Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials

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Received 27 Jun 2015 | Accepted 20 Oct 2015 | Published 2 Dec 2015 DOI: 10.1038/ncomms9947 OPEN
## Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials

Min Sang Kwon 1, Youngchang Yu 1, Caleb Coburn 2, Andrew W. Phillips 3, Kyeongwoon Chung 3,
Apoorv Shanker 3, Jaehun Jung 3, Gunho Kim 1,4, Kevin Pipe 4, Stephen R. Forrest 1,2,5, Ji Ho Youk 1,6,
Johannes Gierschner 7 & Jinsang Kim 1,3,8,9,10

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly

used organometallic phosphors but are generally dimmer and are relatively rare, as, without

heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot

compete with radiationless relaxation processes. Here we present a general design rule

and a method to effectively reduce radiationless transitions and hence greatly enhance

phosphorescence efficiency of metal-free organic materials in a variety of amorphous

polymer matrices, based on the restriction of molecular motions in the proximity of

embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via

Diels–Alder click chemistry is devised as a method. A sharp increase in phosphorescence

quantum efficiency is observed in a variety of polymer matrices with this method, which is

ca. two to five times higher than that of phosphor-doped polymer systems having no such

covalent linkage.

1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. 2 Department of Electrical Engineering,
University of Michigan, Ann Arbor, Michigan 48109, USA. 3 Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor,
Michigan 48109, USA. 4 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. 5 Department of Physics,
University of Michigan, Ann Arbor, Michigan 48109, USA. 6 Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Korea.
7 Madrid Institute for Advanced Studies—IMDEA Nanoscience, Calle Faraday 8, Ciudad Universitaria de Cantoblanco, Madrid 28049, Spain. 8 Department of
Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. 9 Department of Chemistry, University of Michigan, Ann Arbor, Michigan
48109, USA. 10 Biointerfaces Institute, University of Michigan, Ann Arbor, Michigan 48109, USA. Correspondence and requests for materials should be
addressed to J.K. (email: [jinsang@umich.edu).](mailto:jinsang@umich.edu)

etal-free room-temperature phosphorescent (RTP)
materials are attractive alternatives to organometallic
phosphors because of their low cost, earth-abundant
# M
constituent atoms and versatility of molecular design 1 . In
particular, these materials have great potential for high-contrast
bioimaging 2, high-sensitivity optical sensing 3–7, singlet fission
materials 8, unconventional reverse saturable absorption
materials 9 and advanced security imaging 10 . However,
developing highly efficient metal-free RTP materials is very
challenging, as, without heavy-metal atoms, both the rate of
intersystem crossing (ISC) from the lowest excited singlet state
(S1) to the triplet manifold (Tn) and the rate of radiative
deactivation from T1 to the ground state (S0) are typically slow,
and therefore cannot compete with fluorescence, internal
conversion (IC) and/or radiationless relaxation processes from
T1 to S0 via ISC or quenching by the host matrix and/or oxygen,
at ambient conditions (Fig. 1b).

ISC from S1 to Tn can be greatly promoted in special organic
moieties through efficient spin–orbit coupling (SOC) by the
effective mixing of the singlet and triplet states of different
molecular orbital (MO) configurations (El-Sayed rule; for
example, aromatic carbonyls and nitrogen heterocycles 11–13 )
and by heavy halogens 14, and/or through small singlet–triplet
splitting energy (DEST) 15 . However, in such compounds, the rate
constant for phosphorescence is still small (kPB10 2 s � 1 ),
whereas in organometallic complexes, on the other hand, rate
constants are as high as kPB10 5 s � 1 . Therefore, radiationless
relaxation processes from T1 to S0 must be sufficiently suppressed
to achieve bright RTP in metal-free organic phosphors.

A common method to enhance RTP in metal-free phosphors is
to make them a crystal 10,14–19 or to embed them into a carefully
chosen matrix such as poly(vinyl alcohol) 5,20,21, poly(methyl

methacrylate) (PMMA) 4,22–24, steroid analogue 6,9,25, micelles 2,
the cavity of cyclodextrin 26 or inorganic crystals 27,28 . Deuterium
substitution has been another route developed to enhance RTP in
metal-free phosphors, although it requires the rather synthetically
difficult deuterium substitution process 6,9,25 . These studies have
indicated that oxygen permeability, triplet energy and rigidity of
the matrix, and deuterium substitution of the phosphor are all
important contributors to the radiationless transitions. In
particular, rigidity of the matrix is a very important parameter,
which is also supported by the fact that polymer relaxation
behaviour (for example, a-, b- and g-relaxations) largely
influences the phosphorescent emission properties of metal-free
organic phosphors 4,22 . Although the influences of oxygen, the
triplet energy of the host matrix and deuterium substitution of
phosphors on the radiationless transitions have been well
investigated 2,22,23,25, the correlation between the radiationless
relaxation pathways (for example, ISC from T1 to S0 and Dextertype triplet energy transfer (ET) processes) and the rigidity of the
environment is not fully understood, limiting the development of
a general design principle to realize efficient RTP in a variety of
polymer matrices. Moreover, the use of crystals and specific
matrices can significantly reduce the practical utility due to
difficult processing, undesired properties of matrices and
mechanical fragility of those composites. Therefore, finding a
new versatile method that enables the restriction of the
radiationless relaxation pathways in a variety of amorphous
polymer matrices, and that allows for a systematic study of their
dependence on the rigidity of the environment, is highly desired,
yet no such system has been reported.

