Persistent Room Temperature Phosphorescence from Triarylboranes: A Combined Experimental and Theoretical Study

Abstract Achieving highly efficient phosphorescence in purely organic luminophors at room temperature remains a major challenge due to slow intersystem crossing (ISC) rates in combination with effective non‐radiative processes in those systems. Most room temperature phosphorescent (RTP) organic materials have O‐ or N‐lone pairs leading to low lying (n, π*) and (π, π*) excited states which accelerate k isc through El‐Sayed's rule. Herein, we report the first persistent RTP with lifetimes up to 0.5 s from simple triarylboranes which have no lone pairs. RTP is only observed in the crystalline state and in highly doped PMMA films which are indicative of aggregation induced emission (AIE). Detailed crystal structure analysis suggested that intermolecular interactions are important for efficient RTP. Furthermore, photophysical studies of the isolated molecules in a frozen glass, in combination with DFT/MRCI calculations, show that (σ, B p)→(π, B p) transitions accelerate the ISC process. This work provides a new approach for the design of RTP materials without (n, π*) transitions.


Introduction
Luminophores with ultralong room temperature phosphorescence (RTP) have attracted much attention because of av ariety of applications in time-gated biological imaging, [1] anti-counterfeiting, [2] watch dials,s afety signs,a nd optoelectronic devices. [3] Unlike metal-containing materials,i nw hich the heavy atom effect can efficiently accelerate the intersystem crossing (ISC) process from singlet to triplet excited states, [4] RTPfrom purely organic molecules is relatively rare because the formation of the triplet states is usually not efficient as ISC is slow.Inaddition, radiative decay from T 1 to the S 0 ground state is also spin forbidden, and is very slow compared to the non-radiative relaxation from T 1 in an unrestricted environment. [5] Designing purely organic systems showing ultralong RTPi sachallenge. [6] Key approaches involve reducing the nonradiative decay rate (k nr (T 1 )) from T 1 by avoiding collisions with quenching species such as oxygen, and minimizing vibrational relaxation (Figure 1a). [7] For example,T ang and co-workers reported purely organic luminophores which phosphoresce in the crystalline state. [8] Huang and colleagues proposed that effective stabilization of triplet excited states through strong coupling in H-aggregated molecules enables their lifetimes to become orders of magnitude longer than those of conventional organic fluorophores. [9] Adachi and co-workers developed efficient persistent RTPmaterials by minimizing nonradiative decay rates in organic amorphous host-guest materials. [10] Very recently, Wang and co-workers have achieved ultralong RTPf rom Nphenyl-2-naphthylamine by confining it in ac rystalline dibromobiphenyl matrix. [11] To increase the population of triplet excitons,h eteroatoms with lone pairs are usually introduced into organic systems to enhance spin-orbit coupling (El-Sayedsr ule), [12] which is why most RTP phosphors are limited to phenothiazine,c arbazole,a nd naphthylimide derivatives (Figure 1b). [1b, 4d, 13] Thus,i ntersystem crossing usually involves 1 (n, p*)! 3 (p, p*) transitions. Recently,a rylboronic acids and esters,w hich also contain lone pairs on their hydroxy or alkoxy groups,h ave been reported to show RTPwith lifetimes up to several seconds in the solid state. [14] Thus far, ultralong RTPfrom purely organic phosphors without lone pairs has rarely been reported, [15] as k isc is slow.
In fact, organic compounds without lone pairs, [16] such as triarylboranes,can show phosphorescence in afrozen optical glass at 77 K( Figure 1c). [17] This indicates that k isc in aphotoexcited triarylborane molecule can compete with fluorescence,f or which the rate constant is usually on the order of 10 7 s À1 .Therefore,wepropose that k isc can also be accelerated by (s,Bp)!(p,Bp) transitions,which would be the inversion of the normally observed 1 (n, p*)! 3 (p, p*) ISC process ( Figure 1d). However,probably due to the fact that the nonradiative decay rate from T 1 k nr p at RT is usually much faster than the phosphorescence,R TP from triarylboranes has not been reported. Only if k nr p is suppressed to al arge extent, might we observe RTPf rom triarylboranes.I n 1955, Wittig et al. reported that some triarylboranes,i ncluding tris(2methylphenyl)borane,s howed ay ellowish-white emission under UV light. [18] However,n ol ifetimes were reported and, when we prepared tris(2-methylphenyl)borane,i t showed only blue fluorescence;inother words,nophosphorescence at room temperature was detected (Supporting Information, Figures S14 and S15). Given our interest in the linear and nonlinear optical properties of 3-coordinate organoboron compounds, [19] we examined the triarylboranes 1-4 ( Figure 1e). Crystalline samples of 3 (tris(2,6-dimethylphenyl)borane) show ultralong (t p = 478 ms), intense,y ellow phosphorescence under ambient conditions,a nd it is thus,t o the best of our knowledge,t he first triarylboron compound without lone pairs to display ultralong RTP.

