Aref, Diaa (2020) Developing Artificial Multi-Metal Peptide-Based Photocatalysts. [Tesi di dottorato]

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Tipologia del documento:	Tesi di dottorato
Lingua:	English
Titolo:	Developing Artificial Multi-Metal Peptide-Based Photocatalysts
Autori:	Autore Email Aref, Diaa diaa.aref@unina.it
Data:	13 Marzo 2020
Numero di pagine:	187
Istituzione:	Università degli Studi di Napoli Federico II
Dipartimento:	Scienze Chimiche
Dottorato:	Scienze chimiche
Ciclo di dottorato:	32
Coordinatore del Corso di dottorato:	nome email Lombardi, Angelina alombard@unina.it
Tutor:	nome email Lombardi, Angelina [non definito] Pavone, Michele [non definito]
Data:	13 Marzo 2020
Numero di pagine:	187
Parole chiave:	Artificial peptide, Photocatalyst, Photosensitizer, Ruthenium complex, Hydrogen energy
Settori scientifico-disciplinari del MIUR:	Area 03 - Scienze chimiche > CHIM/02 - Chimica fisica Area 03 - Scienze chimiche > CHIM/03 - Chimica generale e inorganica
Depositato il:	27 Mar 2020 11:37
Ultima modifica:	10 Nov 2021 09:49
URI:	http://www.fedoa.unina.it/id/eprint/13087

Abstract

The development of photosynthetic systems is a growing field in chemistry with significant environmental applications. Ruthenium(II)-based polypyridine complexes are thoroughly studied for their capability to perform light-driven catalysis and to photo-induce charge-transfer to diverse organic, inorganic, and biological molecules. However, the development of a powerful and durable photosensitizer, which can be easily conjugated to different biocatalysts, has not been achieved yet. In the field of artificial metalloenzymes, the design of a suitable photosensitizing unit that can be tethered to such compounds is of particular interest, considering that only few multi-cofactor artificial metalloproteins have been reported so far. Among them, [RuII(bpy)3] and [RuII(tpy)2] have been generally adopted. [RuII(bpy)3] possesses well-known photophysical and photochemical properties, such as high chemical stability, high molar absorptivity (~14,600 M-1cm-1), and fluorescence quantum yield. Moreover, an exited-state lifetime in the order of microseconds (~1,100 ns) is crucial for its photochemical application. However, because of its stereogenic metal center, the functionalization of one or more of the coordinated bpy-ligands produces at least two stereoisomers (Lamda and Dalta) with distinct photophysical properties, increasing the synthetic complexity. In contrast, [RuII(bpy)3] has an isomeric defined structure. Nevertheless, poor photophysical properties, such as lower molar absorptivity (<10,000 M-1cm-1) and picosecond excited state-lifetime, hamper its applicability. Several reports have been published that describe tris-heteroleptic complexes as a feasible alternative in order to overcome such limitations. Firstly, we report our contribution in the field of photosensitization. The synthesis and characterization of a new tris-heteroleptic ruthenium(II) complex, [RuII(bpy)(tpy)(pyCOOH)] (bpy = 2,2’-bipyridine, tpy = 2,2’:2”,6’-terpyridine and pyCOOH = 4-carboxylpyridine), will be presented. Such complex has an isomerically-defined structure; it holds a carboxyl moiety that provides a versatile coupling moiety for bioconjugation, and that enhances its photophysical properties thanks to the electron withdrawing effect. The photophysical performance of the ruthenium(II) complex has been characterized by steady-state spectroscopic experiments, and supported by DFT and TD-DFT calculations for further understanding of the observed properties at a molecular scale. The synthesized ruthenium(II) complex has shown an enhanced light absorptivity of approximately 12,000 M-1cm-1 (at λmax = 460 nm) and 13,000 M-1cm-1 (at λmax = 427 nm) in acetonitrile, and a wider range of visible light absorption in comparison to [RuII(bpy)3] (14,600 M-1cm-1; λmax = 460 nm). As expected for this class of terpyridine-based ruthenium(II) complexes, [RuII(bpy)(tpy)(pyCOOH)] has only limited fluorescence quantum yield, approximately tenfold less than [RuII(bpy)3] (Q[RuII(bpy)(tpy)(pyCOOH)])=0.009; Q[RuII(bpy)3])=0.095). The TD-DFT shows that the electronic density of the LUMO, involved in the charge transfer transition, mainly occupies the carboxyl moiety. Such key feature could be extremely positive if we consider that such moiety is involved into the conjugation between the ruthenium(II) complex and other molecular assemblies. The photochemical performance has been studied using a multicomponent system, composed of the ruthenium(II) complex, cytochrome c and aniline as the photosensitizer, electron acceptor and sacrificial electron donor, respectively. The photo-induced electron transfer process was found to be pseudo first-order against ruthenium(II) complex concentration, being 76% as active as [RuII(bpy)3]. Such lower activity may be accounted to the higher activation energy barrier (ca. 3.7 kJ.mol-1) between the two complexes. Secondly, the tethering between the ruthenium(II) complex and artificial peptide has been tested by preparing two prototype systems based on cobalt(III) Mimochrome VI*a (CoIIIMC6*a), which is a rationally designed heme-peptide conjugate. The latter has been reported for its remarkable electrochemical activity, longevity, and stability in electrochemical hydrogen evolution in water under mild conditions. Therefore, CoIIIMC6*a has been used as a model system to test the here-presented complex conjugation. Two conjugation protocols have been characterized using both spectroscopic and chromatographic techniques. The synthetic results show that the ruthenium(II) complex can be easily tethered to the lysine-residue of CoIIIMC6*a either through click-chemistry crosslinking (using DBCO-PEG4-NHS as the spacer) or through direct conjugation (using a one-pot protocol for NHS coupling). Preliminary results of photoinduced hydrogen evolution have shown that it occurred only when [RuII(bpy)3] was used as freely diffusing photosensitizer. Nonetheless, spectroscopic evidence has excluded any inactivation of CoIIIMC6*a by the [RuII(bpy)(tpy)(pyCOOH)] complex, thus confirming the general covalent approach of photosensitization.

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