Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter**

Abstract We report the characterization of rotaxanes based on a carbazole‐benzophenone thermally activated delayed fluorescence luminophore. We find that the mechanical bond leads to an improvement in key photophysical properties of the emitter, notably an increase in photoluminescence quantum yield and a decrease in the energy difference between singlet and triplet states, as well as fine tuning of the emission wavelength, a feat that is difficult to achieve when using covalently bound substituents. Computational simulations, supported by X‐ray crystallography, suggest that this tuning of properties occurs due to weak interactions between the axle and the macrocycle that are enforced by the mechanical bond. This work highlights the benefits of using the mechanical bond to refine existing luminophores, providing a new avenue for emitter optimization that can ultimately increase the performance of these molecules.


Introduction
Organic compounds exhibiting thermally activated delayed fluorescence (TADF) have enjoyed tremendous recent attention due to their ability to undergo efficient spin state changes between the low-lying excited states.T his has led to TADF compounds being exploited as emitters in organic light emitting diodes (OLEDs), [1] where they enable both singlet and triplet excitons to be harvested to achieve high efficiency, as well as photocatalysts for photoredox-based organic transformations, [2] sensors [3] and as bio-imaging agents. [4] One of the major challenges in the optimization of TADF emitters is the inherent contradiction in the parameters that determine their photophysical properties. [5] TADF is the result of two successive processes:r everse intersystem crossing (rISC) from the lowest excited triplet (T 1 )t ot he lowest excited singlet state (S 1 ), followed by the emission from S 1 to the ground state (S 0 ). Thee fficiency of the former can be increased by decreasing the singlet-triplet energy gap, DE ST , which is itself governed by the exchange integral between the HOMO and LUMO; [1a] as maller overlap of the electron density distributions of these two orbitals leads to as maller DE ST .However,the rate of emission from S 1 is proportional to the overlap between these two orbitals.T hus,u pon first inspection, it appears paradoxical to maximize the efficiencies of both simultaneously.
Ther ate of rISC,characterised by the rate constant k rISC , increases exponentially as DE ST decreases. [1b] In purely organic TADF compounds,asmall DE ST is accomplished by spatially separating the orbitals involved in the lowest excited state which minimises the effect of Pauli repulsion (exchange) interactions. [6] Themost common molecular design to achieve this relies upon donor-acceptor (D-A) architectures,w here the HOMO is localized on the donor and the LUMO is localized on the acceptor,resulting in S 1 and T 1 excited states that are predominantly intramolecular charge-transfer (CT) in character. [7] Several strategies to modulate the overlap between the two frontier molecular orbitals (FMOs) have been successfully applied, including inducing al arge torsion between the Da nd Au nits by insertion of bulky substituents, [5b, 6,8] inserting spiro-junctions, [9] and physically separating the FMOs through ah omo-junction that allows through-space interaction of the HOMO and LUMO. [10] Thechallenge of enhancing k rISC by decreasing DE ST whilst maintaining sufficient k r ,t og ive high photoluminescence quantum yield, F PL ,h as motivated as ignificant amount of experimental [11] and computational [12] work focused on both elucidating and resolving the complex interplay between the photophysical properties of TADF materials.T his has demonstrated, with the exception of multi-resonance emitters, [13] the importance of the rotational freedom around the D-A bond to permit vibronic coupling between T 1 and other low lying triplet excited states which aids rISC to the S 1 state, combined with anear 908 8 mean dihedral angle between donor and acceptor to minimise DE ST .
