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

Article abstract of 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. This journal is

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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 benets 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 oen 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 oen 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:
m-wasielewski@northwestern.edu
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 rigidied 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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Scheme 1 Synthesis of brominated naphthalene tetracarboxylic ethyl ester (BrnNTE).
suggested to be the major reason to avoid energy dissipation previously studied bromobenzaldehydes, this system is ex-
through vibrational motions. The proximity of a fourth-row Br pected to have less carbonyl-originated n–p* character in the
element to the C]O group, where the non-bonding electrons low singlet excited states to drive ISC, thus offering a platform to
originate in the n–p* transition, is believed to enhance SOC as highlight the halogen effects. Well-developed synthetic meth-
well.40,41 In fact, in a later study by Kim and Dunietz, it was odologies42–45 were used to introduce multiple halogen-bond
found that moving the Br substituent from the para to the ortho donors (e.g. Br) and acceptors (e.g. O) in naphthalene to
position, closer to the triplet-producing carbonyl functionality, permit multiple non-covalent interactions to occur synergisti-
in benzaldehyde increases SOC on the single-molecule level, cally, enabling phosphorescence from halogen-bonded
which enhances both the rates of ISC kISC and phosphorescence frameworks.46
kPhos signicantly by 5–15 fold.40
Inspired by these ndings as well as other successful
demonstration of halogen-bond-induced phosphorescence in
the solid state, we exploited the naphthalene scaffold, a proto-
Results and discussion
Naphthalene derivatives with halogen-bond accepting carbonyl
functionalities and a various number of halogen-bond donating
Br atoms can be prepared readily from 1,4,5,8-naphthalenete-
tracarboxylic dianhydride (NDA) (Scheme 1).47–49 Bromination of
typical building block in organic optoelectronics, to study the
effect of the halogen substitution and the role of halogen
bonding in mediating triplet formation. Compared to the
Fig. 1 Single crystal X-ray molecular structure of (a) Br0NTE (space group P21/n), (b) Br1NTE (P21), (c) Br2NTE (P21/n), and (d) Br4NTE (P1ꢀ) and the
close neighbours in crystals. Crystals were obtained by diffusing MeOH vapour into the CHCl3 solutions of BrnNTE. Colour code: C ¼ grey, O ¼
red, Br ¼ brown. For Br1NTE, only the major component of the disorder is shown and discussed. The terminal carbon of the ethyl group and all
hydrogen atoms are omitted for clarity. Thermal ellipsoids of the central molecules are shown at the 50% probability level, whereas the
neighbouring molecules shown in stick representation. Non-covalent Br/O and Br/Br interactions are highlighted with cyan dashed lines.
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ꢂ
˚
NDA with 1,3-dibromo-5,5-dimethylhydantoin is slow and can be 3.32(2) A (q(C–Br/O) ¼ 151.7(9) ), barely shorter than the
produce a mixture of NDA with various numbers of Br substit- van der Waals contact distance. At last, no p-stack or short
uents.47 However, the application of excess reagents at elevated contact between C–H and naphthalene was found in the crystals
temperatures for a prolonged reaction time gives tetrabromi- of Br0NTE.
nated NDA (Br4NDA) as the sole product. Esterication of Brn-
While naphthalene and its 2-brominated derivatives display
NDA with ethyl iodide in alkaline ethanol gave a mixture of electronic absorption <300 nm, ester substitution induces
naphthalene tetracarboxylic ethyl esters, BrnNTE (n ¼ 0, 1, 2, 4; a bathochromic shi of the naphthalene-centred transitions by
n ¼ 3 can be isolated but it is not discussed here for ca. 50 nm, extending the absorption bands to 350 nm with
simplicity).50,51 The individual compounds were isolated by maxima at ꢁ300 nm (Fig. 2).54–56 Compared to pristine naph-
column chromatography on SiO2 and their identity was thalene, which has an appreciable uorescence quantum yield
conrmed by NMR, MS, and single-crystal X-ray crystallography. of 23% (40% triplet formation yield),55 no emission was detec-
In the solid state, an extended Br/O network can be ted from all BrnNTE in deaerated CH2Cl2 up to 0.02 M (near
observed for Br2NTE50 (Fig. 1). For each molecule, each pair of saturation) excited at 330 nm.
the peri ester groups interacts with a Br atom of a neighbouring
Despite the non-radiative energy dissipation in solution,
molecule to establish bifurcated, slightly asymmetric halogen crystalline solids of the brominated molecules display visible
˚
bonds with d(C–Br/O]C) ¼ 3.268(2) A, d(C–Br/O–C2H5) ¼ phosphorescence in the 500–700 nm region with millisecond
ꢂ
˚
3.308(2) A and both q(C–Br/O) ꢁ150 (i.e. 152.40(9) and lifetimes, whereas non-brominated Br0NTE remains non-
149.21(8)).38 Reciprocally, each Br atom is interacting with two emissive (Fig. 2 and Table 1). Powdery crystalline solid
peri ester groups of a nearby molecule of Br2NTE. Being samples of BrnNTE, whose powder X-ray diffraction proles
symmetrically substituted with four Br and four ester func- match the pattern based on their single-crystal data, were used
tionalities, Br4NTE is also embedded in a framework of halogen in all phosphorescence measurements. Phosphorescence of
bonds in the solid state. However, likely due to the steric crystalline Br2NTE and Br4NTE feature clear vibrational
requirement of large Br atoms, the same arrangement in progression with a quantum yield of FPhos ¼ 19.6% and 9.3%,
Br2NTE was not observed. Instead, only two out of the four respectively. Much weaker and structureless emission was
esters on the 1 and 5 positions form linear C–Br/O]C short observed for Br1NTE (FPhos ¼ 1.4%).
