Photophysics and applications of organic semiconductors

The work presented in this thesis is motivated by the great commercial impact of organic semiconductors especially in optoelectronics. In particular, we focus our attention on some of the current challenges in organic light-emitting diodes from the point of view of the photophysical properties of materials (i.e. via steady-state and time-resolved photoluminescence characterization) and the device physics. In view of the interest in near-infrared emission, we propose two ways to obtain emission at long wavelength. Firstly, we report a new molecular design of one of the currently best performing polymer for near-infrared light-emitting diodes. We investigate the substitution of sulphur with selenium and find that it is more effective both in terms of photo- and electro-luminescence efficiency than by exploiting higher sulphur-chromophore loadings, while achieving a more important red-shift. Secondly, we explore the tuning of the energy gap through a careful choice of the relative positions of the frontier levels of two organic semiconductors. Following this strategy, we obtain a nearly pure near-infrared electroluminescence with essentially no emission from the single components at any operational voltage. We believe that results obtained are a valuable feedback as they suggest materials design criteria. Given to the potential of phosphorescent materials for obtaining high efficiency OLEDs, we also consider the investigation of a novel emitter based on a wide-gap host co-polymerized with a low-gap phosphorescent emitter for efficient energy transfer. Finally, we report a detailed investigation of the photoluminescence emission at low temperature of the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), widely employed as an electron acceptor in organic solar cells. Owing to the availability of solvent-free single-crystals, whose growth has been recently reported by our group, we are able to investigate PCBM optical properties without solvent dependence. Our attempt is to provide significant information on the ordering and relative importance of the relevant excited states in PCBM

Low-Temperature Photoluminescence Spectroscopy of Solvent-Free PCBM Single-Crystals

PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) is a highly soluble C60 derivative that is extensively used in organic solar cells, enabling power conversion efficiencies above 10%. Here we report, for the first time to the best of our knowledge, the photoluminescence of high-quality solvent-free PCBM crystals between room temperature and 4 K. Interestingly, the PL spectra of these crystals become increasingly structured as the temperature is lowered, with extremely well-resolved emission lines (and a minimum line width of ∼1.3 meV at 1.73 eV). We are able to account for such a structured emission by means of a vibronic coupling model including Franck–Condon, Jahn–Teller and Herzberg–Teller effects. Although optical transitions are not formally forbidden from the low-lying excited states of PCBM, the high symmetry of the electronically active fullerene core limits the intensity of the 0–0 transition, such that Herzberg–Teller transitions which borrow intensity from higher-lying states represent a large part of the observed spectrum. Our simulations suggest that the emissive state of PCBM can be considered as a mixture of the T1g and Hg excited states of C60 and hence that the Hg state plays a larger role in the relaxed excited state of PCBM than in that of C60

Deep-red electrophosphorescence from a platinum(II)–porphyrin complex copolymerised with polyfluorene for efficient energy transfer and triplet harvesting

A series of polyfluorene-based polymers with a range of weight percentages (w/w) of a platinum(II)-containing porphyrin, 5,15-dimesityl-10,20-diphenylporphyrinato platinum(II) (MPP(Pt)), were synthesised and incorporated into organic light-emitting diodes. All polymers showed emission predominantly in the red/NIR region with only those polymers with porphyrin w/w of less than 2% showing residual tails at wavelengths lower than 600 nm, indicating increased emission from the porphyrin as w/w increases. The 2% loading of MPP(Pt) gave the highest efficiency LED (0.48%) and light output (2630 mW/m2)