Designing TADF emitters with the required fine structural control is not trivial. Many publications have focused on either using covalent modifications,s uch as substitution to enhance steric hindrance, [14] or non-covalent interactions [15] to modify the conformational preferences of the D-A bond. An alternative and as yet unexplored approach is to use the crowded, flexible environment of the mechanical bond in mechanically interlocked molecules (MIMs) [16] such ar otaxanes to influence the properties of aT ADF luminophore, although this approach has been used to influence the properties of other radiative processes. [17] Indeed, the investigation of MIMs has expanded dramatically over the last half-century,t hanks largely to the development of high yielding,f lexible synthetic methodologies [16] that make them available for study in ar ange of areas including as sensors [18] and catalysts, [19] as well as their well-known role as components of molecular machines. [20] Herein we report as eries of carbazole-benzophenone-basedr otaxanes that demonstrate the ability of the environment provided by the macrocycles threaded close to the emitting core to fine-tune the photophysical properties of aTADF-active axle in solution and thin films. [21] Results and Discussion

Synthesis of Interlocked TADF Emitters
Thed esign of prototypical interlocked TADF emitters [2]rotaxane 1&2 and [3]rotaxane 1&2 2 ( Figure 1a)was based on the carbazole-benzophenone system developed by Zysman-Colman and co-workers. [22] Therotaxanes were synthesised in good yield using an active template [23] Cu-mediated azide-alkyne cycloaddition (AT-CuAAC) [24] reaction between ab enzophenone (BP) substituted carbazole (Cz) bis-alkyne core and abulky benzylic azide in the presence of macrocycle 2 [25] (see ESI for details,S cheme S2). Non-interlocked axle 1 was synthesized using the CuAAC [26] reaction in the absence of macrocycle 2. 1 HNMR analysis of the purified products revealed the expected differences between the non-interlocked and interlocked emitters ( Figure 1b Single-crystal X-ray diffraction analysis of [2]rotaxane 1&2 [37] suggests that, in keeping with previous reports, [24c] the high ppm chemical shift of the encircled triazole protons is due to CH···N hydrogen bonds to the bipyridine Na toms (Figure 1c). In addition, anetwork of weak C À H···p,C À H···N and C À H···O contacts between the macrocycle and axle are observed in the solid state.V iewed in as pacefill representation, it is clear that the macrocycle impinges significantly on the Cz fragment but does not interact with the BP unit, suggesting that any changes in the photophysical properties of the interlocked structures relative to the non-interlocked axle are likely to arise from modulation of the properties of the donor unit. Photophysical Properties of 1, 1&2a nd 1&2 2 All three emitters exhibited the expected CT UV/Vis absorption band, which is slightly and progressively redshifted with each additional macrocycle encircling the axle ( Figure 2a). Thes ame trend was observed in the broad, steady-state photoluminescence (PL) spectra, with am ore pronounced shift in the emission maxima, l PL ,from 449 nm to 477 nm and 484 nm, for 1, 1&2 and 1&2 2 ,r espectively ( Figure 2b). Abathochromic shift was also observed in more polar solvents,c onsistent with the emissive excited state having significant CT character ( Figure S39). [27] Rotaxanes 1&2 and 1&2 2 displayed higher F PL than the non-interlocked axle 1 under both deoxygenated and ambient conditions (Table 1). F PL was lower in the presence of O 2 as is expected for TADF-active compounds,f or which accessible triplet states play ak ey role.P rompt, t p ,a nd delayed, t d , fluorescence lifetimes were obtained from time-resolved PL decays ( Figure 3a). Theaverage t p and t d values both increase from 1 to 1&2 to 1&2 2 (Table 1). Prompt fluorescence for 1&2 and 1&2 2 at 77 Ks till retains aC Tc haracter,w hile the structured phosphorescence observed is clearly locally excited ( 3 LE) in nature (Figures 3b-d);t he prompt fluorescence for 1 at 77 Kshows amixed CT/LE character.The differences in molecular orbital type between S 1 and T 1 implies that direct rISC is possible between these two states according to El Sayedsr ule. [28] The DE ST values calculated from the difference between the onset of the prompt fluorescence and phosphorescence emission in spectra measured at 77 K decreased from 0.25 eV for 1 to 0.23 eV for 1&2 and 0.21 eV for 1&2 2 .
