S1. Supplementary figures S1-S19 S2-S11 S2. Supplementary methods S12 S3. Bibliography S13

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1 Supporting Information Thermally switchable molecular upconversion emission Giuseppina Massaro, Jordi Hernando, Daniel Ruiz-Molina, # Claudio Roscini,*,# Loredana Latterini*, Department of Chemistry, Biology and Biotechnology, Perugia University, Via Elce di sotto, 8, 06123, Perugia, Italy Departament de Química, UniversitatAutònoma de Barcelona, Edifici C/n, Campus UAB, Cerdanyola del Vallès, Spain # Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, 7 Campus UAB, Bellaterra, Barcelona, Spain loredana.latterini@unipg.it; croscini@cin2.es Contents S1. Supplementary figures S1-S19 S2-S11 S2. Supplementary methods S12 S3. Bibliogray S13 S1

2 Normalized absorption Normalized emission S1. Supplementary figures PtOEP DPA 300 Figure S1. Chemical structures of UC PtOEP-DPA pair and normalized PtOEP absorption (black) and emission spectra of DPA (blue) and PtOEP (red) in toluene. a) UC 273 K 283 K 293 K 303 K b) UC Temperature (K) Figure S2. a) UC ( max = 433 nm) and osorescence () emissions ( max = 647 nm) of a PtOEP-DPA mixture in HD at different temperatures ([PtOEP] = 10 M; [DPA] = 300 M). b) Integrated intensities of the UC and osorescence bands measured for this sample at different temperatures. S2

3 UC Figure S3. Comparison of the emission spectra of a PtOEP-DPA liquid solution in HD at 303 K ([PtOEP] = 10 M; [DPA] = 300 M) before (black line) and after (red line) a cooling-warming cycle. Clearly, both UC ( max = 433 nm) and osorescence ( max = 647 nm) bands recovered their initial intensities after undergoing a liquid-solid-liquid transition. a) UC 273 K 283 K 293 K 303 K b) UC Figure S4. a) UC ( max = 433 nm) and osorescence ( max = 647 nm) emissions of a PtOEP-DPA mixture in HD at different temperatures ([PtOEP] = 1 M; [DPA] = 30 M). b) Comparison of the emission spectra of this sample at 303 K measured before (black line) and after (red line) a cooling-warming cycle. Both UC and osorescence bands recovered their initial intensities after the thermal treatment. S3

4 Intensity (Arb. u.) 303 K 273 K Figure S5. Emission spectra of an aerated PtOEP-DPA mixture ([PtOEP] = 1 M; [DPA] = 30 M) in liquid (T = 303 K) and solid (T = 273 K) HD. No UC emission ( max = 433 nm) was observed in either case, while only very dim osorescence ( max = 647 nm) was registered. a) b) Figure S6. Emission spectra of a deaerated PtOEP solution ([PtOEP] = 1 M) in a) liquid (T = 293 K) and b) solid (T = 273 K) HD. S4

5 Normalized absorption a) b) 255 mw/cm 2 64 mw/cm 2 31 mw/cm mw/cm 2 64 mw/cm 2 31 mw/cm Figure S7. Emission spectra of a deaerated PtOEP-DPA mixture ([PtOEP] = 1 M; [DPA] = 30 M) in a) liquid (T = 293 K) and b) solid (T = 273 K) HD at three different excitation power densities. In the respective insets there are the zooms of the spectra in the range of nm. 273 K 303 K Figure S8. Section of the absorption spectrum of a PtOEP-DPA mixture ([PtOEP] = 10 M, [DPA] = 300 M) in liquid (T = 293 K) and solid (T = 273 K) HD. The solid mixture was measured by using the integrating sere. S5

6 UC UC intensity (Arb. U.) a) b) 10 5 m = m = m = 2.1 I th = 82 mw/cm m = 1.9 I th = 37 mw/cm Power density (mw/cm 2 ) 10 2 Power density (mw/cm 2 ) Figure S9. Double logarithmic plots of the integrated UC emission of a PtOEP-DPA mixture in HD ([PtOEP] = 10 M; [DPA] = 300 M) measured at increasing power densities and exc = 532 nm in a) liquid (T =293 K) and b) solid (T = 273 K) state. Figure S10. UC decays measured at = 433 nm for a HD solution of PtOEP-DPA ([PtOEP] = 1 M, [DPA] = 30 M) in the liquid (T = 293 K) and solid (T = 273 K) state. Black squares and red dots correspond to the experimental decays, while solid lines were obtained by fitting equation S1 to this data (see Section S2). S6

7 Lifetime ( s) UC intenty (Arb. U.) a) b) m = 1.3 m = m = 2.3 I th = 88 mw/cm m = 2.0 I th = 47 mw/cm Power density (mw/cm 2 ) Power density (mw/cm 2 ) Figure S11. Double logarithmic plots of the integrated UC emission of a PtOEP-DPA mixture in EC ([PtOEP] = 10 M; [DPA] = 300 M) measured at increasing power densities and exc = 532 nm in a) liquid (T =323 K) and b) solid (T = 293 K) state. a) PtOEP 313 K PtOEP 303 K PtOEP-DPA 313 K PtOEP-DPA 303 K x x x10-4 Time (s) b) PtOEP PtOEP-DPA Temperature (K) Figure S12. a) Phosorescence decays at λ = 647 nm of deaerated EC solutions of PtOEP ([PtOEP] = 1 M) and PtOEP-DPA ([PtOEP] = 1 M; [DPA] = 30 M) measured at 313 K and 303 K. b) T vs temperature for the EC solutions of PtOEP and PtOEP-DPA. S7

