Chemical Science This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online EDGE ARTICLE View Journal | View Issue Emission color tuning and white-light generation based on photochromic control of energy transfer Cite this: Chem. Sci., 2016, 7, 5867 Open Access Article. Published on 02 June 2016. Downloaded on 6/28/2018 11:48:57 AM. reactions in polymer micelles† Magnus Bälter,a Shiming Li,a Masakazu Morimoto,b Sicheng Tang,c Jordi Hernando,d Gonzalo Guirado,d Masahiro Irie,b Françisco M. Raymoc and Joakim Andréasson*a We encapsulate a fluorescent donor molecule and a photochromic acceptor unit (photoswitch) in polymer micelles and show that the color of the emitted fluorescence is continuously changed from blue to yellow upon light-induced isomerization of the acceptor. Interestingly, white-light generation is achieved in Received 12th April 2016 Accepted 23rd May 2016 between. With the photoswitch in the colorless form, intense blue emission from the donor is observed, while UV-induced isomerization to the colored form induces an energy transfer reaction that quenches DOI: 10.1039/c6sc01623e the donor emission and sensitizes the yellow emission from the colored photoswitch. The process is www.rsc.org/chemicalscience reversed by exposure to visible light, triggering isomerization to the colorless form. donor chromophore.9,10,12,17,18,22,27–30 Due to the intrinsic ability Introduction of photoswitches to change color upon isomerization (both in Molecular systems displaying stimuli-responsive emission color absorption and emission),31 the FRET process and conse- are attracting considerable attention, very oen in the quest for quently, the overall emission color can be controlled by white-light emitting materials.1–3 Emission color tuning and photonic means. Common downsides with the systems white-light generation (WLG) can be achieved by additive reported so far are: (i) very low uorescence quantum yields of mixing, that is, combining two or more uorophores in the the photoswitch, resulting in a dramatically decreased emission correct proportion to generate the desired emission color.4–8 intensity when the photoswitch is in the FRET-active uorescent The major downside with this approach is that the addition of form; (ii) that the photoswitch cannot be fully converted to the material requires physical access, preventing any kind of respective isomeric form, limiting the extent of the emission remote control of the color change. Moreover, the apparent color change; (iii) poor thermal stability of one of the isomeric simplicity of additive mixing using uorescent molecular forms, implying that the system will display emission color systems is misleading, as the resulting emission color is changes even in the absence of photoisomerization; and typically not the “sum” due to excited state communication (iv) WLG is very rare. between the uorophores, e.g., energy transfer reactions. Here, we present a self-assembled photochromic system Several alternative strategies have therefore been devised and where all the above mentioned downsides have been successfully demonstrated lately.9–28 One such approach is the eliminated. We base the approach on 9,10-diphenylanthracene, use of molecular photoswitches (photochromic molecules), DPA, as the FRET donor and a photochromic diarylethene most oen acting as acceptors in uorescence resonance energy derivative, DAE, as the acceptor photoswitch. transfer (FRET) reactions together with a highly uorescent a Chemistry and Chemical Engineering, Physical Chemistry, Chalmers University of Results and discussion Technology, 41296 Göteborg, Sweden. E-mail: a-son@chalmers.se b Scheme 1 shows the structures and the light-induced isomeri- Department of Chemistry and Research Center for Smart Molecules, Rikkyu University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan zations of DAE. The open form (DAEo) is isomerized to the c Laboratory for Molecular Photonics, Department of Chemistry, University of Miami, corresponding closed isomer (DAEc) by UV light exposure 1301 Memorial Drive, Coral Gables, Florida 33146-0431, USA (365 nm used here, isomerization quantum yield ¼ 0.42).32 The d Departament de Quı́mica, Universitat Autònoma de Barcelona, 08193 Cerdanyola del reverse reaction is triggered by visible light (523 nm used in this Vallès, Spain study, isomerization yield ¼ 4.0  104).32 Both the UV- and the † Electronic supplementary information (ESI) available: Experimental section; visible light-induced reactions enrich the sample to 100% of the absorption and emission spectra of DPA and DAE in acetonitrile; steady-state and SPC uorescence measurements at varying concentrations; redox energies respective isomeric form. The thermal reaction DAEc / DAEo and driving forces for PET reactions; and simulations of the intermolecular is extremely slow (no absorption changes observed aer 100 h donor–acceptor distances in the micelles. See DOI: 10.1039/c6sc01623e at 80  C.)32 This journal is © The Royal Society of Chemistry 2016 Chem. Sci., 2016, 7, 5867–5871 | 5867 View Article Online Chemical Science Edge Article to DAEc in the presence of DPA: with the photoswitch in the DAEo isomeric form, unquenched blue emission is observed from the DPA donor upon excitation. In the other extreme, UV induced isomerization DAEo / DAEc is extensive enough for This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DAEc to quench the excited DPA with unity efficiency, resulting in exclusively yellow emission from DAEc, sensitized in the FRET reaction. However, in all intermediate situations, that is, partial isomerization of DAEo to DAEc, the overall emission will be a mix between the blue emission from DPA and the yellow