Antonio Barbon

There is an increasing interest in piezocatalysis, converting mechanical energy (ultrasonic vibrations) into chemical energy in piezoelectrics, separating charge carriers to generate reacative oxygen species through a surface redox process. It is known that hydroxyapatite can be piezoelectric, particularly due to the presence of oxygen vacancies in the structure. In this paper we investigate the piezoelectric properties of sustainable biogenic hydroxyapatite prepared from waste chicken bones calcined in air at 700 or 1000 °C, then further treated at 800 oC in N2 for 2-4 h to induce oxygen vacancies and improve the piezoelectric/piezocatalytic properties. The materials were characterised to determine phase and elemental composition, and the presence of oxygen vacancies confirmed in those treated under N2 by electron paramagnetic resonance (EPR). The piezoelectric properties were assessed by piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM), and those treated under N2 shown to have superior piezoelectric properties and surface potential, particularly when calcined at 1000 °C and then heated under N2 for 2 h. Piezocatalysis measurements showed hydroxyapatite treated under N2 was able to degrade/remove up to 40% of methylene blue dye, using an ultrasonic tip of either 20 kHz or 30 kHz at a power of 40 W.

We studied the intersystem crossing (ISC) and triplet states of thionated perinones to assess how the thionation of the carbonyl group in a chromophore affects these processes or properties. The thionated compounds show red-shifted absorption and strongly quenched fluorescence compared to the oxo-counterparts. Femtosecond transient absorption spectra show fast ISC kinetics (0.31–1.92 ps⁻¹) for the thionated chromophores. Nanosecond transient absorption spectra demonstrated short triplet excited state lifetimes (0.62–11.5 µs) for the thionated perinones, which is unusual for heavy atom-free small organic molecules. Time-resolved electron paramagnetic resonance spectroscopy following photoexcitation shows inversion of the electron spin polarization pattern of the triplet state, which is resulted from the overpopulation of one sublevel and its fast decaying with respect to the other two sublevels of the T₁ state. Spectral simulation shows that the T_z sublevel decays 25-time faster (1.0 × 10⁴ s⁻¹) than the T_x and T_y sublevels (ca. 4.0 × 10² s⁻¹). Interestingly, zero field splitting (ZFS) D parameters (ca. 0.268 cm⁻¹) was observed for S3, which is unusually large for an organic molecule. These results demonstrated the relationship between the thionation and the enhanced ISC, the triplet state lifetimes, as well as the triplet-state electron spin dynamics, it is useful for the development of heavy-atom-free triplet photosensitizers.

A long-lived triplet charge separated state (0.8 µs) was observed in donor–acceptor dyads and the rISC wasn’t accelerated by Se atoms.

The mechanism of proton transfer (PT), and how it is affected by water structure, is a fundamental issue in numerous chemical and biological processes. Formulated more than 200 years ago, a possible model for PT in aqueous media was proposed by Grotthuss, which continues to be actively studied and debated. In this study, we exploit electron paramagnetic resonance to investigate PT in aqueous solutions. Our proposed method employs pH-sensitive stable nitroxyl radicals and makes use of photolysis of 2-nitrobenzaldehyde to generate protons in the sub-nanosecond timescale. This approach was used to study the impact of classical chaotropic compounds on PT as studied in various aqueous solutions, i.e. 8 M urea, 6 M guanidine hydrochloride (Gdn·HCl), and potassium chloride (KCl). Our findings confirm significant impacts on PT rates. For instance, in 6 M Gdn·HCl, PT occurred 40-fold slower than in water. The method’s sensitivity to water structure is demonstrated, highlighting its potential for monitoring the kinetics of PT in ice and in proteins.

We performed thionation of the electron acceptor naphthalimide (NI) moiety in donor–acceptor (D‐A) dyads in a series of thio‐substituted D‐A dyads. Thionation significantly enhances the electron‐accepting ability of the chromophore, facilitating the formation of low‐lying charge‐separated (CS) states. Femtosecond transient absorption spectra revealed intersystem crossing (ISC) rate constants of S‐NI‐PXZ ( k _ISC = 3.8 × 10 ¹¹ s ⁻¹ ) and DS‐NI‐PXZ ( k _ISC = 2.4 × 10 ¹¹ s ⁻¹ ), which are interestingly independent of the number of sulfur atoms. Generally, fluorescence quenching was observed due to the rapid ISC process. Native NI‐PXZ exhibits thermally activated delayed fluorescence (TADF). Thionation increased the singlet‐triplet energy gap, making the TADF vanish. In n ‐hexane, nanosecond transient absorption spectroscopy detected the triplet lifetimes shortened from 11.6 µs of NI‐PXZ to 0.66 µs for S‐NI‐PXZ and 0.25 µs for DS‐NI‐PXZ . Time‐resolved electron paramagnetic resonance revealed the large zero‐field splitting D value (4200–9800 MHz) of the triplet state of the thionated derivatives, which is due to the much larger SOC matrix elements of triplet state of DS‐NI‐PXZ (ca. 135 cm ⁻¹ ) and DS‐NI (ca. 136 cm ⁻¹ ) than the nonthionated NI (3.1 cm ⁻¹ ). Thionation of the carbonyl groups in chromophore is a promising method for design of triplet photosensitizers to achieve fast ISC.