Here we present a general design principle and a method to
effectively reduce radiationless relaxation pathways and hence to
achieve highly efficient RTP of metal-free organic materials in a

a b

Ene

unit

_Sn_

_S_ 1

Exc.

_S_ 0

( _k_ IC)

Fluo.

•

Ene

unit

Phosphorescent

core

H O

Diene Polymer

unit backbone

O O

_High �P_

- Non. rad.

ISC ( _S_ 1 → _Tn_ )

( _k_ ISC)

_Tn_

_T_ 1

Phos.

_k_ ISC _> k_ IC _+ k_ F

_kP > kTS+ kq_

O O

DA1 PFMA

Diels–Alder, 120 °C

( _k_ TS _,k_ q)

X

( _k_ F) ( _k_ P)

H O

Influence of restriction of molecular motions
by cross-linking :

_k_ P
_�P_ = _�ISC_ ·
_k_ P + _k_ q + _k_ nr

Cross-linking suppresses radiationless decay,
resulting in highly efficient RTP

O O

Cross-linked DA1

H O

Br6A (reference)

• _�ISC_

•

_�ISC_ No significant change

_k_ P Increase
_k_ q Significant decrease
(collision frequency ↓)
_k_ TS Significant decrease in _k_ TS

Significant decrease in _k_ TS0 _,_ _P_ TS

(vibronic mixing ↓)

Figure 1 | Chemical structures of designed phosphor and polymer and description of covalent cross-links strategy. (a) Chemical structures of the newly
designed DA1 and PFMA are shown. Diels–Alder click reaction in DA1-doped PFMA blend film forms covalent bonds between DA1 and PFMA, resulting in
greatly enhanced phosphorescence efficiency (FP) compared with Br6A-doped PFMA blend film having no covalent linkages. (b) A general Jablonski
diagram of organic emitters is presented. General rules for efficient RTP emitters are shown in the grey box. The influence of cross-linking via the Diels–Alder
reaction is described in the blue box. The restriction of motions of phosphors and matrices by cross-linking effectively reduces the collision frequency and
hence the Dexter-type triplet ET processes (kq) and the vibronic mixing that directs the reduction of the rate of the ISC process from T1 to S0 (kTS).

variety of amorphous polymer matrices. Covalent cross-linking
between phosphors and polymer matrices is devised as a method
to suppress the molecular motions of embedded phosphors and
polymer matrices. In fact, a cross-linking method has been used
to improve thermal stability and solvent resistance of organometallic phosphor-based light-emitting devices 29 . We chose Diels–
Alder click chemistry as the cross-linking method due to its
modular and efficient nature, leading to convenient preparation
and easy characterization of phosphor-doped polymer films. The
resulting cross-linked phosphor-doped polymer system
comprised of the designed diene-reactive phosphor (DA1) and
ene-reactive polymer matrix showed highly efficient RTP with
phosphorescence quantum efficiency up to ca. 28%, which is ca.
2–5 times higher than that of phosphor (Br6A)–polymer blend
system having no such covalent linkages between the matrix and
the phosphor (Fig. 1a). Detailed spectroscopic studies on both
polymer blend systems reveal that restricting molecular motions
in the proximity of phosphors (increasing the rigidity of their
local environment) by cross-linking effectively reduces the
collision frequency and hence the Dexter-type triplet ET
processes and vibronic mixing between zero-order electronic
states of T1 and S0 that directs the reduction of the rate of the ISC
process (Fig. 1b). Moreover, by developing the DA1-copolymer
systems with various monomers, we demonstrate that our
method is a generally applicable strategy to achieve highly
efficient metal-free RTP in a variety of polymer systems.

Results
Molecular design of DA1. We designed a clickable phosphor,
DA1, through side-chain modification of Br6A, while maintaining the core structure of Br6A. For the side chains, we chose a
maleimide moiety that enables the Diels–Alder click reaction
with a furan group of a designed complementary polymer,
poly(furfuryl methacrylate) (PFMA; Fig. 1a). The bromo/
benzaldehyde design promotes ISC via SOC by the creation of an
np*-type triplet (Tn) state, due to the benzaldehyde moiety, as
well as by the intra- and intermolecular heavy atom effect of
bromine (however, this is moderate with respect to heavy
metals) 16 . In fact, bromine promotes SOC by the direct
participation in the relevant frontier p/p*-type MOs, providing
efficient solid-state phosphorescence (vide infra) 13,14 . A full
description of the synthetic approaches and 1 H-NMR, 13 C-NMR
and mass spectrometric characterizations are described in the
Supplementary Information.

Photophysics of DA1 in solution. We first investigated the
solution-state luminescent properties of DA1 in methanol,
showing distinctively different photophysics from Br6A.
Although Br6A is fluorescent with a quantum yield of FF ¼ 12%
at lmax ¼ 455 nm, DA1 shows negligible fluorescence at room
temperature (RT) and ambient air pressure (Fig. 2b). However, at
77 K, both compounds show noticeable phosphorescence
(Fig. 2c). Time-dependent density functional theory was used to
understand the difference between Br6A and DA1.

According to the calculations, electronic excitation to the
first excited singlet state (S1) of Br6A in methanol is mainly
attributed to a HOMO-LUMO (pp*) transition (Fig. 2d and
Supplementary Table 1), calculated at 3.61 eV (343 nm) with
considerable oscillator strength (f ¼ 0.16), from which Br6A can
efficiently fluoresce. Alternatively, ISC from S1 to Tn can occur.
This process is promoted by the heavy atom effect of bromine
and by the energetically close nature of the T2 state. As the T2
state is np*, ISC from S1(pp*) to T2(np*) is allowed by the
El-Sayed rule and is followed by rapid relaxation to the T1 state
(Fig. 2d). At ambient conditions, T1 effectively deactivates

non-radiatively by ISC and consecutive thermal relaxation to
S0, as well as by quenching by oxygen, enabled by effective
diffusion in solution during the long T1 lifetime of Br6A.
Lowering the temperature to 77 K restricts the ISC pathway and
oxygen diffusion in the frozen environment, allowing for Br6A
phosphorescence.

