Thermal Control of Intermolecular Interactions and Tuning of

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Thermal Control of Intermolecular Interactions and Tuning of Fluorescent State Energies Giuseppina Massaro, Giulia Zampini, Daniel Ruiz-Molina, Jordi Hernando, Claudio Roscini, and Loredana Latterini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09774 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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The Journal of Physical Chemistry

Thermal Control of Intermolecular Interactions and Tuning of Fluorescent State Energies Giuseppina Massaro,†,‡ Giulia Zampini,† Daniel Ruiz-Molina,# Jordi Hernando, Claudio Roscini,#,* Loredana Latterini†,* † Department of Chemistry, Biology and Biotechnology, Perugia University, Via Elce di sotto, 8, 06123 Perugia, Italy # Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain. Departament de Química, UniversitatAutònoma de Barcelona, Edifici C/n, Campus UAB, 08193 Cerdanyola del Vallès, Spain. ‡ Present address: Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans 16, 43007 Tarragona, Spain

Corresponding Authors *Loredana Latterini: [email protected]

* Claudio Roscini: [email protected].

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Abstract The prospect to tune the energy of emitting states through external stimuli opens the possibility to shift the energy of emitting units on demand and control the bimolecular processes they are involved in. To prove this concept, the fluorescence properties of three differently 9,10substituted anthracene derivatives are investigated in a phase change material (eicosane). The liquid-to-solid transition of the medium leads to an increase of the local dye concentration, a shortening of the intermolecular distances and the establishment of excited and ground-state interactions. As a result, a new contribution to the overall luminescence derives from the downshifted emission (up to 0.7 eV) from excimer-like species is observed. The addition of a second dye (a Pt-porphyrin) reduces the efficiency of excited and ground-state complexes between fluorophore units, although does not prevent the formation of multichromophoric aggregates where interactions between Pt-porphyrin and the emissive state of anthracene derivatives are observed. The emission of excimer-like species, formed upon solidification of the medium, can be exploited to further down shift the fluorescence through energy transfer processes to a suitable energy acceptor, like Rubrene.


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Introduction In properly designed systems, the control of homo-chromophoric intermolecular distances or interactions can modify the nature of the lowest excited states, which affects the efficiency, and interestingly, the energy of emitted radiation.1,2 On the other hand, by controlling the energy of the luminescent state, the management of the rate and the efficiency of hetero-chromophoric bimolecular processes involving the excited states can be achieved. For instance, the luminescence originating from non-linear processes, such as the anti-Stokes emission of upconverting molecular materials, can be modulated in this way. In particular, triplet-triplet annihilation based upconversion (TTA-UC) requires intermolecular energy transfer among the excited states of donors and acceptors and these interactions can be manipulated by tuning the energy of their emitting states. This is valid not only for liquid solutions but also for solid materials, where the energy migration through molecular aggregates3,4 emerged as an alternative strategy to overcome the lack of molecular diffusion5,6 and allow TTA-UC in the solid phase. We have previously exploited the aggregation phenomenon of the donor units in aliphatic phase change materials (PCMs) to achieve thermally switchable TTA-UC.7 While negligible energy transfer processes and, as such, red phosphorescence from the donors were observed in the liquid state of the system at relatively low dye concentrations, blue upconverted emission was registered upon solidification of the medium. This was due to chromophore aggregation in the solid PCM, which facilitated the energy transfer processes that underlie TTA-UC. Though we demonstrated the very efficient triplet-triplet energy transfer process between sensitizers and emitters (ET> 95%) in the solid PCM and the role of sensitizer aggregates, we did not explore the occurrence of interactions among the emitter units and their impact on the energy of their emitting state.