contacts with the Br atoms on the 3 and 7 positions of the
The varying luminescent behaviours suggest that the excited-
neighbouring molecules (values taken from two crystallo- state dynamics were modulated in a subtle way by Br-specic
graphically independent molecules: d(Br/O) ¼ 3.074(3) and properties, which is however not directly related to the
ꢂ
˚
3.286(3) A, q(C–Br/O) ¼ 165.5(1) and 168.5(1) ). The remaining number of Br atoms in the molecule. It is conceivable that
two Br atoms on the 2 and 6 positions are engaged in orthog- multi-point halogen bonding provides a geometric framework
onal (“Type II”)52,53 C–Br/Br–C interactions (d(Br/Br) ¼ to strengthen the rigidity of BrnNTE in the crystalline state. This
ꢂ
˚
3.692(1) A, q(C–Br/Br) ¼ 86.4(1) ).
effect is especially substantial for Br2NTE where all the
With only one Br atom per molecule, the ester groups in peripheral substituents engage in the directional Br/O inter-
Br1NTE do not engage in extended halogen-bonded networks. actions, providing the additional factor to the solid-state
In fact, the shortest d(C–Br/O–C2H5) distance is measured to effect6,8 of RTP to impede competitive non-radiative relaxation
Fig. 2 (a) UV-Vis absorption spectra of BrnNTE at (5–8) ꢃ 10ꢀ5 M in CH2Cl2. (b) Normalised phosphorescence spectra of BrnNTE in the crystalline
solid state (solid line) or in PMMA (dashed line). Samples were excited at 300–320 nm. (c) Photographs of solid emission under UV irradiation (365
nm).
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Table 1 Photophysical properties of BrnNTE
In CH2Cl2
Crystalline solids
s(S1 / Tn)a
s(T1 / S0)a
sPhos
FPhos
b
b,c
Br0NTE
Br1NTE
Br2NTE
Br4NTE
111.7 ꢄ 0.8 psd
9.1 ꢄ 0.3 psd
7.5 ꢄ 0.3 ps
29.1 ꢄ 0.1 ms
n.a.
n.a.
1.42 ꢄ 0.01 ms
0.50 ꢄ 0.01 ms
0.0121 ꢄ 0.0006 ms
1.53 ꢄ 0.02 ms
1.94 ꢄ 0.01 ms
1.11 ꢄ 0.01 ms
1.4%
19.6%
9.3%
48.3 ꢄ 0.9 psd
From transient absorption measurements. b From (time-resolved) phosphorescence measurements. c The uncertainty is estimated to be 20% of
a
the measured values. d Preceded by the relaxation of hot S1 in (0.9–1.2)ꢄ0.3 ps.
through intramolecular motions. The highest phosphorescence that the triplet decay in Br1NTE is largely intrinsic to the
quantum yield was thus observed for the crystalline sample of monomeric molecule. However, the vibrational progression of
Br2NTE.
Br4NTE, a signature of chromophore rigidity, became less
The weaker and non-structured phosphorescence observed pronounced, and that of Br2NTE completely disappeared and
for Br1NTE (and Br0NTE) seems to be originated from its looser the overall emission prole resembles very well to that of
solid-state packing. If we dene the volumetric index Vi as the Br1NTE.
57,58
ratio between the Voronoi volume (VVor
)
and the van der
Additional support for the efficient population of the triplet
Waals volume (VwdW) of a molecule in the crystal, smaller Vi ¼ excited state was provided by transient absorption measure-
VVor/VwdW would suggest denser packing. Vi of 1.27–1.30 were ments. Spectroscopically, all BrnNTE exhibit similar excited-
found for Br2NTE and Br4NTE embedded in halogen-bonded state dynamics: following the initial formation of the singlet
frameworks, but the values are signicantly larger for Br0NTE excited state, which displays excited-state absorption (ESA)
and Br1NTE (1.36–1.38). The larger free space available to each peaking at ca. 490 nm and a broad feature in the near infrared
molecule in the Br0NTE and Br1NTE crystals allows the excited region of 800–1000 nm (Fig. 3 and ESI Section 5†), a new excited-
molecules to decay radiatively and non-radiatively on various state species with ESA at ca. 480 nm appears with microsecond
points of the triplet potential energy surface.
lifetimes. This long-lived species was assigned to the triplet of
The signicance of the inter-BrnNTE Br/O interactions is each chromophore based on the lifetime and spectral similarity
further supported by comparing the phosphorescence of crys- to the triplet–triplet absorption of methyl 1-naphthalate59 and 2-
talline BrnNTE with that of the dispersed molecules in poly(- bromonaphthalene.60 Therefore, the decay of the initial state
methyl methacrylate) (PMMA, Mw ꢁ996 kDa; 2 wt% doping). can be ascribed to singlet-to-triplet ISC; time constants on the
The rigid polymer matrix is expected to constrain the molecular order of tens of picosecond were observed for this process
motion at room temperature but disrupt inter-BrnNTE Br/O (Table 1). Compared to the typical uorescence lifetime (1 ns or
halogen bonds. The phosphorescence spectra of Br1NTE longer) of naphthalene derivatives,54,61 the fast ISC process
remained identical in either environment (Fig. 2), indicating suggests a high triplet forming efficiency. Such efficient ISC on
Fig. 3 Transient absorption spectra of BrnNTE (n ¼ 1, 2, and 4) in deaerated CH2Cl2 at various pump–probe delay indicated (excitation ¼ 330 nm,
see ESI Section 5† for n ¼ 0).
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