Intermolecular Electronic Coupling of Organic Units for Efficient Persistent Room-Temperature Phosphorescence

Abstract

Although persistent room-temperature phosphorescence (RTP) emission has been observed for a few pure crystalline organic molecules, there is no consistent mechanism and no universal design strategy for organic persistent RTP (pRTP) materials. A new mechanism for pRTP is presented, based on combining the advantages of different excited-state configurations in coupled intermolecular units, which may be applicable to a wide range of organic molecules. By following this mechanism, we have developed a successful design strategy to obtain bright pRTP by utilizing a heavy halogen atom to further increase the intersystem crossing rate of the coupled units. RTP with a remarkably long lifetime of 0.28 s and a very high quantum efficiency of 5 % was thus obtained under ambient conditions. This strategy represents an important step in the understanding of organic pRTP emission.

Keywords: intersystem crossing; organic materials; phosphorescence; photochemistry; spin-orbit coupling.

Figure 1

Energy level diagram of the relevant photophysical processes for the phosphorescence of organic molecules with a) nπ* excited state configuration (i.e., containing an n unit) and b) ππ* excited state configuration (i.e., containing a π unit). c) Proposed energy level diagram of the relevant photophysical processes for pRTP of coupled intermolecular n and π units in organic crystals, and examples of rationally designed molecular structures for pRTP utilizing the proposed mechanism. S₀=ground state; S₁=lowest singlet excited state; T₁=lowest triplet excited state; T_n=high‐level triplet excited state; ISC=intersystem crossing; k _ST=ISC state; and k_r=radiative rate. The superscripts (‘ and *) indicate excited states with different configurations.

Figure 2

a) The transient photoluminescence (PL) decay image (delay 25 ms) of the Cz‐BP crystal powder sample, the color change from red to green indicates the decrease in emission intensity. Steady‐state PL spectra (open violet circles) and persistent phosphorescence spectra (delay 25 ms; filled orange circles) of different crystal powder samples: a) Cz‐BP and b) Cz‐DPS, BCz‐DPS, and BCz‐BP. Steady‐state PL spectra of the Cz‐BP samples under different conditions: c) in air or in vacuum and d) crystal powder, amorphous, and dilute solution in vacuum. The spectra and images were recorded in air at 300 K unless otherwise stated.

Figure 3

a) Intermolecular electronic coupling of the carbonyl and Cz groups in two Cz‐BP molecules that are in close proximity in a single crystal. b) Energy level diagram of the isolated and coupled Cz‐BP molecule(s). Schematic representations of the TD‐DFT calculated energy levels, main orbital configurations, and possible ISC channels of c) isolated Cz‐BP and d) coupled Cz‐BP at the singlet (S₁) and triplet (T_n) states. H and L refer to HOMO and LUMO, respectively. Schematic representations of the possible intramolecular and intermolecular ISC channels in e) isolated Cz‐BP and f) coupled Cz‐BP molecules. The carbonyl (electron acceptor) and the Cz (electron donor) units are labeled as A and D, respectively. The plain and dashed arrows refer to the major and minor ISC channels in (c), (d), (e), and (f), respectively.

Figure 4

Intermolecular electronic coupling of the n and π units in two neighboring molecules in single crystal structures: a) Cz‐DPS, b) BCz‐BP, and c) BCz‐DPS. The distance from the n group to the coupled π plane Cz plane) is indicated by dashed lines: a) 2.983 Å (O‐Cz plane) and 3.524 Å (C‐Cz plane), b) 3.266 Å (O‐Cz plane), and c) 3.240 Å (O‐Cz plane). The relative angle (∡SON) between the S=O bond and the N atom of the Cz plane in (b) and (c) is also indicated: b) 154.93° and c) 155.82°.

Figure 5

Dual‐responsive security protection applications involving the use of BCz‐BP for color‐coded and time‐resolved applications. When excited with 365 nm ultraviolet irradiation, the amorphous part (light blue) of security letter “π” was clearly distinguished from the crystal part (yellow). After the excitation is turned off, only the crystal part (orange) of the letter “π” can be observed.