Near‐IR to Near‐IR Upconversion Luminescence in Molecular Chromium Ytterbium Salts

Abstract Upconversion photoluminescence in hetero‐oligonuclear metal complex architectures featuring organic ligands is an interesting but still rarely observed phenomenon, despite its great potential from a basic research and application perspective. In this context, a new photonic material consisting of molecular chromium(III) and ytterbium(III) complex ions was developed that exhibits excitation‐power density‐dependent cooperative sensitization of the chromium‐centered 2E/2T1 phosphorescence at approximately 775 nm after excitation of the ytterbium band 2F7/2→2F5/2 at approximately 980 nm in the solid state at ambient temperature. The upconversion process is insensitive to atmospheric oxygen and can be observed in the presence of water molecules in the crystal lattice.


Introduction
Metal-based upconversion (UC) transforming low-energy photons into an anti-Stokes-shifted luminescence is av ery attractive non-linear process for fundamental studies as well as for future applications.E xamples are solid inorganic host matrices with low-phonon energies doped with transition metal or lanthanoid cations,e ither as bulk materials [1] or, more recently,a sn anocrystalline systems. [2] UC was long considered to be impossible in discrete metal-organic com-plexes [3] due to the pronounced non-radiative deactivation of the excited metal states by high-frequencyoscillators present in organic ligands like -OH, -NH or -CH groups. [4] In the past few years,h owever, many advances have been achieved in implementing metal-based UC in molecular complex species, some even at ambient temperature and in solution. [5] This includes metal chelate-organic chromophore combinations, [6] mononuclear metal complexes, [7] and hetero-oligometallic sensitizer-activator architectures. [8,9] Thelatter have shown to hold the greatest potential for efficient UC,e specially for energy transfer upconversion (ETU) but also for cooperatively sensitized upconversion (CSU). Forboth UC schemes, sensitizer metal centers (S) with appropriate energy levels and sufficiently long luminescence lifetime are necessary to successfully populate an activator (A) excited state with approximately twice the energy of the excited sensitizer state at relatively low excitation power densities.A mong the best sensitizing metal centers are Yb 3+ ( 2 F 5/2 at % 10 250 cm À1 , % 976 nm) and Cr 3+ (octahedral geometry: 2 E/ 2 T 1 at % 15 000-12 400 cm À1 , % 665-805 nm depending on the ligand field). This has been demonstrated for several emissive UC activators in molecular systems,f or example the lanthanoids Er 3+ and Tb 3+ . [8,9] Thee arth-abundant metal Cr 3+ has also gained renewed interest as downshifting luminophore/sensitizer, [10] on one hand because of the recently developed class of "molecular ruby" emitters which show very high luminescence quantum yields of the 2 E/ 2 T 1 phosphorescence of up to 30 %a tr oom temperature in solution in the absence of oxygen, [11] and on the other hand as successful antenna moieties for the downshifting sensitization of near-IR lanthanoid luminescence. [12,13] Tw odecades ago,Güdel et al. reported an interesting UC Scheme for the generation of 2 EU Ce mission for solid state hosts such as Y 3 Ga 5 O 12 co-doped with Yb 3+ as sensitizer and Cr 3+ as activator. [14] These compounds operate via CSU where two excited Yb 3+ cooperatively transfer the energy from their 2 F 5/2 states to an excited quartet state of Cr 3+ ( 4 T 2 / 4 T 1 )w hich subsequently populates the emissive 2 Es tate by intersystem crossing (ISC) (Figure 1). This Scheme is particularly interesting because both, excitation and UC emission, are in the near-IR spectral window,increasingly used for bioimaging. [15] In molecular systems,n ear-IR to near-IR upconversion is unknown and the few systems utilizing the couple Yb/Cr reported so far exhibited UC only at very low temperatures (usually below 100 K) in extended solid inorganic matrices.In the past, however,r eports on efficient downshifting energy transfer (EnT) 2 E(Cr 3+ )! 2 F 5/2 (Yb 3+ ), [12,13] that led to deactivation of the UC-emissive 2 Es tate,m ade the successful implementation of this attractive UC Scheme unlikely. Especially Cr 3+ /Yb 3+ -architectures with highly efficient Dexter EnT (here total angular momentum allowed for DJ = 1) [16] in hexacyanidochromate-and oxalato-bridged coordination compounds [13a-c] seemed unsuitable for this purpose.O nt he other hand, dipole-dipole EnT (Fçrster) Cr 3+ !Yb 3+ in oligometallic molecular systems also showed unfavorably high EnT efficiencies of up to ca. 50 %d espite being forbidden by the total angular momentum selection rule (DJ = 2,4,6). [13a, 16] With these challenges of the Cr 3+ /Yb 3+ pair in mind, we revisited the design concept for molecular Yb-Cr-UC.T his led to anew photonic material composed of easily accessible Cr 3+ and Yb 3+ complex ions which shows 2 E/ 2 T 1 UC at room temperature already at relatively low excitation power densities.