In DA1, the HOMO is slightly stabilized due to the presence of
the maleimide groups on the side chains, so that the energetically
lowest core-centred singlet state of pp* nature (S3; Fig. 2e and
Supplementary Table 1), which dominates the absorption
spectrum, is blue shifted by 0.04 eV compared with the
corresponding state of Br6A (that is, S1; Fig. 2d), in a qualitative
agreement with the experimental data (0.10 eV; Fig. 2a). In
contrast to Br6A, an effective quenching pathway for fluorescence
is operative in DA1 due to the presence of the maleimide ligands.
In fact, the lowest unoccupied maleimide MOs are lower in
energy compared with the core-centred one, so that electronic
excitation from the core-centred HOMO leads to nearly
degenerated excited singlet states of S1 and S2 with complete
charge-transfer (CT) character (Fig. 2d). Photoinduced electron
transfer (PeT) from the originally excited, bright Franck–Condon
state (S3) to the optically dark CT state (S1) effectively quenches
the fluorescence of DA1. In any case, the ISC pathway to the
(intra-core centred) triplet manifold in DA1 is identical to that in
Br6A (Fig. 2d), so that at low temperature phosphorescence is
noticeable in both systems, just slightly blue shifted in DA1 in
agreement with experimental data (Fig. 2c).

Characterization of Diels-Alder reaction. We prepared a
DA1-doped PFMA film for characterizing the Diels–Alder
reaction. The synthesized polymer, PFMA (1.0 wt%), was
dissolved in chloroform (CHCl3) and mixed with the phosphor
DA1 (1.2 wt% of DA1 for PFMA). The mixed solution was
drop-cast on a pre-cleaned glass substrate and kept at RT for
10 min. The resulting drop-cast film was thermally annealed at
120 �C for 20 min under nitrogen atmosphere, leading to the
formation of covalent bonding between DA1 and PFMA. For
comparison, a Br6A-doped PFMA film (1.0 wt% of Br6A for
PFMA) was also prepared in the same way.

To characterize the Diels–Alder reaction in the resulting
polymer blend film, ultraviolet–visible spectroscopy was
employed, which is a standard quantitative analysis tool
commonly used for this purpose 30 . The maleimide moiety, M1,
exhibited a typical absorption peak at around 300 nm, as seen in
the M1 spectrum in Fig. 2a, which corresponds to an np*
transition of the conjugated diene as confirmed by timedependent density functional theory calculations (see
Supplementary Fig. 1). Although in the sample before thermal
annealing the M1 band is present, it disappeared after thermal
annealing due to the Diels–Alder reaction (Fig. 3a). The degree of
conversion of the Diels–Alder reaction can be estimated by
calculating the expected DA1 spectrum after thermal annealing
by subtracting the M1 spectrum from the DA1 spectrum before
thermal annealing (for details on the procedure, see
Supplementary Fig. 2); from this, a conversion efficiency of
495% can be estimated. Differential scanning calorimetry further
confirmed the success of the Diels–Alder reaction, showing a
notable increase of the glass transition temperature (Tg) of the
annealed DA1-doped PFMA compared with the unreacted
pristine PFMA or Br6A-doped PFMA (Fig. 3b). This is
attributed to the fact that cross-linking via the Diels–Alder
reaction limits the mobility of polymer chains, resulting in an
increase of Tg. Finally, the success of the cross-linking reaction
can be seen in the rise of fluorescence in the annealed DA1-doped
PFMA film, that is, compared with the DA1-doped PMMA film

a b c

Br6A DA1 350 nm excitation Br6A DA1

0.10

0.08

0.06

0.04

0.02

350 nm excitation

Br6A (0.93×10 –5 M, MeOH)

DA1 (1.1×10 –5 M, MeOH)

M1 (2.1×10 –5 M, MeOH)

Br6A+M1 (MeOH)

355 nm
(3.49 eV)

350 nm excitation
150 μs decay
Br6A in MeOH at 77 K
DA1 in MeOH at 77 K

513 nm
(2.42 eV)

526 nm
(2.36 eV)

450 500 550 600 650

Wavelength (nm)

O M1

345 nm
(3.59 eV)

0.00

275 300 325 350 375 400 425 450

Wavelength (nm)

Br6A
DA1

Br6A + M1

455 nm (2.72 eV)

447 nm (2.77 eV)

400 450 500 550 600 650

Wavelength (nm)

d Br6A DA1 e Core MOs Ligand MOs

_S_ 2 : 3.78

_n�_
_S_ 1 : 3.61

ISC _T_ 3: 3.45

ISC

Intra-core _CT_ Intra-ligand

_S_ 3: 3.65

LUMO+2
–1.89 eV

_S_ 3 : 3.65 eV
_f_ = 0.150

HOMO
–6.16 eV

HOMO-1
–6.99 eV

HOMO-2
–7.11 eV

HOMO-5
–7.90 eV

_�_

_n_

_�_

LUMO+1
–2.67 eV

LUMO
–3.08 eV

_T_ 9: 3.45

_T_ 7: 3.28

_T_ 5: 3.23

_T_ 4: 3.18

_T_ 2: 3.28

PeT

_S_ 1 : 3.03 eV
_f_ = 0.001

HOMO-3
–7.60 eV

_n�_ - _T_ 2: 3.28 _n�_

_T_ 8: 3.28

_n�_

_T_ 6: 3.27 _n�_ - _T_ 5: 3.23

_n_

_�_

_n_
_n_

_T_ 3: 3.05

_T_ 2: 2.95

### ×

Fluo.

