TripletâTriplet Annihilation Photon Upconversion in ... - ACS Publications

Subscriber access provided by The Libraries of the | University of North Dakota

Article

Triplet-triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design Xinpeng Jiang, Xinyan Guo, Jiang Peng, Dahui Zhao, and Yuguo Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01339 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Triplet-triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design Xinpeng Jiang, Xinyan Guo, Jiang Peng, Dahui Zhao,* and Yuguo Ma* Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China

ABSTRACT Efficient visible-to-UV photon upconversion via triplet-triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

distance separating the donor (D) and acceptor (A) units, and incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices. KEYWORDS: photon upconversion, triplet-triplet annihilation, triplet sensitizer, iridium complex, solid matrix INTRODUCTION Boasting the advantage of utilizing noncoherent, low-intensity excitation source, photon-energy upconversion through triplet-triplet annihilation (TTA) attracts increasing research interests in recent years. The potential application of this technique ranges from displaying,1,2 solar energy conversion,3-10 bioimaging,11-14 to photodynamic therapy,15 and more. In a typical TTA process, the photo-excited sensitizers first enable Dexter-type triplet-triplet energy transfer (TTET) to suitable annihilators/emitters, which then undergo bimolecular TTA to generate singlet excitons of higher energy.16-19 As the molecular collisions and interactions among transient species are indispensable for the sensitization and annihilation processes, it is apparently a lot more challenging to realize TTA upconversion in the solid state compared to the solution state, because of the different molecular diffusion rate. However, the fluid and usually volatile nature of solvents pose great difficulties to device fabrications. Therefore, developing TTA systems that are functional under more processing-amiable conditions, e.g. in non-fluid and non-volatile media, is highly desirable for realizing practical applications of TTA.20,21 The first example of


2

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


using polymer blends as the solid matrix was demonstrated by Castellano, et al.22,23 Then, a variety of polymers such as polyurethane (PU), polyacrylate, poly(methylmethacrylate), and poly(phosphoester) were investigated as the hosting materials.24-27 Nonetheless, the upconversion efficiencies so far attainable in these solid matrices are far less satisfactory compared to solution systems. The inferior performance is undoubtedly related to the limited molecular mobility in the solid state, which greatly debilitates the TTET and TTA processes. A plausible solution to this problem is evidently to extend the triplet excited-state lifetimes of the sensitizers and annihilators, thereby allowing more sufficient time for the transient species to diffuse and interact. Compared to the annihilators, which are usually triplet-singlet spinforbidden organic small molecules having relatively long triplet lifetimes, we deem that the triplet sensitizers deserve more attention, because they often possess relatively short triplet lifetimes for capable of efficient intersystem crossing.22-27 Among the various transition-metal complexes applicable as photosensitizers, cyclometalated iridium(III) complexes with highenergy triplet energy levels are suitable candidates for pairing up with various annihilators, including high-energy UV emitters.28-32 An additional important merit of the cyclometalated iridium molecules is their commonly slow nonradiative rate.33,34 However, most conventional cyclometalated iridium complexes are incompetent photosensitizers due to their typically fast radiative decays and hence short triplet lifetimes. Special structural modifications can however significantly prolong the triplet lifetimes.35-38 Recently, our group developed a series of bichromophoric triplet sensitizers featuring triscyclometalated Ir(III), which manifested ultra-long triplet lifetimes up to a few milliseconds but still maintained impressively low nonradiative energy loss. Such properties qualify these molecules for ideal triplet sensitizers.39,40 The highlight of the molecular design laid with an


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

organic triplet acceptor that was covalently tethered to the tris-cyclometalated Ir complex through a non-conjugative spacer. Such a bichromophoric scaffold not only brought about the ultra-long triplet lifetime by establishing an energetically imbalanced yet reversible intramolecular triplet energy transfer (intra-ET) process, 41-49 but it also allowed the triplet levels of the donor (D) and acceptor (A) in the same molecule to be independently tuned, offering flexibility for tuning the photophysical proerties40 as well as accommodating a greater variety of annihilators, including the high-energy emitters. Using such long-lifetime sensitizers, we previously demonstrated that the efficiency of a visible-to-UV TTA system was significantly enhanced in solution, with optimal photon conversion quantum yield achieved even at the very low sensitizer and emitter concentrations.39 In the current work, we further investigate and optimize the sensitizer structure and performance for realizing application in the solid state. It is found that the sensitizing capability in the solid matrix is highly sensitive to the linker joining the metal-complex and the acceptor group, and the bichromophoric sensitizers comprising a shorter D-A spacer offer much more favorable TTA results. Such a phenomenon is contributed to the greatly facilitated intra-ET under the diffusion restricted conditions when the triplet D and A are positioned within a shorter distance. Using the optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene (DBP) as the annihilator/emitter, we demonstrate that efficient visible-to-UV TTA upconversion can be realized in polyurethane (PU) films with a low-power excitation source. A quantum efficiency of 2.6% is reached, presenting the highest value so far achieved with the visible-to-UV TTA in the solid state. Moreover, it is also shown that the overall TTA efficiency is also dependent on the chemical structure of the acceptor moiety, suggesting that both the