Results and Discussion
Thes ynthesis and characterization of all compounds are given in the Supporting Information and the photophysical properties of 1-4 are summarized in Table 1. Thei mportant results of our quantum chemical studies are shown in brackets,a nd complete data are given in Tables S2 and S4 in the Supporting Information. TheU V/Vis absorption and emission spectra were first measured in hexane.C ompounds 1-4 all show ab road first absorption band between 280-350 nm in hexane,which can be assigned to B ! p transitions, that is,atransition from the aryl ring p-systems to the empty p-orbital on the boron atom ( Figure 2a). Our calculations reveal that this band is formed by up to five electronic transitions,S 1 ! S 0 to S 4 ! S 0 in the D 3 -symmetric compound 3 and S 1 ! S 0 to S 5 ! S 0 in the less symmetric compounds 1, 2, and 4 (Supporting Information, Figures S1-S5). Theenergies

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Research Articles of the absorption maxima decrease in the order 1 > 2 > 3 > 4. This indicates that introducing each methyl substituent, aw eak s-donor, on the phenyl ring, redshifts the absorption spectra by 6-12 nm (580-1230 cm À1 ). Thef luorescence spectra of the compounds in hexane show the same trend;t heir maxima redshift 2-7 nm (150-530 cm À1 )f or each methyl group added to the phenyl ring ( Figure 2a). However,t he emission spectra of crystalline 1-4 are not related to their chemical structures in an obvious way ( Figure 2b). In general, the fluorescence spectra of the solid, crystalline samples of compounds 1-4 are all redshifted compared with those in hexane solution. Theb athochromic shift of 3 (2060 cm À1 )i s considerably larger than those of 1, 2,a nd 4.T he bathochromic shifts of 2 (750 cm À1 )a nd 4 (930 cm À1 )a re smaller than that of 1 (1150 cm À1 ). This indicates that intermolecular interactions in crystalline 1 and 3 are larger than those in 2 and 4,w hich is one possible explanation for the slower nonradiative decay (k nr )f rom both S 1 and T 1 ,s ee below.I n addition, compounds 1 and 3 may also have ah igher probability of showing excimer emissions. We noticed that upon exposure to ah and-held UV-lamp (l = 365 nm), crystalline 3 showed violet fluorescence which disappears immediately when the lamp is turned off.P ersistent greenish-yellow phosphorescence emission was then observed, which is visible to the naked eye for almost 4s ( Figure 3). Time-gated emission spectroscopy revealed longlived (t = 478 ms) phosphorescence from crystalline 3,with an emission maximum at 575 nm and as houlder at 540 nm (Figures 2b and S13 in the Supporting Information). To the best of our knowledge,t his is the first triarylborane without any heavy atom [6j] to show long-lived RTP, and one of the rare examples where free electron pairs are absent. In addition to the RTPf rom 3,R TP was also observed from crystalline 1, with ap hosphorescence emission maximum at 515 nm and al ifetime of t = 680 ms.C ompared to compound 3,t he phosphorescence quantum yield (F P )o f1 is 0.26 %, which is lower than that of 3 (1.14 %). We did not observe any phosphorescence from compounds 2 and 4 at room temperature.
Interestingly,w ef ound that the photoluminescence behavior of 3 largely depends on its aggregation state.W e investigated two different kinds of aggregation states,c rystalline sample A and ball-milled sample B.SEM pictures and powder X-ray diffraction (pXRD) patterns clearly revealed the difference between the samples.I nt he SEM pictures of the ball-milled powder,wecan see smaller size particles with al arger surface area (Supporting Information, Figure S21). This is in agreement with the powder X-ray diffraction pattern of the ball-milled sample,w hich shows broader reflections compared to the diffraction pattern obtained from the crystalline sample A (Supporting Information,Figure S29), and this indicates that sample B contains much smaller crystallites than the sample A.W ef ind that the emission maxima differ by 25 nm (1760 cm À1 )a nd that the peak at 350 nm in the excitation spectrum of sample A decreases in intensity as the crystalline domains increase (Supporting Information, Figures S23 and S24). Although the fluorescence lifetime and the time-gated phosphorescence emission spectra remained the same,t he phosphorescence lifetime and quantum yield decreased significantly for the ball-milled sample B compared with crystalline A,from 478 to 340 ms and 1.12 %t o0.2 %, respectively.I nt he ball-milled powder,t he exposed surface area is much larger,a nd phosphorescence is more sensitive to oxygen quenching, compared to the crystalline state.This hypothesis is supported by phosphorescence lifetime measurements under argon, for which the difference between the two samples disappears (Supporting Information, Figures S27 and S28).
