Naphthalimide-phenothiazine dyads: effect of conformational flexibility and matching of the energy of the charge-transfer state and the localized triplet excited state on the thermally activated delayed fluorescence

Abstract

In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (³LE) states on the thermally activated delayed fluorescence (TADF) in electron donor-acceptor molecules, a series of compact electron donor-acceptor dyads and a triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at the 3-position of NI and the N-position of the PTZ moiety via a C-N single bond, or they are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, so that the coupling magnitude between the CT and the ³LE states can be controlled. The singlet oxygen quantum yield (Φ_Δ) of NI-PTZ is moderate in n-hexane (HEX, Φ_Δ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was observed in the nanosecond transient absorption spectra with lifetimes in the 4-48 μs range. Computational investigations show that the orthogonal electron donor-acceptor molecular structure is beneficial for TADF. These calculations indicate small energetic difference between the ³LE and ³CT states, which are helpful for interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the ³LE and ³CT states. Conversely, NI-PTZ-O, which has a higher CT state and bears a much more stabilized ³LE state, does not show TADF.

Keywords: TADF; charge-transfer; electron donor; intersystem crossing; triplet state.

Scheme 1

Synthesis of the compounds^a. ^aKey: (a) phenothiazine, sodium tert-butoxide, dried toluene (TOL), tri-tert-butylphosphine tetrafluoroborate, Pd(OAc)₂, 120 °C, 8 h, 93.1%; (b) H₂O₂ (30%), CH₃COOH, 40 °C, 1 h, yield: 87.2%; (c) similar to step (a), yield: 80.0%; (d) bis(pinacolato)diboron, KOAc, Pd(dppf)Cl₂, toluene, N₂, 110 °C, 16 h, yield: 11.9%.; (e) 1-bromo-4-iodobenzene, Pd(PPh₃)₄, K₂CO₃, TOL, EtOH, H₂O, N₂, reflux, 8 h, yield: 92.9%; (f) similar to step (a), yield: 62.4%; (g) 5-bromo-2-iodo-1,3-dimethylbenzene, Pd(PPh₃)₄, K₂CO₃, TOL, EtOH, H₂O, N₂, 110 °C, 9 h, yield: 60.6%; (h) similar to step (a), yield: 28.3%.

Figure 1

UV–vis absorption spectra of NI-PTZ, NI-PTZ-O, NI-PTZ₂, NI-Ph-PTZ, and NI-PhMe₂-PTZ in HEX. c = 1.0 × 10⁻⁵ M at 20 °C.

Figure 2

Fluorescence spectra of the compounds (a) NI-PTZ; (b) NI-PTZ-O; (c) NI-PTZ₂, and (d) NI-Ph-PTZ in different solvents. The solvents used are: CHX, HEX, TOL and acetonitrile (ACN). Optically matched solutions were used, A = 0.100, λ_ex = 330 nm, 20 °C.

Figure 3

Fluorescence spectra of (a) NI-PTZ, (b) NI-PTZ-O, (c) NI-PTZ₂, and (d) NI-Ph-PTZ in HEX under different atmospheres (N₂, air). Optically matched solutions were used, A = 0.100, λ_ex = 330 nm, c = 1.0 × 10⁻⁵ M, 20 °C.

Figure 4

Fluorescence lifetime of NI-PTZ under (a) N₂ atmosphere and (d) air atmosphere (λ_em = 610 nm, c = 5.0 × 10⁻⁵ M). Fluorescence lifetime of NI-PTZ₂ under (b) N₂ atmosphere and (e) air atmosphere (λ_em = 610 nm, c = 5.0 × 10⁻⁵ M). Fluorescence lifetime of (c) NI-PTZ-O (λ_em = 472 nm, c = 1.0 × 10⁻⁵ M) and (f) NI-Ph-PTZ (λ_em = 482 nm, c = 1.0 × 10⁻⁵ M). Excited with a picoseconds pulsed laser (λ_ex = 340 nm), in HEX, 20 °C.

Figure 5

(a) Phosphorescence spectra of NI-PTZ-O; (b) decay traces of the phosphorescence of the compounds NI-PTZ-O. λ_ex = 340 nm, at 77 K, in 2-methyltetrahydrofuran, c = 1.0 × 10⁻⁵ M.

Figure 6

Cyclic voltammograms of the compounds. NI-PTZ, NI-PTZ₂, NI-Ph-PTZ, and NI-PhMe₂-PTZ were studied in deaerated DCM; NI-PTZ-O in deaerated ACN. Ferrocene (Fc) was used as internal reference (set as 0 V in the cyclic voltammograms). 0.10 M Bu₄NPF₆ as supporting electrolyte. Scan rates: 100 mV/s, c = 1.0 × 10⁻³ M, 20 °C.

Figure 7

Spectroelectrochemistry traces of the UV–vis absorption spectra for (a) NI-PTZ observed from neutral (red) to monoanion (purple) with a potential of −1.83 V applied; (b) NI-PTZ observed from neutral (red) to monocationic (purple) at a potential of 0.53 V applied; (c) NI-PTZ-O observed from neutral (red) to monoanion (purple) at controlled-potential of −1.80 V; (d) NI-PTZ-O observed from neutral (red) to monocationic (purple) at controlled-potential of 2.00 V. In deaerated DCM containing 0.10 M Bu₄[NPF₆] as supporting electrolyte and with Ag/AgNO₃ as reference electrode, 20 °C.

Figure 8

Nanosecond transient absorption spectra of (a) NI-PTZ (c = 1.5 × 10⁻⁴ M), (b) NI-PTZ-O (c = 2.5 × 10⁻⁴ M), and (c) NI-PTZ₂ (c = 5.0 × 10⁻⁵ M). The corresponding decay traces are (d) NI-PTZ (c = 5.0 × 10⁻⁶ M) at 520 nm, (e) NI-PTZ-O (c = 2.0 × 10⁻⁶ M) at 530 nm, and (f) NI-PTZ₂ (c = 6.0 × 10⁻⁶ M) at 510 nm. In deaerated HEX, λ_ex = 355 nm, 20 °C.

Figure 9

Optimized ground state geometry of (a) NI-PTZ, (b) NI-PTZ-O, (c) NI-PTZ₂, (d) NI-Ph-PTZ, and (e) NI-PhMe₂-PTZ; the green and orange sheets show the planes of the donor and the receptor.

Figure 10

Kohn–Sham frontier molecular orbitals (CAM-B3LYP/6-31G(d) in gas phase) involved in S₁, T₁, and T₂ of NI-N-PTZ, NI-PTZ, NI-PTZ-O, NI-PTZ₂, NI-Ph-PTZ, and NI-PhMe₂-PTZ, based on the optimized ground state geometries. An isovalue of 0.02 is used.

Scheme 2

Jablonski diagram of (a) NI-PTZ and (b) NI-PTZ-O, including electron density difference (EDD) at T₂ (³LE) geometry and computed in gas phase. EDD isovalues are 0.02 au. The cyan and blue lobes of the EDD indicate increase and decrease of electron density, respectively. The energies in eV were obtained with single point calculations on optimized S₁, T₁, and T₂ geometry in HEX, DCM, TOL, and ACN.