Fast and long-range triplet exciton diffusion in metal–

1

Fast and long-range triplet exciton diffusion in metal–

organic frameworks for low power photon upconversion

Prasenjit Mahato,

Angelo Monguzzi,

Nobuhiro Yanai,

Teppei Yamada,

and Nobuo

Kimizuka

1

Department of Chemistry and Biochemistry, Graduate School of Engineering, Center

for Molecular Systems (CMS), Kyushu University, Fukuoka-819-0395, Japan.

2

Dipartimento di Scienza dei Materiali, Università degli Studi Milano-Bicocca, via R.

Cozzi 55, 20135 Milano, Italy.

3

JST, PRESTO, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.

*

Correspondence and requests for materials should be addressed to N.Y. (email:

yanai@mail.cstm.kyushu-u.ac.jp) or to N.K. (e-mail: n-kimi@mail.cstm.kyushu-u.ac.jp).

2

Abstract:

The controlled generation and conversion of excited triplet states play a crucial

role in many areas of photochemistry and photophysics, as represented by triplet-triplet

annihilation-assisted photon upconversion (TTA-UC). Although the TTA-UC has

emerged as a promising wavelength-shifting technology, its application has been

significantly hampered by the slow diffusion of excited triplet molecules in solid matrices.

Here, we introduced metal–organic frameworks (MOFs) to promote TTA-UC by taking

advantage of triplet exciton migration among the regularly aligned emitter chromophores

having a controlled spatial orientation. We synthesized anthracene-containing MOFs with

different chromophore orientations, and the analysis of TTA-UC emission kinetics revealed a high triplet diffusion rate and a remarkably large diffusion length of 13 m.

This allowed the construction of molecular-diffusion-free solid-state upconverter by

donor modification on MOF surfaces, which shows a near 100% TTA efficiency under

an excitation intensity weaker than solar irradiance and a reasonably high UC quantum

yield of 2.5% at low excitation power. Here, we show that efficient triplet energy

migration and upconversion in defined MOF architectures lay the foundation for triplet

exciton engineering, which will be useful in many disciplines.

3

Main text:

The capability to control triplet exciton dynamics in the solid state is crucial to

obtain high performances in various organic devices

One of the important applications

involves photon upconversion based on triplet–triplet annihilation (TTA-UC), which has

recently been recognized as a promising wavelength-shifting methodology for enhancing

the efficiency of sunlight-powered devices, such as photovoltaics and photocatalysts

.

As schematically shown in Figure 1, the collision of two acceptor (emitter) triplets,

A*,

populated by triplet–triplet energy transfer (TTET) from donor (sensitizer) molecules,

3

D*, leads to TTA within their lifetimes. The annihilation generates a higher energy

excited singlet state,

A*, from which upconverted fluorescence is obtained. Because the

TTA process proceeds via an electron exchange mechanism, the excited triplet molecules

need to come to within a distance of 1 nm to allow the overlap of each molecular

orbital

. To achieve TTA-UC in the solid state, conventional approaches use rubbery

polymer matrices and rely on the diffusion of excited triplet molecules therein. The

polymers inevitably restrain the diffusion of any embedded dye molecules

and

consequently, an undesirably high incident light intensity exceeding solar irradiance (1

sun) is required to maximize the UC efficiency.

To solve these fundamental problems in TTA-UC, we propose a triplet energy

4

harvesting approach that employs regularly preorganized chromophores. To achieve

efficient UC even under low excitation photon densities below 1 sun

, we introduce fast

triplet energy migration among aligned acceptor molecules, which is a feature distinct

from classical molecular diffusion-based TTA-UC. In an organic crystalline lattice,

photogenerated excitons generally move by hopping between resonant energy centres

(Frenkel-type exciton). Triplet diffusion in a crystal depends on the overlap of wave

functions and the highest diffusivity for triplet excitons would only be realized under the

condition where the chromophores are regularly aligned in a crystalline order with

properly controlled mutual orientation

. However, to date, efficient low power TTA-UC

has not been achieved in crystals

and there are no clues concerning how to achieve

it.

Here, we show the first example of TTA-UC utilizing crystalline metal–organic

frameworks (MOFs) that contain emitter chromophores as bridging ligands (Fig. 1b).

MOFs offer the advantages of designability, structural regularity, and rigidity

, and the

use of bidentate ligands that contain an acceptor group allows us to control their

intermolecular distance, mutual orientation, and consequent orbital overlap precisely

.

Thus, it is expected that MOFs will serve as an ideal platform to reveal the relationship

between dye arrangements and triplet exciton dynamics that lead to TTA-UC. With this

5

in mind, we prepared MOFs with different acceptor alignments and sensitize triplets in

MOF crystals by the TTET from a donor. The sensitized triplet excitons moved within the acceptor crystalline network with a high diffusivity of up to 2.5  10

cm

s

, with a surprisingly large diffusion length of 13

constructed by modifying donor molecules on the MOF surfaces, which lead to a

quantitative TTA yield and reasonably high UC efficiency at low excitation intensity in

the solid state.

