Ultralong Organic Phosphorescence in the Solid State: The Case of Triphenylene Cocrystals with Halo- and Dihalo-penta/tetrafluorobenzene

The polycyclic aromatic hydrocarbon (PAH) triphenylene (TP) has been reacted with halo-pentafluorobenzene (XF5, X = Br, I) and 1,4-dihalo-tetrafluorobenzene (X2F4, X = Br, I) to yield the corresponding cocrystals TP·BrF5, TP·Br2F4, TP·IF5, and TP·I2F4 form I. These materials have been synthesized by dissolving TP into an excess of liquid or molten coformer, and single crystals have been grown via seeding chloroform solutions. They have been fully characterized by a combination of techniques including X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy in the solid state. TP·I2F4 form I was found to undergo a single-crystal to single-crystal (SCSC) polymorphic phase transition induced by temperature (when cooled down to 100 K) leading to the new form TP·I2F4 form II, which is transformed back into the first structure when brought again at RT. This behavior was confirmed also by Raman spectroscopy. Upon cocrystallization and as a result of the external heavy atom effect, all crystalline mat...

In the specific, persistent RTP has received considerable attention in the last decade due to significant advantages and potential over fluorescence for application in bioelectronic and optoelectronic devices. [18][19][20] Phosphorescence, indeed, may enable highly sensitive bioimaging because of the exclusion of background and endogenous fluorescence, and active layers based on phosphorescent materials have shown a quantum efficiency up to 3-fold higher with respect to fluorescent ones in OLED devices.
Crystal engineering, [21][22][23] which has as ultimate goal the tailoring of physical and chemical properties of solids through the study and exploitation of intermolecular interactions within the context of the crystal packing, offers the powerful tool of cocrystallization for designing of materials with desirable properties.
Organic cocrystals are multi-component systems wherein a molecule of interest and a co-former, purposely chosen to alter the packing of the former, self-assemble together within a unique crystalline structure through non-covalent interactions. Though simple, this approach 24 has proved to be a successful tool for achieving optical properties of use for several applications, such as NIR photothermal conversion, 25 white-light emission, 26 development of thermo-and photochromic materials, [27][28][29] fluorescence color modification [29][30][31][32] and enhancement 33,34 as well as for promoting delayed-fluorescence [35][36][37] and room temperature phosphorescence from purely organic systems. [38][39][40][41] The purpose of this study is to design and investigate organic cocrystals that could display phosphorescence in the solid state and at RT. This approach is based on a strategy that relies on cocrystal formation via π-hole···π interactions 37,42 and halogen bond (XB) of the kind X···π [43][44][45] and makes use of organic co-formers containing heavy atoms (halo-perfluorobenzene). These play two key roles within the cocrystals: (i) they serve as solid diluents to prevent the emitting molecule from aggregating or self-quenching, and (ii) they act as perturbants, inducing phosphorescence emission in the emitting chromophore. In particular, our study focuses on the investigation of the role of the halogen involved in cocrystal formation in tuning the optical properties of the organic photoactive component, by using both iodide-and bromide-containing co-formers.

X-ray Diffraction
Single crystal data for all cocrystals were collected at 100K, except TP·I2F4 which was collected either at 100 K and RT (300 K), on an Oxford XCalibur S CCD diffractometer equipped with a graphite monochromator (Mo-Kα radiation, λ = 0.71073Å). Data collection and refinement details are listed in Table 1. All non-hydrogen atoms were refined anisotropically. Single crystals of the coformer Br2F4 were also obtained and its structure redetermined. 49 For this compound, crystal structure and data and details are reported in Figure SI-1, and Table SI-1. For TP·BrF5 and TP·IF5, the halo-pentafluorobenzene co-former was found to be disordered over two positions (see Figure   SI-2) and refined anisotropically applying EADP constraints.
Site occupancy factors (SOFs) were also refined by adding a second free variable in the FVAR command line. Two twin unit cells were indexed for TP·I2F4 form II, and the reflection data were integrated with the default configuration for twinned crystals of CrysAlisPro software. Subsequent structure solution and refinement were performed using the HKLF4 file containing non-overlapped reflections. HCH atoms for all compounds were added in calculated positions and refined riding on their respective carbon atoms. SHELX97 50 was used for structure solution and refinement on F2.