To understand further the relationship between molecular structure and phosphorescence,w em easured the emission spectra of 1-4 in af rozen methylcyclohexane optical glass at 77 K( Supporting Information, Figures S30 and S31), where we can assume that there are no intermolecular interactions present (c < 10 À5 mol L À1 ). All four compounds show two well-separated emission bands.W eo bserved phosphorescence emissions (400-600 nm), which are all hypsochromically shifted in comparison to the emission from the solid at room temperature (by 5230-5670 cm À1 ). All compounds show similar vibrational fine structures except compound 1,b ecause 1 has low-frequencyvibrational modes according to our calculations,w hich broaden the emission bands.I na ddition, there are high energy fluorescence emission bands (330-400 nm), which show less vibrational fine structure (Supporting Information, Figure S30). Them axima of the computed emission spectra (Supporting Information, Figures S8 and S11) are redshifted (by 1220-1820 cm À1 )w ith respect to the experimental spectra in af rozen glass while the energies of the 0-0 transitions agree well. Theredshifts of the maxima are partially attributed to the harmonic oscillator approximation, which overestimates the intensities at the long wavelength tail of the emission spectrum that stems from electric dipole transitions between the vibrational ground state of the electronically excited state and vibrationally excited levels of the electronic ground state.The calculated values for k isc of 1-4 are circa 10 7 s À1 ,thus ISC can compete with fluorescence. Noticeably,t he major components of the phosphorescence lifetimes of all four compounds are similar, with av alue of circa 1.5 s( Supporting Information, Figure S32). Up to six triplet states are located energetically below or very close to the S 1 state as shown in Figures S6 and S7 in the Supporting Information. Some of the triplet potential energy surfaces cross the S 1 energy profile along the linear interpolated path connecting the Franck-Condon point with the minimum of the S 1 state.I SC is nevertheless fastest for at ransition between S 1 and T 2 in 1, 2, 3,and 4.T ounderstand the origin of the non-negligible spin-orbit coupling (SOC) between these states,w ec omputed and plotted the differences of the electron densities between the ground and excited-state wave functions.S 1 and T 1 of compound 3,f or example (Figure 4), result from similar (p,Bp) excitations,w ith T 1 showing additional contributions from local (p, p*) excitations on xylyl ring a. Forthis transition, [12] the SOC is very small. Comparing the difference densities of S 1 and T 2 instead, we see two major differences.First, in T 2 ,most of the electron density has been transferred from the other two xylyl rings ba nd c. As the largest SOCs result from one-center terms,e xcitations from different p systems to the same boron orbital yield negligible interaction matrix elements.The second, and more important, difference with regard to SOC is acontribution to the T 2 wave function in which charge is transferred from a s-type orbital connecting xylyl ring aw ith boron. Thec hange of orbital angular momentum on this carbon atom leads to stronger SOC than expected in the absence of (n, p*) excitations.This evidence clearly demonstrates that k isc can also be accelerated by (s,Bp)!(p,Bp) and (p,Bp ) !(s,Bp )t ransitions.A n electronic matrix element j< T 2 j H SO j S 1 >j % 1cm À1 is sufficient for ISC to proceed at ar ate of circa 10 7 s À1 (for more details,see Tables S1 and S2 in the Supporting Information). As the calculated fluorescence and ISC rate constants are of the same order of magnitude,t he competition between the two processes is easily explained. T 2 and T 1 form aJ ahn-Te ller pair which is degenerate in D 3 -symmetric geometries.

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Thes tates are thus coupled by strong vibronic interactions that facilitate fast T 2 ! T 1 internal conversion. Although triplet formation is likely to occur in all compounds,n o phosphorescence was detected in solution at room temperature,likely due to rapid nonradiative decay k nr (T 1 )compared to slow k p .O ur data indicate that triplet excited states are formed in all compounds 1-4 after excitation.
