Carborane‐Induced Excimer Emission of Severely Twisted Bis‐o‐Carboranyl Chrysene

Abstract The synthesis of a highly twisted chrysene derivative incorporating two electron deficient o‐carboranyl groups is reported. The molecule exhibits a complex, excitation‐dependent photoluminescence, including aggregation‐induced emission (AIE) with good quantum efficiency and an exceptionally long singlet excited state lifetime. Through a combination of detailed optical studies and theoretical calculations, the excited state species are identified, including an unusual excimer induced by the presence of o‐carborane. This is the first time that o‐carborane has been shown to induce excimer formation ab initio, as well as the first observation of excimer emission by a chrysene‐based small molecule in solution. Bis‐o‐carboranyl chrysene is thus an initial member of a new family of o‐carboranyl phenacenes exhibiting a novel architecture for highly‐efficient multi‐luminescent fluorophores.

principally o-carborane (C 2 B 10 H 12 ), have now been incorporated in aw ide range of molecular and polymeric semiconductors.  Ak ey property of o-carborane is its ability to accept an excited charge via intramolecular charge transfer (ICT) from an aryl donor group,o ccurring most efficiently when the carborane has its C À Cb ond perpendicular to the aromatic plane.T hese ICT states are notable for their flexible carboranyl CÀCb ond, which can vibrate and relax nonradiatively,o rr adiatively with ah ighly red-shifted emission. [24][25][26][27][28][29][30] Such systems often exhibit aggregation-induced emission (AIE), [13-15,17-20, 22, 31, 32] that is,i na na ggregated state where molecular motion is restricted, non-radiative relaxation pathways from the ICT state are inhibited, leading to as ignificant increase in emission efficiency. It has been demonstrated that incorporation of bulky carboranes on the long edges of linear acenes creates al arge steric hinderance between the carboranes and proximal protons,l eading to aconsiderable deformation of the ring system. [15][16][17] With this comes alarge increase in the carborane rotation barrier, and generally am ore rigid structure;aproperty known to be useful for AIE.
o-Carborane has also been shown, to av ery limited extent, to aid excimer emission. When di-substituted with two naphthyl donor groups,t he imposed proximity of the aryl systems allows for an overlap of the aromatic wavefunctions, such that dimerization occurs readily when one naphthalene is in the excited state. [27] In other cases where o-carborane has been attached to pyrene,t he bulky carborane moiety was found not to inhibit the p-p stacking required for pyrene excimer formation. [19,20,23] In these few cases,h owever, the carborane can be considered essentially electronically isolated from the aromatic excimer species,s ave for some electron-withdrawing inductive effects.
To explore whether o-carborane could induce excimer emission ab initio,w ef ocused our attention on chrysene, an on-linear phenacene fluorophore.C hrysene is notable amongst polycyclica romatic hydrocarbons for its lack of excimer emission. Fordecades it was thought chrysene could only form excimers in the solid state,f ollowing morphology changes at very high pressure. [33,34] Only recently have these excimers been observed in solution, requiring supramolecular DNAs caffolds to force chrysene units sufficiently close together in af ace-to-face orientation such that excimer formation can occur. [35] Inspired by these results,w ep lanned to incorporate phenyl-o-carborane into the bay area of chrysene,theorizing that the bulky carborane would cause aromatic deformation to such an extent that p-delocalization would be inhibited. As ac onsequence,t he intermolecular repulsion potential would be lowered, conceivably allowing chrysene monomers to approach one another and support dimer formation. [33] Also, we proposed that the p-p interactions between the phenyl group of the carborane and chrysene would help to lock the carboranyl CÀCbond perpendicular to the aromatic plane in ag eometry favorable for ICT,a nd in doing so create an ew class of severely twisted, multi-emissive luminescent materials. [36] Herein, we present the synthesis of bis(phenyl-o-carborane)chrysene.W es how through single-crystal analysis the unique structure of this compound, including by far the largest a and b deformation angles reported to date for carboranyl donor-acceptor systems.Ahost of photoluminescence properties are displayed, which include localized emission (LE), ICT,and AIE, and excimer emission. Significantly,t his is the first reported excimer of ac hrysene based small molecule in solution, but most remarkably,itisthe first example of an excimer in an o-carborane containing molecule where the carborane is electronically involved, via ac harge transfer state,i nt he dimerized species.
Thes tructure of 2 was elucidated by X-ray diffraction of asingle crystal grown from slow evaporation from abiphasic mixture of dichloromethane and hexane ( Figure 1).
Thei ncorporation of the carborane results in significant distortion of the usually planar chrysene core,a ffording ah ighly asymmetrically distorted ring system (compare the planar chrysene core of 1,S upporting Information, Figures S8-10). Borrowing from the deformation parameters of anthracene-based molecules, a and b,a nd adding an ew parameter g,t he distortion of 2 is summarized in the Supporting Information, Figure S11. Thec arboranes are displaced out of the aromatic plane to ad egree much larger than other recently reported carborane-containing aromatic systems (Supporting Information, Figure S12). [15][16][17] The g parameter highlights further distortion, with the carboranes shifted perpendicular to the plane of the rings and also parallel to them, away from the bay-area protons and the medge C À Cb ond.