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Notably, the fluorescence of the emitters is able to report on intermolecular interactions occurring on the singlet state but also on the triplet manifold. For instance, bimolecular interactions or extended aggregation can stabilize singlet and triplet states thus fastening energy transfer or energy migration. These interactions can enhance the efficiency of triplet energy transfer (up to TTET

0.9) involved in TTA-UC and explain the high UC quantum yield (UC 0.06)7 observed below

the PCM melting point (Tm). Furthermore, the outcome of careful analysis of the fluorescence data could be easily extended to many fundamental processes. In view of this, we investigate herein the fluorescence properties of anthracene dyes (frequently used as emitters in TTA-UC) as a function of the PCM phase and we further extend these studies in the presence of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP), usually employed as an antenna to sensitize TTA-UC, and Rubrene, a dye potentially suitable to receive the energy from the excimer species of anthracene derivatives. The modifications of the fluorescence spectra and decay times should provide information on the ground and/or excited state intermolecular interactions and document changes of their singlet state energies, which have the same behavior as the corresponding triplet states, according to literature data.8-11 To this aim, we select three anthracene derivatives with different 9,10-substitutents (Scheme S1); the 9,10substitutions can significantly alter the - intermolecular interactions occurring in the ground and/or excited state12-15 but, according to experimental data and DFT calculations, only induce minimal changes of the energies either in the singlet and triplet manifold.16 Eicosane (EC), an aliphatic paraffin already used in our previous work, is chosen as a PCM (Tm=37 ºC).17


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Experimental section Materials. 9,10-diphenylanthracene (DPA), 9,10-dimethylanthracene (DMA, 99%, EGA Chemie), anthracene (ANT, 99%, J. T. Baker), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP, 98%, Sigma Aldrich) and rubrene (RUB, 99%, J. T. Baker) were used without further purification. Dichloromethane (DCM, ≥ 99.5%) has been provided by Alfa Aesar. Eicosane (EC, 98%, melting temperature, Tm = 37 °C), hexadecane (HD, 98%, Tm = 18 °C) were purchased from Merkel and Sigma Aldrich, respectively, and used as received. Solutions preparation. Stock solutions of DPA ([DPA]0 = 1.35 mM), DMA ([DMA]0 = 3.5 mM), ANT ([ANT]0 = 0.53 mM), PtOEP ([PtOEP]0 = 0.12 mM) and RUB ([RUB]0 = 0.92 mM) were prepared in DCM and stored in the dark to prevent any photodegradation. The proper amount of the stock solution was added to the weighted quantity of EC (EC20ºC = 0.7886 mg/mL18) to obtain the desired concentration of the dyes in EC. After the addition of DCM to EC, the mixture was stirred and warmed above the melting temperature of the PCM, thus allowing the homogenization of the system. The complete removal of volatile DCM was thus achieved. The samples were transferred into cuvettes to perform the photophysical analysis. In the sample containing both DPA (or, alternatively, DMA or ANT) and PtOEP, the molar ratio between the two dyes was set at 30:1. For the mixture DMA-RUB, the molar ratio was 3:1. Photophysical characterization. The fluorescence emission and excitation spectra have been recorded on a Fluorolog-Spex F112AI spectrofluorometer equipped with a Xenon lamp as an excitation source and a temperature-controlled sample holder. The spectra have been acquired in front face configuration (with a 23° angle between the excitation light and the detector) and have been corrected for the instrumental response. The emission spectra have been corrected for the


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self-absorption effect, as reported in ref. 19. Fluorescence decay times were measured by the single photon counting method using an Edinburgh Instrument 199S setup. A 370 nm nanoLED with a 0.2 ns pulse duration was used as an excitation source and the signal was acquired by a Hamamatsu R7400U-03 detector. The decays have been collected using a 45° configuration between the excitation light and the detector. Solid EC samples were measured at 20°C while the measurements for the EC liquid solutions were performed at 50°C. The HD solutions were measured at 10, 20 and 50°C.

