Effect of π···π Interactions of Donor Rings on Persistent Room-Temperature Phosphorescence in D4-A Conjugates and Data Security Application

Abstract

Organic room-temperature phosphorescence (RTP) materials with persistent RTP (PRTP) have attracted huge interest in inks, bioimaging, and photodynamic therapy. However, the design principle to increase the lifetime of organic molecules is underdeveloped. Herein, we show donor(D₄)-acceptor(A) molecules (TOEPh, TOCPh, TOMPh, TOF and TOPh) with similar orientation of donor rings in aggregates that cause a large number of noncovalent interactions. We observed that TOEPh, TOCPh, TOMPh and TOF showed PRTP, whereas TOPh showed only phosphorescence emission (Φ_P = ∼11%) with no PRTP property at ambient conditions. The spectroscopic and single-crystal X-ray analyses confirm the molecular assembly via J-aggregation with a face-to-face orientation of the donor rings. The crystal structure analysis (TOEPh, TOCPh, TOMPh, TOF) reveals that moderate π···π interactions (3.706 to 4.065 Å) between the donor rings cause the enhancement of the phosphorescence lifetime (26 to 245 ms), whereas the short phosphorescence lifetime (12 ms) of TOPh was observed because of the absence of π···π interactions. We found that TOEPh shows a long lifetime (245 ms) as compared to other derivatives because of the presence of ethoxy (-OEt) groups that enables spin-orbit coupling caused by strong lone pair (O)···π interactions present in the molecule. Utilizing the PRTP feature of TOEPh and the fluorescence emission of TOPh, we have shown data security applications in poly(methyl methacrylate).

Figure 1

Molecular structures of TOPh, TOEPh, TOCPh, TOMPh and TOF.

Figure 2

Absorption spectra of (a) TOPh and (b) TOEPh with disparate polarity of solvents.

Figure 3

Steady-state emission spectra of (a) TOPh and (b) TOEPh in solutions. λ_ex = 365 nm.

Figure 4

Steady-state and phosphorescence emission spectra of (a) TOPh and (b) TOEPh in powder. The phosphorescence decay of (c) TOPh and (d) TOEPh.

Figure 5

Crystal structure of (a) TOPh, (b) TOEPh, (c) TOCPh, (d) TOMPh and (e) TOF.

Figure 6

Distance between the centroids of the donor rings in (a) TOPh and (b) TOEPh (c) TOCPh and (d) TOEPh. Hydrogen atoms are omitted for clarity. Distances are measured in Å.

Figure 7

Absorption spectra of (a) TOPh and (b) TOEPh in THF and 90% (v/v) THF–H₂O.

Figure 8

(a) PRTP feature of TOPh, TOEPh, TOCPh and TOMPh in powder under ambient conditions. (b) Data security application patterns of “E” and “I” under illumination with a 365 nm lamp using the PMMA films of TOPh and TOEPh.