4

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intramolecular and intermolecular TTET processes are necessarily optimized for the best upconversion performance. RESULTS AND DISCUSSION Sensitizer designs and syntheses. Given the observation that a previously developed sensitizer Ir(ppy-F-DBP)40 exhibited nearly identical emission spectrum to that of its analogue Ir(ppy-F) without the acceptor unit in glassy 2-methyltetrahydrofuran (MeTHF) at 77 K (Figure S1), we suspected that intra-ET from the iridium chromophore to the appended DBP was inefficient with Ir(ppy-F-DBP) in such a solid matrix. According to the conclusion drawn from our prior work,39 this intra-ET process was pivotal for attaining favorable sensitizing performance. Hence, the intra-ET efficiency must be improved for achieving effective TTA upconversion in the solid state. We thus speculated that the relatively long spacer separating the acceptor moiety from the iridium center in Ir(ppy-F-DBP) might have hampered the intra-ET under the diffusion-limited conditions, and that a shortened spatial distance between the D and A units may be beneficial to this Dexter-type process. Accordingly, we further designed two different bichromophoric analogues Ir(ppy-DBP) and Ir(ppy-pyr), which comprised a shorter spacer between the Ir(ppy) chromophore and acceptor moiety, i.e., DBP or the plain pyrene (Chart 1). Ir(ppy-DBP) and Ir(ppy-pyr) were then prepared through ligand exchange reactions between a phenylpyridine ligand tethered with pyrene or DBP and a chlorine-bridged iridium dimer (see the Supporting Information for synthetic details). The structures of these new compounds were confirmed by 1H &

13

C NMR spectra and high-resolution mass spectrometry.

The photophysical properties and sensitizing performances of these newly designed sensitizers were then studied and compared to their appendent-free parent molecule Ir(ppy), as well as another bichromophoric analogue Ir(ppy-F-DBP) with a different D-A spacer.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

Chart 1. Structures of Studied Sensitizers t-Bu 1

R

R

t-Bu

2

O

N t-Bu N

Ir

Ir(ppy-F-DBP)

N

R1 = H

R2 =

t-Bu t-Bu

Ir(ppy)

R 1 = t-Bu

R2 = H

Ir(ppy-DBP)

R 1 = t-Bu

R2 =

Ir(ppy-F)

R1 = H

R2 =

Ir(ppy-pyr)

R1 = t-Bu

R2 =

t-Bu

Photophysical properties. The absorption spectra of Ir(ppy-DBP) and Ir(ppy-pyr) clearly showed contributions from the appended pyrene and DBP units between 300 and 360 nm, with well-resolved vibronic structures. Similar to the properties of previously studied Ir(ppy-F-DBP) and Ir(ppy-F),40 identical metal-to-ligand charge-transfer (MLCT) bands were manifested beyond 390 nm by the two new sensitizers and Ir(ppy) (Figure 1). Such spectral features indicated that in the ground state the metal-complex chromophore was completely electronic decoupled from the tethered pyrene and DBP due to the saturated aliphatic linker.

Figure 1. UV-vis absorption and normalized photoluminescence (excited at 390 nm) spectra of studied molecules in toluene at 298 K.


6

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The photoluminescence properties were first studied at room temperature. Previous studies had shown that the strong emissions from Ir(ppy-F) and Ir(ppy) under deoxygenated conditions were given off upon the relaxation of their triplet states of admixed 3MLCT-LC (ligand-centered) characteristics.33,34 After a pyrene or DBP unit was tethered to these iridium complexes, very similar phosphorescence bands were observed (Figure 1), suggesting that the triplet energy level of the iridium chromophore remained nearly unchanged in the presence of these appendents. However, compared to the moderately reduced emission quantum yield (Φ') observed with Ir(ppy-F-DBP) relative to Ir(ppy-F), significantly larger magnitudes of Φ' reduction were exhibited by Ir(ppy-DBP) and Ir(ppy-pyr) (Table 1). Compared to the highly emissive prototype molecule Ir(ppy), the Φ' values of Ir(ppy-DBP) and Ir(ppy-pyr) attached with DBP or pyrene via a short linker were diminished to 2~3%. Then, the time-resolved characterizations revealed more insightful information about the photophysical properties. Similar to Ir(ppy-FDBP),

both

Ir(ppy-DBP)

and

Ir(ppy-pyr)

displayed

two

dramatically

different

phosphorescence lifetimes. Similar shorter lifetime components (τ1) of about 2 nanosecond were determined with Ir(ppy-DBP) and Ir(ppy-pyr), which were noticeably truncated compared to τ1 of Ir(ppy-F-DBP). Furthermore, great disparity was detected with the longer component (τ2) for the two complexes with similar shorter linker. With τ2 ≈ 3 ms shown by Ir(ppy-DBP) a much smaller τ2 of 0.7 ms was displayed by Ir(ppy-pyr) (Table 1). With all these collected photophysical data and using the same method40 previously applied with Ir(ppy-F-DBP), the forward and backward intra-ET rates from the donor Ir(ppy) chromophore to the acceptor moiety (DBP or pyrene) could be estimated for these new bichromophoric sensitizers (Table 1). The results showed that, compared to Ir(ppy-F-DBP), the forward-energy transfer rate (kf) was noticeably accelerated while the back-energy transfer rate