Results and Discussion Anthracene (ANT) in liquid EC presents well-structured emission and excitation spectra (Figure 1) in agreement with the literature data.16 The liquid-to-solid transition of the medium results in dramatic spectral changes. The emission spectrum of ANT in solid EC preserves its vibronic structure, though the relative intensities of the bands are strongly altered and a new broad band centered at 510 nm appears (Figure 1b). The excitation spectrum loses its characteristic vibronic structure and becomes broadened (Figure 1a). The comparison with literature data supports the assignment of the 510 nm band to the ANT excimer,20 whose formation is also confirmed by the overlap of the excitation spectra recorded at 443 and 510 nm (Figure S1). The phase change of the medium also affects the fluorescence decay of ANT (Figure S2 and Table S1). The mono-exponential behavior observed in liquid EC (τ = 5.1 ns) is attributable to the monomeric species, while in the solid sample the decay has a bi-exponential behavior with decay times that vary with the emission wavelength. The shorter decay time for the monomeric emission


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(0.8 and 2.2 ns at 418 nm), was ascribed to the quenching of the excited state due to the formation of excimer species, to which corresponds longer decay times at 510 nm (1.4 and 5.2 ns). Since the liquid-solid phase transition of the solvent is achieved by temperature variations, doubts may arise on the temperature impact on the spectroscopic behavior of the compounds under examination. The attribution of the significant changes in the fluorescence behavior exclusively to the solidification of EC, rather than to temperature variation is firstly supported by literature data; these data which evidence negligible temperature effects on the radiative and non-radiative decay rates of anthracene derivatives in hydrocarbon solvents, in the investigated range of temperature (20-50 °C).21,22 Moreover, steady state fluorescence spectra of ANT recorded in hexadecane (HD, 16-carbon paraffin, Tm =18 °C) at 20 and 50°C, at which HD is always liquid, are practically superimposable (Figure S3), while spectral variations similar to those observed in solid EC are observed at 10°C (at which HD is solid). Thus, upon EC solidification, the emission spectrum of ANT documents that excimer-like species are formed due to the establishment of excited state intermolecular interactions,23 although ground state aggregation cannot be excluded to account for the changes in the excitation spectra. Considering the emission maximum of the monomer in liquid eicosane (3.1 eV) and the maximum of the excimer emission (2.4 eV), obtained in the solid medium, we conclude that a new emitting state, 0.7 eV downshifted compared to the monomer, raises upon PCM solidification.


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Figure 1. (a) Normalized excitation (λem = 418 nm) and (b) emission spectra (λexc = 326 nm) of ANT ([ANT] = 0.11 mM) in liquid and solid EC.

When the emitter has bulky substituents tethered to the anthracene core, as in the case of 9,10diphenylanthracene (DPA), the liquid to solid transition of EC produces notable but dissimilar changes of the fluorescence behaviour. Once EC solidifies, the excitation spectrum (Figure S4a) broadens and loses the vibronic structure characteristic of monomeric DPA,24 while the emission spectrum (Figure S4b) is essentially unaltered (only a very small red shift of the spectrum is observed upon liquid-solid transition of the medium; a similar behaviour to what obtained upon solidification of DPA in HD, Figure S5). In contrast to ANT, this suggests that, although groundstate interactions do take place between DPA molecules in solid EC,25 neither excimer-like species formation nor significant emission energy variation occur. We ascribe these results to the steric hindrance imparted by the large phenyl substituents,26,27,14 which impede achieving sufficiently short inter-chromophoric distances as to enable for excited state interactions. Actually, the ground


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state aggregation of DPA-analogues has been already observed,25 but at fluorophore concentrations at least one order of magnitude higher. When 9,10-dimethylanthracene (DMA) is used as an emitting unit, both ground-state aggregates and excimers are formed in solid EC. In liquid EC, the structured monomeric emission is observed (Figure 2a) and it has a mono-exponential decay whose decay time (10.3 ns) well matches the value reported for DMA in homogeneous solutions.28 After cooling the PCM below the Tm, in addition to the structured emission spectrum (390 - 520 nm), the appearance of a broader band centered at 550 nm is observed (Figure 2a). The latter is due to excimer-like species,29 as indicated by the good overlap of the excitation spectra collected at 427 and 540 nm (Figure 2b). Furthermore, the concomitant broadening of the excitation spectra stands for the presence of ground-state aggregates in the solid medium. Also in the case of DMA, fluorescence spectral features are not related to the temperature variations, in the explored interval, but rather to the liquid-solid transition of the medium. This is supported by the spectral changes observed for DMA in liquidsolid HD (Figure S6), which resemble those observed in EC, but occur above and below the Tm of HD. Thus, in the case of DMA, the energy of the lowest fluorescence state shifts from 2.9 eV to 2.2 eV going from liquid to solid EC.