8 Normalized absorption 323 K 293 K Figure S13. Section of the absorption spectrum of a PtOEP-DPA mixture ([PtOEP] = 10 M, [DPA] = 300 M) in liquid (T = 323 K) and solid (T = 293 K) EC. The solid mixture absorption was measured by using the integrating sere. a) b) Figure S14. Emission spectra of a deaerated PtOEP solution ([PtOEP] = 10 M) in a) liquid (T = 323 K) and b) solid (T = 293 K) EC. S8

9 a) b) 255 mw/cm 2 64 mw/cm mw/cm2 64 mw/cm Figure S15. Emission spectra of a deaerated PtOEP-DPA mixture ([PtOEP] = 1 M; [DPA] = 30 M) in a) liquid (T = 323 K) and b) solid (T = 293 K) EC at two different excitation power densities. In the respective insets there are the zooms of the spectra in the range of nm. a) 258 K 273 K 293 K 323 K b) UC UC Temperature (K) Figure S16. a) UC ( max = 433 nm) and osorescence ( max = 647 nm) emissions of a PtOEP-DPA mixture in EC at different temperatures ([PtOEP] = 0.1 M; [DPA] = 30 M). b) Integrated intensities of the UC and osorescence bands for this sample at different temperatures. S9

10 Figure S17. Representative snapshots (1 to 6) of the color change of the PtOEP-DPA mixture in EC during heating ([PtOEP] = 1 M; [DPA] = 30 M) from room temperature (298 K) to 323 K. The blue-to-red colour emission change during EC melting can be appreciated in the supported video recorded in real time. The sample was irradiated at 532 nm. Heating was induced by immersing the sample cuvette in a pre-warmed water bath (323 K). Figure S18. a) Emission spectra of a PMMA polymer film doped with a PtOEP-DPA mixture at 298 K ([PtOEP] = 1 M, [DPA] = 30 M). Clearly, no UC emission ( max = 433 nm) was measured at the same concentrations at which this enomenon was observed for solid HD and EC samples. Instead, only osorescence ( max = 647 nm) from the PtOEP sensitizer was registered. S10

11 DMA TPPy Figure S19. Chemical structures of other investigated TTA-UC pairs. S11

12 S2. Supplementary methods a) Fitting of UC emission decays in hexadecane solution The UC emission decays in Figure S10 were fitted using equation S1: β I UC = I 0 ( T exp(k DPA t) β ) T In this equation I 0 stands for the UC emission intensity right after the excitation pulse, k DPA is the unimolecular decay rate constant of DPA triplet states ( 3 DPA), and is a parameter that quantifies the initial fraction of DPA triplet states that decay via triplet-triplet annihilation characterized by the bimolecular rate constant k TTA : = k T DPA 3 k TTA [ DPA] 0 + k TTA [ 3 (S2) DPA] 0 T T Table S1 gives the k DPA and values obtained from our fits. Clearly, k DPA remained essentially constant by lowering the temperature and crystallizing the HD solvent, while notably increased indicating a larger contribution of the TTA processes required for UC in the solid matrix. (S1) T Table S1. k DPA and values obtained from fitting the UC emission decays in Figure S10 to equation S1. Temperature (K) T k DPA (s -1 ) 293 K K b) Estimation of the HD liquid-to-solid transition in the bimolecular TTA rate constant The TTA bimolecular rate constant k TTA can be determined from the I th value for UC emission using equation S3, 2 T provided that the corresponding unimolecular decay rate constant k DPA, the quantum yield of the donor-acceptor triplet-triplet energy transfer Φ TTET, and the absorption coefficient α(e) of the donor at the excitation wavelength are known. I th = ( T k DPA ) 2 Φ TTET α(e) γ TTA (S3) In our case we used equation S3 to estimate the change in k TTA occurring in PtOEP-DPA mixtures in HD 273 K 293 upon solvent solidification (k TTA / k K TTA ). For this we used the values of I th (I th = 37 and 82 mw/cm 2 at T T 273 and 293 K, respectively), Φ TTET (Φ TTET = 0.97 and 0.77 at 273 and 293 K, respectively) and k DPA (k DPA = and s -1 at 273 and 293 K, respectively) previously determined as well as assumed α(e) to 273 K 293 K negligibly vary along the K thermal range. In this way, we obtained that k TTA / k TTA = 1.75, which clearly indicates that TTA intermolecular processes are also favored in solid HD due to chromoore aggregation. S12

13 S3. Bibliogray 1) Cheng, Y. Y.; Fùckel, B.; Khoury, T.; Clady, R. G. C. R.; Tayebjee, M. J. Y.; Ekins-Daukes, N. J.; Crossley, M. J.; Schimidt, T. M. Kinetic Analysis of Photochemical Upconversion by Triplet-Triplet Annihilation: Beyond any Spin Statistical Limit, J. Phys. Chem. Lett. 2010, 1, ) Monguzzi, A.; Mezyk, J. Tubino, R.; Meinardi, F. Upconversion-induced Fluorescence in Multicomponent Systems: Steady-state excitation power threshold, Phys. Rev. B 2008, 78, S13