Decreasing DE ST is very desirable for TADF.The decrease across the series superficially appears to contradict the increase in t d as lower DE ST is expected to increase k rISC .This apparent contradiction can be resolved by considering that emission lifetimes depend on both the rate of emission and the rate of non-radiative decay,w hich both deplete the excited state population. Theh igher F PL of 1&2 and 1&2 2 compared with 1 despite the longer measured lifetimes suggests that the mechanical bond serves to supress nonradiative decay,outweighing the effect of any increase in k rISC on the emission lifetime.T his effect of the mechanical bond appears to extend to the photostability of the compounds; upon continuous irradiation at 325 nm of an aerated toluene solution the emission spectra of 1 evolved to one that contained am ore pronounced LE character,w hich implies photodegradation of the emitter,w hereas the emission profiles of rotaxanes 1&2 and 1&2 2 saw only am odest decrease in intensity over the same period ( Figure S38). [29] TheH OMO and LUMO levels of 1, 1&2 and 1&2 2 were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in MeCN (Table 1). TheC z-centred   oxidation waves and BP-centred reduction waves were found to be reversible for all three molecules ( Figure S39). The oxidation potentials, E 1/2 ox ,f or 1 (1.20 V), 1&2 (1.05 V) and 1&2 2 (0.99 V) and the corresponding trend in the HOMO levels are consistent with the bathochromic shift observed in both the absorption and PL spectra. Thereduction potentials, E red ,f or 1 (À1.72 V), 1&2 (À1.74 V) and 1&2 2 (À1.75 V) are very similar, suggesting that the LUMO levels are largely unaffected by the mechanical bond, consistent with the solidstate structure of 1&2 that suggests there is little interaction between the BP acceptor with the macrocycle components of the rotaxanes.
Computational Modelling of 1, 1&2a nd 1&2 2 To aid in the interpretation of the photophysical and electrochemical data, and shed light on the role of the mechanical bond in determining the properties of 1, 1&2 and 1&2 2 ,density functional theory (DFT) and ab initio molecular dynamics (MD) simulations were performed using the Q-Chem [ 30 ] and Te rachem [31] softwares,respectively.Consistent with previous results, [22] DFT (PBE0) [32] analysis of the lowest energy conformer of the ground state (S 0 )s tructures found the HOMO to be centred on the Cz donor moiety in all three cases,w ith the LUMO centred on the BP acceptor unit (Figure 4). In all structures,b oth triazole moieties were also found to contribute to the HOMO.T he calculated HOMO/ LUMO energy levels ( Table 2) agree well with those measured by electrochemistry and, importantly,r eproduce the trend for an increasing red-shift in the CT absorption maximum (l abs 1 > 1&2 > 1&2 2 ).  Thecalculations also shed light on the origin of this effect. Whereas the LUMO energy levels are largely unaffected by the mechanical bond, the HOMO is destabilised due to ahydrogen bond between the bipyridine Ndonors and the C À Ho ft he triazole,a so bserved in the solid-state structure of 1&2.T his serves to increase the donation of electron density from the triazole units to the carbazole core and accounts for the large difference in HOMO level (0.27 eV) between 1, which lacks this interaction, and 1&2,a nd also the less pronounced difference (0.06 eV) between 1&2 and 1&2 2 .
Bond lengths and angles in the S 0 state were remarkably similar for all three emitters.T he Cz-BP bond was found to vary between 1.403 and 1.406 and minima at 408 8 and 1408 8 were found when the Cz-BP dihedral was scanned, with ar elatively low barrier ( % 0.1 eV) to conformational exchange ( Figure S48). Them ost striking structural difference between 1&2 and 1&2 2 is that the Tz units of the latter adopt a syn-syn orientation whereas those of 1&2 preferentially adopt a syn-anti arrangement, the syn oriented unit being that encircled by the macrocycle. [33] This difference can be explained by observing that the syn conformation of Tz units minimises steric repulsion between an encircling macrocycle and the Cz unit. A syn-anti preference was also observed in the case of axle 1 as this minimises repulsion between the dipoles associated with the Tz rings. [34] In all cases the Cz-Tz distance was found to be identical (1.46 ).
