Abstract
Development of purely organic materials displaying room-temperature phosphorescence (RTP) will expand the toolbox of inorganic phosphors for imaging, sensing or display applications. While molecular solids were found to suppress non-radiative energy dissipation and make the RTP process kinetically favourable, such an effect should be enhanced by the presence of multivalent directional non-covalent interactions. Here we report phosphorescence of a series of fast triplet-forming tetraethyl naphthalene-1,4,5,8-tetracarboxylates. Various numbers of bromo substituents were introduced to modulate intermolecular halogen-bonding interactions. Bright RTP with quantum yields up to 20% was observed when the molecule is surrounded by a Br⋯O halogen-bonded network. Spectroscopic and computational analyses revealed that judicious heavy-atom positioning suppresses non-radiative relaxation and enhances intersystem crossing at the same time. The latter effect was found to be facilitated by the orbital angular momentum change, in addition to the conventional heavy-atom effect. Our results suggest the potential of multivalent non-covalent interactions for excited-state conformation and electronic control.
Highlights
Room temperature phosphorescence (RTP) has received increasing interest due to the potential it presents for photonic devices, bio-imaging, anti-counterfeiting, and night-vision applications.[1,2,3] Until recent years, the main sources of RTP luminophores have been inorganic or organometallic complexes, due to the presence of metal atoms being able to promote singlet-to-triplet intersystem crossing (ISC) in the excited states
Esteri cation of BrnNDA with ethyl iodide in alkaline ethanol gave a mixture of naphthalene tetracarboxylic ethyl esters, brominated naphthalene tetracarboxylic ethyl ester (BrnNTE) (n 1⁄4 0, 1, 2, 4; n 1⁄4 3 can be isolated but it is not discussed here for simplicity).[50,51]
The decay of the initial state can be ascribed to singlet-to-triplet ISC; time constants on the order of tens of picosecond were observed for this process (Table 1)
Summary
Room temperature phosphorescence (RTP) has received increasing interest due to the potential it presents for photonic devices, bio-imaging, anti-counterfeiting, and night-vision applications.[1,2,3] Until recent years, the main sources of RTP luminophores have been inorganic or organometallic complexes, due to the presence of metal atoms being able to promote singlet-to-triplet intersystem crossing (ISC) in the excited states. The proximity of a fourth-row Br element to the C]O group, where the non-bonding electrons originate in the n–p* transition, is believed to enhance SOC as well.[40,41] in a later study by Kim and Dunietz, it was found that moving the Br substituent from the para to the ortho position, closer to the triplet-producing carbonyl functionality, in benzaldehyde increases SOC on the single-molecule level, which enhances both the rates of ISC kISC and phosphorescence kPhos signi cantly by 5–15 fold.[40]
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