Results and Discussion

Preparation and characterization of regular emitter arrays in MOFs. The structure–

property relationships were systematically investigated by controlling the spatial

arrangements of the acceptor ligand in the MOFs. According to the bench-mark TTA-UC

combination of the emitter 9,10-diphenylanthracene (DPA) and sensitizer Pt(II)

octaethylporphyrin (PtOEP), we employed 4,4-(anthracene-9,10-diyl)dibenzoate (ADB) as an emitter ligand. We synthesized a new MOF using 4,4-bipyridine (bpy), [Zn

(adb)

bpy]

(1), and prepared a MOF with 1,4-diazabicyclo[2.2.2]octane (dabco),

[Zn

(adb)

dabco]

(2), which was originally reported by Kaskel et al

. A new MOF

without a co-ligand, [Zn(adb)(DEF)

]

(3, DEF, N,N-diethylformamide), was also

6

synthesized. The purity of the MOF samples was confirmed from elemental analysis and

X-ray powder diffraction (XRPD) measurements (Supplementary Fig. 1). Figure 2 shows

the MOF structures determined from single-crystal X-ray diffraction measurements. The

basic three-dimensional connectivity of MOFs 1 and 2 is similar, with the ADB ligands

coordinated to pairs of Zn

ions to form two-dimensional sheets that are pillared by either

bpy or dabco units. The three-dimensional frameworks in 1 and 2 interpenetrate in a

threefold and twofold fashion, respectively. In MOF 3, the Zn

ions are connected by

ADB ligands to form one-dimensional chains. The closest centre-to-centre distances

between the anthracene moieties are 5.0 Å and 7.5 Å for 1, 4.8 Å for 2, and 9.0 Å for 3.

The MOF crystals were dispersed in benzene and the fluorescence spectra were

measured under an excitation wavelength of 365 nm (Supplementary Fig. 2). The

fluorescence spectra of these compounds exhibited an emission maximum at 440 nm,

which did not show any significant changes when compared with a benzene solution containing the ligand alone, 4,4-(anthracene-9,10-diyl)dibenzoic acid (ADBA). This indicates that the singlet energy level of the ADB chromophore is basically preserved in

these MOFs, which would also be the case for the triplet level. Fluorescence decay

measurements were carried out on ADBA and MOF crystals in benzene (Supplementary

Fig. 3). The photoluminescence of an ADBA solution at 440 nm showed a single

7

exponential decay with a characteristic lifetime of  = 4.1 ns. These MOFs showed multi- exponential decays with shorter lifetimes, which is a common feature of the condensed

molecular systems. A singlet excitation energy migrating in a molecular assembly is often

trapped by defects, where the excitation energy is then dissipated through non-radiative

decay paths

. These results support the notion that the fluorescence of the MOF

dispersions originates from the ligands in the crystalline frameworks, and not from any

ligands that have leached out in the solution. The absence of any leached ligands in the

solution was further confirmed by centrifugation of the benzene dispersions to remove

any MOF crystals, where the supernatant liquid showed no detectable fluorescence. The absolute fluorescence quantum yield 

of MOFs dispersed in benzene was determined to be 19%, 17%, and 45% for MOFs 1, 2, and 3, respectively.

Low power TTA-UC from fast and long-range triplet exciton diffusion in MOFs. A

series of spectroscopic studies were carried out on deaerated samples to investigate the

triplet migration properties of each MOF structure. The MOF crystals were dispersed in

a benzene solution containing the sensitizer PtOEP and the mixtures were deaerated using

repeated freeze–pump–thaw cycles ([PtOEP] = 18-30

occurrence of donor-to-acceptor TTET was verified by monitoring the donor

8

phosphorescence lifetimes. The phosphorescence decay at 645 nm of 100 M PtOEP in deaerated benzene showed a single exponential decay with a lifetime of

= 85

(Supplementary Fig. 4). In the presence of MOF crystals, the phosphorescence lifetime,, decreased and the energy transfer efficiency, 

= 1 – /

were determined to be 12%, 8%, and 59% for MOFs 1, 2, and 3, respectively. Although the value of

can be

improved by controlling factors such as crystal size, morphology, and surface

functionality of the MOFs, we did not pursue this line of work further, because the current

transfer efficiency was high enough to study the intrinsic triplet properties in the MOFs.

Interestingly, all the MOF dispersions showed upconverted blue emission around

440 nm on irradiation with 532 nm light (Figs. 3a-3c). Although studies on triplet

diffusion and light harvesting in MOFs have been reported previously by Lin et al. and

Hupp et al.

, TTA-UC in MOFs has been unprecedented to date. Note that the donor

PtOEP was not included in the MOF pores, as confirmed by the filtration test; the benzene

dispersion of MOF and PtOEP was incubated for 12 h, filtrated, and re-dispersed in

deaerated benzene, and the obtained re-dispersed samples did not show any PtOEP

phosphorescence at 645 nm nor upconverted emission at 440 nm (Supplementary Fig. 5).

The analysis of TTA-UC kinetics as a function of excitation power density provides an

important parameter, the threshold excitation intensity (I

), which classifies the

9

performance of a given TTA-UC system

. The upconverted emission intensity shows

a quadratic power dependence on the excitation power density, which changes to a linear

dependence beyond I

. In the low power excitation regime, acceptor triplets decay

spontaneously, and this results in a quadratic dependence of the UC luminescence

intensity relative to the incident light intensity. This is typically observed for two-photon

processes, such as TTA

. Meanwhile, at power excitation regimes greater than I

, TTA

becomes a main triplet decay channel, and consequently, the UC luminescence changes

linearly with the excitation intensity. In this high-intensity regime, the upconversion quantum yield 

reaches its maximum. At I

, the spontaneous decay rate of excited acceptor triplets becomes equal to the TTA rate

. To realize the highest 

value under

a weak excitation power, such as in sunlight, it is essential that the I

values are small,

having a magnitude of a few mW cm

. Figure 3d shows a log-log plot of the UC emission

intensity as a function of the excitation power density. A transition from quadratic (slope

= 2) to linear (slope = 1) was observed for all the MOF dispersions. Interestingly, the

measured I

values of 2.4, 1.5, and 1.5 mW cm

for MOFs 1, 2, and 3, respectively, are

lower than those reported for the polymer-based systems (4.3

20 mW cm

)

, and are

close to the values using solar irradiance of 1.6 mW cm

at 532

5 nm. On further

increasing the excitation power well above I

, the UC emission intensity deviates from a

10

linear relationship, suggesting the appearance of additional decay channels competing

with the TTA. These additional decay channels are expected from homo-molecular

annihilation between donor triplets (Supplementary Fig. 5). However, as this process

takes place well above the solar irradiance and the observed I

values, it does not affect

the performance of MOFs in low power UC systems.

