Thermally Activated Delayed Fluorescence in CuI Complexes Originating from Restricted Molecular Vibrations.

The mechanism of thermally activated delayed fluorescence (TADF) in molecules in aggregated or condensed solid states has been rarely studied and is not well understood. Nevertheless, many applications of TADF emitters are strongly affected by their luminescence properties in the aggregated state. In this study, two new isomeric tetradentate CuI complexes which simultaneously show aggregation induced emission (AIE) and TADF characteristics are reported for the first time. We provide direct evidence that effectively restricting the vibrations of individual molecules is a key requisite for TADF in these two CuI complexes through in-depth photophysical measurements combined with kinetic methods, single crystal analysis and theoretical calculations. These findings should stimulate new molecular engineering endeavours in the design of AIE-TADF active materials with highly emissive aggregated states.

Recent developments of thermally activated delayed fluorescence (TADF) materials have attracted tremendous attention as they can be used as highly efficient emittersf or organic light emitting diodes (OLEDs). [1] TADF is ap romising mechanism for convertinge lectricity into light with an internalq uantum efficiency (IQE) of nearly 100 %, whichu tilizes the up-conversion from triplet excitons to singlet states by reverse intersystem crossing( RISC). [2] However,as tubborn problem for TADF materials in practical applications is that most TADF emitterss uffer from aggregation-causedq uenching (ACQ) in the condensed phase. Therefore,e ffective TADF can be realized in ad oped host-guest system only with careful control of their concentrations to restrictthe nonradiative deactivation processes.Aggregation-induced emission (AIE) materials are, therefore, able to respond to these challenges. AIE molecules were first reported by Ta ng et al, [3] and while they showw eak emissioni nd ilute solution,u pon aggregation they emit strongly,d ue to restriction of intramolecularr otation.R ecently,Y asuda et al. reported as eries of o-carborane derivatives that simultaneously show AIE and TADF characteristics. [4] OLEDsu tilizing these molecules as an ondoped emission layer exhibited maximum EQEs as high as 11 %. Thus, AIE-TADF active materials are promising luminescent materials and are widely used in numerouso ptoelectronic applicationse specially forn on-doped OLEDs. However,t he mechanism of TADF in aggregated molecules is still unclear and has not been studied in detail,although it is essential for many of the desired applications of these emitters. Fundamental factors thatl ead to enhancede mission of dyes in viscous and solid-state environments are of ongoing interest. [5] Cu I complexes with TADF characteristics have been used in OLEDsa nd some derivatives show impressivee lectroluminescent performance. [6] Recently,d ual-emissive Cu I complexes that can simultaneously show TADF and phosphorescent emission have been demonstrated. [7] Cu I complexes have the practical advantages of the relativelyl ow-cost and high abundance of copper,inc ontrasttor are metals such as iridium.
The aim of the present study is to explore the coexistence of TADF and AIE in the new copperc omplexes 1 and 2.W e demonstrate that both complexes show almostn oe mission in solution,b ut strong luminescence and obvious TADF is observedw ith an extremely short delayedl ifetimei nt heir aggregateds tate. To our knowledge,t hese combined photophysical phenomenah ave not been previously reportedi nC u I complexes, althought he coexistence of both TADF and AIE in allorganicm olecules has been reported in af ew cases. [8] The relationship between AIE and TADF is still not clear.H erein, we address the pertinent question:d oes aggregation induce or promote TADF?T his topic is of fundamentali mportancef or a better understanding of the mechanism of AIE-TADFa nd further designing this kind of material for use in OLEDs.The kinetics of TADF are investigated in two Cu I complexes in pristine film state and doped in PMMA matrices. The PMMA matrices of different molecular weight, doped with complexes 1 and 2, have been used in solid-state "dilution" experiments to probe the role of intramolecular vibrationso nt he observation of TADF.The similar TADF behavior presented in both high molecular weighth ost (H-PMMA) and in pristine films clearly reveals that effectively restricting specific vibrations of individual emitter molecules is the origin of TADF for these Cu I complexes. Thus, ag uideline for designing AIE-TADF active materials is presented.