_S_ 2: 3.06

_S_ 1: 3.03

Phos.

ISC

_T_ 1: 2.68

Phos.

_T_ 1: 2.72

ISC

HOMO-4
–7.80 eV

HOMO-6
–8.01 eV

Figure 2 | Photophysical properties of DA1 in a methanol solution. (a) Ultraviolet–visible absorption spectra of Br6A (0.93 � 10 � 5 M; blue line), DA1
(1.1 � 10 � 5 M; red line), M1 (2.1 � 10 � 5 M; black line) and Br6A/M1 (0.93 � 10 � 5 M/2.1 � 10 � 5 M; green line) in a methanol solution. Calculated
ultraviolet–visible spectra of Br6A and DA1 using time-dependent density functional theory (TD-DFT) are shown in Supplementary Fig. S1, which are in
good qualitative agreement with the experimental data. Inset shows the chemical structure of M1. (b) Photoluminescence (PL) spectra of methanol
solutions of Br6A, DA1 and Br6A/M1 excited at lex ¼ 350 nm. The inset shows PL images of methanol solutions of Br6A and DA1 at RT under a 365-nm
hand-held ultraviolet light. (c) Gated PL spectra of methanol solutions of Br6A and DA1 excited at lex ¼ 350 nm at 77 K. The inset shows PL images of
methanol solutions of Br6A and DA1 at 77 K under a 365-nm hand-held ultraviolet light. (d) Term diagrams of Br6A and DA1 in methanol as calculated by
TD-DFT; np*-type states are indicated, whereas the other states are all of pp* character. Energies are given in eV. (e) MO diagram of DA1, as calculated at
the DFT (B3LYP//BHLYP/6-311G*) level of theory.

and solution, which demonstrates the shutdown of the PeT
process via the CT state by the Diels-Alder reaction.

Phosphorescence properties of the polymer blend film. To
identify the cross-linking effect on the phosphorescence properties, gated photoluminescence spectra, phosphorescence quantum
yield (FP) and phosphorescence lifetime (tP) of DA1- and Br6Adoped polymer films were measured at RT (Fig. 3c–e and
Table 1). To minimize oxygen effects on the phosphorescence
processes, all processes for sample preparation from drop-casting
to packaging with an epoxy-seal were done under a nitrogen
atmosphere (in a controlled atmosphere). To avoid aggregationinduced quenching, we optimized the doping concentration of
phosphors (1.2 wt% of DA1 and 1.0 wt% of Br6A for polymers,
respectively) through the measurement of FP of phosphor-doped
polymer films at different doping concentration of phosphors

(Supplementary Fig. 3). It should be noted that DA1-doped
polymer films were thermally annealed at 120 �C for 20 min, to
induce cross-linking between DA1 and polymers. For comparison, Br6A-doped polymer films were prepared in exactly the
same way as the DA1-doped polymer films. Additional details for
sample preparation and measurement procedure are described in
the Supplementary Information.

The resulting Br6A- and DA1-doped PFMA films showed
noticeable phosphorescence at 526 nm (2.36 eV) and 513 nm
(2.42 eV) with lifetimes of 2.0 and 2.6 ms, respectively (Fig. 3c,e).
The observed blue shift of 0.06 eV in the emission spectrum of
DA1-doped PFMA film is due to the presence of maleimide
moieties at the side chain, corresponding well to that in methanol
solution as mentioned in the section ‘Photophysics of DA1 in
solution’. The rigid environment of the polymer matrix and
exclusion of oxygen allow for noticeable RTP in both Br6A- and
DA1-doped PFMA films, in contrast to those in methanol

a b c

DA1-doped (1.2 wt%) PFMA

Before annealing
After annealing
Expected absorption spectrum

_n�_       - of maleimide

250 300 350 400 450

Wavelength (nm)

350 nm excit.
150 μs delay

Br6A-doped (1.0 wt%) PFMA
DA1-doped (1.2 wt%) PFMA

64.0 °C

59.7 °C

59.3 °C

PFMA

Br6A-doped (1.0 wt%) PFMA
DA1-doped (1.2 wt%) PFMA

73.9 °C

_T_ g= 69.0 °C

513 nm
(2.42 eV)

526 nm
(2.36 eV)

69.6 °C

_T_ g= 64.7 °C

70.3 °C _T_ g= 64.9 °C

55 60 65 70 75 80

T (°C)

d e f

400 450 500 550 600 650 700

Wavelength (nm)

Br6A
100 DA1

60
0 20 40 60 80 100

n m

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Br6A
DA1

O O

P(FMA- _r_ -MMA)

Br6A (1.0 wt%)

DA1 (1.2 wt%)

Br6A (1.0 wt%) + M2 (0.7 wt%)

In P(FMA- _r_ -MMA)

PeT

0
0 20 40 60 80 100

0.0
0 20 40 60 80 100

100 × _�_ MMA (%) 100 × _�_ MMA (%) 100 × _�_ MMA (%)