The observed low I

values of the MOFs were further scrutinized by quantifying

the triplet exciton diffusion constants, D

, in the crystalline frameworks. The threshold is

expressed as a function of the system’s fundamental parameters using

𝐼

= (𝛼𝛷

𝛾

)

(𝜏

)

, (1)

where

,

is the lifetime of the acceptor triplet, and 

The triplet diffusion constant was calculated from the above using

𝐷

= (𝛾

)(8𝜋𝑎

)

, (2)

where a

is the annihilation distance of the triplets (9.1 Å for DPA triplets)

. These

parameters of the three MOF systems are summarized in Supplementary Tables 1-3.

The

values were measured from the decay time of the upconverted emission by considering the relationship I

(t)

) = exp(–2t/

) in the longer timescale

region, when the annihilation efficiency becomes negligible compared with the

11

spontaneous decay of the triplets (Supplementary Figs. 8, 10, and 12)

. From our

calculations, the triplet diffusion constant, D

, and the length, L

= (D

, in each MOF

material were obtained. We repeated the experiments using the MOFs synthesized in

different batches, and repeatedly observed the similar D

and L

values, confirming the

good reproducibility of the measurements (Supplementary Figs. 7-12, Tables 1 and 2).

Averaged D

values of 1.2  10

, 2.4  10

, and 1.6  10

cm

s

were obtained for

MOF 1, 2, and 3, respectively, and averaged L

values were 9.4, 13, and 4.0 m for MOF

1, 2, and 3, respectively. Significantly, all the D

values in MOFs were larger than the

diffusivity of DPA in the low viscosity solvent (1.2

10

cm

s

)

. MOF 2 showed the

best diffusivity among the three MOFs, which would be due to the largest overlap

between the anthracene rings with a short intermolecular distance of 4.8 Å. In contrast,

MOF 3 showed a smaller D

value compared with that observed for MOFs 1 and 2, which

is ascribed to the large intermolecular displacement (9.0 Å) that made the interaction between adjacent anthracene moieties weaker. There are – stacking interactions with an interplane distance of 3.2 Å between the edges of the anthracene rings in MOF 3, but

the overlap between the anthracene rings is small. These comparisons highlight the

importance of controlling the intermolecular distance, spatial organization, and overlap

of the  orbitals to achieve an efficient triplet exciton diffusion. The overlap between the

12

aromatic planes seems to be the key factor for enhancing triplet diffusivity. Although it is better for the inter-plane distance to be short, the presence of strong – stacking is not an essential element. The significantly long, micrometer-scale diffusion lengths were

observed for the MOF crystals. This extremely long exciton diffusion length was realized

by a combination of the long triplet exciton lifetime in the rigid framework and the fast

triplet migration within the chromophores, whose spatial alignment was controlled

properly. The triplet diffusion in MOFs 1 and 2 is faster than that reported for pure

anthracene (1.5

10

cm

s

)

, and their diffusion lengths are longer than that

exhibited by rubrene crystals (L

= 5

. However, it needs special attention to

compare the diffusivities with these previous reports, because they were obtained by the

different experimental methods and the diffusion constant and length may depend on the

employed techniques

.

The TTA-UC quantum yield, 

, was then determined using the DPA

PtOEP mixed solution as a standard. Note that the TTA-UC process uses two photons to produce

one photon, and therefore the theoretical maximum of 

is considered as 50%

. The

UC

values obtained for MOFs 1, 2, and 3 were 0.43%, 0.34%, and 4.3%, respectively, in the high excitation regime where the conversion yield is constant (slope =1 for the data

in Fig. 3d). To extract the intrinsic parameters of the MOFs, these

values were

13

normalized to donor-to-acceptor energy transfer efficiency, 

. The normalized values of 3.6%, 4.3%, and 7.3% obtained for MOFs 1, 2, and 3 are comparable to other efficient

TTA-UC systems

. This was the first attempts to affirm the good potential of MOFs as

light-harvesting platforms for TTA-UC, and we presume that the UC efficiency can be

improved further by employing recently developed strongly emissive MOFs with higher

FL

values close to unity

.

Solid state TTA-UC with extremely low I

. Taking advantage of the effective triplet

exciton diffusion in the MOF structures, we successfully achieved a low power solid state

TTA-UC system without the help of molecular diffusion. The most important challenge

is the successful integration of the sensitizer and the MOF into a upconverting solid. To

achieve this, we modified crystal surfaces of MOFs with donor molecules (Fig. 4a). It has

been recently reported that surface carboxylate ligands of MOF crystals can be easily

exchanged by carboxylate groups attached to fluorescent dye

. In this sense, we modified

surfaces by dispersing the MOF particles in a solution of Pd(II) mesoporphyrin IX

(PdMesoIX) containing carboxylate groups (Fig. 4a). It is expected that the donor-to-

acceptor TTET takes place at the crystal surfaces, followed by triplet exciton migration

and TTA-UC in the nanocrystals. As MOF crystals, we employed MOF 3 because it has

14

the highest fluorescence quantum yield (45%) among the three MOFs. It would be better

to have a large surface-to-volume ratio for increasing the triplet concentration in crystals,

and thus we synthesized nano-sized MOF 3 crystals by changing the synthetic condition.