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Figure 2. (a) Normalized emission spectra (λexc = 372 nm) of DMA in solid and liquid EC. (b) Normalized excitation spectra recorded at the monomer (λem = 427 nm) and excimer (λem = 540 nm) emission maxima in solid EC ([DMA] = 1.50 mM). Time-resolved measurements confirmed that DMA excimer-like species are formed when EC solidifies. Below Tm the monomer average lifetime decreases significantly (mean = 5.5 ns) and its decay curve could be reproduced by a bi-exponential function (Figure 3, Table S2), confirming the interaction of the excited state, similarly to what observed for ANT. On the other hand, a nonexponential emission decay with much longer average lifetime (53.2 ns) was collected in the excimer region (λem = 560 nm), in agreement with the literature data.30


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DMA in: liquid EC; em= 427 nm solid EC; em= 427 nm

Counts

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solid EC; em= 560 nm

2

10

1

10

0

60

  ns

120

180

Figure 3. Emission decay curves of DMA in liquid and solid EC. The green lines represent the fitting curves of the experimental points ([DMA] = 1.50 mM; λexc = 370 nm).

The spectral and decay times modification observed for ANT, DMA and, in a lesser extent, DPA when passing from liquid to solid EC can be attributed to the shortening of the intermolecular distances and to the increase of local fluorophore concentration enabling the occurrence of ground and/or excited state interactions, as already reported for aromatic molecules in aliphatic media.31 In fact, the PCM liquid-to-solid transition brings spectral modifications analogous to those obtained when increasing the dye concentration (Figure S7-S10). The presence of other types of chromophores can interfere with the arrangement of the emitting units in the aggregates formed in solid phase EC, thus altering the resulting intermolecular interactions and, as a result, impacting on the energy and kinetic properties of the fluorophore excited states. To verify this hypothesis, we decided to introduce PtOEP (a typical sensitizer for


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TTA-UC) as an additional chromophore to the anthracene derivatives/EC solutions, choosing a PtOEP/emitter molar concentration ratio (1/30) normally used in TTA-UC systems.32 In the presence of PtOEP, the emission spectrum of ANT in solid EC lacks the 510 nm band (Figure 4a-b), indicating that the formation of excimer-like species is inhibited by addition of the porphyrin molecules. At these conditions, however, the establishment of interactions between PtOEP and the excited state of ANT is confirmed by the shortening of the fluorescence lifetime (quenching) (Figure S11 and Table S3), which reveals that chromophore aggregation does still take place. The partial structuration of the DPA excitation spectrum and the minor modification of the DPA emission spectrum observed in the presence of PtOEP indicate that porphyrin addition also reduces the DPA ground-state interactions resulting from aggregation (Figure 4c-d). On the other hand, porphyrin effect on the DPA excited singlet manifold is negligible, since the emission spectrum is not modified by the presence of PtOEP. The present data indicate that the efficient UC-emission observed in solid PCM for PtOEP-DPA mixtures7 is mainly due to the presence of ground-state heterocomplexes which can facilitate energy transfer processes (e.g. via triplet migration).3,4 The nature of these ground-state complexes and their interaction mechanism deserve deeper experimental and theoretical investigations, which are however above the aims of the present work.


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Figure 4. (a) Normalized excitation (λem = 443 nm) and (b) emission (λexc = 326 nm) spectra of ANT and PtOEP-ANT ([ANT] = 0.11 mM; [PtOEP] = 3.7 µM). (c) Normalized excitation (λem = 430 nm) and (d) emission spectra (λexc = 372 nm) of DPA and PtOEP-DPA in solid EC ([DPA] = 1.35 mM; [PtOEP] = 45 µM). (e) Normalized excitation (λexc = 430 nm) and (f) emission (λexc = 372 nm) spectra of DMA in solid EC in the absence and presence of PtOEP ([DMA] = 1.5 mM; [PtOEP] = 45 µM). (The valley at 380 nm is due to the re-absorption of PtOEP).