Figure 3 | Photophysical and thermal characterization of Br6A- and DA1-doped polymer blend films. (a) Ultraviolet–visible absorption spectra of DA1doped PFMA (1.2 wt% of DA1 for PFMA polymer) films before (black line) and after (red line) thermal annealing. Blue line shows the expected absorption
spectrum of DA1-doped PFMA. (b) Differential scanning calorimetry (DSC) curves of Br6A- and DA1-doped PFMA (blue and red line, respectively) and
PFMA (black line). (c) Gated PL spectra of Br6A- and DA1-doped PFMA at RT. (d) Phosphorescence quantum yield (FP) of Br6A- (blue line), DA1-doped
(red line), Br6A and M2-doped (black line) P(FMA-r-MMA) at different wMMA. Chemical structures of P(FMA-r-MMA) and M2 are given in inset.
(e) Phosphorescence lifetime (tP) and (f) glass transition temperature (Tg) of Br6A- (blue line) and DA1-doped (red line) P(FMA-r-MMA) at different
wMMA.

Table 1 | Photophysical data of Br6A- and DA1-doped P(FMA-r-MMA) blend films for wMMA ¼ 0.88 at RT.

System UP/% sP/ms UISC/% kP/s � 1 knr/s � 1 kq/s � 1 kTTS /s - 1 k0TS/s - 1 DEq/eV Pq/ DETS/eV PTS/
Br6A 15.6 2.08 43.3 173.5 308.0 223.5 41.5 39.4 0.311 4.74 � 10 7 0.077 8.59 � 10 2

DA1 26.0 2.51 40.6 255.1 143.5 65.7 61.2 15.2 0.152 269 � 10 4 0.044 3.45 � 10 2

DA1, diene-reactive phosphor; kq, rate contant for quenching processes; knr, rate contant for nonradiative decay; kP, rate constants for phosphorescence; RT, room temperature;
Measured FP, tP and FISC, and the calculated kP, knr, kq and temperature-(in)dependent intersystem crossing from T1 to S0 (kTTS and kTS0 ). Activation energies (DETS and DEq) and pre-exponential factors
(PTS and Pq) are extracted from Arrhenius plots of kq versus T and kTS versus T based on equations (5) and (6), respectively.

solution. Notably, FP was measured to be 13% for the
DA1-doped PFMA film, which is ca. 2.5 times larger than that
of the Br6A-doped PFMA film having no covalent linkages
(FP ¼ 5%; Fig. 3d). As discussed in more detail below, we
attribute this large enhancement in FP to the fact that covalent
linkages restrict the molecular motions in the proximity
of embedded phosphors and hence effectively suppress the
radiationless relaxation pathways.

We first investigated the effect of molecular weight of
phosphors on the emission efficiency to confirm that the
enhancement of FP purely originates from the formation of
cross-linking. To study the effect of molecular weight, we
prepared DA1 analogues, DA2 and DA3, through side-chain
modification of DA1, while maintaining the core structure of
DA1 (Supplementary Fig. 4). Newly synthesized DA2 and DA3
with the extended length of alkyl chain have a larger molecular

weight than DA1. As shown in Supplementary Fig. 4, in the crosslinked systems phosphorescence quantum efficiency decreases
with increasing molecular weight. On the other hands, DA1
showed much higher efficiency than that of Br6A, although
DA1 has a larger molecular weight than Br6A. Thus, we
could conclude that molecular weight does not affect the
phosphorescence efficiency.

The rigidity of glassy polymers is usually represented by Tg
(ref. 31). We therefore further investigated the effect of Tg of the
polymer matrix on the phosphorescence of phosphor-doped
polymer films by synthesizing ene-reactive random copolymers,
P(FMA-r-MMA), comprising FMA and MMA with various mole
ratios, allowing for specific control of Tg of the synthesized
copolymers (Fig. 3d–f and Supplementary Fig. 5). Tg are higher in
copolymers with a larger MMA mole ratio (wMMA ¼ number of
MMA/number of MMA and FMA) due to the fact that the

relatively short and stiff structure of MMA limits the mobility of
polymer chains 32 . Figure 3d,e summarize the measured FP and tP
of Br6A- and DA1-doped P(FMA-r-MMA) films with various
wMMA. As the wMMAincreases, the measured FP and tP increase
in both Br6A- and DA1-doped P(FMA-r-MMA) films,
corresponding to a decrease of the non-radiative decay,
consistent with the increase of Tg. Remarkably, the measured
FP of DA1-doped P(FMA-r-MMA) films is ca. two times larger
than that of Br6A-doped P(FMA-r-MMA) films at all wMMA,
which correlates well with the increase of Tg in that range (it is
noteworthy that the exceptionally low FP of DA1-doped PMMA
film arises from the activation of PeT process by the maleimide
moieity; see section ‘Photophysics of DA1 in solution’). It
should be stressed that FP of the DA1-doped P(FMA-r-MMA)
film for wMMA ¼ 0.88 reaches ca. 28%, which is comparable
to FP of crystals of phosphorescent materials reported in the
literature 10,14–16 . However, the increased Tg cannot fully
explain the enhancement of FP, because FP of the DA1-doped
P(FMA-r-MMA) films for wMMA ¼ 0.72 and 0.88 is higher than
that of the Br6A-doped PMMA film having higher Tg (Fig. 3d,f),
indicating that the molecular motions in the vicinity of phosphors
are of high importance.