Results and Discussion
Themain idea was to avoid Dexter EnT from 2 E(Cr 3+ )to 2 F 5/2 (Yb 3+ )a nd opt for as ystem, where Cr 3+ !Yb 3+ EnT was only possible by al ess efficient Fçrster mechanism. Therefore,w eu tilized spatially separated metal centers in discrete coordination environments.F or the realization of this design, we chose the complex mer-[Cr(ddpd . This Cr 3+ complex shows av ery high phosphorescence quantum yield F of up to 30 %ina rgon-saturated CD 3 CN solution at room temperature (298 K) and even remains quite emissive in airsaturated water with F = 2.1 %. [11c,d] Despite earlier reports on the complex [Yb(dpa) 3 ] 3À (dpa = 2,6-pyridine-dicarboxylate) and the only moderately long lifetime of its excited 2 F 5/2 energy level in the solid state (solid 1-Yb at 295 K: t = 2.9 ms), [3, 13c] we chose this anion as counterpart for the Cr 3+ complex because of its straightforward synthetic accessibility and its good match with [Cr(ddpd) 2 ] 3+ in terms of comparable size and opposite charge.T he latter parameters were ex-pected to facilitate the crystallization of the desired Cr/Yb ionic solid, where only intermolecular p-p-stacking interactions between the different ions occurs.T he synthesis of our novel photonic material 3-Yb was achieved by mixing Na 3 -[Yb(dpa) 3 ]·6 H 2 O(1-Yb) [17] with [Cr(ddpd) 2 ]Cl 3 (2,see SI for details) in an alcoholic solution (Scheme 1). We also prepared the reference compound 3-Lu as as tructural analogue of 3-Yb,thereby utilizing the photoinactive nature of Lu 3+ with its 4f 14 electronic configuration. 3-Yb and 3-Lu were obtained as bright yellow solids in good to excellent yields (57-88 %). Both, complex anion and cation, [18] are chiral but were used as racemates.E lemental analysis of both compounds revealed large amounts of lattice water and methanol in the material (see SI for details). To suppress potentially severe nonradiative deactivation of both the 2 E/ 2 T 1 and 2 F 5/2 excited states via multiphonon relaxation by C-H and O-H oscillators, [4] thes yntheses were also repeated with [D 4 ]-MeOH/ [D 8 ]-i PrOH. TheX-ray structural analysis of single crystals of 3-Ln grown from MeOH/ i PrOH mixtures confirmed that all salts are isostructural, racemic mixtures of the complex ions ( Figure 2, see also Table S1 and Figure S1 in the SI). [19] As intended, in the solid material, downshifting EnT in 3-Yb should only be possible by the forbidden Fçrster mechanism. In our crystal, each Cr 3+ activator is surrounded   [20] 50 %probability level). Lattice solvent molecules and hydrogen atoms are omitted for clarity. by five Yb 3+ sensitizers as nearest neighbors with ad istance distribution of 8.75 < r Cr-Yb < 9.07 ( Figure S2 in the SI). Taking into account the distance relationship for S!AE nT (k EnT / r À6 )and assuming similar contributions from all other parameters (e.g. orientation of the chromophores,d ipole moments etc.), similar energy transfer rates to the central Cr 3+ activator were expected for the five nearest sensitizers that should hence only vary by af actor of up to (8.75/9.07) À6 = 1.24. Selective excitation of 3-Ln at l exc = 435 nm into the 4 A 2 ! 4 T 2 band [11c,d] of [Cr(ddpd) 2 ] 3+ produces the expected chromium phosphorescence 2 E/ 2 T 1 with an emission maximum around 780 nm. For 3-Yb,e xcitation at 435 nm leads not only to the Cr 3+ emission (Figure 3) but also to the appearance of aY b 3+ luminescence ( 2 F 5/2 ! 2 F 7/2 )a ta round 1000 nm (Figure 3). Since the chromium-free precursor 1-Yb is not emissive upon excitation at 435 nm ( Figure S3), this clearly indicated undesired Cr!Yb EnT in 3-Yb.F urther evidence for adownshifting EnT between Cr 3+ and Yb 3+ was obtained by time-resolved luminescence measurements under the same conditions (Table 1).