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

(kb) was greatly reduced in Ir(ppy-DBP) and Ir(ppy-pyr). These properties were in agreement with the much larger D-A energy gap (∆E > 0.35 eV) presented by Ir(ppy-DBP) and Ir(ppy-pyr) than Ir(ppy-F-DBP). Relevant triplet energy gap values could easily be deduced from the emission maxima of relevant molecules at 77 K (Figure 2). The fluorene unit in conjugation with ligand ppy perceptibly lowered the donor triplet level in Ir(ppy-F-DBP). The enlarged D-A energy gap in Ir(ppy-DBP) and Ir(ppy-pyr) could reasonably induce an increase in kf. Moreover, it should be noted that the significantly raised energy transfer rates should also have benefitted from the shorter D-A spatial distance, since the kf values of Ir(ppy-DBP) and Ir(ppypyr) were still considerably greater than that of a previously studied bichromophoric sensitizer possessing similarly large D-A energy gap but longer D-A linker.40 Table 1. Photophysical Properties of Studied Ir Complexes τ0/µs (kr | knr / 106 s-1)c

λem /nm (298 K/77 K)

Φ or Φ'

Ir(ppy-F)a

560/545

0.92

Ir(ppy-F-DBP)a

563/544

0.60

Ir(ppy)

510/496

0.97

Ir(ppy-DBP)

511/589

0.02

0.0024

2.8 × 103

4 × 108 | 4 × 103

Ir(ppy-pyr)

510/598

0.03

0.0020

6.8 × 102

5 × 108 | 3 × 104

b

c

τ1 (µs)

c

τ2 (µs)

kf | kb (s-1)d

2.2 (0.42 | 0.036) 0.0105

1.0 × 103

1 × 108 | 1.5 × 105

1.3 (0.75 | 0.023)

a

Data was from ref 40. b Phosphorescence quantum yields of the bichromophoric (Φ') and monochromophoric model complexes (Φ); Ir(ppy)3 (Φ = 0.97, ref 50) was used as the standard for all Φ and Φ' measurements. c Lifetimes shorter than 10 µs were measured by the time-correlated single-photon counting method using NanoLED of 389 nm as the excitation light source; lifetimes longer than 40 µs were measured using a pulsed Xe lamp as the excitation light source. d Estimated forward and backward intramolecular energy transfer rates from the Ir-complex chromophore to DBP or pyrene, determined using the method described in ref 40.


8

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Normalized photoluminescence spectra of different complexes at 77 K in glassy MeTHF (λex = 400 nm; the spectra of Ir(ppy-F) and Ir(ppy-F-DBP) were reproduced from ref 40). On the other hand, because a much longer τ2 value was exhibited by Ir(ppy-DBP) than Ir(ppy-pyr), a much slower back-energy transfer rate kb was derived for Ir(ppy-DBP) than Ir(ppy-pyr) (Table 1). This was a bit surprising since Ir(ppy-DBP) possessed a slightly smaller D-A energy gap than Ir(ppy-pyr). As Ir(ppy-DBP) and Ir(ppy-pyr) have nearly identical chemical structures except for the two tert-butyl group on pyrene, we suspect that the impeded back-energy transfer in Ir(ppy-DBP) likely resulted either from the lower molecular mobility of DBP than pyrene due to the larger size, or from the steric hindrance caused by the two bulky groups which frustrated the intimate contact of DBP with the Ir(ppy) moiety. In addition to providing the energy gap values, the emission spectra at 77 K revealed pertinent information regarding the efficiency of intra-ET in the solid state. The emissions of Ir(ppy-DBP) and Ir(ppy-pyr) were dominated by the phosphorescence from DBP and pyrene units at 77 K, with the emission of the Ir(ppy) chromophore being significantly quenched and shown as a residual band of minimal intensity in the shorter wavelength range (Figure 2). Such


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

emission properties indicated that even in the glassy MeTHF matrix the intra-ET was taking place very effectively with Ir(ppy-DBP) and Ir(ppy-pyr). Apparently, such favorable results were mainly owing to the shorter D-A linker in the two molecules, since the other bichromophoric molecule Ir(ppy-F-DBP) manifested mainly the emission band of the iridium chromophore, evidencing the much less efficient intra-ET in the glassy state. Additionally, transient absorption characterizations provided further evidence for the occurrence of intra-ET and the generation of triplet pyrene and DBP in Ir(ppy-DBP) and Ir(ppy-pyr) (Figure S2). The absorption bands assignable to the triplet pyrene or DBP were clearly detectable, while their ground states were significantly bleached. TTA upconversion in solution. The sensitizing abilities of these new bichromophoric complexes were first tested in solution by examining the TTA upconversion performance. In these experiments, UV emitter DBP was used as the annihilator. Quadratic dependence of the upconverted emission intensity on the excitation power density were demonstrated for all examined sensitizers (Figure S3), proving the occurrence of nonlinear TTA processes. It was found that at the same sensitizer and emitter concentrations a noticeably higher upconversion quantum efficiency was detected for Ir(ppy-DBP) than Ir(ppy-F-DBP) in toluene solution (Figure 3). Furthermore, compared to Ir(ppy-F-DBP) producing 7 times stronger emission intensity than its appendent-free analogue Ir(ppy-F), the upconversion emission intensity of Ir(ppy-DBP)-sensitized solution increased by 19 times relative to that of Ir(ppy)-sensitized solution. However, in contrast to such greatly enhanced upconversion efficiency enabled by Ir(ppyDBP), very similar TTA emission intensity was detected from Ir(ppy-pyr) and Ir(ppy) (Figure 3). Since the phosphorescence emission of Ir(ppy-pyr) was vastly quenched in toluene solution,