Raman Spectroscopy
Raman spectra were recorded on a Bruker MultiRam FT-Raman spectrometer equipped with a cooled Ge-diode detector. The excitation source was a Nd 3+ -YAG laser (1064 nm) in the backscattering (180°) configuration. The focused laser beam diameter was about 100 μm and the spectral resolution 4 cm -1 . The reported spectra were recorded with a laser power at the sample of about 20 mW.

Optical Spectroscopy
Room temperature measurements were performed on powder samples placed inside two quartz slides or on single crystals placed on the top of a quartz slide. For 77 K determinations the samples were placed inside quartz capillary tubes and immersed in liquid nitrogen in a homemade quartz Dewar. Reflectance spectra were acquired with a PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer equipped with a 100 mm integrating sphere and converted in absorption spectra using the Kubelka-Munk function. 33 A thin layer of powder was used (absorbance values below 0.2) for luminescence measurements.
Emission spectra were collected in front-face mode with an Edinburgh FLS920 fluorimeter equipped with a Peltier-cooled Hamamatsu R928 PMT (200−850 nm) and corrected for the wavelength dependent phototube response. Absolute emission quantum yields were determined according to the method reported by Ishida et al., 53 each measurement was repeated 3 times. The limit of detection of the system is 0.020.
Gated emission spectra were acquired in front-face mode with the same fluorimeter by using a timegated spectral scanning mode and a μF920H Xenon flash lamp (pulse width < 2 μs, repetition rate between 0.1 and 100 Hz) as excitation source. Spectra were corrected for the wavelength dependent phototube response. Phosphorescence decays were acquired with the same apparatus in multichannel scaling mode.
Fluorescence lifetimes were measured by using an IBH time-correlated single-photon counting (TCSPC) apparatus by using a pulsed NanoLED excitation source at 331 nm. Analysis of the luminescence decay profiles against time was accomplished with the Decay Analysis Software DAS6 provided by the manufacturer. Estimated errors are 10% on lifetimes and 2 nm on emission and absorption peaks.

Structural Description and Synthesis of Polycrystalline Samples
Single crystal XRD analysis for all cocrystals had to be carried out at low temperature (100K) because all samples suffered degradation under the X-ray beam except TP·I2F4 formI, for which it was possible to collect an additional dataset at RT. Interestingly, TP·I2F4 form I was found to undergo a phase transition upon cooling (see below) to a new polymorph denoted as TP·I2F4 form II.
The crystal structures of TP·BrF5, TP·IF5, TP·Br2F4, TP·I2F4 form I and II show some common features and differences. Extended stacks in which molecules of TP alternate with the corresponding halo-perfluorobenzene in an ···A···B···A···B··· fashion characterize all cocrystals, see Figure 1 and Table SI-2 for geometrical parameters. However, as a consequence of the different nature of the co-formers, the inter-planar distances and tilted angles between the triphenylene and the halo-perfluorobenzene are slightly different (Table SI- Table   SI-2. These X··· π intermolecular interactions can be seen as a particular case of directional halogen bonding where the electron depleted polar regions of the polarized halogens reach out toward the electron-rich π-orbitals of an aromatic compound.   Considering that the π-hole···π interactions supply a "milder external heavy atom effect" 37,42 we expected to quench TP fluorescence more than its phosphorescence, achieving thus UOP from TPbased cocrystals. Cocrystallization results in bright RT phosphorescence, clearly visible at naked eye, which lasts for a few seconds after removing the excitation light source (see Figure 4 and a video in the Supporting Information). To circumvent these drawbacks, we decided to attempt the synthesis of cocrystals TP·Br2F4 and TP·I2F4 form I from the melt. However, even this approach showed some critical points. Due to the sublimation of the co-formers, we had to start from an excess of X2F4 to achieve complete conversion of TP into the corresponding cocrystal ( Figure SI-5). The so-obtained samples were then used for the spectroscopic and photophysical characterization (vide infra). Scheme 2 depicts a comparison between the different synthetic strategies.