The chemical structures of two new isomeric complexes 1 and 2 are shown in Figures 1a and 1b. Their synthesis and characterization data are presented in the Supporting Information. The UV/visa bsorption ande mission spectra of 1 and 2 in degassed solution (CH 2 Cl 2 )a nd pristine film are depictedi n Figure S6 (Supporting Information). Upon photoexcitation, 1 and 2 are almostn on-emissive in pure CH 2 Cl 2 solution at room temperature. However,b right emission is observedi nt he pristine film state with as hort luminescence decay time. This short-lived luminescencei si nc ontrast with conventional Cu I complexes,w here luminescence is observed with relatively long decay times (phosphorescence decay times of several 100 msu pt oafew ms). [9] The luminescence decays of complexes 1 and 2 in their pristine films tate are even shorter than the recently reported decay times in mono-nuclear Cu I complexes with TADF. [2c, 6a-c, 7] This implies that 1 and 2 may be outstanding TADF materials with highreverse intersystem crossing rate (k RISC )b ye fficiently eliminating the triplet-triplet annihilation and triplet-polaron quenching in their pristine film state. [9] The detailed photophysical data of 1 and 2 are summarized in Ta ble S1.T he energy levels of the complexes calculatedf rom their electrochemical and photophysical data are listed in Ta ble S2.
To investigate the effect of molecular vibrations on the emission of 1 and 2 in their aggregated state, steady-state and time-resolved photoluminescence experiments were performed in their pristine film state, and in composite films of both complexes dispersed in PMMA hosts. TwoP MMA samples, one with low molecular weight, L-PMMA (Mn = 15,000), and the other with high molecular weight, H-PMMA (Mn = 960,000), were used to fabricate thin films of 1 and 2.T hermal gravimetric analysis(TGA) data showedthat H-PMMAhas asignificantly higher Tg (130 8C) than L-PMMA (86 8C) consistent with H-PMMA being as tiffer material. The temperature dependence of the steady-state emissions pectra in these films is compared with the temperature dependence of the emission in pristine films of 1 and 2 that are used as references. This allows the roles of thermal vibrations and intermoleculari nter-actions on the excited state dynamics in these two complexes to be determined.
The two PMMA hosts with different molecular weight appear to be able to suppress molecular vibrations to different degrees. This results in marked differences of the temperature dependenceo ft he photoluminescence of 1 and 2,c ollected in pristine films:F igures 1c and 1d;i nL-PMMAF igures 1e and 1f;a nd in H-PMMA Figures 1g and 1h.A dditionally,i nF ig- The steady-state fluorescence from 320 to 80 Ki np ristinefilm for c) 1 and d) 2, respectively.T he steady-state fluorescence from 320 to 80 Ki nL-PMMAf or e) 1 and f) 2,respectively. The steady-state fluorescence from 320 to 80 Ki n H-PMMA for g) 1 and h) 2,r espectively.T emperature dependence of the integrated luminescence intensityasaf unction of temperature (K) for i) 1 and j) 2,integratedi ntensity normalized at 80 K. ures 1i and 1j the temperature dependence of the photoluminescencei nt he three samples shows clear differences in the temperature region above 170 K, but agree extremelyw ell at temperatures below 170 K.
In the pristine film, the photoluminescence spectra of 1 and 2 show ad ominant TADF mechanism between 320-170K (Figures 1c and 1d). In this temperature range the emission intensity first increases and then decreases with decreasing temperature, showingatypical bell-shapet hat is characteristic of the equilibrium between the singlet and triplets tates, mediated by the k RISC ,w hich drives the up-conversion of triplets back to the singlet manifold, and the thermal vibrations (internal conversion:IC) that quenchthe excited state population. [10] Below 170 K, the luminescence intensity is dominated by phosphorescence from the lowest T 1 state, and is thus controlled by the decrease in the internal conversion, and as this continues decreasing with temperature, the steady-state emission slowly increases as observed in Figure1i).N ote also the change observed in the emission spectra in Figures 1c and 1d. With decreasing temperature l max shifts to longerw avelengths. This is consistentw ith the emissionb eing dominated by TADF at high temperatures, that is, emission from the singlet excited state, and by phosphorescence at low temperatures, that is, emission from the low energy triplet state. Remarkably,this behavior is not observed for the films of 1 and 2 in L-PMMA host. Here, the emission comesm ainly from the triplet state and the contribution of TADF is practically non-existent.I nt his host the vibrational quenching dominates. The emission intensity,t herefore, increases with decreasing temperature in the entire temperature range, and more markedlyf rom 320 to 170 K, where the effect of suppressing vibrationsi sm ore pronounced. This is presentedi nF igures 1e and 1i.
In H-PMMA ( Figures 1g and 1h), as in L-PMMA, the intermolecular interactions between 1 (or 2)m olecules are practically non-existent. This, therefore, indicates that the H-PMMA host is more abletos uppress vibrations, in the same way as the intermolecular interactions between 1 (or 2)m olecules do in the pristinef ilm. This enables the reverse intersystem crossingt o become operative and to activate the TADF emission. However, in L-PMMA this is not the case, and the internal conversion dominates in the entire temperature range.