10

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicating effective intra-ET was happening with this complex, we tentatively contributed the poor TTA performance to the less efficient intermolecular TTET process between the covalently tethered pyrene of Ir(ppy-pyr) and the free DBP annihilators, likely because they have imperfectly matched structure causing a large re-organization energy, or alternatively the triplet pyrene underwent much faster nonradiative decay than DBP in solution.

Figure 3. (a) Upconverted emission spectra and (b) upconversion quantum yields of deaerated toluene solutions of DBP mixed with different sensitizers at 298 K (excited by monochromic light of 450 nm from Xe lamp of ca. 16.5 mW/cm2, [sensitizers] = 20 µM and [DBP] = 60 µM). TTA upconversion in polyurethane films. Polyurethane (PU) was used as the hosting material for our solid-state TTA experiments because of its favorable mechanical and optical properties, as well as the suitable processability.24 The absorption and emission spectra of PU films containing various sensitizers were first examined to obtain relevant photophysical information (Figures S5 and S6). The absorption spectra collected from the studied sensitizers dispersed in PU films were very similar to those exhibited in toluene solution. The steady-state emission properties in the films also very much resembled those observed in the solutions, with similar Φ (Φ') values determined for both mono- and bichromophoric complex series (Table S1), suggesting that no significant quenching effect was induced by the PU hosting materials, neither


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

was the self-quenching effect particularly pronounced with these complexes under such lowdoping concentrations. However, different behaviors were revealed by the time-resolved data. Compared to an emission lifetime of 1.6 µs shown by Ir(ppy-F), the shorter lifetime component of Ir(ppy-FDBP) was found only slightly reduced to 1.0 µs in the PU film. In stark contrast, much shorter nanosecond lifetimes were still detectable from both Ir(ppy-DBP) and Ir(ppy-pyr) (Figure 4 and Table S1). Such results clearly indicated that the intramolecular triplet energy transfer was vastly frustrated for Ir(ppy-F-DBP) but still proceeding quite properly for Ir(ppy-DBP) and Ir(ppy-pyr) in PU films. We assume this is because the longer spacer separating the D and A chromophores in Ir(ppy-F-DBP) caused difficulties with the intra-ET under diffusion-limited conditions, while the D and A chromophores confined to a closer distance by a shorter linker was affected to a much lesser extent. Consequently, such differences in the intra-ET efficiency entailed perceivable consequences to the TTA process in PU films (vide infra).

Figure 4. Time-resolved emission decay curves of the bichromophoric complexes dispersed in PU films (~1.5 mM) in the nanosecond (a) and microsecond (b) ranges under ambient conditions. Subsequently, the TTA experiments were conducted in PU thin films to investigate the performance of newly designed sensitizers in the solid matrix. PU films of ca. 150 µm thick


12

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


doped with various sensitizers and DBP (at about 1.5 and 15 mM respectively in the solid PU “solution”) were prepared by drop-casting. Under ambient conditions, when such PU films were irradiated with monochromic noncoherent light of 450 nm from a xenon lamp, evident upconverted DBP emission signals were detected with films sensitized by any of the bichromophoric molecules (Figure 5, a & b). In contrast, emissions were hardly observable from thin films doped with monochromophoric model complexes and DBP. Quadratic dependences of the UC emission intensity on the excitation power density were obtained for all bichromophores, verifying the TTA nature of the emission signals from the films (Figure 5c). Among the three bichromophoric sensitizers, Ir(ppy-DBP) exhibited the highest upconversion efficiency. However, dissimilar to the observations made in toluene solutions, much stronger emission signals were detected from Ir(ppy-pyr)-sensitized PU films than those doped with Ir(ppy-FDBP). These results confirmed that a closer spatial distance between D and A chromophores was a very important factor for realizing TTA upconversion in the solid state. In spite the less efficient TTET from the tethered pyrene to the free DBP as above mentioned, Ir(ppy-pyr) still achieved better TTA result than Ir(ppy-F-DBP) in PU films (Figure 5a). Moreover, quantitative experiments with varied DBP concentrations were also performed with the most competent sensitizer Ir(ppy-DBP) (Figure 5d), which showed that the upconversion quantum efficiency increased rapidly with increasing annihilator concentration in the lower DBP concentration range, but after reaching a plateau at about 60 mM, the quantum yield started to decline at even higher DBP concentration, likely resulting from the self-quenching effect of the emitter (Figure S7). To the best of our knowledge, a maximum quantum yield of 2.6% reached by our Ir(ppy-


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

DBP)/DBP system is so far the best visible-to-UV upconversion efficiency attained in the solid matrices.