Models of the excited S 1 state revealed an umber of important changes.F irstly,i na ll cases,t he Cz-BP dihedral angle of the lowest energy conformation was found to be % 908 8 and the Cz-BP bond lengthened (1.44 ), consistent with aslight weakening of this bond, as is typically observed in D-A TADF emitters. [35] In the excited state structures of 1 and 1&2,one of the Cz-Tz bonds was found to contract (D = 0.022 and 0.029 ,r espectively) while the other remained largely unchanged, causing the electron density on the donor to exhibit as light asymmetry.I nc ontrast the Cz-Tz bonds of 1&2 2 both contracted but remained very similar (1.442 and 1.447 ). In all cases,b oth Cz-Tz dihedral angles were reduced in S 1 compared with S 0 ,c onsistent with increased donation from the Tz moieties to the Cz core in the excited state.
In the ground state geometry,time dependent-DFT (TD-DFT) calculations of 1 identified two triplet states below the S 1 energy level, both of which exhibit am ixed 3 CT and acceptor-based 3 LE character ( Table 2). For 1&2 and 1&2 2 the acceptor-based 3 LE appears slightly above the S 1 state.T his presence of a 3 LE state is common for high performance TADF emitters [11a] and consistent with the structured luminescence spectra (Figure 3). At the excited state geometry, TD-DFT calculations found that the lowest S 1 and T 1 states exhibit pure CT character.Although the 3 LE triplet state lies % 0.3 eV higher in energy,i ti se xpected that this gap is overestimate due to the challenges associated with TD-DFT calculations predicting the absolute energy of CT states. [36] Theo scillator strength (f)f or the S 1 -S 0 transition was calculated to be 0.316, 0.272 and 0.271 for 1, 1&2 and 1&2 2 respectively at the ground state geometry but fell to 0f or all structures in their computed excited state geometry.G iven that we experimentally observed photoluminescence with a ms lifetime in all cases,weset out to explore the role of molecular flexibility on the photophysical properties using ab initio MD simulations.
Simulations were performed over 10 ps ( Figure S47) in both the ground (S 0 )a nd emissive excited state (S 1 )t o investigate the dynamic properties of the emitters.Inkeeping with the DFT results,comparable average values of D-A bond length were found for 1, 1&2 and 1&2 2 in both the ground and excited states,c onfirming this value is unaffected by the mechanical bond. Similarly,i nt he ground state,p eaks were found in the distribution of Cz-BP dihedral angles at % 408 8 and 1408 8,i nk eeping with the low barrier to conformational exchange predicted above.Incontrast, in the excited state,the distribution of the Cz-BP dihedral angle is subtly influenced by the interlocked macrocycles,i ndicated by am ean and standard deviation of 89 AE 178 8,9 0 AE 148 8 and 90 AE 118 8,o bserved for 1, 1&2 and 1&2 2 ,respectively.Although subtle,the reduced standard deviation observed upon adding the macrocycle is indicative of as light rigidification of this conformational mode due to the ability of the encircling macrocycles to exert fine conformational control of the excited state dynamics.Asimilar effect is also observed for the dihedral angle between the Cz and Tz moieties,which is 3 AE 168 8 for 1, 2 AE 118 8 (encircled) and 5 AE 178 8 (free) for 1&2 and 2 AE 68 8 for 1&2 2 .N ote,f or 1&2 we report two dihedral angles and only the one encircled by the macrocycle is altered relative to the axle alone.
Thec umulative effect of the subtle dynamic differences enforced by the mechanical bond can be observed in the key parameters defining TADF performance,n amely DE ST and the average oscillator strength for the S 1 !S 0 transition (f S1 ). Firstly,the trend in the mean Cz-BP dihedral angles found by MD leads to an energy gap between the 1 CT and 3 CT states, which follows the trend 1 > 1&2 > 1&2 2 as angles closer to 908 8 are associated with reduced splitting between states of the same character.A lthough the reduction of DE ST observed experimentally is between the 1 CT and 3 LE and due to the stronger electron donating strength introduced by the rotaxanes,t he reduced gap between the two CT states is still expected to increase k rISC.