We found that the rapid mixing of a 100

·2H

O (47.4

mM) and a 3 mL DEF solution of ADBA (1.59 mM) at the room temperature for 30 sec

gave MOF 3 nanocrystals. Transmission electron microscopy (TEM) image of the

obtained sample showed that the crystal size is around 60 nm (Fig. 4b). A XRPD pattern

of this nano-sized MOF 3 showed the good agreement with the simulation pattern

obtained by the crystal structure of MOF 3 (Supplementary Fig. 13).

We modified surfaces by incubating the nanocrystals of MOF 3 (2.5 wt%) in a

DEF solution of PdMesoIX (1.4 mM) at the room temperature for 24 h, washed several

times with DEF using centrifugation, and dried under vacuum at the room temperature

.

This PdMesoIX-modified MOF 3 powder was sandwiched between quartz plates and

sealed under Ar atmosphere (oxygen concentration < 0.1 ppm). Upon excitation with 532

nm laser, this system showed nice upconverted emissions, but several repetitions of the

measurement gradually decreased the UC emission intensity, probably due to the local

heating and bleaching of the donor dyes. To overcome this effect, the donor-modified

nanocrystals were dispersed in poly(butyl acrylate) (PBA, M

= 99,000, T

= –49

15

with the expectation of heat transfer to the surrounding polymer. The PdMesoIX-modified

MOF 3 nanocrystals were added to a toluene solution of PBA, and this dispersion was

cast on a quartz slide, dried under vacuum, and then sealed in the Ar atmosphere. A stable

blue upconverted emission was observed from this solid sample upon excitation with 532 nm laser (Fig. 4c). The absorption coefficient  was obtained as 1.8 cm

from absorption spectra of PdMesoIX-MOF 3 in PBA (Supplementary Fig. 14). Donor phosphorescence

lifetime measurements showed that the surface-anchored PdMesoIX molecules could

transfer the triplet energy to the acceptor crystals with an energy transfer efficiency of

ET

= 37% (Supplementary Fig. 15a,b).

long times of UC emission under 532 nm pulsed excitation. The decay followed a single exponential function, and the characteristic decay corresponded to a triplet lifetime of 

= 5.8 ms, which is longer than the lifetime of MOF 3 in benzene (1.0 ms, Supplementary

Fig. 12). This might be attributed to the less efficient vibrational quenching of the

metastable excitons in the polymeric host, unlike in the organic solvent. Inserting the

experimental values into equation (1), we estimated a notably low I

of 0.047 mW cm

,

which is more than 30 times smaller than the solar irradiance value at the excitation

wavelength (1.6 mW cm

at 5325 nm). This I

value was too low to observe it directly

using our experimental set-up, but the UC emission intensity in the solid system obeyed

16

a linear dependency on the excitation power over the entire excitation intensity range

studied (Fig. 4d).

This behavior suggests the saturation of TTA-UC quantum yield 

at a low excitation power. We measured absolute UC quantum yield Φ

of the PdMesoIX-

modified MOF 3 nanocrystals embedded in PBA by using an integrating sphere and 532

nm laser as excitation source. The details of the measurement setup have been reported

previously

. Remarkably, we observed a reasonably high

of 2.5% at a low laser

power density around 5 mW cm

(Fig. 4e). It was difficult to precisely measure

below this laser intensity owing to the sensitivity limit of the measurement setup. The

UC

value remained constant with further increase of the excitation power, confirming the quantitative TTA process along with the low I

value. This saturation of 

at such

a low excitation power comparable to the solar irradiance has been unprecedented. The

long-time problem of solid state TTA-UC, i.e., too high I

values, was solved here by

combining the MOF surface modification with donor and the fast triplet exciton diffusion

in the optimized chromophore arrangements.

Conclusions:

By exploiting MOF crystalline systems, we successfully maximized UC

17

efficiency under excitation intensities weaker than solar irradiance. This breakthrough

was based on the ability to control the spatial organization of the acceptor molecules in

crystalline MOFs, which led to surprisingly fast and long-range triplet exciton diffusion.

Our results indicate that precise control of the overlap between the aromatic  planes is of paramount importance in rationally designing triplet energy-harvesting materials with

a high triplet exciton diffusivity. Based on the effective triplet diffusion in the MOFs, we

realized a quantitative TTA process in the solid state with an efficiency of 100% even

under excitation intensities much lower than solar irradiance, thus leading the way to

sensitized UC materials workable in solar energy technologies.

A number of promising directions for future work can be pursued to achieve the

theoretical maximum photon upconversion yield of 50% under solar irradiance. The donor-to-acceptor TTET efficiency, 

, can be improved by constructing MOFs with larger pore sizes and accommodating the donor molecules in the nanospaces. Together

with this technique, higher UC quantum yields can be obtained by employing MOFs with

enhanced fluorescence efficiency

. We envisage using recently developed MOF material

processing to finely control the crystal orientation and position, and therefore, tune the

exciton transport properties

. The fast triplet exciton diffusion in suitably designed

MOFs would offer a new avenue for MOF-based optoelectronics that would be applicable

18

in many research areas, including photovoltaics, photocatalysts, and light-emitting diodes.