Figure 5. (a) Comparison of upconversion emission intensity of PU films doped with different sensitizers and DBP. (b) Upconversion emission signal of Ir(ppy-DBP)/DBP in PU films in comparison to the control experiments (picture showing an Ir(ppy-DBP)/DBP-doped PU film). (c) Integrated upconverted emission intensity as a function of excitation power density ([sensitizer] ≈ 1.5 mM and [DBP] ≈ 15 mM in polyurethane under ambient conditions). (d) Upconversion efficiencies of Ir(ppy-DBP)-sensitized PU films co-doped with DBP at different concentrations ([Ir(ppy-DBP)] ≈ 1.5 mM, excitation power density ≈ 16.5 mW/cm2).


14

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A schematic energy diagram of the sensitizing and TTA processes realized by Ir(ppy-DBP) in cooperation with DBP, showing estimated kinetic data, is depicted in Figure 6. Basically, in a diffusion-limited environment, the intermolecular TTET between a monochromophoric iridium complex and DBP takes place at about 104 s-1 (Figure S8), which is unable to compete with the inherent excited-state relaxations (~106 s-1) of the molecule. By tethering a triplet acceptor to the metal complex, an intra-ET occurred and proceeded at ca. 108 s-1, thus quite capable of outcompeting the relaxations of the iridium chromophore. The long-lived triplet exciton resulting from intra-ET, mostly localized with the acceptor moiety (with slow relaxation at ~102 s-1), may greatly facilitate the TTA process by providing sufficient time for encountering a free DBP under the diffusion-limited conditions. An important conclusion elucidated by the current work is that under the highly diffusion-limited conditions, the intra-ET process may as well be prohibited if the D and A were spatially separated by a large distance, which should be avoided by closely tethering the D-A moieties with a short spacer.

Figure 6. Representative Jablonski diagram showing pertinent energy levels of Ir(ppy-DBP) and DBP with estimated kinetic rate constants involved in a TTA upconversion process taking place in the PU film.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

CONCLUSION In summary, a triplet photosensitizer with competent performance in both solution and solid matrix has been developed, as evidenced by the accomplishment of efficient visible-to-UV TTA upconversion in elastic polyurethane thin films as well as toluene. Specifically, an upconversion quantum yield of 2.6% was achieved in the solid films by the optimized sensitizer structure of Ir(ppy-DBP) with DBP as the annihilator/emitter. In addition to confirm that a bichromophoric sensitizer design, featuring an organic acceptor tethered to a tris-cyclometalated Ir complex, can greatly improve the TTA upconversion efficiency through the generation of a long-lived triplet exciton via intramolecular energy transfer, the current work further demonstrates that, in the solid matrix where the molecular diffusion is highly impeded, fine tuning of sensitizer structure is necessary in order to ensure that the intra-ET can proceed. The currently designed sensitizer features a very short linker between the triplet donor (tris-cyclometalated Ir complex) and acceptor (DBP), which is proven indispensable for allowing the intra-ET to occur and sufficient long-lived exciton to be generated in the solid matrix. Moreover, the sensitizers tethered with different acceptor structures also show that both the intra- and intermolecular energy transfer processes are necessarily optimized for attaining the most favorable TTA upconversion efficiency. The results illustrated herein about developing potent photosensitizers applicable under diffusion-limited conditions may as well be of use to other technologies entailing sensitization in the solid-state media. EXPERIMENTAL SECTION Photophysical Measurements. Absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Corrected emission spectra were obtained with a Horiba Jobin Yvon FluoroMax-4P spectrofluometer equipped with an R928 photomultiplier tube (180-850 nm). A


16

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 W ozone-free xenon arc lamp was used as the excitation light source. Toluene solutions were deoxygenated with three freeze-pump-thaw cycles. Emission quantum yields were determined by comparison with a dilute Ir(ppy)3 standard in deaerated toluene (Φ = 0.97).50 Photoluminescence spectra at 77 K were recorded in glassy MeTHF with a quartz Dewar flask filled with liquid nitrogen. All time-resolved phosphorescence decay curves were collected on Horiba Jobin Yvon FluoroMax-4P spectrofluometer. Solutions were deoxygenated prior to the lifetime measurements, while polymer film samples were measured under ambient conditions. Lifetimes shorter than 10 µs were determined with the single-photon counting technique with NanoLED of 389 nm as the excitation source. Analyses of the decay profiles were accomplished with Horiba Jobin Yvon DAS6 software. Lifetimes longer than 40 µs were measured with FluoroMax-4P spectrofluometer with programmed and pulsed xenon lamp as the excitation source, and 390 nm was chosen as the excitation wavelength. Fitting of the decay curves was performed using the Origin software. Nanosecond transient absorption spectra were recorded on a LP-920 laser flash photolysis spectrometer (Edinburgh). Samples were excited with 410 nm laser pulses at repetition rate of 1 Hz produced by a Nd:YAG laser/OPO system from Opotek (Vibrant 355 II). TTA upconversion. Toluene solutions were deaerated through three freeze-pump-thaw cycles and the flask was sealed under vacuum, while PU films were processed under ambient conditions. The PU films coated on silica glass slides were prepared using the reported method.24 The doping concentrations of sensitizers and emitter/annihilator in PU films are expressed in mole concentrations in the main text, taking PU as an elastic “solvent”. Upconversion spectra were recorded on a Fluorolog-3-2-iHR320 spectrofluorometer with a 450 W xenon lamp as the