[12b] Secondly,n arrowing of the distribution of dihedral angles around 908 8 for Cz-BP bond is expected to decrease the overall oscillator strength for the S 1 !S 0 transition. Indeed, the average values of f S1 determined by ab initio MD are 0.0260, 0.0095 and 0.0088 for 1, 1&2 and 1&2 2 respectively,m eaning that although the rotaxanes have reduced DE ST ,t hey retain enough oscillator strength to be efficient emitters.Finally,the more conformationally restricted molecular framework, as indicated by narrower distribution of the Cz-BP and Cz-Tz dihedral angles,c ould also be expected to decrease the rate of non-radiative decay and so lead to ahigher photoluminescence quantum yield, consistent with the 1 > 1&2 > 1&2 2 trend observed.

Luminescent Properties of 1, 1&2and 1&2 2 in Thin Films
We next performed ap reliminary photophysical investigation to determine whether the effect of the mechanical bond was maintained in thin films.Spin-coated 10 wt %doped films were prepared from chlorobenzene solutions of the three emitters with poly(methyl methacrylate) (PMMA) ( Table 3).
Although the emission of the 1, 1&2 and 1&2 2 in PMMA films was blue-shifted compared to the PL spectra in PhMe, as imilar red-shift was also observed along the series (Figure 5a), which suggests that the hydrogen bonds between the bipyridine unit and the Tz moieties are maintained in the film. Theshift in emission wavelength increased incrementally with each additional interlocked macrocycle.The PL spectra of the PMMA films are sharper than those in solution, as would be expected in arigid medium that reduces both conformational freedom and inhibits reorganization of the system. As with the solution-state measurements, t d progressively increases from 1 to 1&2 and 1&2 2 (Figure 5b).
Finally,the effect of the mechanical bond was maintained in different host media. Replacing PMMA with bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) or 1,3-bis(Ncarbazolyl)benzene (mCP) gave qualitatively similar results (see ESI for details);aprogressive red-shift was observed across both series,w ith ac orresponding decrease in DE ST . TADF was observed in all hosts.

Conclusion
We have designed and synthesized as eries of carbazolebenzophenone-based TADF emitters,t wo of which are mechanically interlocked. Thee lectrochemical and photophysical properties of these molecules were mainly inves-tigated in solution, with the conclusion that the mechanical bond can tune important properties of such TADF emitters. First, we find that the mechanical bond leads to al arge increase in photoluminescence quantum yield, which is avery important property for light-emitting materials.S econd, we find that the mechanical bond reduces DE ST ,w hich is highly desirable for TADF OLED emitters because it facilitates harvesting of triplet excitons.T hird, the mechanical bond increases the photostability of the emitters under continuous UV excitation. Fourth, the mechanical bond provides away of fine-tuning the HOMO energy level, making it shallower, an adjustment that would facilitate hole injection in OLEDs. Fifth, we find that it leads to fine-tuning of the emission spectrum, moving it slightly to the red, thereby providing aw ay of optimising the emission colour. Our DFT and ab initio MD simulations show that the fine-tuning of the HOMO energy,s hift in emission energy and reduction in the flexibility of the molecule is caused by weak interactions between the bipyridine Ndonors and the CÀHofthe triazole. They also demonstrate that the mechanical bond subtly alters the conformation of the TADF-active axle,which reduces the energy gap between the two CT states and decreases the oscillator strength of the S 1 state,k ey parameters defining TADF performance.T he effects on the TADF parameters were maintained in thin films,reinforcing the hypothesis that the mechanical bond can be exploited to optimize the efficiency of such emitters.
Given the exceptional interest TADF emitters have recently gained, combined with increasing availability of rotaxanes and other mechanically interlocked molecules,the use of the mechanical bond to engineer existing and novel luminophores is ap romising approach for TADF emitter optimization and, ultimately,anew approach for increasing device efficiency.I na ddition, given that modern computational chemistry is able to simulate the properties of these large structures to predict trends and provide insights on the origin of proposed effects,i ts hould be possible to generate predictive data for future targets.
[d] Average lifetime value obtained from time-resolved PL decay spectra measured at 298 Kof10wt% PMMA film (l exc = 378 nm). NF171163. We are also grateful for financial support from the University of St Andrews Restarting Research Funding Scheme (SARRF), which is funded through the Scottish Funding Council grant reference SFC/AN/08/020.

Conflict of interest
Theauthors declare no conflict of interest.