19

Figures:

Figure 1 | A Schematic representation of TTA-UC by triplet exciton diffusion in MOFs. (a) Scheme for the mechanism of TTA-UC. A triplet state of donor ³D*, formed by intersystem crossing (ISC) from the photo-excited (green arrow) singlet state ¹D*, experiences triplet-triplet energy transfer (TTET) to an acceptor triplet ³A*. Two acceptor excited triplets annihilate to form a higher singlet energy level ¹A*, which consequently produces upconverted delayed fluorescence (blue arrow). (b) The self-assembly of bridging ligands containing an acceptor moiety (cyan) with metal ions and co-ligands forms crystalline MOF structures in which the acceptor units are regularly arranged. Excitation (green arrow) of donor molecules (pink) is followed by a sequence of TTET, triplet exciton diffusion in the acceptor arrays, TTA between the excited acceptors, and finally higher energy UC emission (blue arrow).

20

Figure 2 | Systematic tuning of acceptor arrangements in MOFs. The crystal structures of MOF (a) 1, (b) 2, and (c) 3. Arrangements of the acceptor ligand in MOF (d) 1, (e) 2, and (f) 3.

Hydrogen atoms and guest molecules are omitted for clarity.

21

Figure 3 | First observation of photon upconversion in MOFs. Photoluminescence spectra of MOF (a) 1, (b) 2, and (c) 3 dispersed in deaerated benzene solution of PtOEP with different incident power density of 532 nm laser. (d) TTA-UC emission intensity for the deaerated benzene dispersion of MOF 1 (black), 2 (blue), and 3 (red) with PtOEP as a function of the excitation intensity(λex = 532 nm). The linear fits with slopes 2 and 1 in the lower and higher excitation power regimes are shown. Intersections between the two lines provide the threshold excitation intensity (Ith).

22

Figure 4 | Molecular-diffusion-free solid-state TTA-UC with quantitative TTA efficiency from very low excitation power. (a) Schematic representation of molecular-diffusion –free TTA- UC in donor-modified MOF nanocrystals. The surface of MOF nanocrystals is modified with a carboxylate-containing donor (PdMesoIX). Excitation (green arrow) of donor molecules is followed by a sequence of TTET, triplet exciton diffusion in the acceptor arrays, TTA between the excited acceptors, and finally higher energy UC emission (blue arrow). (b) TEM image of the MOF 3 nanocrystals. (c) Photoluminescence spectra of the PdMesoIX-modified MOF 3 nanocrystals in PBA with different incident power density of 532 nm laser. (d) TTA-UC emission intensity for the PdMesoIX-modified MOF 3 nanocrystals in PBA as a function of the excitation

23

intensity(λex = 532 nm). The linear fit with a slope 1 is shown. (e) TTA-UC quantum yield UC

of the PdMesoIX-modified MOF 3 nanocrystals in PBA with different incident power density of 532 nm laser.

24

Methods:

Materials. All reagents and solvents were used as received otherwise noted. Pt(II)

octaethylporphyrin (PtOEP) and poly(butyl acrylate) (PBA) solution were purchased

from Sigma Aldrich. For spectroscopic measurements, we used spectral grade benzene

purchased from TCI, Japan.

Synthesis. The ligand 4,4’-(anthracene-9,10-diyl)dibenzoic acid (adba) and MOF 2 were

synthesized following the literature procedures

. The synthesis of MOF 1: a mixture of

adba (20 mg, 0.048 mmol), Zn(NO

)

·6H

O (14 mg, 0.048 mmol), 4,4’-bipyridine (bpy)

(3.75 mg, 0.024 mmol), and 2.5 mL N,N’-dimethylformamide (DMF) were placed in a

teflon autoclave and heated at 120 °C for 48 h, then cooled to room temperature at

1 °C/min. Obtained light yellow crystals were filtered and washed several times with

DMF and dried at the room temperature under vacuum. Yield = 20 mg (34.5%). Elemental

analysis for [Zn

(C

H

O

)

(C

H

N

)]·DMF·H

O. Calcd. (%): C 68.44, H 4.08, N 3.47;

found (%): C 67.94, H 4.08, N 3.52. The synthesis of MOF 3: A mixture of adba (20 mg,

0.048 mmol), Zn(NO

)

·6H

O (14 mg, 0.048 mmol), and 2.5 mL N,N-diethylformamide

(DEF) were placed in a teflon autoclave and heated at 120 °C for 24 h, then cooled to the

room temperature at 1 °C/min. Obtained light yellow crystals were filtered and washed

several times with DEF and finally dried at the room temperature under vacuum. Yield =

11.5 mg (35%). Elemental analysis for [Zn(C

H

O

)(C

H

NO)

]. Calcd. (%): C 66.72,

25

H 5.6, N 4.1; found (%): C 66.23, H 5.55, N 4.15.

Sample preparation for TTA-UC emission measurements. For liquid dispersion

samples, we synthesized MOF particles by using microwave instead of solvothermal

method since it gives better particle dispersibility probably due to the smaller crystal size.