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The data collected on the PtOEP-DMA system (Figure 4e-f) demonstrate that, in the presence of the porphyrin, both the excimer and the monomer emission bands are preserved, though they appear modified. The presence of PtOEP mainly affects the excited states of the emitter. Decay time measurements (Figure S12, Table S2) performed in the presence of PtOEP reveal that whereas in liquid EC the DMA decay is not affected by the porphyrin, a clear shortening of excimer emission (up to 4 times) in solid EC is observed. This behaviour documents the interactions between the excited DMA (monomer and excimer) and PtOEP, when the dyes are at short distance. This means that the solidification of EC not only induces the formation of excited state (or ground-state) homomolecular complexes, but also fosters interactions between molecules of different nature (i.e. PtOEP and DMA hetero-complexes). The possibility to establish these hetero-molecular interactions inspired us to exploit the solid/liquid transition of EC to reversibly control the energy transfer between DMA excimer-like species and dyes with suitable absorption and fluorescence properties. As proof-of-concept we selected the highly absorbing (ε528nm = 1.1104mol-1 dm3 cm-1 33) and emitting (f = 0.98 16) rubrene (RUB) as energy acceptor because of its good absorption overlap with the emission of the DMA excimer-like species (500-550 nm) and because the excitation and emission spectra of RUB show, under the experimental conditions used, negligible variations (see Figure S13) in the liquid to solid phase transition. In particular, the EC solidification should make operative the intermolecular interactions between DMA and RUB and consequently the RUB emission is expected to be activated upon DMA excitation through energy transfer processes involving DMA excimer-like species. Indeed, when RUB is added to the DMA solution, the selective DMA excitation (at exc = 400 nm) resulted in the detection of RUB fluorescence in the range 550-700 nm.


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As expected this emission is only observed when the medium is solid (Figure 5). The changes evidenced by the fluorescence decay parameters (see Table S4) enable to hypothesize the occurrence of non-radiative energy transfer processes from DMA excimer-like species to Rubrene.

Figure5. (a) Normalized emission spectra of DMA and (b) DMA in the presence of RUB in liquid (red line) and solid (black line) EC. ([DMA] = 0.15 mM; [RUB] = 46 µM; λexc = 400 nm).

This result shows that PCMs can be used not only to thermally and reversibly control the homoand hetero-molecular interactions of dyes, and therefore tune the energy of the emitted radiations, but also, upon suitable design of the dye mixture, to achieve wide emission down-shifts (from UV to red region). Conclusions In summary, we have investigated the fluorescence behaviour of three anthracene derivatives to rationalize the impact of medium phase change on the inter-chromophoric ground and excited state


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electronic interactions. While in the liquid state the dyes all show the monomeric fluorescence features, indicating lack of intermolecular interactions, in the solid state, these interactions are established at low dye concentrations (mM). The negligible temperature effect, in the explored range (20-50 °C), on the stationary emission properties and on the rate of the decays of anthracene derivatives, supports the idea that the solidification of the medium is the driving force for the establishment of intermolecular interactions able to lower the energy of the emitting state. The electronic state involved depends on the substituents: ground-state aggregates are observed for DPA while both ground-state aggregates and excimers are detected for ANT and DMA, with the latter forming excimers predominantly. In the latter case, the formation of excimer-like species results in changes in the energy of the emitting state of about 0.7 eV and this shift is preserved even in the presence of an interfering dye. We demonstrate that by simply changing the temperature from above to below the melting point of PCM the tuning between monomer to excimer (and/or aggregate) emission can be produced. Furthermore, in the presence of suitable dyes, like Rubrene, the excimer-like species can transfer the excitation energy and shift the emitted light down in the red-region of the spectrum.