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

excitation source. The samples were excited at 450 ± 5 nm. A long-pass filter (400 nm) was placed between the lamp and sample to avoid the interferences of shorter-wavelength light. Different excitation power was realized with a series of neutral density filters. Upconversion quantum yields were determined using the following equation: ߔ௎஼ = 2ߔ௦௧ௗ ൬

‫ܣ‬௦௧ௗ ‫ܫ‬௎஼ ߟ௎஼ ଶ ൰൬ ൰൬ ൰ ‫ܣ‬௎஼ ‫ܫ‬௦௧ௗ ߟ௦௧ௗ

where Φ, A, I and η represent the quantum yield, absorbance at 450 nm, integrated photoluminescence intensity and refractive index of the medium, respectively. The subscripts UC and std denote the parameters of the upconversion and standard systems. The equation is multiplied by a factor of 2 in order to guarantee a maximum quantum yield of unity.16 For the determination of upconversion quantum yields in toluene, Ru(bpy)3Cl2 (ΦP = 4.0%, in air-equilibrated water) was used as the standard. For the determination of upconversion quantum yields in thin films, PU films doped with only Ir(ppy) (1.5 mM) were used as the standard. The absorbance of the standard is the same as that of the UC films at the 450 nm excitation wavelength. Since the standard and upconversion films were both made of PU, the same refractive index values were applied. Therefore, under our experimental conditions, the aforementioned equation can be simplified to: ߔ௎஼ = 2ߔ௦௧ௗ ൬

‫ܫ‬௎஼ ൰ ‫ܫ‬௦௧ௗ

ASSOCIATED CONTENT Supporting Information. Synthetic procedures, additional photophysical data and TTA upconversion characterization results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Authors *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (No. 51473003 and 51573002) and the Ministry of Science and Technology (2013CB933501) of China. REFERENCES 1. Baluschev, S.; Yakutkin, V.; Miteva, T.; Wegner, G.; Roberts, T.; Nelles, G.; Yasuda, A.; Chernov, S.; Aleshchenkov, S.; Cheprakov, A. A General Approach for Non-coherently Excited Annihilation Up-conversion: Transforming the Solar-Spectrum. New J. Phys. 2008, 10, 013007. 2. Miteva, T.; Yakutkin, V.; Nelles, G.; Baluschev, S. Annihilation Assisted Upconversion: AllOrganic Flexible and Transparent Multicolour Display. New J. Phys. 2008, 10, 103002. 3. Schulze, T. F.; Czolk, J.; Cheng, Y.-Y.; Fückel, B.; MacQueen, R. W.; Khoury, T.; Crossley, M. J.; Stannowski, B.; Lips, K.; Lemmer, U.; Colsmann, A.; Schmidt, T. W. Efficiency Enhancement of Organic and Thin-Film Silicon Solar Cells with Photochemical Upconversion. J. Phys. Chem. C 2012, 116, 22794-22801. 4. Nattestad, A; Cheng, Y.-Y.; MacQueen, R. W.; Schulze, T. F.; Thompson, F. W.; Mozer, A. J.; Fückel, B.; Khoury, T.; Crossley, M. J.; Lips, K.; Wallace, G. G.; Schmidt, T. W. DyeSensitized Solar Cell with Integrated Triplet-Triplet Annihilation Upconversion System. J. Phys. Chem. Lett. 2013, 4, 2073-2078.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

5. Schulze, T. F.; Schmidt, T. W. Photochemical Upconversion: Present Status and Prospects for Its Application to Solar Energy Conversion. Energy Environ. Sci. 2015, 8, 103-125. 6. Monguzzi, A.; Braga, D.; Gandini, M.; Holmberg, V. C.; Kim, D. K.; Sahu, A.; Norris, D. J.; Meinardi, F. Broadband Up-Conversion at Subsolar Irradiance: Triplet-Triplet Annihilation Boosted by Fluorescent Semiconductor Nanocrystals. Nano Lett. 2014, 14, 6644-6650. 7. Khnayzer, R. S.; Blumhoff, J.; Harrington, J. A.; Haefele, A.; Deng, F.; Castellano, F. N. Upconversion-Powered Photoelectrochemistry. Chem. Commun. 2012, 48, 209-211. 8. 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. 2012, 134, 17478-17481. 9. Cates, E. L.; Chinnapongse, S. L.; Kim, J.-H.; Kim, J.-H. Engineering Light: Advances in Wavelength Conversion Materials for Energy and Environmental Technologies. Environ. Sci.Technol. 2012, 46, 12316-12328. 10. Kwon, O. S.; Kim, J.-H.; Cho, J. K.; Kim, J.-H. Triplet-Triplet Annihilation Upconversion in CdS-Decorated SiO2 Nanocapsules for Sub-Bandgap Photocatalysis. Acs Appl. Mater. Interfaces 2015, 7, 318-325. 11. Wohnhaas, C.; Mailänder, V.; Dröge, M.; Filatov, M. A.; Busko, D.; Avlasevich, Y. Baluschev, S.; Miteva, T.; Landfester, K.; Turshatov, A. Triplet-Triplet Annihilation Upconversion Based Nanocapsules for Bioimaging Under Excitation by Red and Deep-Red Light. Macromol. Biosci. 2013, 13, 1422-1430. 12. Liu, Q.; Yang, T.; Feng, W.; Li, F. Blue-Emissive Upconversion Nanoparticles for LowPower-Excited Bioimaging in Vivo. J. Am. Chem. Soc. 2012, 134, 5390-5397.