To examine our hypothesis, we measured the FP of DA1doped P(FMA-r-MMA) with different doping concentration of
DA1. As DA1 successfully forms cross-links with polymer matrix
through Diels–Alder click reaction, we can control the extent of
cross-links by changing the doping concentration of DA1. Br6Adoped P(FMA-r-MMA) films were prepared for comparison. As
shown in Supplementary Fig. 3, the phosphorescence quantum
efficiency decreases with increasing doping concentration of DA1.
The aggregation-induced emission quenching by increased
doping concentration may affect dominantly the emission
efficiency (based on the results of Br6A, we clearly conclude that
aggregation-induced quenching is the main reason for the
reduction of phosphorescence quantum yield) and further
increase of cross-links (the increase of matrix rigidity) merely
affect the phosphorescence efficiency. This result also indicates
that rigidity in the proximity of phosphors is much more
important than the matrix rigidity represented by the glass
transition temperature.

To further confirm our argument, we additionally measured
FP of Br6A (1.0 wt% of Br6A for polymers) and M2 (0.72 wt% of
M2 for polymers)-doped P(FMA-r-MMA) films (Fig. 3d). M2
only acts as a cross-linker, as the core structure of DA1 is not
incorporated in M2. As shown in Fig. 3d, Br6A and M2-doped
P(FMA-r-MMA) films showed almost the same value of FP with
Br6A-doped P(FMA-r-MMA) films at all wMMA, which is much
smaller than DA1-doped P(FMA-r-MMA). Therefore, we clearly
conclude that the restricted molecular motions in the vicinity of
phosphors by cross-linking between phosphors and polymer
matrix is the most important factor to suppress the radiationless
decay and hence, enhance the phosphorescence efficiency.

Excited state kinetics analysis. To fully understand the underlying principles of FP enhancement of DA1-doped PFMA and
P(FMA-r-MMA) films, we performed temperature (T)-dependent measurements of the photoluminescent properties for Br6Aand DA1-doped P(FMA-r-MMA) films of wMMA ¼ 0.88, as
shown in Fig. 4a–c. We analysed the characteristics of the photophysical processes from the measured photophysical values of
FP, tP and FF, as a function of T. To obtain the quantum efficiency for ISC from S1 to Tn (FISC), the value of (1–FF)/FF for the
Br6A- and DA1-doped P(FMA-r-MMA) films is plotted as a
function of T (Fig. 4d). The resulting plots can be fitted well by a
bi-exponential decay model, allowing for the extraction of the

ratio of the rate constants for IC (kIC) and ISC from S1 to Tn
(kISC). Then, FISC can be determined at a given temperature
based on equation (1) (Fig. 4e; the detailed procedure for
extracting the ratio kIC/kISC by curve fitting and deriving the
equation (1) are given in the Supplementary Information).

1 � FFðTÞ
FISCðTÞ ¼ ð1Þ
1 þ ðkICðTÞ=kISCðTÞÞ

Having FISC at hand, the rate constants for phosphorescence
process (kP) and non-radiative deactivation processes (knr) are
then obtained through equations (2) and (3) (Fig. 4f and Table 1).

FPðTÞ
kPðTÞ ¼ ð2Þ
FISCðTÞ � tPðTÞ

1
knrðTÞ ¼ tPðTÞ � kPðTÞ ð3Þ

Interestingly, the calculated non-radiative deactivation of the
triplet at RT is much smaller in the DA1-doped P(FMA-r-MMA)
film (knr ¼ 143.5 s � 1 ) compared with the Br6A-doped P(FMA-rMMA) film (knr ¼ 308.0 s � 1 ), indicating that the enhancement of
FP by cross-linking is mainly due to the suppression of the
radiationless relaxation pathways (Table 1). Although the reduced
non-radiative rate is the major contributor to the phosphorescence enhancement, contribution of an increased radiative rate is
also non-negligible (Table 1; the contribution of radiative and
non-radiative processes on the phosphorescence enhancement is
calculated in the Supplementary Information). The reason for the
increase in kP cannot be found in the nature of T1, which is
suggested to be very similar in energy (both from experiment and
calculations), nor in the electronic composition (CI and MO
topology). Thus, the most probable reason for the difference is the
screening by the local environment, as in a first approximation
the radiative rate should scale with n 2 (with n being the refractive
index), which should be higher in DA1 due to the presence of
polar maleimide groups.

To analyse the radiationless deactivation processes in more
detail, the non-radiative decay for the Br6A- and DA1-doped
P(FMA-r-MMA) films are plotted as a function of T (Fig. 4f),
which displays a clear bi-exponential decay, allowing for the
extraction of the rate constant for ISC from T1 to S0 (kTS) and the
rate constant for the quenching processes of triplet state of an
embedded phosphor by interaction with the host matrix and/or
oxygen (kq) based on equations (4–6) (Fig. 5a,b).

knrðTÞ ¼ kqðTÞ þ kTSðTÞ ð4Þ

kqðTÞ ¼ Pq � e � DEq=kBT ð5Þ

where kTST is the rate constant for T-dependent ISC from T1 to S0,
kTS0 is the rate constant for T-independent ISC from T1 to S0, Pq
and DEq are the pre-exponential factor and activation energy of
the quenching processes, respectively, PTS and DETS are the preexponential factor and activation energy of T-dependent ISC
from T1 to S0, respectively, and kB is the Boltzmann constant. As
detailed below, kTS contributes mainly to the radiationless
processes at lower temperatures, whereas kq dominates at higher
temperatures.

