Thermodynamic Programming of Erbium(III) Coordination Complexes for Dual Visible/Near‐Infrared Luminescence

Abstract

Intrigued by the unexpected room-temperature dual visible/near-infrared (NIR) luminescence observed for fast-relaxing erbium complexes embedded in triple-stranded helicates, in this contribution, we explore a series of six tridentate N-donor receptors L4-L9 with variable aromaticities and alkyl substituents to extricate the stereoelectronic features responsible for such scarce optical signatures. Detailed solid-state (X-ray diffraction, differential scanning calorimetry, optical spectroscopy) and solution (speciations and thermodynamic stabilities, spectrophotometry, NMR and optical spectroscopy) studies of mononuclear unsaturated [Er(Lk) ] and saturated triple-helical [Er(Lk) ] model complexes reveal that the stereoelectronic changes induced by the organic ligands affect inter- and intramolecular interactions to such an extent that 1) melting temperatures in solids, 2) the affinity for trivalent erbium in solution, and 3) optical properties in luminescent complexes can be rationally varied and controlled. With this toolkit in hand, mononuclear erbium complexes with low stabilities displaying only NIR emission can be transformed into molecular-based dual Er-centered visible/NIR emitters operating at room temperature in both solid and solution states.

Room‐Temperature Linear Light Upconversion in a Mononuclear Erbium Molecular Complex

“…The absorption spectrum (Figure b) displays the expected Er‐centered transitions with non‐negligible (for f–f transitions) molar absorption coefficients (2≤ ϵ ≤7 L mol −1 cm −1 ), which are still compatible with millisecond‐range radiative lifetimes (Figure b; Supporting Information, Table S1) . The emission spectrum of [Er( L4 ) 3 ] 3+ , which was recorded upon ligand‐centered excitation, displays residual 1 π*→ 1 π ligand‐centered fluorescence together with dual visible Er( 4 S 3/2 → 4 I 15/2 ) and near‐infrared Er( 4 I 13/2 → 4 I 15/2 ) erbium‐centered radiative relaxations as previously observed in the parent [CrErCr( L1 ) 3 ] 9+ helicate (Figure c) . Given that 1) one intermediate Er‐centered excited state is sufficiently long‐lived to act as a relay ( τ (Er( 4 I 13/2 ))=5.57(6) μs at 298 K) and 2) the high‐energy Er( 4 S 3/2 ) excited level is luminescent, [Er( L4 ) 3 ] 3+ fulfills the minimum requirements for potential Er‐centered upconversion via the ESA mechanism.…”

“…To decipher and program the rare dual Er‐centered emission, which is essential for molecular upconversion, the two capping chromium units found in [CrErCr( L1 ) 3 ] 9+ were removed. The resulting solution‐stable mononuclear chromophore [Er( L4 ) 3 ] 3+ was shown to exist as pseudo‐ D 3 symmetrical triple helical complex, the crystal structure of which displayed pseudo‐tricapped trigonal nine‐coordinate Er 3+ sites (Figure a), separated from their neighbors by intermetallic distances larger than 1 nm (Supporting Information, Figure S1) . The absorption spectrum (Figure b) displays the expected Er‐centered transitions with non‐negligible (for f–f transitions) molar absorption coefficients (2≤ ϵ ≤7 L mol −1 cm −1 ), which are still compatible with millisecond‐range radiative lifetimes (Figure b; Supporting Information, Table S1) .…”

Room‐Temperature Linear Light Upconversion in a Mononuclear Erbium Molecular Complex

“…Theresulting solution-stable mononuclear chromophore [Er(L4) 3 ] 3+ was shown to exist as pseudo-D 3 symmetrical triple helical complex, the crystal structure of which displayed pseudo-tricapped trigonal nine-coordinate Er 3+ sites (Figure 1a), separated from their neighbors by intermetallic distances larger than 1nm( Supporting Information, Figure S1). [17] Thea bsorption spectrum (Figure 1b)d isplays the expected Er-centered transitions with non-negligible (for f-f transitions) molar absorption coefficients (2 e 7Lmol À1 cm À1 ), which are still compatible with millisecondrange radiative lifetimes (Figure 1b;Supporting Information, Table S1). [18] Theemission spectrum of [Er(L4) 3 ] 3+ ,which was recorded upon ligand-centered excitation, displays residual 1 p*!…”

“…4 I 15/2 ) erbium-centered radiative relaxations as previously observed in the parent [CrErCr(L1) 3 ] 9+ helicate (Figure 1c). [17] Given Scheme 2. Erbium-based coordination complexesexhibiting linear upconversion processes followingthe ETU mechanism.…”

“…[16] 3 ]-(ClO 4 ) 3 ·1.5 CH 3 CN, and corresponding b) absorption spectrum (3 mm in CH 3 CN with radiative lifetime t rad /ms given in parentheses) and c) emissions pectrum (solid state, l exc = 370 nm) recorded at room temperature (inset:NIR region of the emission spectrum). [17] that 1) one intermediate Er-centered excited state is sufficiently long-lived to act as arelay (t(Er( 4 I 13/2 )) = 5.57(6) msat 298 K) [17] and 2) the high-energy Er( 4 S 3/2 )e xcited level is luminescent, [Er(L4) 3 ] 3+ fulfills the minimum requirements for potential Er-centered upconversion via the ESA mechanism.…”