A mixture of 4,4’-(anthracene-9,10-diyl)dibenzoic acid (adba), Zn(NO

)

·6H

O, co- ligand, and N,N’-dimethylformamide (DMF) at 120 °C for 3 h in microwave, washing several times with DMF, filtration through 200 nm filter to remove small crystals, and

subsequent drying under vacuum at the room temperature. The phase purity of the MOF

crystals was confirmed by the good match between XRPD patterns and simulation patters

obtained by the crystal structures (Supplementary Figure 1). The dried MOF crystals were dispersed in benzene solution of PtOEP ([PtOEP] = 18-30 M, [MOF] = 3 mg/mL). The dispersions were placed in a standard freeze-pump-thaw cell that connects a small Shlenk

flask with 1 mm quartz cell, and cleaned from oxygen by repeated freeze-pump-thaw

cycles. The triplet diffusion constant was obtained by using the equations (1) and (2)

.

The

light scattering from the MOF particles, we measured the absorption spectra after leaving

the degassed dispersion for longer than 12 hours to let the particles sediment to the bottom

(Supplementary Fig. 16 shows the typical spectra). The

value was obtained by

26

measuring the donor (PtOEP) phosphorescence lifetime without (

) and with () acceptor MOFs and using the equation 

= 1 – /

value can be determined as the intersection of the two fitting lines of slope 2 and 1 in the

low and high power regimes, respectively, in double logarithm plots for the UC emission

intensity as a function of incident light power density (Fig. 3, Supplementary Figs. 7, 8,

and 11). The triplet diffusion length was calculated using the relationship L

= (D

)

41

.

Measurements. Elemental analysis was conducted at the Elemental Analysis Center,

Kyushu University. Powder XRD analysis was conducted on a RIGAKU smart-lab with

a copper K-alpha source. Single crystal X-ray data were collected on a CCD

diffractometer (Rigaku Saturn VariMax) with graphite-monochromated Mo Ka radiation (λ = 0.71070 Å). UV−vis absorption spectra were recorded on a JASCO V-670 spectrophotometer at 25 °C. Quartz cell with 1 mm path length was used. Emission

spectra were recorded on PerkinElmer LS55 spectrophotometer with 1 mm path length

cell. The samples were excited with an incidence angle of 45º to the quartz cell surface

and the fluorescence was detected along the normal. Emission spectra were recorded with

excitation wavelength of 375 nm or 510 nm. The absolute quantum yields were measured

using a Hamamatsu C9920-02G setup with integrating sphere. Time-resolved

27

fluorescence lifetime measurements were carried out by using time-correlated single

photon counting lifetime spectroscopy system, HAMAMATSU Quantaurus-Tau C11367-

02 (for fluorescence lifetime)/C11567-01(for delayed luminescence lifetime). The up-

conversion luminescence emission spectra were recorded on Otsuka Electronics MCPD-

7000 instrument with the excitation source using an external, adjustable 532 nm

semiconductor laser.

The TTA-UC emission quantum yield (Φ

) was determined by relative

quantum yield measurement taking the PtOEP-DPA as a standard with its Φ

= 0.25

.

The following formula has been used by taking the intensity of converted light above the

threshold for both systems:

𝛷

= 𝛷

(

𝐼_𝑈𝐶

) (

𝐴_𝑈𝐶

) (

𝐸_𝑠𝑡𝑑

) (

𝜂_𝑠𝑡𝑑

)

where Φ, I, A, E, and η represent the quantum yield, excitation intensity at 532 nm,

absorbance at 532 nm, integrated photoluminescence spectral profile, and refractive index

of the solvent. The subscripts UC and std denote the parameters of the MOF and standard

systems.

Acknowledgments

This work was supported by a Grants-in-Aid for Scientific Research (S) (25220805), a

28

Grants-in-Aid for Young Scientists (B) (26810036), a Grant-in-Aid for Scientific

Research on Innovative Area (No. 26104529) from the Ministry of Education, Culture

Sports, Science and Technology of Japan, and the JSPS-NSF International Collaborations

in Chemistry (ICC) program. P.M. and A.M. acknowledge JSPS postdoctoral fellowships

for foreign researchers.

Author contributions

N.Y. and N.K. conceived and designed the project; P.M., A.M. and N.Y. performed the

experiments and analyzed the data; T.Y. assisted the crystallographic study; P.M., A.M.,

N.Y. and N.K. co-wrote the paper

Additional information

Supplementary information is available in the online version of the paper. Reprints and

permissions information is available online at www.nature.com/reprints. Supplementary

crystallographic data for this paper has been deposited at the Cambridge Crystallographic

Data Centre under deposition numbers CCDC 1030618 (MOF 1) and CCDC 1030619

(MOF 3). Correspondence and requests for materials should be addressed to N.Y. or N.K.

References:

1 Köhler, A. & Bässler, H. What controls triplet exciton transfer in organic semiconductors? J. Mater. Chem. 21, 4003-4011, doi:Doi

10.1039/C0jm02886j (2011).

29

2 Trupke, T., Green, M. A. & Würfel, P. Improving solar cell efficiencies by up-conversion of sub-band-gap light. Journal of Applied Physics 92, 4117- 4122, doi:doi:http://dx.doi.org/10.1063/1.1505677 (2002).

3 Baluschev, S. et al. Up-conversion fluorescence: Noncoherent excitation by sunlight. Phys. Rev. Lett. 97, 143903, doi:Artn 143903

Doi 10.1103/Physrevlett.97.143903 (2006).

4 Ginley, D., Green, M. A. & Collins, R. Solar energy conversion toward 1 terawatt. MRS Bull. 33, 355-364, doi:Doi 10.1557/Mrs2008.71 (2008).

5 Singh-Rachford, T. N. & Castellano, F. N. Photon upconversion based on sensitized triplet-triplet annihilation. Coordination Chemistry Reviews 254, 2560-2573, doi:10.1016/j.ccr.2010.01.003 (2010).