Thes everely twisted geometry induced by the bulky carboranes results in ac omplex array of emissive species,a s revealed by photoluminescence emission (PL) measurements, shown in Figure 2.
Thev ibronic progression around 450 nm is characteristic of LE originating from chrysene, [44] but curiously,for the THF solution, the LE spectrum of 2 shows some excitation dependence (Supporting Information, Figure S13). While the different emission efficiencies offer some insight into the long-wavelength peaks shown in Figure 2, their distinct natures are not immediately apparent from the PL spectra. Monitoring photoluminescence intensity at the emission peaks,t hat is,a t5 80 nm (AIE) and 660 nm, by photoluminescence excitation (PLE) spectroscopy (Supporting Information, Figures S14 and S15) reveals ac ommonality between the two peaks,f aithfully mirroring the ground state absorption profile obtained from UV/Vis spectroscopy (Supporting Information, Figures S16-S20), spectrally similar to that expected from chrysene. [45] Theresults suggest ageometry rearrangement is required post-absorption for these emissions to occur, consistent with both ICT,where the carborane CÀCbond elongates after accepting an excited charge, [25] and excimer formation. Both mechanisms are also consistent with the large Stokes shift observed. Time-resolved measurements (Supporting Information, Figures S21 and S22) highlight further differences.T he lifetimes of the aggregated solution (t = 23 ns; l em = 580 nm) and film (t = 17 ns; l em = 580 nm) are significantly longer to those found in the THF solution (t = 0.18 and 3.1 ns; l em = 436 and 657 nm, respectively); indeed al ifetime of 23 ns is extremely long-lived for singlet emission from an organic fluorophore. [46] Similar long lifetimes have been observed for other carborane-containing AIE-active compounds for which aggregated emissions are assigned to ICT species. [15][16][17][18][19][20] To further probe the character of the long wavelength emissions,atemperature dependent PL study was undertaken (Figure 3). At room temperature the solution of 2 in CHCl 3 is dual-emissive,a nd the emissive profile shows little change until the freezing point of CHCl 3 is reached (ca. 210 K). Upon decreasing the temperature towards the solvent freezing point, the long wavelength peak red-shifts and reduces in intensity.Incontrast, and for temperatures below the solvent freezing point, the trend has reversed, revealing ad efinite blue-shift along with as ignificant increase of the AIE peak. Furthermore,u pon freezing (T = 210-150 K), an otable increase in peak width is initially observed and consistent with the co-existence of two emission species (580 nm and 660 nm) as af raction of 2 is educed out of the solution, (Supporting Information, Figure S24). Similar observations are found in av ariable temperature study of 2 in THF (Supporting Information, Figure S25), and in aH 2 O/THF solvent composition study ( Supporting Information, Figure S26). Thes harp change in emission profile seen around the solvent freezing point in the former is mirrored at an approximate 50/50 H 2 O/THF composition in the latter.T he transition is also easily detected with the naked eye (Supporting Information, Figure S27). From this data, we infer that the 580 nm AIE peak is the result of ac ommon excited-state species present in all samples,and significantly,incontrast to similar carboranyl donor-acceptor compounds, [15,16,18] it appears to be distinct in origin to the long wavelength emission observed in good solvents.
Thee xcited state character can be investigated by solvatochromism with the aid of aL ippert-Mataga plot, as shown in the Supporting Information, Figure S31. [47] The samples of 2 in good solvents,w ith Hildebrand solubility parameters around 9cal 1/2 cm À3/2 ,f ollow ap ositive linear trend in Stokes shift with the Hansen polarity parameter for the long wavelength peak around 660 nm, [48] characteristic of ac harge-transfer nature.P oor solvent solutions of 2 do not conform to this trend, which is most likely due to the occurrence of local aggregation. [49] Fort hese cases,i ti s possible there is no longer one dominant emission species, rather two similarly contributing and competing emissions as evidenced by broadened emission peaks.T his is akin to the PL profile exhibited by the 50/50 H 2 O/THF solution composition of 2;h owever, without observing the dynamic change between the AIE and 660 nm peaks,w ec annot wholly discount the presence of other emissive species.Nevertheless, the study clearly demonstrates the 660 nm emission has as trong delocalized, charge-transfer character.F inally,w e note the Stokes shift for the LE emission is virtually independent of solvent quality,f urther supporting this is indeed emission originating from the chrysene core.