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Liu, Q.; Yin, B.; Yang, T.; Yang, Y.; Shen, Z.; Yao, P.; Li, F. A General Strategy for Biocompatible, High-Effective Upconversion Nanocapsules Based on Triplet-Triplet Annihilation. J. Am. Chem. Soc. 2013, 135, 5029-5037. 14. Zhou, J.; Liu, Q.; Feng, W.; Sun, F.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395-465. 15. Askes, S. H. C.; Bahreman, A.; Bonnet, S. Activation of a Photodissociative Ruthenium Complex by Triplet-Triplet Annihilation Upconversion in Liposomes. Angew. Chem. Int. Ed. 2014, 53, 1029-1033. 16. Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized TripletTriplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560-2573. 17. Ceroni, P. Energy Up-Conversion by Low-Power Excitation: New Applications of an Old Concept. Chem. Eur. J. 2011, 17, 9560-9564. 18. Zhao, J.; Ji, S.; Guo, H. Triplet-Triplet Annihilation Based Upconversion: from Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields. RSC Adv. 2011, 1, 937950. 19. Schmidt, T. W.; Castellano, F. N. Photochemical Upconversion: The Primacy of Kinetics. J. Phys. Chem. Lett. 2014, 5, 4062-4072. 20. Vadrucci, R.; Weder, C.; Simon, Y. C. Organogels for Low-Power Light Upconversion. Mater. Horiz. 2015, 2, 120-124. 21. Sripathy, K.; MacQueen, R. W.; Peterson, J. R.; Cheng, Y. Y.; Dvořák, M; McCamey, D. R.; Treat, N. D.; Stingelin, N.; Schmidt, T. W. Highly Efficient Photochemical Upconversion in a Quasi-Solid Organogel. J. Mater. Chem. C 2015, 3, 616-622.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

22. Islangulov, R. R.; Lott, J.; Weder, C.; Castellano, F. N. Noncoherent Low-Power Upconversion in Solid Polymer Films. J. Am. Chem. Soc. 2007, 129, 12652-12653. 23. Singh-Rachford, T. N.; Lott, J.; Weder, C.; Castellano, F. N. Influence of Temperature on Low-Power Upconversion in Rubbery Polymer Blends. J. Am. Chem. Soc. 2009, 131, 1200712014. 24. Kim, J.-H.; Deng, F.; Castellano, F. N.; Kim, J.-H. High Efficiency Low-Power Upconverting Soft Materials. Chem. Mater. 2012, 24, 2250-2252. 25. Monguzzi, A.; Bianchi, F.; Bianchi, A.; Mauri, M.; Simonutti, R.; Ruffo, R.; Tubino, R.; Meinardi, F. High Efficiency Up-Converting Single Phase Elastomers for Photon Managing Applications. Adv. Energy Mater. 2013, 3, 680-686. 26. Lee, S. H.; Lott, J. R.; Simon, Y. C.; Weder, C. Melt-processed Polymer Glasses for LowPower Upconversion via Sensitized Triplet-Triplet Annihilation. J. Mater. Chem. C 2013, 1, 5142-5148. 27. Marsico, F.; Turshatov, A.; Peköz, R.; Avlasevich, Y.; Wagner, M.; Weber, K.; Donadio, D.; Landfester, K.; Baluschev, S.; Wurm, F. R. Hyperbranched Unsaturated Polyphosphates as a Protective Matrix for Long-Term Photon Upconversion in Air. J. Am. Chem. Soc. 2014, 136, 11057-11064. 28. Zhao, W.; Castellano, F. N. Upconverted Emission from Pyrene and Di-tert-butylpyrene Using Ir(ppy)3 as Triplet Sensitizer. J. Phys. Chem. A 2006, 110, 11440-11445. 29. El-Ballouli, A. O.; Khnayzer, R. S.; Khalife, J. C.; Fonari, A.; Hallal, K. M.; Timofeeva, T. V.; Patra, D.; Castellano, F. N.; Wex, B.; Kaafarani, B. R. Diarylpyrenes vs. Diaryltetrahydropyrenes:

Crystal

Structures,

Fluorescence,

and

Upconversion

Photochemistry. J. Photochem. Photobiol. A 2013, 272, 48-57.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Duan, P.; Yanai, N.; Kimizuka, N. A Bis-Cyclometalated Iridium Complexes as a Benchmark Sensitizer for Efficient Visible-to-UV Photon Upconversion. Chem. Commun. 2014, 50, 13111-13113. 31. Singh-Rachford, T. N.; Castellano, F. N. Low-Power Visible-to-UV Upconversion. J. Phys. Chem. A 2009, 113, 5912-5917. 32. Merkel. P. B.; Dinnocenzo, J. P. Low-Power Green-to-Blue and Blue-to-UV Upconversion in Rigid Polymer Films. J. Lumin. 2009, 129, 303-306. 33. Yan, Q.; Yue, K.; Yu, C.; Zhao, D. Oligo- and Polyfluorene-Tethered fac-Ir(ppy)3: Substitution Effects. Macromolecules 2010, 43, 8479-8487. 34. Yan, Q.; Fan, Y.; Zhao, D. Unusual Temperature-Dependent Photophysics of OligofluoreneSubstituted Tris-Cyclometalated Iridium Complexes. Macromolecules 2012, 45, 133-141. 35. Sun, J.; Wu, W.; Zhao, J. Long-Lived Room-Temperature Deep-Red-Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and Its Application in Triplet-Triplet Annihilation-Based Upconversion. Chem. Eur. J. 2012, 18, 8100-8112. 36. Ma, L.; Guo, S.; Sun, J.; Zhang, C.; Zhao, J.; Guo, H. Green Light-Excitable Naphthalenediimide Acetylide-Containing Cyclometalated Ir(III) Complex with Long-Lived Triplet Excited States as Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversion. Dalton Trans. 2013, 42, 6478-6488. 37. Sun, J.; Zhong, F.; Yi, X.; Zhao, J. Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet-Triplet Annihilation Upconversion. Inorg. Chem. 2013, 52, 62996310.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

38. Yi, X.; Zhang, C.; Guo, S.; Ma, J.; Zhao, J. Strongly Emissive Long-Lived 3IL Excited State of Coumarins in Cyclometalated Ir(III) Complexes Used as Triplet Photosensitizers and Application in Triplet-Triplet Annihilation Upconversion. Dalton Trans. 2014, 43, 16721683. 39. Peng, J.; Jiang, X.; Guo, X.; Zhao, D.; Ma, Y. Sensitizer Design for Efficient Triplet-Triplet Annihilation Upconversion: Annihilator-Appended Tris-Cyclometalated Ir(III) Complexes. Chem. Commun. 2014, 50, 7828-7830. 40. Jiang, X.; Peng, J.; Wang, J.; Guo, X.; Zhao, D.; Ma, Y. Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes. ACS Appl. Mater. Interfaces 2016, 8, 3591-3600. 41. Ford, W. E.; Rodgers, M. A. J. Reversible Triplet-Triplet Energy Transfer within a Covalently Linked Bichromophoric Molecule. J. Phys. Chem. 1992, 96, 2917-2920. 42. Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Intramolecular Triplet Energy Transfer in Pyrene-Metal Polypyridine Dyads: A Strategy for Extending the Triplet Lifetime of the Metal Complex. Chem. Eur. J. 1999, 5, 3366-3381. 43. Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y.-Q.; Hanna, G. S. In Search of Ruthenium(II) Complexes Based on Tridentate Polypyridine Ligands that Feature Long-lived Room-Temperature Luminescence: The Multichromophore Approach. Angew. Chem. Int. Ed. 2003, 42, 1608-1611. 44. Wang, X.-Y.; Guerzo, A. D.; Schmehl, R. H. Photophysical Behaviors of Transition Metal Complexes Having Interacting Ligand Localized and Metal-to-Ligand Charge Transfer States. J. Photochem. Photobiol. C 2004, 5, 55-77.


24

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45. McClenaghan, N. D.; Leydet, Y.; Maubert, B.; Indelli, M. T.; Campagna, S. Excited-State Equilibration: A Process Leading to Long-Lived Metal-to-Ligand Charge Transfer Luminescence in Supramolecular Systems. Coord. Chem. Rev. 2005, 249, 1336-1350. 46. Lavie-Cambot, A.; Lincheneau, C.; Cantuel, M.; Leydet, Y.; McClenaghan, N. D. Reversible Electronic Energy Transfer: A Means to Govern Excited-State Properties of Supramolecular Systems. Chem. Soc. Rev. 2010, 39, 506-515. 47. Ragazzon, G.; Verwilst, P.; Denisov, S. A.; Credi, A.; Jonusauskas, G.; McClenaghan, N. D. Ruthenium(II) Complexes Based on Tridentate Polypyridine Ligand That Feature LongLived Room-Temperature Luminescence. Chem. Commun. 2013, 49, 9110-9112. 48. Denisov, S. A.; Cudré, Y.; Verwilst, P.; Jonusauskas, G.; Marín-Suárez, M.; FernándezSánchez, J. F.; Baranoff, E.; McClenaghan, N. D. Direct Observation of Reversible Electronic Energy Transfer Involving an Iridium Center. Inorg. Chem. 2014, 53, 2677-2682. 49. Castellano, F. N. Altering Molecular Photophysics by Merging Organic and Inorganic Chromophores. Acc. Chem. Res. 2015, 48, 828-839. 50. Sajoto, T.; Djurovich, P. I.; Tamayo, A. B.; Oxgaard, J.; Goddard, W. A.; Thompson, M. E. Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes. J. Am. Chem. Soc. 2009, 131, 9813-9822.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

Graphical Abstract


26

TripletâTriplet Annihilation Photon Upconversion in ... - ACS Publications

Recommend Documents

TripletâTriplet Annihilation Photon Upconversion in ... - ACS Publications