We further noticed that the RTPe mission in crystalline samples of 3 is noticeably redshifted by 5230 cm À1 when compared to that in the frozen glass ( Figure 2d). Such alarge shift makes it unlikely that it results from the suppression of the internal conversion (temperature effect) in the excited state,o rb yaless polar environment (environment effect) in the frozen glass.T oexamine how temperature influences the luminescent behavior of crystalline samples,w ea lso measured the emission spectra of crystalline 1-4 at 77 K (Supporting Information, Figure S33). In crystalline 3,asharp fluorescence peak appears at 415 nm at 77 K, which is almost identical to the fluorescence in the frozen glass.H owever, av ery broad phosphorescent emission ranging from 430 to 720 nm (Figure 2d)i so bserved, which we assign to two phosphorescence bands,one at 488 nm and asecond ranging from 500 to 720 nm. We noticed that the band at 488 nm is only visible at low temperature and is most likely not av ibrational band of the 500-720 nm emission, for which the range is identical to the spectrum at room temperature ( Figure 2d). We observe two lifetimes,o ne of 1.64 s, and asecond of 0.52 s, which further support the existence of two independent triplet states.W enote that the longer lifetime is almost identical to the lifetime in the frozen glass,inwhich we can assume the absence of any intermolecular interaction except with solvent matrix molecules.W ea ssume that the band at 488 nm is phosphorescence which is caused by the population of the T 1 state of the triarylboranes and which is only visible when the non-radiative decay is suppressed. Therefore,i tc annot be observed at higher temperatures,a t which k nr (T 1 )d ominate.T his emission is also found in the frozen glass in which it is shifted by 67 nm (3260 cm À1 ), which is ar easonable shift if one considers the different environments of the frozen glass matrix and the crystalline sample. Theemission between 500-720 nm, however, is the real RTP emission which is an aggregation induced phenomenon, in contrast to the phosphorescence at 488 nm. It is important to note that this emission is absent in the dilute frozen glass,i n which we can assume that the emission resembles that of the isolated molecules.F urthermore,w hen 3 is embedded in ap oly(methyl methacrylate) (PMMA) matrix, RTPi so nly observed in very highly doped films (! 50 wt %, Figure 2c), further confirming the critical role of aggregation for this emission (Supporting Information, Figures S16-S20).
To understand the effect of the solid-state structures and the intermolecular packing on the luminescence properties, the crystal structures of compounds 1-4 were obtained by single-crystal X-ray diffraction (Supporting Information, Figures S39-S42). If we compare the molecular geometries of compounds 1-4 in their crystal structures,w ec an observe the influence of additional methyl groups on the phenyl rings close to the central boron atom. While the B À Cb ond distances lie in as imilar range for the bulkier m-xylyl and mesityl groups (1.576-1.587 ), the BÀC( aryl) distances to the o-tolyl group (BÀC = 1.570(2) in compound 2)and the phenyl ring (BÀC = 1.569(2) in compound 1)a re slightly shorter (Supporting Information, Table S7). Theeffect of the bulkiness of the substituent and, hence,r epulsion between methyl groups is further observed in the torsion angles between the aryl groups and the BC3 planes.W hile the torsion angles are in as imilar range (50.0-54.98 8)f or the mxylyl and mesityl groups in compounds 3 and 4,asignificantly smaller torsion angle (41.98 8)isobserved for the o-tolyl group in compound 2,and avery small torsion angle of only 16.18 8 is observed for the phenyl group in compound 1.T hese smaller torsion angles are compensated by larger torsion angles (56.7-65.38 8)for the m-xylyl groups in compounds 1 and 2 compared to those of compounds 3 and 4 (Table S7).
In order to compare and classify the types and magnitudes of the intermolecular interactions within single crystals of these four triarylboranes,which organize in acomplex threedimensional arrangement, the concept of Hirshfeld surface analysis was applied (see Supporting Information for more details). [20] TheH irshfeld surface is as pecial isosurface defined by the weighting function w(r) = 0.5 for ap articular molecule.T his means that the Hirshfeld surface envelops the volume within which the particular molecule contributes more than half of the electron density.Hence,italso includes information on the nearest neighbors and closest contacts to the molecule.T he molecules are most densely packed in compound 2,a si sc lear from both the crystal packing coefficient c k ,w hich corresponds to the ratio of volume

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Research Articles occupied by all molecules in the unit cell to the unit cell volume,and the surface of the crystalsvoid per formula unit, which is obtained from the Hirshfeld analysis (Supporting Information, Table S8). [21] Interestingly,c ompounds 1 and 3 show similar, intermediate packing densities,while compound 4 seems to have the loosest packing.While the surfaces of the voids seem to be spread well throughout the unit cells of compounds 1, 2,a nd 4,alarger void of 9 3 is present in compound 3 around the origin of the unit cell ( Figure 5). From comparison of the fluorescence emissions of compounds 1-4,w ecan conclude that the RTPi sn ot correlated with the packing density,ascompound 2 is the densest packed compound. Ad eeper insight into the intermolecular interactions is required in order to provide an interpretation of the observed differences in emission behavior. Fingerprint analysis of the Hirshfeld surface and its breakdown into the individual relative contributions in crystals of 1-4, [22] exhibited as trong contribution of H···H interactions (75-83 %), followed by as ignificant amount of C···H interactions (17-25 %) in all four compounds (Supporting Information, Figures S4 and S44). Only av ery weak contribution of C···C interactions is observed for compound 3 (0.2 %). While this analysis shows the relative contributions of the different types of intermolecular interactions,weare now interested in their strengths in the individual crystal structures.C ompounds 1 and 3 exhibit several significant intermolecular CÀH···C interactions,i ncluding strong, nearly linear interactions (C···H = 2.835-2.841 ,C À H···C = 164-1688 8,T able S9 in the Supporting Information). In addition, compound 1 has ashort H···H contact (2.241 )between two aryl rings,which is also demonstrated by the spike in the bottom left corner of its fingerprint plot (Supporting Information, Figure S44).