Below 170 Kt he temperature dependence of the photoluminescencei ss imilar in all samples (Figures 1i and 1j). This shows that once k RISC is suppressed, and as the effect of vibrations decreasesd ue to lowering the temperature, the differences in the host are not so important to the outcome of the emission. These experiments highlight the fact that the nonradiative pathways playacrucial role in the triplet-harvesting mechanism in complexes 1 and 2.M oreover,i nt he case of 1,a nd to lesser extent in 2,t he intermolecular interactions presenti n the pristine films, due to the molecular packing, are able to effectively suppress vibrations that otherwise will quenchT ADF.
Time resolved spectroscopy was used to follow the excited state dynamics in 1 and 2.T he luminescence decays of 1 and 2 at 300 Ki np ristine films and dispersed in L-PMMA are shown in Figures S7a and S7b. Both luminescence decayso f1 at 300 Ka re characterizedb yafast component around 5a nd 7.9 ns in pristine film and L-PMMA,r espectively. These fast components are followed by al ong bi-exponential decay,w ith time constants of 0.9 and 5 msi nt he pristine film, and of 9a nd 30 msi nL-PMMA. The lifetimei nL-PMMA is thus longert han in the pristine film. This reflects the effecto fT ADF in the pristine film, which at 300 Km ore rapidly quenches the triplet state, whereas in L-PMMA the emission occurs mainly directly from the triplet state, for example, phosphorescence. These observations are consistentw ith the more intense emission in the pristine film below 1 ms, where TADF dominates. Complex 2 shows similarb ehavior ( Figure S7b). Importantly,t his reveals also how the presence of TADF accelerates the excited state decay in Cu I complexesa nd minimizes the roll-off effect in devices.
The decayso btained as af unction of temperature for the pristine films, L-PMMAa nd H-PMMA films, and crystalline samples are entirely consistent with the relative importance of TADF.I np ristine films, H-PMMA and crystalline samples both complexes 1 and 2 exhibit clear temperature dependence for the delayed fluorescencew hich is characteristic of the TADF mechanism (Figures 2c,2d and SupportingI nformation Figure S8). Here with decreasing temperature the emission intensity decreases andt he lifetime increases, consistent with a slower depopulation of the triplet state. In contrast, in the L-PMMA host (Figures 2a and 2b) the photoluminescence decay is dominated by the nonradiative constants. Thisiso bserved in the temperature dependence of the prompt fluorescence component which increases in intensity with decreasing the temperature, and the increase of the phosphorescence lifetime in both compounds, as ad irect response of suppressing the nonradiatived ecay pathways. Note that no TADF component is observed on these decays. In Figures 2e and 2f the power dependence of the delayed fluorescencef or both compounds confirms the origin of the mechanism responsible for the longlived fluorescence as TADF (slope 1) and not triplet-triplet annihilation (TTA) (slope 2). [10] These experiments werep erformed on pristine films, where the TADF component is stronger.
The two isomeric complexes 1 and 2 differ only in the substitutionp attern on the tetrazolate ring, and this is shown to have ap rofound effect on the supramolecular structure and photophysical properties. It is instructive to comparet he single-crystal X-ray structures of the two complexes. They both exhibit highly distorted tetragonal coordination,t ypical of Cu(N N)(POP) + complexes (Figures 3, S10 and S11). [6c] Crystals of 1 revealed multiple modes of intra-and intermolecular interactions leading to af ascinating 3D supramolecular structure ( Figure 3). Important interactions are summarized in Ta bleS5. Generally,t he presence of strongi ntra-and intermolecular interactions would supply am ore rigid environment and thereby effectively limit molecular distortions in the excited state, leading to bright emission through restricting the non-radiative processes. [6d, 11] Furthermore, due to the small DE(S 1 ÀT 1 )v alue obtained from photophysical experiments,a nd from the DFT calculations (see below),T ADF is triggered in the pristine aggregation state of complex 1,w hichi ss imilart ot he situation in H-PMMA. It is worth noting that intermolecular aromatic stacking has been shown to offer ac harge-transfer pathway and to enhancet he carrier-transport ability,w hich is essential for excellent electroluminescence materials. [12] In the crystal structure of 2,m ultiple modes of intra-and intermolecular interactions lead to ao ne-dimensional supramolecular chain structure ( Figure S11). [13] Due to the relatively weak intermolecular interactions in 2,c ompared with 1 ( Figure 3) complex 2 showed less distortion of the tetragonal coordination in the ground state. For the excited state, Riesgo et al. stated that the distortion of Cu complexes is directly related to their "Stokes-like'' shift, that is, the difference in wavelength between absorption and emission spectra. [14] Indeed, the smaller Stokes-like shift for 1 compared with 2 ( Figure S6 and Table S1) indicates that excited-state distortion is suppressed more effectively in 1 which,i nt urn, means that for 1 the non-radiative process is suppressed. It could, therefore, be expected that the photoluminescence quantum yield (PLQY) for 1 is substantially higher than 2.I ndeed PLQYs of 47.1 and 9.4 %w ere measured in the crystalline state for 1 and 2,r espectively (Table S1).
