Enhanced Solid-State Fluorescence of Flavin Derivatives by Incorporation in the Metal-Organic Frameworks MIL-53(Al) and MOF-5

Abstract

The flavin derivatives 10-methyl-isoalloxazine (MIA) and 6-fluoro-10-methyl-isoalloxazine (6F-MIA) were incorporated in two alternative metal-organic frameworks, (MOFs) MIL-53(Al) and MOF-5. We used a post-synthetic, diffusion-based incorporation into microcrystalline MIL-53 powders with one-dimensional (1D) pores and an in-situ approach during the synthesis of MOF-5 with its 3D channel network. The maximum amount of flavin dye incorporation is 3.9 wt% for MIA@MIL-53(Al) and 1.5 wt% for 6F-MIA@MIL-53(Al), 0.85 wt% for MIA@MOF-5 and 5.2 wt% for 6F-MIA@MOF-5. For the high incorporation yields the probability to have more than one dye molecule in a pore volume is significant. As compared to the flavins in solution, the fluorescence spectrum of these flavin@MOF composites is broadened at the bathocromic side especially for MIA. Time-resolved spectroscopy showed that multi-exponential fluorescence lifetimes were needed to describe the decays. The fluorescence-weighted lifetime of flavin@MOF of 4 ± 1 ns also corresponds to those in solution but is significantly prolonged compared to the solid flavin dyes with less than 1 ns, thereby confirming the concept of "solid solutions" for dye@MOF composites. The fluorescence quantum yield (Φ_F) of the flavin@MOF composites is about half of the solution but is significantly higher compared to the solid flavin dyes. Both the fluorescence lifetime and quantum yield of flavin@MOF decrease with the flavin loading in MIL-53 due to the formation of various J-aggregates. Theoretical calculations using plane-wave and QM/MM methods are in good correspondence with the experimental results and explain the electronic structures as well as the photophysical properties of crystalline MIA and the flavin@MOF composites. In the solid flavins, π-stacking interactions of the molecules lead to a charge transfer state with low oscillator strength resulting in aggregation-caused quenching (ACQ) with low lifetimes and quantum yields. In the MOF pores, single flavin molecules represent a major population and the computed MIA@MOF structures do not find π-stacking interactions with the pore walls but only weak van-der-Waals contacts which reasons the enhanced fluorescence lifetime and quantum yield of the flavins in the composites compared to their neat solid state. To analyze the orientation of flavins in MOFs, we measured fluorescence anisotropy images of single flavin@MOF-5 crystals and a static ensemble flavin@MIL53 microcrystals, respectively. Based on image information, anisotropy distributions and overall curve of the time-resolved anisotropy curves combined with theoretical calculations, we can prove that all fluorescent flavins species have a defined and rather homogeneous orientation in the MOF framework. In MIL-53, the transition dipole moments of flavins are orientated along the 1D channel axis, whereas in MOF-5 we resolved an average orientation that is tilted with respect to the cubic crystal lattice. Notably, the more hydrophobic 6F-MIA exhibits a higher degree order than MIA. The flexible MOF MIL-53(Al) was optimized essentially to the experimental large-pore form in the guest-free state with QuantumEspresso (QE) and with MIA molecules in the pores the structure contracted to close to the experimental narrow-pore form which was also confirmed by PXRD. In summary, the incorporation of flavins in MOFs yields solid-state materials with enhanced rigidity, stabilized conformation, defined orientation and reduced aggregations of the flavins, leading to increased fluorescence lifetime and quantum yield as controllable photo-luminescent and photo-physical properties.

Keywords: 10-methyl-isoalloxazine; MIL-53; MOF-5; dye anisotropy; flavin@MOF; flavins; fluorescence; fluorescence lifetime; metal-organic framework (MOF); multiparametric fluorescence microscopy; solid solution.

Figure 1

10-Methyl-isoalloxazine (MIA) and the mono-fluorinated derivative 6-fluoro-10-methyl-isoalloxazine (6F-MIA) [17].