Supporting Information. Schemes of molecular structures, emission and excitation spectra, tables reporting the fitting parameters of the fluorescence decay times, is available free of charge on http://pubs.acs.org AUTHOR INFORMATION Corresponding authors: e-mail: [email protected];


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[email protected] ORCID: Loredana Latterini - 0000-0002-1021-9423 Claudio Roscini - 0000-0002-0157-8934 Daniel Ruiz Molina - 0000-0002-6844-8421 Giuseppina Massaro - 0000-0002-3971-8453 Giulia Zampini - 0000-0002-2684-1604 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The University of Perugia and the Ministero Istruzione dell’Università e della Ricerca (MIUR) are thanked for the financial support through the program “Dipartimenti di Eccellenza 2018-2022” (grant AMIS). This work was supported by project MAT2015-70615-R and CTQ2015-65439-R from the Spanish Government and by FEDER funds. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). Funded by the CERCA Programme/Generalitat de Catalunya.

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(23) The spectra display that the formation of ANT excimer-like species in solid EC is concentration dependent, as observed in liquid solutions; thus the band in the 450-550 nm region is not observed until a defined concentration (ca. 0.11 mM for ANT), at which the excimer emission is detectable. When the ANT concentration is further increased above this value, the formation of ground states aggregates competes with the excimer formation, as shown by the excitation spectra. An analogous effect is also observed for DMA, although the threshold concentration is 1.50 mM (see SI figure S10). For DPA, no excimerlike species are observed (figure S9), probably due to the formation of ground-state aggregates is favored. (24) Hamal, S.; Hirayama, F. Actinometric Determination of Absolute Fluorescence Quantum Yields. J. Phys. Chem.1983, 87, 83-89. (25) Oyamada, T.; Akiyama, S.; Yahiro, M.; Saigou, M.; Shiro, M.; Sasabe, H.; Adachi, C. Unusual photoluminescence characteristics of tetraphenylpyrene (TPPy) in various aggregated morphologies. Chem. Phys. Lett., 2006, 421, 295–299. (26) Barnes, R. L.; Birks, J. B., Flowers, B. H.Excimer' Fluorescence. X. Spectral Studies of 9-Methyl and 9, 10-Dimethyl Anthracene, Proc. R. Soc. Lond. A Math. Phys. Sci., 1966, 291, 570–582. (27) Hinoue, T.; Shigenoi, Y.; Sugino, M.; Mizobe, Y.; Hisaki, I.; Miyata, M.; Tohnai, N. Regulation of π‐Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission Chem. Eur. J., 2012, 18, 4634 – 4643. (28) Scully, A. D.; Yasuda, H.; Okamoto, M.; Hirayama, S. The effect of pressure on the dynamic quenching by oxygen of the excited singlet state of 9, 10-dimethylanthracene in solution. Chem. Phys., 1991, 157, 271 – 278. (29) Birks, J. B. Excimers. Rep. Prog. Phys., 1975, 38, 903-974. (30) Mataga, N.; Torihashi, Y.; Ota, Y. Studies on the fluorescence decay times of anthracene and perylene excimers in rigid matrices at low temperatures in relation to the structures of excimers Chem. Phys. Lett., 1967, 1, 385-387


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(31) Duan, P.; 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., 2015, 137, 1887−1894; (32) Penconi, M.; Ortica, F.; Elisei, F.; Gentili, P.L. New molecular pairs for low power noncoherent triplet–triplet annihilation based upconversion: dependence on the triplet energies of sensitizer and emitter. J. Lumin., 2013, 135, 265-270. (33) Lima, C. F. R. A. C.; Costa, J. S. C.; Lima, L. M. S. S.; Melo, A.; Silva, A. S. M.; Santos, L. M. N. B. F. Energetic and Structural Insights into the Molecular and Supramolecular Properties of Rubrene ChemistrySelect., 2017, 2, 1759 – 1769.


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Thermal Control of Intermolecular Interactions and Tuning of

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