To probe the role,ifany,ofexcimers, Figure 4outlines the effect of varying concentration on the emission of 2 in THF. Of interest here is the stark reduction in the 660 nm peak, relative to the LE peak, below ac ritical concentration of about 10 À6 m.A tac oncentration of 10 À8 m,t he 660 nm peak has all but disappeared, while asmall amount of blue-shifted emission (Figure 4b)remains.W eattribute this to the 580 nm emissive species,which appears to be present in tiny amounts (Supporting Information, Figure S13). The660 nm emission is consistent with an excited state dimerization process where, above acritical concentration (ca. 10 À6 m), emission from the excimer dominates,w hilst below this most excited state relaxation occurs prior to excimer formation. On this basis and supported by evidence from the PL and solvatochromism study,weattribute the 660 nm emission to an excimer species formed from aground state and an excited molecule of 2;this is the first time excimer emission has been observed in ac hrysene-based small molecule. [33,35,44] Thenovelty and significance of this excimer can be further appreciated by comparing the degree of Stokes shift from the peak emission (l max = 660 nm) of 2 with that of the pristine chrysene excimer (l max = 475 nm). [35] Given the severe twisting of the chrysene backbone and steric hindrance induced by carboranyl groups,t he excimer of 2 is distinct to that previously observed, not solely originating from ad imer of the chrysene core.Rather, the lower energy emission points to am ore electronically delocalized excimer species,c onsistent with dimerization where at least one of the molecules exhibits ICT character, with delocalization across the chrysene core onto the carboranyl groups.
Thep resence of the 580 nm peak even at very low concentrations suggests au nimolecular excited state is responsible for this emission. To investigate this,atheoretical study employing density functional theory (DFT) and timedimensional DFT (TD-DFT) was performed, detailed in SI Experimental. Emission wavelengths were calculated, as af unction of the relative dihedral angles (f)o fC b1 and Cb2 following vertical excitation with solvent relaxation, and presented in full in the Supporting Information, Figures S33  and S35, with selected data shown in Figure 5. Thefirst point of note is that, except "Cb1 (f), Cb2(f)" of "08 8,0 8 8", all geometries exhibit localization of electron density onto the chrysene core as indicated by the frontier molecular orbitals (MOs) of the S 1 excited state.T he calculated emission wavelengths of these all span the range of 296-500 nm, in good agreement with observed LE PL emission. It appears that, depending on the relative dihedral geometries of the carboranes,chrysene is twisted to varying degrees,leading to significant differences in absorption and emission energies, that is,h igher degrees of twisting leads to shorter absorption and emissive wavelengths and vice versa. [50] As such, the complexity of the vibronic region of 2 is explained by the flexing of the chrysene core as the carboranes rotate around f. Secondly,t he "08 8,0 8 8"g lobal minimum ground state geometry (with carborane C À Cbonds virtually perpendicular to the aromatic plane) is the only geometry which shows astrong ICT character for the S 1 excited state,that is,from the chrysene donor to Cb1. Accordingly,asignificantly lower emissive energy is calculated in comparison with all other geometries,s pectrally consistent with the observed AIE  emission. Furthermore,adihedral potential energy scan (PES) carried out in the crystal bulk using an ONIOM model shows adeeper potential energy well around the global minimum geometry,o nt he order of that of 1,1'-binaphthol (BINOL), amolecule with asteric-induced axis of chirality at room temperature. [51] This suggests that in aggregated states the ground state geometry is significantly more favorable than other geometries,further favoring ICT emission.
We have sought to clarify two important points regarding the long-wavelength emission species.F irst, given the ICT state of 2 involves acharge transfer from the chrysene to the carborane,itseems reasonable to expect the excimer of 2 has asimilar ICT character.Remarkably,this is,tothe best of our knowledge,the first example of an excimer species involving ac arborane with ICT character.S econdly,a nd in line with many similar carborane-containing fluorophores, [13-15, 17-20, 22, 31, 32] the AIE observed for 2 is the result of aunimolecular ICT species.T hese two points may also be used to rationalize the differences between the emission efficiencies recorded from the two long-wavelength emissions.Webelieve that establishing these two distinct processes is as ignificant result, highlighting o-carborane as an ovel excimer-inducible element block, [52] and consequently,t hat ICT is not necessarily as ufficient explanation for long wavelength emission observed from carborane-containing donor-acceptor systems.
In conclusion, we have synthesized ab is-o-carborane based chrysene derivative,the first member of anew class of severely twisted luminescent phenacene-based carboranes, exhibiting the highest a and b deformation angles of any reported carborane-containing compound. Optical studies revealed ac omplex, excitation dependent, multi-emissive profile,d emonstrating AIE with good quantum efficiency (F PL = 0.32) and exceptionally long singlet excited state lifetimes (t = 23 ns). Thenature of the two broad, featureless and highly red-shifted emissions was initially ambiguous,but solvatochromism studies revealed the 660 nm peak to have significant charge-transfer character and concentration studies confirmed the emissive species to be an excimer.While the origin of the AIE emissive species was not definitively confirmed, ICT is believed to be responsible.S ignificantly, this is the first reported excimer to electronically involve ac arborane as well as the first solution-based chrysene excimer not requiring as upramolecular scaffold. Through innovative architectural design, we were able to induce excimer formation and AIE by locking the molecule in ag eometry highly favorable for ICT formation.