Emission spectra of complexes 1 and 2 in amorphous pristine films and crystalline forms were collected in vacuum and in the presenceo fo xygen,s howing only small variations of the luminescence intensity once oxygen has been removed ( Figure S9). This indicates that oxygen quenching is not the cause fort he lower PLQY of both complexes in their amorphousf ilms, when compared with their crystalline forms, and in particular for the lower PLQY of complex 2 when compared with complex 1.I nstead, the origin of the lower PLQYi n2 is due to the weakeri ntermolecular interactions that are prevalent in complex 2 compared with the highly ordered three-dimensional supramolecular chain structure observed in 1,b ut not in 2.
To further probe the interesting optoelectronic phenomena discussed above, we performed time-dependent density functional theory( TD-DFT) calculations on complexes 1 and 2.T he electronic structures based on the optimized geometries of 1 and 2 plottedi nF igure S12 show the clearly separate electronic occupations in the HOMO and the LUMO for each complex, facilitating as mall DE(S 1 ÀT 1 )w hich is ac haracteristic of TADF materials. [15]   Since the value of DE(S 1 ÀT 1 )i su sually used to assess the ability of reverse intersystem crossing from T 1 to S 1 , [16] it was evaluated here for every structure (IM, LPM and CM)p resented in Figure4 to explaint he change of luminescence properties in the different situations mentioned above.T he nomenclature IM, LPM and CM defines the optimized isolated molecule, the simulatedm olecule in low PMMA (L-PMMA) host, and the molecule in the crystal structure, respectively.A sm entioned above,t he complex in L-PMMAh ost has more freedom to rotate the phenyl group than in the crystal structure. Therefore, an important dihedral angle q,w hich corresponds to the nearest phenyl group to the tetrazole ring, as ar epresentative angle,w as changeda nd scanned for 1 and 2.T he resultsc ollected in Ta ble S6 show: (i)both the IM and CM structures have as maller DE(S 1 ÀT 1 )v alue, further verifying that TADF may occur in isolated molecules (solution state) and in the aggregation state (crystal structure). However,t he huge difference in optimized geometriesb etween ground state and S 1 along with T 1 collected in Ta ble S7 also illustrate the large nonradiatived ecay of single molecules of both complexes in solution. This is crucial for the quenching of TADF in the solution state. (ii)With the increase of q, DE(S 1 ÀT 1 )b ecomes larger for both 1 and 2,s uggesting the rotation of the nearest phenyl group to the tetrazole ring raises DE(S 1 ÀT 1 )relative to the crystal structure, which might greatly reducet he ability for transfer from the T 1 to the S 1 state. Thus for 1 and 2 phosphorescent emission, rathert han TADF,a ppears in the L-PMMA environment where the molecules are free to rotate their phenyl groups.H ence, the relationship between restriction of molecular vibrations and the luminescent properties of the two Cu I complexes are summarized in Figure 4b.
Moreover,as inglet-triplet gap around0 .2 eV,a sp redicted in the calculations, is in excellent agreement with the energy difference determined from the onset of the emissions pectra in pristine films (see Figure 1c and1 d).
In summary,w eh ave studied two new AIE-TADFa ctive isomeric Cu I complexes.S olid-state "dilution" experiments investigated their photophysical properties at different degrees of suppression of molecularv ibrations, revealing phosphorescence in low MWt PMMA (L-PMMA), but efficient TADF in high MWt PMMA (H-PMMA). The similarT ADF behavior in H-PMMA and in the pristine aggregateds tate (neatt hin films) suggests that TADF behavior is strongly influenced by effectivelyrestricting the vibration of individual molecules throughi ntermolecular interactions. TD-DFT calculations are in excellent agreement with the observedp hotophysical properties of the complexes. Our findings may stimulate new moleculare ngineering endeavours in the design of AIE-TADF active materials with highly emissive aggregated states.M oreover,t he extremely short delayedl ifetime in pristine film state suggests that both complexes 1 and 2 are candidates for non-doped OLEDs.