of simple parabolic PESs with equal force constants, which is a crucial assumption of Marcus theory, EA can be analytically expressed as: (10) Section S2.Photophysics of TADF emitters.Table S3 The       Section S9. fs-TA measurements.
The TDBA-Ac and PzTDBA samples were purchased from Luminescence Technology Corp. and used without further purification.All involved organic solvents are HPLC grade and used as received.For fabricating PS doping films of TDBA-Ac and PzTDBA, PS solutions were firstly prepared by dissolving 1 g PS in 11.5 mL toluene.The resulted PS solution (2 mL) was then mixed with 3 mg TDBA-Ac or PzTDBA powder, corresponding to 2wt% doping concentration.The two-step dissolving was performed with stirring (400 rpm) at room temperature.The PS solution of TDBA-Ac or PzTDBA were further spincoated (1000 rpm, 1 min) on 1 mm thick quartz substrates with 1000 rpm/s acceleration, resulting in doping films with absorbance of 0.2-0.3 at excitation wavelength of TDBA-Ac and PzTDBA.Spectroscopic experiments.Steady and tr-FL.The steady-state UV/Vis absorption and photoluminescence spectra were recorded on U-3900 (Hitachi, Japan) spectrophotometer and F-4700 (Hitachi, Japan) fluorescence spectrometer, respectively.The fluorescence quantum yield (F) of solution was determined by using Coumarin 153 (F = 0.38, in EtOH) as a reference.1For oxygenfree measurements, solutions were bubbled with nitrogen flow for 15 min before measurements.The F of PS doping films were measurements by an integrating sphere with CW excitation at 365 nm.The fluorescence time traces of solution and doping films were recorded with a time correlated single-photon counting (TCSPC) spectrometer (PTI Quanta Master 800, HORIBA) equipped with a 330 nm nano-LED excitation source, which leads to an instrument response function (IRF) value of 0.6 ns.

Fig. S7
Fig. S7 Fluorescence decay kinetics of TDBA-Ac in different solutions and PS doping film

Fig. S8
Fig. S8 Fluorescence decay kinetics of PzTDBA in different solutions and PS doping film

Fig. S9
Fig. S9 Measured fs-TA spectra of TDBA-Ac in CHX solution (a) and PS doping film (b)

Fig. S10
Fig. S10 Measured fs-TA spectra of PzTDBA in CHX solution (a) and PS doping film (b)

Table S1
Physical properties of involved solvents.

Table S5
Calculated kISC and kRISC of TDBA-Ac and PzTDBA by Adachi's method, thermal vibration correlation function (TVCF) and semi-classical Marcus approaches.
a calculated by method described by Adachi et al.; b TVCF calculated by MOMAP program; c calculated by semi-classical Marcus approaches according to eq.6-10.