Light‐Harvesting Antennae using the Host–Guest Chemistry of Mesoporous Organosilica

Abstract

This study presents two series of new host–guest chromophoric systems, where BODIPY dyes are organized into mesoporous silica. The dyes self‐assemble with surfactants to generate micellar templates that can direct the formation of the silica networks. The materials were characterized by means of small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) to elucidate their structure, and by UV/Vis absorption spectroscopy to determine their optical properties. Dye‐loaded COK‐12 materials retain an ordered structure and exhibit a green fluorescence that slightly red‐shifts and undergoes quenching as the dye loading increases. A second system is based on MCM‐41 silica and works as a polychromatic antenna, where a high energy species forms within the template and drive excitation energy transfer in timescales down to 20 ps. Such systems show promising performances for the realization of photonic antennae, to be used as sensitizers for solar cells and photocatalytic devices.

Mesoscopic FRET Antenna Materials by Self‐Assembling Iridium(III) Complexes and BODIPY Dyes

“…The cyclometalated heteroleptic iridium(III) complex, Ir(F 2 ppy) 2 (p-Br-ptap) (IrD), and the BODIPY compound N,N'-difluoroboryl-2,8-diethyl-1,3,7,9-tetramethyl-5-(4-w-methoxy-PEG 2000 -phenyl)-dipyrrin (JM)w ere synthesised according to previously reported procedures. [47,66] (see also the Experimental Sectionf or furtherd etails). The UV-visible absorption and emission properties were also formerlye lucidated and are recalled in Figure 2, with ap articular focus on the spectralo verlap between IrD emission and JM absorption, ak ey requisitef or resonanceenergy transfer.…”

“…The silica hybrid materials containing the two dyes were prepared using the modified templateds ynthesis depicted in Figure 1, where JM,h aving an average number of 45 -CH 2 CH 2 O-units, is embedded along the folded P123 chains, whereas the iridium(III) complex is expected to be localised inside the core hydrophobic regionofthe micelles,b eing totally insoluble in water.D onor-acceptor (D-A) 1:1r atios were used in the construction of two HGCs whichw erep repared using D:A:P123 molarp ercentages of 1:1:98 and 5:5:90, yielding the samples namedh ereafter 1%IrD-1%JM and 5%IrD-5%JM,r espectively.O verall dye loadings highert han 10 % were avoided for two main reasons:i )tom aintain the ordered hexagonal arrangement of the cylindrical micelles within the silica, which starts losing its long-scale order beyondt he above dye loading, [47] and in turn to ensure the reliability of the geometricalm odel used to interpret the optical properties, and ii)tom inimise the occurrence of potentialt riplet-triplet annihilation between IrD molecules, incurred by the closer proximity of dye molecules as their concentration increases, which would be detrimental for the efficiency of energy transfer between IrD and JM dyes. Al ow concentration of these molecules naturally leads to ag reater average distance between them and reduces aggregation overall, makingt he assumption of as tatistical distribution of intermolecular distances arising from ar andom spatiald ispersion of dyes, which has indeed been appliedi nt he literature to rationalise dye speciesb ehaviour in polymericm atrices; [55] additionally,t he complexity of the excited state deactivation dynamics is expected to generally decrease with ar eduction in aggregation processes,a s fewer new radiative de-excitation pathways should arise.…”

“…Reference samples were also prepared for each of the two donor-acceptor systems, containing the donor species only (1%IrD and 5%IrD)a nd the acceptoro nly (1%JM and 5%JM), the latter two having been previously characterised. [47] Alls yntheses were carried out using ac itrate buffer at pH 6.5, which does not alter the stability of the dyes, and the formation of mesoporoussilica was achieved upon addition of as odium silicate solution to the respective micellar phases (see Experimental for details). The resulting powdery insoluble materials were extracted and purified each time using vacuum filtration, aqueous washinga nd desiccation at ambient temperature.…”

“…Ar elatively recent approachr egardingt he synthesis of photoactive HGCs is to position photoactive organic chromophores as part of the template and then allow the dyes to remain inside the mesopores. [47,48] In such strategy,i ti sn ecessary to select an appropriate host framework which uses a template large enough so that can not be inordinately disrupted by the presence of the guest dye species and that can be synthesised under adequately mild conditions, avoiding the risk of chemical decomposition of the dye. As uitable host is the mesoporouss ilica named COK-12, whose synthesis utilises cylindrical micelles of the gemini surfactant Pluronic-P123 (see Figure 1), at riblock copolymer consisting of ah ydrophobic polypropylene glycol (PPG) core (about 70 units)a nd ah ydrophilic polyethylene glycol (PEG) tail (20 units) on either end of the chain, and quasi-neutral conditions (pH 6.5) to polymerise the silica precursor,s odium metasilicate, circumventing the need for stronga cidic or basic conditions when using alkoxysilanes.…”

“…[49] Recently,w er eported on the successful synthesis of analogous COK-12-based structures employing aB ODIPY fluorophore embedded into Pluronic-P123 micelles. [47] Known for their excellent optical properties, namely large molar extinction coefficients (over 10 5 m À1 cm À1 ), highf luorescenceq uantum yields and smallS tokes shifts, BODIPYs have found aw ide range of applications including as popular biological labelling reagents,chemosensors and laser dyes, and are promising candidates to be utilised as light-harvesting pigments in artificial photosynthetic systems, light-emitters in OLEDs, or as fluorescent switches. [50][51][52][53][54][55] When incorporated into mesoporous silica at low loadings of 1-5 %, the green-emitting BODIPY derivative JM (see Figure 2) retains its exceptionalp roperties, with fluorescence quantum yields up to 20 %, and the hybrid material maintains its structural order,i ndicatingah omogeneous dispersiono ft he dye into the structure.…”