Molecular-Level Understanding of Dual-RTP via Host-Sensitized Multiple Triplet-to-Triplet Energy Transfers and Data Security Application

Abstract

Dual-room-temperature phosphorescence (DRTP) from organic molecules is of utmost importance in chemical physics. The Dexter-type triplet-to-triplet energy transfer mechanism can therefore be used to achieve DRTP at ambient conditions. Here, we report two donor-acceptor (D-A)-based guests (CQN1, CQN2) in which the donor (D) and acceptor (A) parts are held in angular orientation around the C-N single bond. Spectroscopic analysis along with computational calculations revealed that both guests are incapable of emitting either thermally activated delayed fluorescence (TADF) or RTP at ambient conditions due to large singlet-triplet gaps, which are presented to show host (benzophenone, BP)-sensitized DRTP via multiple intermolecular triplet-to-triplet energy transfer (TTET) channels that originate from the triplet state (T₁ ^BP) of BP to the triplet states (T₁ ^D, T₁ ^A) of the D and A parts (TTET-I:T₁ ^BP → T₁ ^D; TTET-II:T₁ ^BP → T₁ ^A). In addition, an intramolecular TTET channel that occurs from the T₁ ^D to T₁ ^A states of the D and A parts of CQN2 is also activated due to the low triplet (T₁ ^D)-triplet (T₁ ^A) gap at ambient conditions. The efficiency of TTET processes was found to be 100%. The phosphorescence quantum yields (ϕ_P) and lifetimes (τ_P) were shown to be 13-20% and 0.48-0.55 s, respectively. Given the high lifetime of the DRTP feature of both host-guest systems (1000:1 molar ratio), a data security application is achieved. This design principle provides the first solid proof that DRTP via radiative decay of the dark triplet states of the D and A parts of D-A-based non-TADF systems is possible, revealing a method to increase the efficiency and lifetime of DRTP.

Figure 1

(a) Energy diagram for DRTP via radiative decay of the donor and acceptor parts in the proposed molecular design. (b) Molecular structures of the guest molecules.

Figure 2

Oak Ridge thermal ellipsoid plots (50% probability ellipsoids) of (a) CQN1 and (b) CQN2. Selected torsions of (c) CQN1 and (d) CQN2. The protons are removed for the sake of clarity.

Figure 3

(a) Solvent-dependent (MCH, methyl cyclohexane; Tol, toluene; THF, tetrahydrofuran; DMF, dimethylformamide) absorption and steady-state (SS) emission of CQN1; absorption of CQN1 (1 wt % mCP film); and absorption of the donor (Cz) and acceptor (QCN) in toluene solutions. (b) Steady-state emission spectra of CQN1 under degassed and oxygenated conditions. (c) Fluorescence decay analysis of CQN1 under degassed and oxygenated conditions. (d) Phosphorescence spectra of CQN1, CQN2, and QCN in toluene at 77 K (λ_ex = 320 nm).

Figure 4

Absorption spectra of (a) CQN1 in toluene (10 μM), mCP (1 wt %), BP (1000:1 host–guest molar ratio), and BP (neat). (b) Steady-state (SS) emission (λ_ex = 425 nm) and RTP (λ_ex = 415 nm) (1.0 ms detector delay) of CQN1/BP and CQN2/BP in a 1:1000 molar ratio at ambient conditions. (c,d) Temperature-dependent phosphorescence decays of CQN1/BP (1:1000 molar ratio) (λ_ex = 415 nm).

Figure 5

Delay-dependent TRES of CQN1/BP (λ_ex = 415 nm) at (a) 300 K, (b) 150 K, and (c) 10 K. Delay-dependent TRES of CQN2/BP at (d) 300 K, (e) 150 K, and (f) 10 K (λ_ex = 390 nm).

Figure 6

Schematic representation of DRTP in the host–guest systems of (a) CQN1/BP and (b) CQN2/BP at ambient conditions.

Figure 7

(a) CQN1 and CQN1/BP and (b) CQN2 and CQN2/BP showing an afterglow “V” pattern when the UV lamp (365 nm) was switched off (web-enhanced objects).