6 Cheng, Y. Y. et al. On the efficiency limit of triplet-triplet annihilation for photochemical upconversion. Phys. Chem. Chem. Phys. 12, 66-71, doi:Doi 10.1039/B913243k (2010).

7 Tanaka, K., Inafuku, K. & Chujo, Y. Environment-responsive upconversion based on dendrimer-supported efficient triplet-triplet annihilation in

aqueous media. Chem. Commun. 46, 4378-4380, doi:Doi 10.1039/C0cc00266f (2010).

8 Zhao, J. Z., Ji, S. M. & Guo, H. M. Triplet-triplet annihilation based

upconversion: from triplet sensitizers and triplet acceptors to upconversion quantum yields. Rsc Adv. 1, 937-950, doi:Doi 10.1039/C1ra00469g (2011).

9 Monguzzi, A., Tubino, R., Hoseinkhani, S., Campione, M. & Meinardi, F.

Low power, non-coherent sensitized photon up-conversion: modelling and perspectives. Physical Chemistry Chemical Physics 14, 4322-4332, doi:Doi 10.1039/C2cp23900k (2012).

10 Simon, Y. C. & Weder, C. Low-power photon upconversion through triplet- triplet annihilation in polymers. J. Mater. Chem. 22, 20817-20830, doi:Doi 10.1039/C2jm33654e (2012).

11 Kim, J. H. & Kim, J. H. Encapsulated Triplet-Triplet Annihilation-Based Upconversion in the Aqueous Phase for Sub-Band-Gap Semiconductor Photocatalysis. J. Am. Chem. Soc. 134, 17478-17481, doi:Doi

10.1021/Ja308789u (2012).

12 Liu, Q. et al. A General Strategy for Biocompatible, High-Effective

Upconversion Nanocapsules Based on Triplet-Triplet Annihilation. J. Am.

Chem. Soc. 135, 5029-5037, doi:Doi 10.1021/Ja3104268 (2013).

13 Duan, P. F., Yanai, N. & Kimizuka, N. Photon Upconverting Liquids:

30

Matrix-Free Molecular Upconversion Systems Functioning in Air. J. Am.

Chem. Soc. 135, 19056-19059, doi:Doi 10.1021/Ja411316s (2013).

14 Gray, V., Dzebo, D., Abrahamsson, M., Albinsson, B. & Moth-Poulsen, K.

Triplet-triplet annihilation photon-upconversion: towards solar energy applications. Phys. Chem. Chem. Phys. 16, 10345-10352, doi:Doi

10.1039/C4cp00744a (2014).

15 Inokuti, M. & Hirayama, F. Influence of Energy Transfer by Exchange Mechanism on Donor Luminescence. J. Chem. Phys. 43, 1978-1989, doi:Doi 10.1063/1.1697063 (1965).

16 Scholes, G. D. Long-range resonance energy transfer in molecular systems.

Annu. Rev. Phys. Chem. 54, 57-87, doi:DOI

10.1146/annurev.physchem.54.011002.103746 (2003).

17 Islangulov, R. R., Lott, J., Weder, C. & Castellano, F. N. Noncoherent low- power upconversion in solid polymer films. Journal of the American Chemical Society 129, 12652-+, doi:10.1021/ja075014k (2007).

18 Kim, J. H., Deng, F., Castellano, F. N. & Kim, J. H. High Efficiency Low- Power Upconverting Soft Materials. Chem. Mater. 24, 2250-2252, doi:Doi 10.1021/Cm3012414 (2012).

19 Jiang, Z., Xu, M., Li, F. Y. & Yu, Y. L. Red-Light-Controllable Liquid- Crystal Soft Actuators via Low-Power Excited Upconversion Based on Triplet-Triplet Annihilation. J. Am. Chem. Soc. 135, 16446-16453, doi:Doi 10.1021/Ja406020r (2013).

20 Monguzzi, A. et al. High Efficiency Up-Converting Single Phase Elastomers for Photon Managing Applications. Adv. Ener. Mater. 3, 680-686, doi:DOI 10.1002/aenm.201200897 (2013).

21 Zhang, C., Zheng, J. Y., Zhao, Y. S. & Yao, J. N. Organic core-shell

nanostructures: microemulsion synthesis and upconverted emission. Chem.

Commun. 46, 4959-4961, doi:Doi 10.1039/C0cc00347f (2010).

22 Zhang, C., Zheng, J. Y., Zhao, Y. S. & Yao, J. N. Self-Modulated White Light Outcoupling in Doped Organic Nanowire Waveguides via the Fluctuations of Singlet and Triplet Excitons During Propagation. Adv.

Mater. 23, 1380-1384, doi:DOI 10.1002/adma.201003829 (2011).

23 Yaghi, O. M. et al. Reticular synthesis and the design of new materials.

Nature 423, 705-714 (2003).

24 Kitagawa, S., Kitaura, R. & Noro, S.-i. Functional porous coordination

polymers. Angew. Chem. Int. Ed. 43, 2334-2375 (2004).

31

25 Férey, G. & Serre, C. Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequences. Chem. Soc. Rev. 38, 1380-1399, doi:Doi 10.1039/B804302g (2009).

26 Borisov, S. M., Larndorfer, C. & Klimant, I. Triplet-Triplet Annihilation- Based Anti-Stokes Oxygen Sensing Materials with a Very Broad Dynamic Range. Advanced Functional Materials 22, doi:10.1002/adfm.201200794 (2012).

27 Zhang, T. & Lin, W. Metal-Organic Frameworks for Artificial

Photosynthesis and Photocatalysis. Chem. Soc. Rev. 43, 5982-5993 (2014).