Compound 2,although more densely packed than 1 and 3, shows significantly fewer and weaker intermolecular C À H···C interactions.I na ddition, it shows an early linear, weak C À H···p interaction towards the centroid of an m-xylyl ring (H···p = 2.907 )a nd two close C···C contacts (C···C = 3.334 and 3.384 ), astrong one between two aryl rings,and aweak one between the same aryl and am ethyl group (Table S9). These results are consistent with our analysis of the fluorescence emission in the crystalline states,w herein compounds 1 and 2 have the same emission maxima although one more methyl group is introduced to the phenyl ring in compound 2. This may be explained by the presence of more and stronger interactions in 1 than in 2.I nc rystals of compound 4, intermolecular interactions are the weakest (Table S9). This is in agreement with the loosest packing mode.I na ddition to the strong C À H···C interactions,compound 3 also has astrong C···C interaction (C6···C6 = 3.319 )b etween two aryl rings with an approximately parallel alignment of their planes.This is the shortest nearest-neighbor (nn) C···C distance in all of the compounds.T he interplanar separation between the aryl planes is only 2.980 ;h owever, the offset shift is large (4.221 ), resulting in ac entroid-to-centroid distance of 5.167 ,t he latter two values being too large for at ypical offset face-to-face p···p stacking interaction between two arenes (Supporting Information, Table S10), which typically have values ranging from 3.3-3.8 for the interplanar separation, < 4.0 for the offset, and < 5.0 for the centroid-to-centroid distance. [23] There is another arrangement of nearly parallel aryl rings,w hich has al onger C···C distance (3.495 )a nd interplanar separation (3.397 ), but as maller shift (3.493 )a nd, hence,asmaller centroid-tocentroid distance of 4.872 ,a ll of those values being within the typical range of weak p···p interactions.The aryl rings,and hence the p···p interaction, are situated close to the voids, which are around the origin ( Figure 5). It is proposed that, on compression of the crystal structure,the voids may shrink and, hence,t he offset may also be reduced, enhancing the p···p interaction between these aryl rings.O nt he other hand, expansion of the molecule may also bring the rings closer together and enhance the p···p interaction. We assume that the aggregation of molecules forming C À H···C and p···p interactions is important for effective RTPi nc ompounds 1 and 3.AC···C offset aryl-aryl interaction is also present in both compounds 2 and 4 (Supporting Information, Table S10);however, the CÀH···C interactions are much weaker in these compounds.Insummary,the presence of both strong C À H···C and C···C contacts as well as weak p···p interactions in compound 3,t ogether with the void accumulation at the origin of the unit cell ( Figure 5) may be the reason for the strong redshift and persistence of the aggregation-induced phosphorescence emission of these crystals at room temperature and in highly doped PMMA-films.

Conclusion
We have prepared triarylboranes without lone pairs which exhibit long-lived room-temperature phosphorescence in the crystalline state and in highly doped PMMA films.T heoret- ical calculations revealed that the ISC process can be accelerated by transitions between local s and p excitation, which is consistent with photophysical studies of the isolated molecules in af rozen glass and is an extension of El-Sayeds rule.Moreover,the phosphorescent compounds 1 and 3 have the strongest interactions,e specially when considering CÀ H···C interactions,which appear to play an important role in achieving persistent RTPa nd, at the same time,s uppressing nonradiative decay.However,compounds 2 and 4 have fewer and weaker contacts in their crystalline states,a nd their nonradiative decay is fast, even though compound 2 has the densest packing.T hus,w ed on ot observe RTPf rom crystals of compounds 2 and 4.T his study on triarylboranes provides an interesting example of how to expand the scope of purely organic phosphorescent materials.