Synthesis and photophysical characteristics of polyfluorene polyrotaxanes

Two alternating polyfluorene polyrotaxanes (3·TM-βCD and 3·TM-γCD) have been synthesized by the coupling of 2,7-dibromofluorene encapsulated into 2,3,6-tri-O-methyl-β- or γ-cyclodextrin (TM-βCD, TM-γCD) cavities with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. Their optical, electrochemical and morphological properties have been evaluated and compared to those of the non-rotaxane counterpart 3. The influence of TM-βCD or TM-γCD encapsulation on the thermal stability, solubility in common organic solvents, film forming ability was also investigated. Polyrotaxane 3·TM-βCD exhibits a hypsochromic shift, while 3·TM-γCD displays a bathochromic with respect to the non-rotaxane 3 counterpart. For the diluted CHCl3 solutions the fluorescence lifetimes of all compounds follow a mono-exponential decay with a time constant of ≈0.6 ns. At higher concentration the fluorescence decay remains mono-exponential for 3·TM-βCD and polymers 3, with a lifetime τ = 0.7 ns and 0.8 ns, whereas the 3·TM-γCD polyrotaxane shows a bi-exponential decay consisting of a main component (with a weight of 98% of the total luminescence) with a relatively short decay constant of τ1 = 0.7 ns and a minor component with a longer lifetime of τ2 = 5.4 ns (2%). The electrochemical band gap (ΔEg) of 3·TM-βCD polyrotaxane is smaller than that of 3·TM-γCD and 3, respectively. The lower ΔEg value for 3·TM-βCD suggests that the encapsulation has a greater effect on the reduction process, which affects the LUMO energy level value. Based on AFM analysis, 3·TM-βCD and 3·TM-γCD polyrotaxane compounds exhibit a granular morphology with lower dispersity and smaller roughness exponent of the film surfaces in comparison with those of the neat copolymer 3

Traceable atomic force microscopy of high-quality solvent-free crystals of [6,6]-phenyl-C61-butyric acid methyl ester

We report high-resolution, traceable atomic force microscopymeasurements of high-quality, solvent-free single crystals of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These were grown by drop-casting PCBM solutions onto the spectrosil substrates and by removing the residual solvent in a vacuum. A home-built atomic force microscope featuring a plane mirror differential optical interferometer, fiber-fed from a frequency-stabilized laser (emitting at 632.8 nm), was used to measure the crystals' height. The optical interferometer together with the stabilized laser provides traceability (via the laser wavelength) of the vertical measurements made with the atomic force microscope. We find that the crystals can conform to the surface topography, thanks to their height being significantly smaller compared to their lateral dimensions (namely, heights between about 50 nm and 140 nm, for the crystals analysed, vs. several tens of microns lateral dimensions). The vast majority of the crystals are flat, but an isolated, non-flat crystal provides insights into the growth mechanism and allows identification of “molecular terraces” whose height corresponds to one of the lattice constants of the single PCBM crystal (1.4 nm) as measured with X-ray diffraction

Traceable atomic force microscopy of high-quality solvent-free crystals of [6,6]-phenyl-C-61-butyric acid methyl ester

We report high-resolution, traceable atomic force microscopymeasurements of high-quality, solvent-free single crystals of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These were grown by drop-casting PCBM solutions onto the spectrosil substrates and by removing the residual solvent in a vacuum. A home-built atomic force microscope featuring a plane mirror differential optical interferometer, fiber-fed from a frequency-stabilized laser (emitting at 632.8 nm), was used to measure the crystals' height. The optical interferometer together with the stabilized laser provides traceability (via the laser wavelength) of the vertical measurements made with the atomic force microscope. We find that the crystals can conform to the surface topography, thanks to their height being significantly smaller compared to their lateral dimensions (namely, heights between about 50 nm and 140 nm, for the crystals analysed, vs. several tens of microns lateral dimensions). The vast majority of the crystals are flat, but an isolated, non-flat crystal provides insights into the growth mechanism and allows identification of “molecular terraces” whose height corresponds to one of the lattice constants of the single PCBM crystal (1.4 nm) as measured with X-ray diffraction

Increased luminescence efficiency by synergistic exploitation of lipo/hydrophilic co-solvency and supramolecular design

We use steady-state and time-resolved photoluminescence (PL) spectroscopy to investigate the luminescent properties of a sulfonated poly(diphenylenevinylene) lithium salt (PDV.Li) in water/propanol solutions at different concentrations, with a view to assessing its aggregation behavior. In particular, we compare results from uninsulated PDV.Li and cyclodextrin-threaded PDV.Li polyrotaxane (PDV.Li⊂β-CD). We find that addition of 1-propanol (≥20 weight%) leads to a significant blue-shift (of ∼0.20 eV) of the PL spectra, that we assign to suppressed interchain aggregation in PDV.Li solutions, with a concomitant fourfold increase in the fluorescence quantum efficiency (i.e. from 14 to 60%). Surprisingly, a moderate concentration of propanol increases further the luminescence efficiency even for PDV.Li⊂β-CD, whose supramolecular encapsulation already provides a shield against aggregation. Indeed, addition of propanol reduces the solvent polarity, and therefore helps solubilizing these materials that are still largely aromatic in nature. Interestingly, however, both uninsulated PDV.Li and polyrotaxane solutions exhibit signs of aggregation at high propanol fraction (>70%) with a distinctively weaker coupling than that of interchain states in PDV.Li at high water concentration and in pure water in particular. While we ascribe such behavior to a poor solvation of the polar moieties, we also report a different strength of aggregation for PDV.Li and PDV.Li⊂β-CD that can be attributed to the presence of the cyclodextrin rings. In PDV.Li⊂β-CD hydrogen bonding between the cyclodextrin rings may lead to closer packing between the polymer chains. We therefore suggest that a content of propanol between 30 and 70% provides a good balance of hydrophobic and hydrophilic interactions both for PDV.Li and PDV.Li⊂β-CD