28 Deria, P. et al. Beyond post-synthesis modification: evolution of metal- organic frameworks via building block replacement. Chem. Soc. Rev. 43, 5896-5912 (2014).

29 Hauptvogel, I. M. et al. Flexible and Hydrophobic Zn-Based Metal-Organic Framework. Inorg. Chem. 50, 8367-8374, doi:Doi 10.1021/Ic200937u (2011).

30 Zhang, G. H. & Thomas, J. K. Transport of Singlet Excitation in Solid Aromatic Polymers. J. Phys. Chem. 99, 11203-11215, doi:Doi

10.1021/J100028a023 (1995).

31 Kent, C. A. et al. Energy Transfer Dynamics in Metal-Organic Frameworks.

J. Am. Chem. Soc. 132, 12767-12769, doi:Doi 10.1021/Ja102804s (2010).

32 Lin, J. X. et al. Triplet Excitation Energy Dynamics in Metal-Organic Frameworks. J. Phys. Chem. C 117, 22250-22259, doi:Doi

10.1021/Jp401515r (2013).

33 Son, H. J. et al. Light-Harvesting and Ultrafast Energy Migration in Porphyrin-Based Metal-Organic Frameworks. J. Am. Chem. Soc. 135, 862- 869, doi:Doi 10.1021/Ja310596a (2013).

34 Monguzzi, A., Mezyk, J., Scotognella, F., Tubino, R. & Meinardi, F.

Upconversion-induced fluorescence in multicomponent systems: Steady- state excitation power threshold. Phys. Rev. B 78, 195112, doi:Artn 195112 Doi 10.1103/Physrevb.78.195112 (2008).

35 Haefele, A., Blumhoff, J., Khnayzer, R. S. & Castellano, F. N. Getting to the (Square) Root of the Problem: How to Make Noncoherent Pumped

Upconversion Linear. J. Phys. Chem. Lett. 3, 299-303, doi:Doi 10.1021/Jz300012u (2012).

36 Jortner, J., Katz, J. L., Rice, S. A. & Choi, S. I. Triplet Energy Transfer and

Triplet-Triplet Interaction in Aromatic Crystals. Phys. Rev. Lett. 11, 323-

326, doi:DOI 10.1103/PhysRevLett.11.323 (1963).

32

37 Pope, M. & Swenberg, C. E. Electronic Processes in Organic Crystals, Clarendon Press (1982).

38 Monguzzi, A., Tubino, R. & Meinardi, F. Upconversion-induced delayed fluorescence in multicomponent organic systems: Role of Dexter energy transfer. Phys. Rev. B 77, 155122, doi:Artn 155122

Doi 10.1103/Physrevb.77.155122 (2008).

39 Ern, V. Anisotropy of Triplet Exciton Diffusion in Anthracene. Phys. Rev.

Lett. 22, 343-345, doi:DOI 10.1103/PhysRevLett.22.343 (1969).

40 Grisanti, L. et al. Roles of local and nonlocal electron-phonon couplings in triplet exciton diffusion in the anthracene crystal. Phys. Rev. B 88, 035450, doi:Artn 035450

Doi 10.1103/Physrevb.88.035450 (2013).

41 Najafov, H., Lee, B., Zhou, Q., Feldman, L. C. & Podzorov, V. Observation of long-range exciton diffusion in highly ordered organic semiconductors.

Nat. Mater. 9, 938-943, doi:Doi 10.1038/Nmat2872 (2010).

42 Lin, J. D. A. et al. Systematic study of exciton diffusion length in organic semiconductors by six experimental methods. Mater. Horiz. 1, 280-285, doi:Doi 10.1039/C3mh00089c (2014).

43 Wei, Z. et al. Rigidifying fluorescent linkers by metal-organic framework formation for fluorescence blue shift and quantum yield enhancement. J.

Am. Chem. Soc. 136, 8269-8276, doi:10.1021/ja5006866 (2014).

44 Kondo, M., Furukawa, S., Hirai, K. & Kitagawa, S. Coordinatively

Immobilized Monolayers on Porous Coordination Polymer Crystals. Angew.

Chem. Int. Ed. 49, 5327-5330, doi:DOI 10.1002/anie.201001063 (2010).

45 Yanai, N. & Granick, S. Directional Self-Assembly of a Colloidal Metal- Organic Framework. Angew. Chem. Int. Ed. 51, 5638-5641, doi:DOI 10.1002/anie.201109132 (2012).

46 Duan, P. F., Yanai, N., Nagatomi, H. & Kimizuka, N. Photon Upconversion in Supramolecular Gel Matrixes: Spontaneous Accumulation of Light- Harvesting Donor–Acceptor Arrays in Nanofibers and Acquired Air Stability. J. Am. Chem. Soc. 137, 1887-1894 (2015).

47 Furukawa, H., Cordova, K. E., O'Keeffe, M. & Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 341, 974, doi:DOI 10.1126/science.1230444 (2013).

48 Sindoro, M., Yanai, N., Jee, A. Y. & Granick, S. Colloidal-Sized Metal-

Organic Frameworks: Synthesis and Applications. Acc. Chem. Res. 47, 459-

33

469, doi:Doi 10.1021/Ar400151n (2014).

49 Furukawa, S., Reboul, J., Diring, S., Sumida, K. & Kitagawa, S.

Structuring of metal-organic frameworks at the mesoscopic/macroscopic

scale. Chem. Soc. Rev. 43, 5700-5734 (2014).