At RT, kq might include quenching processes by endothermic
triplet–triplet ET from phosphors to the host matrix 21,23, through
aggregation formation, or by the presence of oxygen 2 . However,
in this study we could minimize the quenching processes by
oxygen and aggregation, because the experiments were conducted
under inert atmosphere and at low doping concentration
(ca. 1 wt% of phosphors for copolymers). Thus, kq should be

kTSðTÞ ¼ k T

T TS þ k0

0 TS ¼ PTS � e � DETS=kBT þ k0

TS 0 ð6Þ

a b c

Br6A
DA1

50 100 150 200 250 300

_T_ (K)

Br6A
Fit curve

DA1
Fit curve

_k_ q dominant _k_ TS dominant

2 4 6 8 10 12 14

100

Br6A-doped (1.0 wt%)
DA1-doped (1.2 wt%)

In P(FMA- _r_ -MMA) at _�_ MMA = 0.88

Br6A
DA1

0
50 100 150 200 250 300

_T_ (K)

0
50 100 150 200 250 300

_T_ (K)

d e f

10 3

10 2

10 1

Br6A
DA1

2 4 6 8 10 12 14

10 2

10 1

Br6A
Fit curve

DA1
Fit curve

kIC dominant _k_ ISC dominant

2 4 6 8 10 12 14

100

1,000/ _T_ (K –1 ) 1,000/ _T_ (K –1 ) 1,000/ _T_ (K –1 )

Figure 4 | Temperature-dependent measurements of photophysical properties of Br6A- and DA1-doped P(FMA-r-MMA) blend films. Temperaturedependent plots of Br6A- (blue dots) and DA1-doped (red dots) P(FMA-r-MMA) blend films at wMMA ¼ 0.88 for (a) FP, (b) tP, (c) fluorescence quantum
yield (FF) and (e) ISC quantum yield (FISC) are shown. (d) Temperature-dependent plots of Br6A- (blue dots) and DA1-doped (red dots) P(FMA-r-MMA)
blend films at wMMA ¼ 0.88 for (1–FF)/FF are shown. The fitted curves (blue and red lines, respectively) by a bi-exponential decay model are shown
together. (f) The non-radiative decay rates (knr) of Br6A- (blue dots) and DA1-doped (red dots) films calculated from equations (2) and (3) are plotted
versus 1,000/T. The fitted curves (blue and red lines, respectively) by a bi-exponential decay model are shown together.

a b

10 8

Δ _E_ q

_k_ B

10 3

10 2

10 7

10 6

10 5

10 4

10 3

10 2

y-intercept = _P_ q

Collision frequency

Slope = –

_k_ q (Br6A)

_k_ q (DA1)

y-intercept = _P_ TS
Vibronic mixing

Slope = –

_k_ _T_ TS (Br6A)

_k_ 0 TS

Δ _E_ TS

_k_ B

_k_ T TS (DA1)

_k_ 0 TS

10 1 10 1

0 2 4 6 8

1,000/ _T_ (K –1 ) 1,000/ _T_ (K –1 )

Figure 5 | The Arrhenius plots of kTS and kq versus 1,000/T. (a) kq and (b) kTTS (solid lines) and kTS0 (dotted lines), extracted by fitting curves of knr versus
T (Fig. 4f), are plotted versus 1,000/T. Pre-exponential factors (Pq and PTS) and activation energy (DEq and DETS) of Dexter-type triplet ET process (left)
and ISC from T1 to S0 (right) were determined by equations (5) and (6), respectively.

caused by Dexter-type ET from T1 of the phosphors to triplet
states of the polymer matrix and to small amount of residual
oxygen (o0.1 p.p.m.) via collision between the reacting partners.
Notably, the evaluated kq at RT is greatly reduced in the DA1doped P(FMA-r-MMA) film (kq ¼ 65.7 s � 1 ), being 3.4 times
smaller than that of the Br6A-doped P(FMA-r-MMA) film

(kq ¼ 223.5 s � 1 ; Fig. 5a and Table 1). The large reduction of
kq(RT) is mainly attributed to a decrease in the pre-exponential
factor (Pq; Fig. 5a and Table 1) by three orders of magnitude, as
evaluated from Arrhenius plots of kq versus T based on
equation (5), which represents the collision frequency according
to diffusion-controlled triplet ET theory 33 . We could clearly

conclude that the restriction of diffusion/translational motion of
phosphors and of polymer chains effectively suppresses the
Dexter-type triplet ET process by minimizing the number of
collisions between reacting partners and hence significantly
increases FP.

The activation energies of Dexter-type triplet ET (DEq) can be
determined from the slope of the Arrhenius plot of equation (5)
(kq versus T; Fig. 5a and Table 1). However, the calculated
activation energies (0.311 and 0.152 eV for Br6A and DA1,
respectively) are much smaller than the difference between the T1
energy of the phosphors (ca. 2.36 and 2.42 eV for Br6A and DA1,
respectively) and T1 energy of the polymer matrices (ca. 3.1 and
3.85 eV for PMMA 22 and 2-methylfuran 34, respectively). We thus
hypothesize that quenching may occur via a dark CT state formed
between the phosphors and the carbonyl groups of polymers, as
previously described in literature 35 .