emission from DAEc. Here, the relative contributions depend on the degree of isomerization from DAEo to DAEc – the larger Open Access Article. Published on 02 June 2016. Downloaded on 6/28/2018 11:48:57 AM. the isomerized fraction, the more yellow the emission appears. Both DPA and DAE are essentially insoluble in water, but are effectively dissolved in the presence of the ST-7-4 copolymer34 in aqueous solution by encapsulation into polymer micelles. The structure of the copolymer is displayed in Scheme 1. This particular macromolecular construct incorporates hydrophobic decyl and hydrophilic oligo(ethylene glycol) side chains randomly distributed along a common poly(methacrylate) backbone. Such structural composition promotes the self-assembly of multiple macromolecules into single micelles in aqueous solution. The resulting supramolecular assemblies can encapsulate hydrophobic chromophores in their hydrophobic interior and force them to be in close proximity. As a result, this relatively simple and highly modular noncovalent strategy can be exploited to maintain the complementary components DPA and DAE at distances compatible with FRET without the need for the tedious synthetic procedures that would, otherwise, be required to constrain them covalently.35,36 From dynamic light scattering measurements, the average diameter of the micelles was determined to 15 nm. Compared to acetonitrile, both DPA and DAE retain their photophysical/ Scheme 1 Isomerization scheme for the DAE derivative used in spectral properties in the micelles, aside from a small overall this study. Also shown is the structure of the ST-7-4 copolymer bathochromic shi (see Fig. 1). Most importantly, the uores- (m : n ¼ 6 : 1). cence quantum yields are not signicantly affected, and the photochromic properties of the DAE photoswitch are preserved. Fig. 1 shows the changes in the overall emission spectrum of a water dispersion of the polymer micelles (total micelle DPA is a blue emitter with an emission spectrum centered concentration ca. 1.35 mM) containing DPA (total concentration between 400 nm and 450 nm and a uorescent quantum yield of ca. 3.5 mM) and DAE (total concentration ca. 2 mM) upon gradual unity (see Fig. S1 in the ESI† for the spectra of all implicated exposure to 365 nm UV light.37 It is obvious that the emission species in acetonitrile). DAEo displays absorption only at intensity of DAEc in the micellar “cocktail” does indeed increase wavelengths shorter than 400 nm. This implies that there is with an increasing UV dose, at the expense of the DPA emission. virtually no spectral overlap between DPA emission and DAEo It is also seen that the nal DAEc population cannot quench absorption and, hence, that DAEo cannot quench the excited DPA to 100%, as the emission intensity levels off at a non-zero state of DPA by FRET. On the contrary, there is a substantial value (see below for a comprehensive evaluation of the overlap between DPA emission and the absorption of DAEc. The quenching efficiencies). R0-value for this FRET-pair in acetonitrile was determined to Judging from only the spectral evolution, the perceived color 52 Å, assuming k2 ¼ 2/3. The uorescence of DAEc spans the changes of the overall emission is by no means obvious. wavelength region from 500 nm to 700 nm, with an emission However, converting each spectrum to a CIE coordinate and maximum at 535 nm. The corresponding quantum yield was mapping the coordinates in the corresponding CIE diagram are determined to be 0.75 in acetonitrile. This value is dramatically a straightforward means to depict the color changes. The result higher compared to all other photoswitches from the most is shown in Fig. 2. Here it is seen that the emission color common photochromic families, where the typical uorescence gradually changes from blue to yellow upon continuous quantum yields are below 0.05.33 UV-induced isomerization from DAEo to DAEc. In fact, the From the above mentioned spectral features, we anticipated emission color of the cocktail virtually spans the entire region the following scenario upon UV-induced isomerization of DAEo between what is observed for DPA alone and DAEc alone, which 5868 | Chem. Sci., 2016, 7, 5867–5871 This journal is © The Royal Society of Chemistry 2016 View Article Online Edge Article Chemical Science the sense that each system can only generate one emission color. Our approach, however, represents a means where one and the same cocktail of DPA and DAE can be used to generate emission colors from blue to yellow, and as we hit the coordi- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. nate (0.32, 0.33) along the way, we come very close to the generation of perfect white light. It is also very encouraging to see that the blue-to-yellow emission color change is reversible, by exposure to visible light (523 nm used here) that triggers the isomerization DAEc / DAEo. This process is illustrated in the CIE diagram in Fig. 2 by the magenta hollow triangles.38 Exposing the cocktail to another UV dose again shis the CIE Open Access Article. Published on 02 June 2016. Downloaded on 6/28/2018 11:48:57 AM. coordinate very close to that observed for DAEc (cyan hollow square in Fig. 2), proving that the emission color changes are indeed due to the reversible isomerization of the DAE photo- switch rather than the corresponding photodecomposition. Fig. 3 compiles the spectra of the extreme situations (the cocktail with no isomerization and complete isomerization DAEo / DAEc) as well as the reference micelles with DPA and DAEc alone, each at the same concentration as in the cocktail. Fig. 1 Emission spectra (lexc ¼ 375 nm) of a water dispersion of polymer micelles containing DPA (total concentration 3.5 mM) and DAE From this data, it is seen that the emission intensity of DPA is (total concentration 2 mM) at different degrees of UV-induced isom- decreased by 43% and 96% with the cocktail in 100% DAEo and erization DAEo / DAEc (365 nm, 1.5 mW cm2). The initial spectrum 100% DAEc, respectively. Hence, it is clear that DAEo is (black line) was excited at 415 nm to avoid any isomerization induced quenching DPA. As FRET quenching is excluded, likely expla- by the excitation light. The intensity was subsequently scaled to nations include the formation of a non-emissive ground state represent the corresponding intensity after excitation at 375 nm. complex between DPA and DAEo (static quenching) or the introduction of dynamic quenching processes, e.g., photoinduced electron transfer (PET), a thermodynamically favorable process for this system (see ESI† for redox data). The results from time-resolved single photon counting (SPC) measurements are consistent with the latter, as the quenching is also reected in the average uorescence lifetime of DPA, Fig. 2 CIE diagram showing the trajectories of the color changes upon UV exposure (black hollow squares) and visible light exposure (magenta hollow triangles) of the micellar cocktail containing DPA and DAE. Also shown are the coordinates for DPA alone (red solid triangle), DAEc alone (blue solid triangle), and the cocktail after a second dose of UV light, again triggering isomerization to 100% DAEc (cyan hollow square). implies that the “dynamic range” of the color switching is close to being maximized. Perfect white light corresponds to a CIE coordinate of Fig. 3 Emission spectra of DPA alone (black line), DAEc alone (blue line), the cocktail DPA + DAEo (green line), and DPA + DAEc (red line), (0.33, 0.33), and a common strategy of generating this type of all in water dispersions of the polymer micelles. The concentration of light is to mix a blue emitter and a yellow emitter in adequate the respective species was the same in all experiments (3.5 mM for DPA proportions. As mentioned above, this approach is “static”, in and 2 mM for DAE). This journal is © The Royal Society of Chemistry 2016 Chem. Sci., 2016, 7, 5867–5871 | 5869 View Article Online Chemical Science Edge Article decreasing from 7.5 ns to 4.2 ns by the introduction of DAEo. increase of the overall emission intensity. This is ascribed to The formation of a non-emissive ground state complex would activation of FRET sensitization, which in turn decreases the instead have no effect on the lifetime. It is also encouraging to efficiency of non-radiative PET losses from DPA*. In addition, note that the quenching efficiency judged by the decrease in the direct excitation of DAEc is also contributing to the increase. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DPA average lifetime, 44%, agrees very well with the The subsequent minor decrease is explained by the onset of PET corresponding result from the steady-state emission from DPA* to DAEc, competing with the FRET process. measurements, 43%. Finally, choosing the DAE concentration implies a trade-off At the other extreme, where 100% of the DAE population is between the decreased overall emission intensity resulting from isomerized to DAEc, the overall emission from this species non-radiative losses from PET reactions, and maximizing the arises from direct excitation as well as sensitization via FRET dynamic range of the CIE trajectory. At 2 mM DAE, the resulting from the excited state of DPA (DPA*). Aer compensation for initial emission intensity of DPA in the cocktail corresponds to Open Access Article. Published on 02 June 2016. Downloaded on 6/28/2018 11:48:57 AM. the directly excited fraction (cf. blue line in Fig. 3) the FRET a quantum yield of 0.57. We nd this satisfactory, as it both sensitization efficiency is estimated to 45%, that is, signicantly allows for coverage of all emission colors between pure DPA and lower than the corresponding quenching efficiency of DPA of DAEc, as well as maintaining a relatively constant overall 96%. Again, this discrepancy is ascribed to thermodynamically emission intensity irrespective of the degree of DAE favorable PET processes originating from DPA*, rather than the isomerization. corresponding processes from DAEc* (see below). The results from SPC agree well with the steady-state observations, as the Conclusions quenching efficiency from the time-resolved data is determined to 91% (average DPA lifetime decreasing from 7.5 ns to 0.7 ns by We have presented a FRET-based self-assembled construct the introduction of DAEc), which is close to the 96% from where the emission color is reversibly controlled by means of steady-state measurements. The subsequent decay of DAEc* is photonic stimuli. As the system exhibits the CIE coordinates not affected by the presence of DPA (average lifetime of 2.5 ns (0.32, 0.33), we achieve virtually perfect white-light generation. for DAEc* alone and 2.5 ns in the cocktail). Hence, we can Due to the strongly emissive nature of the photochromic exclude PET processes contributing to the overall deactivation acceptor, the resulting uorescence output remains high of DAEc*, fully consistent with the much less favorable driving throughout operation within the whole range of emission color. force for these processes (see ESI† for a comprehensive evaluation of the SPC data and redox energies). Acknowledgements The variation of the overall emission intensity with increasing isomerization DAEo / DAEc is worth commenting This work was nanced by the Swedish Research Council VR on (see Fig. 4). Initially, the emission quantum yield is 0.57. 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