The time dependence of the light-induced spin polarization of a series of porphyrins is reported. The porphyrins contain central elements from three distinct groups in the Periodic Table with different oxidation states, types, and numbers of axial ligands, as well as different peripheral substituents on the porphyrin ring. Shortly after photoexcitation, in all cases, the primary multiplet polarization of the porphyrin triplet state evolves into long-lived net polarization whose lifetime is determined by the lifetime of the triplet state. The zero-field splitting parameters and sign of the multiplet polarization vary significantly among the porphyrins, but the transient EPR spectra taken at later times are remarkably similar. The development of long-lived net polarization is proposed to involve the dynamic Jahn–Teller effect, in which hopping between the two lowest triplet states occurs.

Thermally activated delayed fluorescence (TADF) molecules have been widely investigated in organic light emitting diodes (OLED), organic lasing, etc. Small singlet‐triplet energy gap (ΔE_ST) and high radiative rate constants (k_F) are highly desired to utilize triplet excitons efficiently and are beneficial to reduce efficiency roll‐off of devices of OLED devices. The prevalent TADF molecules are via donor‐acceptor molecular design, for which the decreasing of the ΔE_ST is often at the expense of reducing the k_F. Herein, we demonstrated a new ΔE_ST modulation approach to construct TADF with high k_F, based on triplet blocking effect, i.e., the extension of π‐conjugation of a triplet constrainer (IB) leads to a gradually red‐shifted S₁ but a constant T₁ energy and therefore reduced ΔE_ST controlled from monomer (IB), monomer‐linker (IB‐BF₂), to dimer of IB‐BF₂‐IB. The natural transition orbital analysis indicates that S₁ state is delocalized while T₁ state is localized as confirmed by time resolved electron paramagnetic resonance spectroscopy. Therefore, the ΔE_ST is reduced from 0.60 eV, 0.46 eV to 0.25 eV, while keeping faster radiation rate (around 10⁸ s⁻¹) than that of conventional donor‐acceptor molecules (10⁶∼10⁷ s⁻¹). As a result, the emission mechanisms are regulated from fluorescence for IB, phosphorescence/TADF dual emissions for IB‐BF₂ to TADF for IB‐BF₂‐IB. This paper proposed a new approach of ΔE_ST modulation and a new type of TADF molecule with high radiation rate, which is crucial for fundamental photophysics as well as material science.

Thermally activated delayed fluorescence (TADF) molecules have been widely investigated in organic light emitting diodes (OLED), organic lasing, etc. Small singlet‐triplet energy gap (ΔE_ST) and high radiative rate constants (k_F) are highly desired to utilize triplet excitons efficiently and are beneficial to reduce efficiency roll‐off of devices of OLED devices. The prevalent TADF molecules are via donor‐acceptor molecular design, for which the decreasing of the ΔE_ST is often at the expense of reducing the k_F. Herein, we demonstrated a new ΔE_ST modulation approach to construct TADF with high k_F, based on triplet blocking effect, i.e., the extension of π‐conjugation of a triplet constrainer (IB) leads to a gradually red‐shifted S₁ but a constant T₁ energy and therefore reduced ΔE_ST controlled from monomer (IB), monomer‐linker (IB‐BF₂), to dimer of IB‐BF₂‐IB. The natural transition orbital analysis indicates that S₁ state is delocalized while T₁ state is localized as confirmed by time resolved electron paramagnetic resonance spectroscopy. Therefore, the ΔE_ST is reduced from 0.60 eV, 0.46 eV to 0.25 eV, while keeping faster radiation rate (around 10⁸ s⁻¹) than that of conventional donor‐acceptor molecules (10⁶∼10⁷ s⁻¹). As a result, the emission mechanisms are regulated from fluorescence for IB, phosphorescence/TADF dual emissions for IB‐BF₂ to TADF for IB‐BF₂‐IB. This paper proposed a new approach of ΔE_ST modulation and a new type of TADF molecule with high radiation rate, which is crucial for fundamental photophysics as well as material science.

A borazine derivative functionalized by nitroxide free radicals, N,N',N″-(tris(4-Bromophenyl))-B,B',B″-tris((2,6-dimethyl-4-(N-tert-butyl-N-oxyamino)phenyl) borazine (TriBNit), was synthesized as a milestone of open-shell inorganic benzene. The crystal structure determined from X-ray diffraction on a single crystal ascertains the grafting of three nitroxide radicals. The temperature dependence of the magnetic susceptibility evidences weak intramolecular antiferromagnetic interactions between the radicals with strong intermolecular antiferromagnetic interactions between two nitroxide moieties of two neighboring molecules. EPR spectroscopy at 80 K on a frozen glassy solution evidences the coexistence of S = 1/2 and S = 3/2 ground-spin state species. This is ascribed to the nitroxide radicals having different orientations with respect to the borazine core giving rise either to antiferromagnetic interaction with a low ground-spin state S = 1/2 or to ferromagnetic interaction with a high ground-spin state S = 3/2 as supported by theoretical data. At room temperature, because of nitroxide mobility, the EPR spectrum is averaged to a ground-spin state S = 1/2.