Interestingly, the pre-exponential factor (PTS) of the DA1doped P(FMA-r-MMA) film evaluated from the Arrhenius plots
of kTS versus T based on equation (6) is ca. 2.5 times smaller than
that of the Br6A-doped P(FMA-r-MMA) film (Fig. 5b and
Table 1). The T-independent ISC process (k0TS) is also
significantly suppressed in the DA1-doped P(FMA-r-MMA) film
(Fig. 5b and Table 1). We believe that the restriction of specific
vibrational motions (for example, out-of-plane vibrational modes,
vide infra) of the phosphor that are effective in mixing of zeroorder electronic states of T1 and S0 (that is, vibronic mixing
through Herzberg–Teller vibronic coupling and/or diabatic (nonBorn–Oppenheimer) coupling caused by the nuclear momentum 36–39 ), by cross-linking, may be attributed to the decrease in
PTS and k0TS. In fact, recent quantum chemical calculations of a
related molecule (that is, psoralen) indicate that out-of-plane
vibration modes allow for efficient ISC from a pp*-type singlet to
a pp*-type triplet, driven by Herzberg–Teller vibronic coupling,
which is in good support of our argument 38 . Despite the
meaningful reduction in PTS and k0TS, kTS(RT) did not

_n_ _m_ _n_ _m_

_n_ _m_

O O

HN O

O O

H2N O

P(FMA- _r_ -NiPAM) O P(FMA- _r_ -S) O P(FMA- _r_ -AM)

_n_ _m_

O O

_n_ _m_

O N O

_n_ _m_

O O

P(FMA- _r_ -AP) P(FMA- _r_ -AN) P(FMA- _r_ -VBC)

40
Br6A

DA1

Figure 6 | Phosphorescence QY of various Br6A- and DA1-doped
copolymers. (a) Chemical structures of various copolymers are shown.
(b) Phosphorescence QYof various Br6A- (sky blue) and DA1-doped (pink)
copolymers are presented. DA1-doped copolymers show higher
phosphorescence QY than those of Br6A-doped copolymers.

significantly change due to an unexpected decrease of DETS in
the DA1-doped P(FMA-r-MMA) film. Further optimized
material development through elaborate engineering of DETS is
required to additionally enhance FP.

Phosphorescent properties in a variety of copolymers. To test
the applicability of our approach, we designed a series of copolymers containing a variety of monomers (Fig. 6a). A full
description of the syntheses and characterizations of designed
copolymers is described in the Supplementary Information. As
anticipated, DA1-doped copolymers all exhibited significantly
enhanced FP compared with Br6A-doped copolymers (Fig. 6b),
indicating that our method can be generally applied for enhanced
RTP of metal-free organic materials.

Discussion
In summary, we have developed a new design rule for highly
efficient RTP in metal-free organic materials. Covalent linking
between phosphors and a polymer matrix not only provides
convenient access to enhanced RTP of metal-free organic
materials in a variety of polymer matrices but also enables better
fundamental understanding of the influence of molecular
motions on the radiationless relaxation pathways. Our experimental and theoretical study might allow for the development of
next-generation organic phosphorescent materials that may give
new impetus to the field of organic optoelectronics, bio-imaging,
sensing and data security.

Methods
Preparation of Br6A- and DA1-doped polymer films for photophysical
measurements at RT. PMMA, PFMA, P(FMA-r-MMA), P(FMA-r-S), P(FMA-rNiPAM), P(FMA-r-AM), P(FMA-r-AP), P(FMA-r-AN) and P(FMA-r-VBC)
(1 wt%) were dissolved in chloroform (CHCl3) and mixed with the phosphor
(1.0 wt% of Br6A and 1.2 wt% of DA1 for polymers, respectively). The mixed
solutions were drop-cast on a pre-cleaned glass substrate and kept at RT for
10 min. The resulting drop-cast films were thermally annealed at 120 �C for 20 min
and kept in a vacuum chamber for 30 min, to completely remove residual solvent
and oxygen. The resulting polymer films were packaged by attaching a glass lid to
the polymer films using an epoxy seal (EPOXY TECHNOLOGY 305) around the
perimeter and kept at RT for 24 h. All processes from drop-casting to packaging
were done in a nitrogen-filled glove box.

Preparation of Br6A- and DA1-doped polymer films for temperaturedependent measurements. P(FMA-r-MMA) (1 wt%) with wMMA ¼ 0.88 was
dissolved in chloroform (CHCl3) and mixed with the phosphor (1.0 wt% of Br6A
and 1.2 wt% of DA1 for polymers, respectively). The mixed solutions were
drop-cast on sapphire substrates and kept at RT for 10 min. The resulting drop-cast
films were thermally annealed at 120 �C for 20 min and kept in a vacuum chamber
for 30 min, to remove residual solvent and oxygen.

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Acknowledgements
We gratefully acknowledge useful discussions with Kyungmin Lee at Cornell University.
This work was supported by the National Science Foundation (DMREF DMR 1435965)
and Samsung GRO grant. The work in Madrid was supported by the Spanish Science
Ministry (project MultiCrom, grant number CTQ2014-58801) and by the Campus of
International Excellence (CEI) UAM þ CSIC.

Author contributions
M.S.K. and J.K. conceived the project. M.S.K. designed and performed most of experiments. M.S.K., J.G. and J.K. analysed the data. M.S.K., Y.Y., A.W.P., J.J. and J.H.Y carried
out the syntheses. M.S.K., C.C. and S.R.F. conducted temperature-dependent measurements. K.C., A.S., G.K. and K.P. performed differential scanning calorimetry measurements. J.G. performed DFT calculations. M.S.K. prepared the manuscript. J.G. and J.K.
revised the manuscript. All the authors discussed the results and commented on the
manuscript. J.K. supervised the project.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
[naturecommunications](http://www.nature.com/naturecommunications)

Competing financial interests: The authors declare no competing financial interests.

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How to cite this article: Kwon, M. S. et al. Suppressing molecular motions for enhanced
room-temperature phosphorescence of metal-free organic materials. Nat. Commun.
6:8947 doi: 10.1038/ncomms9947 (2015).

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