Figure 2

Schematic presentation of the periodic channel structures in (a) MIL-53, [Al(OH)(bdc)] and (b) MOF-5, [Zn₄O(bdc)₃] (bdc = benzene-1,4-dicarboxylate) (The inorganic building units {AlO₆} and {Zn₄(O)(O₂)₆}, respectively, are represented in cyan and the bdc linker schematically as a grey rod; see Section S3, Supplementary materials for structure details). (Structure images were drawn with Diamond [57] from the deposited cif files under CCDC-no./Refcode 220476/SABVUN [41] and 256966/SAHYOQ [52,58]).

Figure 3

Confocal images of MIA@MOF-5. (a) Confocal laser scanning microscopy 3D profile, (b) sectional plane at 200 µm from top, and (c) stack of sectional planes of MIA@MOF-5 (cw excitation at λ_ex = 458 nm, λ_em = 530–555 nm). Every sectional plane corresponds to a measurement thickness of 4 μm, which corresponds to the optical resolution of the used objective UPLSAPO10X/0.4NA.

Figure 4

(a) Confocal laser scanning microscopy 3D profile for a height z of 25 µm, (b) line profile along z of 6F-MIA@MOF-5 (cw excitation at λ_ex = 458 nm, λ_em = 530–555 nm, objective UPLSAPO10X/0.4NA). The fluorescence intensity decreases with the depth due to the inner filter effect.

Figure 5

Normalized fluorescence spectra at room temperature of (a) MIA and (b) 6F-MIA in their neat solid state, in 1:1 DCM/MeOH solution and for the flavin@MOF composites; wt% refers to the flavin loading. Since the fluorescence signal of flavin@MOF composites with low loading is weak, the contributions of scattered excitation light at short emission wavelengths becomes relevant, which causes additional shoulders in the spectra at short wavelengths.

Figure 6

Images of MIA and 6F-MIA in two MOFs obtained by confocal multi-parameter fluorescence image spectroscopy (MFIS) (see Section 3). (Panel 1) Image of fluorescence-weighted average lifetime; (Panel 2) Image of experimental steady state fluorescence anisotropy r; (Panel 3) Interrelation ${〈\tau 〉}_F-r$ in the 2D diagram of the parameter images in panels 1 and 2 with a full horizontal line for the average anisotropy $\stackrel{-}{r}$ of the sample and second dashed line of other MOF composite as reference; and (Panel 4) Time-resolved fluorescence anisotropy curves r(t) to resolve the fundamental anisotropy r₀ and the depolarization time τ_depol that are displayed in the individual panels 4. (a) MIA@MIL-53 (3.9 wt%), (b) MIA@MOF-5, (c) 6F-MIA@MIL-53 (1.5 wt%) and (d) 6F-MIA@MOF-5. For flavin@MOF-5 five slices in the center of the z-stack were selected to generate the decay histograms, for flavin@MIL53 all photons from the images were used. The acquisition conditions were λ_ex = 440 nm (pulsed@32MHz, objective UPLSAPO10X/0.4NA for MIL-53 and objective UPLSAPO20X/0.75NA for MOF-5), λ_F = 502–538 nm, under air.

Figure 7

π−π stacking modes in crystalline MIA with centroid-centroid distances (graphics were drawn from the deposited cif file with CCDC Refcode MISALX [60]). The numbers 1, 2 and 3 differentiate the molecules for their different stacking interactions (see text).

Figure 8

Computed structure of MIA@MIL-53 viewed from both ends of the MIA molecule with indication of the weak C-H···O, C-H···π, C-H···C, N-H···O, O-H···O and O-H···C interactions (H···X distances in Å, distances above 3 Å are not shown).

Figure 9

(a) Alternating neighboring pores in MOF-5 with different cages for binding MIA. Orange pore at left: Linker phenyl hydrogens face inwards toward the center of the pore. Green pore at right: outward position of linker phenyls. (b) Computed MIA molecule in the inward-cage in MOF-5 with the only two supramolecular contacts below 3.0 Å (two C-H···H-C contacts) indicated as dashed orange lines. (c) Computed MIA molecule in the outward-cage for which there are no supramolecular contacts below 3.0 Å.

Figure 10

DFT/MRCI computed absorption (full lines) and emission (dashed lines) signatures of MIA in vacuum, neat crystal and MOF environments.