Thedecay curve of the Cr 3+ -centered 2 E/ 2 T 1 emission of 3-Yb exhibited monoexponential decay kinetics with al ong lifetime t = 390 ms. Thed ecay profile of the Yb 3+ emission revealed biexponential kinetics with al ong luminescence lifetime of 369 ms, uncharacteristic for molecular Yb 3+ species [4] which normally show luminescence lifetimes in the low ms-range.T he lifetime of 369 msc losely matches the 2 E/ 2 T 1 lifetime of the Cr 3+ emission of 390 ms. In addition, an oticeable rise time component (t = 9 ms) was present. These observations are all typical for EnT from the long-lived 2 E state to Yb 3+ . [13d-f] As detailed before,t his EnT could reduce the efficiency of the 2 Eu pconversion luminescence by nonradiatively depopulating this state.T oq uantify the potential loss in efficiency,wedetermined the quantum yield of the 2 E/ 2 T 1 phosphorescence of 3-Yb and 3-Lu upon excitation at 435 nm (Table 1). These measurements yielded F values of 6.8 %a nd 5.8 %f or deuterated 3-Lu and 3-Yb,r espectively, and hence revealed only am oderate decrease of 15 % 2 E quantum yield for 3-Yb relative to 3-Lu.This is favorably low compared to the loss due to Fçrster EnT reported for analogous downshifting systems in the literature (ca. 20-50 %), [13d-f] especially when considering that in our case each Cr 3+ has considerably more next Yb 3+ neighbors (here 5, previously 1a ts imilar distances r Cr-Yb )a sE nT acceptors. Surprisingly,n either the crystallization of 3-Yb from deuterated solvents nor the presence of oxygen significantly affected the luminescence decay kinetics of Cr 3+ in 3-Yb and 3-Lu (Table 1). Thed ecay profile of the Cr 3+ 2 E/ 2 T 1 emission in deuterated 3-Lu in air also showed biexponential decay kinetics and revealed considerably longer lifetimes than observed for 3-Yb (Table 1. Deuterated 3-Lu: t 1 = 660 ms, 89 %a nd t 2 = 280 ms, 11 %).
Finally,U Cm easurements of 3-Yb and 3-Lu were performed at 298 Ku nder ambient atmosphere.E xpectedly, 3-Lu did not yield any UC emission upon excitation at 976 nm. In contrast, excitation of the Yb 3+ sensitizers in 3-Yb produced intense 2 E/ 2 T 1 UC emission of the activator Cr 3+ with am aximum around l em = 780 nm ( Figure 4). Timeresolved studies confirmed successful UC in 3-Yb and deuterated 3-Yb,while no luminescence signal was observed for 3-Lu ( Figure S9). For 3-Yb,excitation power densities (P)  n.a. 6.8 [a] Lifetimes are fitted mono-or biexponentially,p ercentages in parentheses give relative amplitudes of the components, estimated uncertainty of t AE 5%. [b] Measuredu sing an integrating sphere setup Quantaurus-QY C11347-11 (see SupportingInformation for details), estimated uncertainty AE 5%. as low as P % 67 Wcm À2 were sufficient for the observation of UC which is areasonably low threshold for UC by anormally not very efficient CSU mechanism. [5] The P dependence of the UC emission intensity depicted in Figure 5s hows two distinct regions.Below P % 494 Wcm À2 ,the number of excited Yb 3+ is low and UC depends quadratically on P indicating abiphotonic process (log-log plot:slope or photonic order of 1.99). At higher P,s ensitizer saturation slowly occurs as indicated by ap hotonic order below 2w hich eventually approaches 1a si stypical for ao ne-photon process (slope or photonic order of 1.05). [21]

Conclusion
In conclusion, by carefully revisiting earlier downshifting Cr 3+ /Yb 3+ systems,w er ealized an ovel near-IR to near-IR upconversion (UC) material by simply combining Cr 3+ and Yb 3+ complexes in an ionic solid. This expands the small number of molecular UC examples by anew pair of sensitizer/ activator metal complexes.I mportantly,U Cc an be realized with synthetically easily accessible non-deuterated/non-halogenated building blocks at room temperature in the presence of oxygen and water molecules.T his proof-of-concept study will pave the way to an ew class of photonic materials and enable new possibilities for the field of molecular UC.