Thermal control of intermolecular interactions and tuning of fluorescent state energies

The prospect of tuning the energy of emitting states through external stimuli opens the possibility of shifting the energy of emitting units on demand and controlling the bimolecular processes they are involved in. To prove this concept, the fluorescence properties of three differently 9,10-substituted anthracene (ANT) 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 that 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 it does not prevent the formation of multichromophoric aggregates where interactions between Pt-porphyrin and the emissiv...


Introduction
In properly designed systems, the control of homochromophoric 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 heterochromophoric 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 aggregates 3,4 emerged as an alternative strategy to overcome the lack of molecular diffusion 5,6 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.
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 (T m ). 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. 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 behaviour as the corresponding triplet states, according to literature data. [8][9][10][11] To this aim, we select three anthracene derivatives with different 9,10-substitutents (Scheme S1); the 9,10-substitutions can significantly alter the - intermolecular interactions occurring in the ground and/or excited state but, according to experimental data and DFT calculations, only induce minimal changes of the energies either in the singlet and triplet manifold. Eicosane (EC), an aliphatic paraffin already used in our previous work, is chosen as a PCM (T m =37 ºC). 12

Results and Discussion
Anthracene (ANT) in liquid EC presents well-structured emission and excitation spectra ( Figure 1) in agreement with the literature data. 15 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, 16 whose formation is also confirmed by the overlap of the excitation spectra recorded at 443 and 510 nm ( Figure S2). The phase change of the medium also affects the fluorescence decay of ANT ( Figure S1 and Table S1). The mono-exponential behavior observed in liquid EC (τ = 5.  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 S3a) broadens and loses the vibronic structure characteristic of monomeric DPA, 17 while the emission spectrum ( Figure S3b) is essentially unaltered. In contrast to ANT, this suggests that, although ground-state interactions do take place between DPA molecules in solid EC, 18 neither excimerlike species formation nor emission energy variation occur. We ascribe these results to the steric hindrance imparted by the large phenyl substituents, which impede achieving sufficiently short interchromophoric distances as to enable for excited state interactions. Actually, the ground state aggregation of DPA-analogues has been already observed, 18 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 monoexponential decay whose decay time (10.3 ns) well matches the value reported for DMA in homogeneous solutions. 19 After cooling the PCM below the T m , 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, 20 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. 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.  interfering dye. We demonstrate that by simply changing the temperature from above to below