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Organic room-temperature phosphorescence from
halogen-bonded organic frameworks: hidden
electronic effects in rigidified chromophores†
Cite this: DOI: 10.1039/d0sc04646a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
b
c
Jiawang Zhou,
Ljiljana Stojanovic,
Andrey A. Berezin,
´
c
c
cd
Tommaso Battisti,
Rachel Crespo-Otero,
Abigail Gill,c Benson M. Kariuki,
Davide Bonifazi,
and Yi-Lin Wu
b
a
c
*
*
*
Michael R. Wasielewski
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.
Received 22nd August 2020
Accepted 4th November 2020
DOI: 10.1039/d0sc04646a
rsc.li/chemical-science
cheaper to acquire, and environmentally safer.4,5 While there
are many benets of organic phosphors compared to those
Introduction
containing heavy metals, achieving RTP from purely organic
molecules has proven a challenge on account of slow ISC rates
and competitive non-radiative processes, in particular.
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.1–3 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. However, heavy metal complexes or inorganic
materials can oen be toxic and expensive; through the study of
purely organic phosphors, the applications of phosphorescence
materials can expand by becoming more biocompatible,
In recent decades, organic phosphorescence has become
a more widely explored topic due to the discovery of long-lasting
RTP by utilising crystallisation,6–8 aggregation,9,10 halogen
bonding,11–14 heavy atoms,15 and carbonyl substituents16–18 to
circumvent the aforementioned issues.19–28 Although spin–orbit
coupling (SOC) in organic molecules is usually small (on the
order of 1 cmꢀ1, cf. 102 to 103 cmꢀ1 for organometallic
complexes), the introduction of a carbonyl functionality to
aromatic rings oen opens up a 1(n–p*) / 3(p–p*) (or 1(p–p*)
29–33
/
3(n–p*)) channel with SOC ꢁ100 cmꢀ1
.
Such a small
aDepartment of Chemistry, Institute for Sustainability and Energy at Northwestern,
Northwestern University, Evanston, Illinois 60208-3113, USA. E-mail:
increase is sufficient to allow efficient ISC and populate the
triplet of, for instance, benzophenone or benzaldehyde with
a near-unitary quantum efficiency.34,35 The structure of the as-
generated triplet states can be rigidied in the solid state with
the aid of non-covalent interactions (e.g. hydrogen and halogen
bonds)11–13,20 to suppress non-radiative vibrational relaxation,
resulting in nearly quantitative RTP quantum yields in the solid
state.19,36
bSchool of Biological and Chemical Sciences, Queen Mary University of London,
London E1 4NS, UK. E-mail: r.crespo-otero@qmul.ac.uk
cSchool of Chemistry, Cardiff University, Cardiff CF10 3AT, UK. E-mail: WuYL@
cardiff.ac.uk
d
¨
Institute of Organic Chemistry, Faculty of Chemistry, University of Vienna, Wahringer
Str. 38, Vienna, 1090, Austria
† Electronic supplementary information (ESI) available: Experimental procedures,
additional transient absorption data, synthetic and computational details, and
X-ray crystallographic data. CCDC 1949875, 1949880 and 1949883. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc04646a
Combining these design principles, the Kim group reported
seminal work on efficient RTP luminophores based on 2,5-
bis(hexyloxy)-4-bromobenzaldehyde.37 The linear C]O/Br
halogen-bonding interactions38,39 present in the solid state were
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