Organic room-temperature phosphorescence from halogen-bonded organic frameworks: hidden electronic effects in rigidified chromophores

• Published in: Chemical science (Cambridge) Vol. 12; no. 2; pp. 767 - 773
• Main Authors: Zhou, Jiawang, Stojanovi, Ljiljana, Berezin, Andrey A, Battisti, Tommaso, Gill, Abigail, Kariuki, Benson M, Bonifazi, Davide, Crespo-Otero, Rachel, Wasielewski, Michael R, Wu, Yi-Lin
• Format: Journal Article
• Language: English
• Published: CAMBRIDGE Royal Soc Chemistry 21.01.2021; Royal Society of Chemistry; Royal Society of Chemistry (RSC); The Royal Society of Chemistry
• Subjects: Angular momentum; Chemistry; Chemistry, Multidisciplinary; Chromophores; Crystallography; Electronic control; Energy dissipation; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Naphthalene; Organic materials; Phosphorescence; Phosphors; Physical Sciences; Room temperature; Science & Technology; PERSISTENT; STATES; C-H ARYLATION; NANOCRYSTALS; OPTICAL-PROPERTIES; NAPHTHALENE DIIMIDES; EMISSION; SPIN-ORBIT COUPLINGS; 2-LASER; SINGLET
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc04646a

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. The number and position of halogen substituents in purely organic π-π* chromophores critically affect the efficiency of phosphorescence.