Scheme 2.
Comparison between different synthetic approaches highlighting why cocrystallization from the melt is more effective for the obtainment of polycrystalline samples suitable for spectroscopic analyses.

Raman Spectroscopy
The spectrum of the TP·I2F4 form I cocrystal ( Figure 5) was dominated by the bands of TP, although the bands of I2F4 were clearly visible and identified. Wavenumbers and assignments [58][59][60][61] of the main Raman bands of the cocrystal, TP and I2F4 are reported in Table SI-3. Going from the co-formers to the cocrystal, several bands underwent wavenumber shifts and changes in relative intensities, as shown in Figure 5 and Many TP bands shifted upon adduct formation; the blue shift observed for the TP C-H stretching band at 3051 cm -1 (which shifted to 3056 cm -1 in the cocrystal) was reported also by Shen et al. 65 for the cocrystals of I2F4 with naphthalene or phenantrene. The red shift of the TP C-H in plane bending at 1247 cm -1 (which shifted to 1245 cm -1 in the cocrystal) was observed also for I2F4pyrene cocrystals. 64 These changes as well as the other observable in Figure 5 (and detailed in Table SI-3) may be assigned to the intermolecular interactions between I2F4 and TP; actually, as can be easily seen in were stronger in the experimental spectrum than in the theoretical one, the latter to a significant extent, confirming to be the main marker band to detect the halogen bond occurrence.
The low-wavenumber range appeared the most suitable to confirm the occurrence of the reversible polymorphic transformation of TP·I2F4 form I into its form II upon cooling to 77 K. As can be seen from the spectra reported in Figure SI  By comparing the Raman spectrum of the TP·Br2F4 cocrystal with those of the coformers, i.e. TP and Br2F4 (Figure 6 and Table SI-4 for wavenumbers and assignments), 59-63 trends similar to those above reported for TP·I2F4 form I were observed. Also in this case, the experimental spectrum was not the superposition of the spectra of the single components, and was significantly different from the theoretical one ( Figure SI-10), suggesting the occurrence of distinct interactions. The differences between experimental and theoretical spectra of TP·Br2F4 were observed in the same spectral ranges as for TP·I2F4 form I cocrystal ( Figure SI-8).
De Santis et al. 66 have compared the shifts of the I2F4 and Br2F4 bands at 159 and 211 cm -1 , respectively, upon halogen bond formation with dipyridyl derivatives; they have reported that the former band shifted to a higher extent than the latter and have ascribed this behavior to the stronger halogen-bonding donating character of I2F4, i.e. to its higher acidity character.

