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. 2024 Sep 4;4(6):604-611.
doi: 10.1021/acsmaterialsau.4c00036. eCollection 2024 Nov 13.

Role of Pyridine Nitrogen Position on the Moisture Sensitivity of Organic Emitters

Gyana Prakash Nanda  1 , Rajan Suraksha  1 , Pachaiyappan Rajamalli  1
Affiliations

Role of Pyridine Nitrogen Position on the Moisture Sensitivity of Organic Emitters

Gyana Prakash Nanda et al. ACS Mater Au. .

Abstract

Moisture-sensitive fluorescent emitters are a class of smart materials that can change their emission behavior upon exposure to water. In this study, we have synthesized two highly fluorescent organic emitters, 4BPy-PTA and 2BPy-PTA, and showed how moisture sensitivity can be enhanced by molecular design modification. Owing to the different nitrogen atom positions in the acceptor units, the emitters show different degrees of moisture sensitivity. Upon moisture exposure, both emitters change their emission color from greenish-yellow to blue, but a larger shift was witnessed in 4BPy-PTA (81 nm) than in 2BPy-PTA (68 nm). Moisture exposure enhances the photoluminescence quantum yield (PLQY) of 4BPy-PTA from 37 to 48%, whereas it suppresses the PLQY of 2BPy-PTA from 59 to 15%. A shorter moisture sensing time, large emission color shift, and enhanced PLQY make 4BPy-PTA a better moisture-sensitive material than 2BPy-PTA. Interestingly, the emission colors of the emitters can be completely regained by heating and partially by applying mechanical force to the moisture-exposed solids. In addition, these emitters also show mechanochromic luminescence behavior with a completely reversible emission color switch between blue and green.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Molecular Structure and Synthesis Procedure of 4BPy-PTA and 2BPy-PTA
Figure 1
Figure 1
Single crystal structure of (a) 2BP-PTA and (c) 4BPy-PTA showing the dihedral angle between the pyridine unit and central benzene ring; interplanar distance of (b) 2BPy-PTA and (d) 4BPy-PTA obtained from single crystal XRD. [Color code; gray = carbon; white = hydrogen; blue = nitrogen; and red = oxygen].
Figure 2
Figure 2
Optimized molecular structures, dihedral angle between pyridine and central benzene ring, HOMO-LUMO distribution, and S1 energy levels and optical band gaps of (a) 4BPy-PTA and (b) 2BPy-PTA evaluated from TD-DFT using the B3LYP 6-311G(d,p) basis set.
Figure 3
Figure 3
(a) Normalized UV–visible absorbance and PL measurements of 4BPy-PTA and 2BPy-PTA in 0.01 mM toluene solution, λex = 380 nm; solvatochromic study of (b) 4BPy-PTA and (c) 2BPy-PTA in cyclohexane, hexane, toluene, 1,4-dioxane, chlorobenzene, tetrahydrofuran, and chloroform solutions.
Figure 4
Figure 4
(a) Moisture sensing study of 4BPy-PTA; (b) moisture sensing study of 2BPy-PTA; (c) schematic representation of moisture sensing set up; (d) FTIR of pristine and moisture exposed solids of 4BPy-PTA; (e) FTIR of pristine and moisture exposed solids of 2BPy-PTA; (f) digital photographs showing the emission colors of pristine solids, moisture exposed solids, and ground forms of 4BPy-PTA and 2BPy-PTA.
Figure 5
Figure 5
(a) MCL study of 4BPy-PTA; (b) reversible switching of emission color in 4BPy-PTA during MCL study; (c) powder XRDs of different solids of 4BPy-PTA obtained during the MCL experiment; (d) MCL study of 2BPy-PTA; (e) reversible switching of emission color in 2BPy-PTA during MCL study; (f) powder XRDs of different solids of 2BPy-PTA obtained during the MCL experiment.
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