Excited State Dynamics of Thermally Activated Delayed Fluorescence from an Excited State Intramolecular Proton Transfer System

We describe the photophysical processes that give rise to thermally activated delayed fluorescence in the excited state intramolecular proton transfer (ESIPT) molecule, triquinolonobenzene (TQB). Using transient absorption and time-resolved photoluminescence spectroscopy, we fully characterize prompt and delayed emission, phosphorescence, and oxygen quenching to reveal the reverse intersystem crossing mechanism (rISC). After photoexcitation and rapid ESIPT to the TQB-TB tautomer, emission from S1 is found to compete with thermally activated ISC to an upper triplet state, T2, very close in energy to S1 and limiting photoluminescence quantum yield. T2 slowly decays to the lowest triplet state, T1, via internal conversion. In the presence of oxygen, T2 is quenched to the ground state of the double proton transferred TQB-TC tautomer. Our measurements demonstrate that rISC in TQB occurs from T2 to S1 driven by thermally activated reverse internal conversion from T1 to T2 and support recent calculations by Cao et al. (CaoY.; EngJ.; PenfoldT. J.Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules. J. Phys. Chem. A2019, 123, 2640−264930848598).

Compound 2 was synthesized by modifying the literature 1 A solution of 1-bromo-4-hexylbenzene (16.4 g, 68.1 mmol) in sulfuric acid (35 ml) was cooled in an ice bath. To this solution was added nitric acid (d = 1.38, 60-61 wt%, 5.2 ml, 68.3 mmol). The ice bath was removed, and the mixture was stirred at room temperature for 20 min. A further portion of nitric acid was added (0.2 ml, 2.63 mmol) and the mixture was stirred for a further 20 min. The mixture was poured into K 2 CO 3 aq, which was extracted with EtOAc. The organic phase was washed with saturated K 2 CO 3 aq, water and brine, then dried and concentrated. The crude product was purified by column chromatography on a silica gel using hexane/EtOAc (20:1 v/v) mixture to give isomeric product 2 and byproduct 2' as colorless liquid.
Procedure for the synthesis of 5 COOH C 6 H 13 NH 2

5
Compound 5 was synthesized by modifying the literature 2 Palladium 10% on Carbon (Pd/C, 0.28 g) was added to a solution of compound 4 (5.11 g, 20.3 mmol) in EtOAc (100 ml), and the solution was saturated with hydrogen for 12 h at room temperature. The reaction mixture was filtered through a Celite pad and the solution was concentrated to dryness. The crude material was purified by column chromatography on a silica gel using hexane/EtOAc (1:1 v/v) mixture (R f = 0.27, fluorescent) as eluent to
Toluene was added and then tri-tert-butylphosphine (0.265 g, 1.31 mmol) was added. The reaction mixture was refluxed for 3 days. After cooling, the mixture was filtered, washed with toluene and an aqueous solution of NH 4 Cl.

MS
The film of 'regular' (non-hexyl) TQB in zeonex matrix was made by doctor blade coating onto transparent sapphire substrate. Stock solution of TQB was prepared by dissolving the material in toluene at a nominal 1mg/ml concentration. Due to poor solubility, the solution was then filtered to remove undissolved solids before being blended with an equal volume of zeonex solution (180mg/ml) and blade cast. The films produced in this way were therefore lower than 0.5 wt%. Despite this low concentration we find that the additional film thickness associated with blade coating gave sufficient absorption to allow optical measurements.
-Photophysical characterisation Absorption spectra for films and solutions were collected using a double beam Shimadzu UV-3600 UV/VIS/NIR spectrophotometer. Steady state photoluminescence spectra were collected using a Jobin-Yvon Fluoromax-4 fluorimeter.
-Transient Absorption and Time-Resolved Photoluminescence Transient absorption measurements were carried out using a pump-probe spectrometer, in which an actinic YAG 355 nm laser (with pulse width 4 ns) acted as the pump source for measuring dynamics on the nanosecond scale, whilst for picosecond measurements, the pump beam was supplied by a Pharos Yb:KGW femtosecond laser directed using an Optical Delay Line (from Thorlabs.) The probe beam was a white light continuum generated by passing the Pharos Yb:KGW femtosecond laser beam through a sapphire plate, with measuring range between 500-800 nm. Time-resolved photoluminescence spectra and decays were measured using a nanosecond gated spectrograph-coupled iCCD (Stanford) using an Nd:YAG laser emitting at 355 nm (EKSPLA) as an excitation source, along with a gated 4 Picos iCCD camera. S16 Figure S2: Comparison of structures, absorption (dashed line) and emission (solid line, 340nm excitation), and emission lifetimes of TQB and hexyl-TQN in toluene. Emission decays were collected using a Hamamatsu Photonics Quantaurus-Tau C11367-03 with 340nm excitation and 520nm emission collection. Emission was detected with a photomultiplier after being dispersed with a monochromator. Time to amplifier converter (TAC) was used with a multichannel analyser (MCA) to perform TCSPC S17 Figure S3: Normalised time-resolved emission spectra of TQB in DPEPO at 300K (top) and 80K (bottom). Although signal to noise ratios vary at different times, the emission bandshape and onset do not vary over the timescales investigated (ns-ms). S18 Figure S4: Emission decay kinetics and spectra of plain (non-hexyl) TQB in zeonex: a) comparison of emission decays at 300K and 80K b) representative PF and DF spectra at 300K c) representative PF and DF spectra at 80K, with band onsets indicated to give the singlet and triplet energies d) exponential lifetime fitting of emission decay at 300K.
We suggest that the observation of phosphorescence solely in zeonex may be due to its intermediate rigidity -flexible enough to allow vibrational modes associated SOC and T1 emission (modes restricted in DPEPO), but rigid enough to prevent vibrational relaxation of T1 that quenches phosphorescence in liquid toluene.