Optical spectroscopy
Absorption and emission properties of the stable cocrystals TP·I2F4 form I and TP·Br2F4 have been characterized at room temperature in the solid state and compared to those of bare TP, studied as reference compound. TP·I2F4 luminescence has also been characterized at 77K, to probe the effects of the thermally triggered phase transformation leading to TP·I2F4 form II.
Solid TP showed a bright fluorescence in the 400-500 nm region with a quantum yield of 37%.
( Figure 7 and Table 2). The vibronic structure resembled that reported for TP fluorescence in solution, 67,69 but with an overall bathochromic shift of ca. 40 nm. A similar trend has been described for TP spin-coated films and attributed to π-stacking of the planar molecules. 42 The fluorescence decay could be satisfactorily fitted by a bi-exponential function, with a main component in the order of 46 ns (Table 2, Figure SI-11a). TP also exhibited a weak phosphorescence that could be detected only by means of a gated detection ( Figure SI-12). It can be noticed that, as well as fluorescence, the observed phosphorescence is largely red-shifted (ca. 90 nm) with respect to that detected for TP in solution at low temperature. [69][70][71] The phosphorescence decay showed a bi-exponential behavior with lifetimes of ca. 90 ms and 290 ms (Table 2, Figure SI-14a). (single crystals, olive) in the solid state at room temperature. exc = 310 nm. mean values from two measurements. Representative decays are shown in Figure SI In contrast to pure TP, the luminescence of cocrystal TP·I2F4 form I was characterized by an intense phosphorescence, being the fluorescence of TP almost completely suppressed (Figure 7).
The phosphorescence spectrum, isolated by means of a gated detection, is reported in Figure SI-13.
This outcome can be ascribed to the external heavy atom effect produced by the iodine atoms of the co-former which interact with the π system of the TP molecule in the crystal lattice through weak I···π interactions. The phosphorescence lifetime is reduced to ca. 10 ms since, as well as intersystem crossing, the radiative deactivation rate of the triplet state is increased by the presence of the heavy atom. The overall luminescence quantum yield, which is dominated by phosphorescence emission, is 36% (Table 2), i.e. comparable with the fluorescence quantum yield of the pure TP. This is a remarkable value for organic phosphorescence at room temperature and in aerated conditions. At low temperature TP·I2F4 form II showed a similar behavior, with a weak fluorescence and an intense phosphorescence, the latter characterized by a higher spectral resolution with respect to room temperature ( Figure SI-15). The luminescence data at 77 K are summarized in Table SI (Table 2). § It can be noticed that, while the vibronic structure of fluorescence was well comparable among the three compounds (Table 2) and halo-perfluorobenzene (XF5 and X2F4, where X = I or Br) co-formers were successfully assembled by π-hole···π, a subclass of π-stacking, and further stabilized by a combination of weak X···π and X···X interactions.
Cocrystals TP·BrF5 and TP·IF5 display disorder of the co-former and are unstable once taken out from the mother liquor, making further spectroscopic characterization unfeasible. Cocrystal TP·I2F4 exists in two different polymorphs as a function of temperature, namely form I and II for RT and LT, respectively. As confirmed by variable temperature SCXRD, transitions between these two phases occur reversibly in a single crystal to single crystal fashion, and, as proved by variable temperature Raman spectroscopy, also in polycrystalline samples.
The luminescence properties of the cocrystals TP·Br2F4, TP·I2F4 form I and TP·I2F4 form II in the solid state were studied and compared with those of pure TP. Interestingly, at a difference with TP which is exclusively fluorescent at room temperature (a weak phosphorescence could be detected only in gated mode), the cocrystals TP·I2F4 form I and TP·Br2F4 were found to be also phosphorescent, but with different properties in their triplet emission depending on the nature of the involved halogen atom. The higher spin-orbit coupling constant of iodine with respect to bromine ( = 5069 and 2460 cm -1 , respectively), 72 led to an almost quantitative intersystem crossing in the   organic units of TP·I2F4 form I, with the result of a nearly exclusive bright phosphorescence emission. Yet, because of the same process, the triplet excited state lifetime was found to be of the order of 10 ms. The interaction of bromine with the π-system of TP resulted in an effective but not complete quenching of the fluorescence, with the interesting outcome of a phosphorescence lifetime of 208 ms in single crystals. The orange phosphorescence of these crystals, in fact, could be observed by naked eye lasting for ca.

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources
Financial support from MIUR, and the University of Bologna is acknowledged. Italian CNR (Project "PHEEL") and MIUR-CNR project "Nanomax" N-CHEM are gratefully acknowledged.

Notes
The authors declare no competing financial interest. § The CIE 1931 spectral chromaticity coordinates of the luminescence of TP, TP·I2F4 form I and