NUP98-fusion transcripts characterize different biological entities within acute myeloid leukemia: A report from the AIEOP-AML group.

In the last years, collaborative studies have joined to link the degree of genetic heterogeneity of acute myeloid leukemia (AML) to clinical outcome,1, 2 allowing risk stratification before therapy and guiding post-induction treatment of children with AML. So far, still half of these patients, whose disease is usually characterized by a grim prognosis, lack a known biomarker offering opportunities of targeted treatment

CD56, HLA-DR, and CD45 recognize a subtype of childhood AML harboring CBFA2T3-GLIS2 fusion transcript

The presence of CBFA2T3‐GLIS2 fusion gene has been identified in childhood Acute Myeloid Leukemia (AML). In view of the genomic studies indicating a distinct gene expression profile, we evaluated the role of immunophenotyping in characterizing a rare subtype of AML‐CBFA2T3‐GLIS2 rearranged. Immunophenotypic data were obtained by studying a cohort of 20 pediatric CBFA2T3‐GLIS2‐AML and 77 AML patients not carrying the fusion transcript. Enrolled cases were included in the Associazione Italiana di Ematologia Oncologia Pediatrica (AIEOP) AML trials and immunophenotypes were compared using different statistical approaches. By multiple computational procedures, we identified two main core antigens responsible for the identification of the CBFA2T3‐GLIS2‐AML. CD56 showed the highest performance in single marker evaluation (AUC = 0.89) and granted the most accurate prediction when used in combination with HLA‐DR (AUC = 0.97) displaying a 93% sensitivity and 99% specificity. We also observed a weak‐to‐negative CD45 expression, being exceptional in AML. We here provide evidence that the combination of HLA‐DR negativity and intense bright CD56 expression detects a rare and aggressive pediatric AML genetic lesion improving the diagnosis performance

Osteomalacia: The Missing Link in the Pathogenesis of Bisphosphonate-Related Osteonecrosis of the Jaws?

Background. Bisphosphonate-related osteonecrosis of the jaw (BRONJ) is a well-documented adverse event from treatment with nitrogen-containing bisphosphonates (NBPs). During a preliminary histomorphometric study aimed at assessing the rate of bone remodeling in the jaws of patient with surgically resected BRONJ, we found a defect of bone mineralization (unpublished data). We hypothesized that osteomalacia could be a risk factor for BRONJ in patients taking NBPs. Therefore, we looked for static and dynamic histomorphometric evidence of osteomalacia in biopsies from subjects with and without BRONJ.Methods. This case-control study used histomorphometric analysis of bone specimens of patients using NBPs (22 patients with BRONJ and 21 patients without BRONJ) who required oral surgical interventions for the treatment/prevention of osteonecrosis. Patients were given tetracycline hydrochloride according to a standardized protocol before taking bone biopsies from their jaws. Biopsies with evidence of osteomyelitis or necrosis at histology were excluded from the study. Osteomalacia was defined as a mineralization lag time >100 days, a corrected mean osteoid thickness >12.5 mm, and an osteoid volume >10%.Results. In all, 77% of patients with BRONJ were osteomalacic compared with 5% of patients without BRONJ, according to histomorphometry. Because osteomalacia was found almost exclusively in NBP users with BRONJ, this is likely to be a generalized process in which the use of NBPs further deteriorates mechanisms of bone repair.Conclusions. Osteomalacia represents a new and previously unreported risk factor for disease development. This finding may contribute to a better understanding of the pathogenesis of this disease and help with the development of strategies to increase the safety of NBP administration