Circularly polarized luminescence: from spectroscopy to applications

Chiral molecules can emit left and right polarized light with different intensities; this phenomenon is known as circularly polarized luminescence (CPL). CPL can be observed in photoluminescence and in electroluminescence. In general CPL spectroscopy can be an interesting tool to investigate the chirality of the excited state or it can be employed in technological applications such as OLEDs able to directly emit circularly polarized electroluminescence (CP-OLEDs). The first part of this work concerns chiral lanthanide complexes, which can emit CPL with a degree of circular polarization which is almost precluded to non- aggregated purely organic molecules. In this part, we present a new chiral Eu complex with highly polarized emission and, a series of CP-OLEDs based on a lanthanide complex. Following this strategy, the highest polarization degree reported to date is obtained. Moreover, we discuss and rationalize the factors affecting the polarization inside a device. In the second part, we investigate the CPL properties of chiral organic molecules and we apply CPL spectroscopy as a qualitative analytical tool in pH-dependent chiroptical switch and to signal the interaction between fluorescent stains and protein-based matrices used in paintings

Sintesi e caratterizzazione di complessi chirali di lantanidi via NMR e spettroscopia chiroottica.

I lantanidi mostrano caratteristiche spettroscopiche peculiari dovute alla natura degli orbitali 4f; in emissione presentano righe strette, comparabili con le righe dell'atomo isolato, e non hanno una forte dipendenza dall'ambiente intorno al metallo determinato dalla presenza dei leganti. I leganti svolgono però il loro ruolo assorbendo la radiazione luminosa e trasferendo al lantanide l'energia, secondo i meccanismi descritti in seguito. Inoltre complessi di lantanidi hanno suscitato interesse per le loro possibili applicazioni come emettitori di luce circolarmente polarizzata (CPL) in quanto mostrano fattori di anisotropia di luminescenza (glum) talvolta dell'ordine di 0,1-0,5 (0 corrisponde all'emissione di un fascio racemo mentre |2| è il massimo teorico) e perfino di 1,38 nel caso del complesso eterobimetallico CsEu(hfbc)4$. Recentemente è stato visto che un meccanismo di coupling dinamico in cui i momenti elettrici di transizione indotto sul legante si accoppiano con il momento magnetico generato dalla transizione sul metallo, può spiegare gli alti valori di glum} osservati per questi sistemi; in un sistema ottacoordinato di geometria C4 la situazione antiprismatica è quella che fornisce le migliori condizioni per l'instaurarsi di questo tipo di meccanismo. La conoscenza della struttura del complesso indagato anche in soluzione diventa dunque necessaria per interpretarne le caratteristiche spettroscopiche; è possibile conoscere la struttura, o almeno avere qualche indizio sulla inerzia del complesso in soluzione, via NMR: gli shift di pseudocontatto sono contributi anisotropi (eccetto che per il Gd3+ in cui il contributo di pseudocontatto è nullo) e i tempi di rilassamento dipendono dalla distanza tra il centro paramagnetico ed il nucleo osservato, da tali valori sperimentali, ricavati utilizzando sapientemente le tecniche che la moderna spettroscopia NMR mette a disposizione, è dunque possibile ottenere le informazioni strutturali necessarie applicando attentamente i principi e le equazioni che regolano tali fenomeni. Nel corso di questo lavoro ci siamo occupati nella progettazione e nella sintesi di nuovi leganti da impiegare nella sintesi di nuovi complessi che presentassero: -buone proprietà ottiche per garantire emissioni elevate -una geometria adatta per garantire un valore di glum in accordo con il meccanismo di coupling dinamico. Dei complessi ottenuti sono state indagate le proprietà ottiche sia in assorbimento (UV-VIS) che in emissione sia le proprietà chiroottiche (ECD, CPL) e le proprietà strutturali tramite NMR, sintetizzando complessi utilizzando altri lantanidi per verificare la presenza di isostrutturalità lungo la serie lantanidica o lungo una porzione di ess

Design of Lanthanide-Based OLEDs with Remarkable Circularly Polarized Electroluminescence

Organic light-emitting diodes (OLEDs) able to directly emit circularly polarized (CP) electroluminescence (CP-OLEDs) are rapidly gaining much interest, due to their possible applications in displays with antiglare filters and 3D displays. Development of more efficient CP-OLEDs can open their use also in point-of-care and personalized diagnostic tools, since CP light alteration can be related to health state of irradiated tissues. In this work it is shown that the performance of chiral europium complex-based CP-OLEDs can be improved both in terms of external quantum efficiency (measured on all the Eu bands) and degree of polarization of emitted photons (as measured by the dissymmetry factor gEL), by proper active layer formulation and through a fine tuning of the architecture of the device. Polarization performances (gEL = −1) are obtained about three times higher than for any other CP-OLED reported so far. Moreover, for the first time, it is shown that the position of the recombination zone (RZ) plays a major role on the polarization outcomes. In order to rationalize these results the level of light polarization is related to the position of the RZ allied with the reflection on the cathode through a simple mathematical model. The values predicted by this model are in qualitative agreement with the experimental ones

Supramolecular organization of new chiral π-conjugated oligomers: synthesis and spectroscopic study

π-Conjugated polymers and oligomers are widely used as organic semiconductors in devices such as field-effect transistors (OFET), light-emitting diodes (OLED) and solar cells (OPV)1; in the last years, a very important role in this field has been played by olygothiophenes2. Their optoelectronic properties (structure of the absorption bands, fluorescence efficiency, charge and exciton transport) depend not only on the chemical nature and the conformation assumed, but also on their supramolecular organization in the solid state. In particular, the introduction of chiral groups can be used to drive their self-assembly. We shall summarize our recent results about the supramolecular aggregation of three new chiral π- conjugated oligomers, consisting of an aromatic central ring (1,4-hydroquinone, benzo[1,2-b:4,5-b′]dithiophene- 4,8-diol, 1,4-diketopyrrolo[3,4-c]pyrrole) functionalized with two (S)-3,7-dimethyl-1-octyl groups and connected to two 2,2'-bithienyl units. In particular, we shall discuss: a) the synthetic route developed for the preparation of these new molecules;b) the characterization of their supramolecular aggregates in solution and of their thin films through optical (UV-VIS, fluorescence) and chiroptical (electronic circular dichroism) spectroscopies, in connection with optical microscopy investigation

Circularly Polarised Luminescence in Enantiopure Samarium and Europium Cryptates

Circularly polarised luminescence (CPL) is a chiroptical phenomenon gaining more and more attention, as the availability of the necessary spectrometers is getting better and first applications in bioimaging or for the preparation of OLEDs (organic light emitting diodes) are coming within range. Until now most examples of distinctly CPL-active compounds were europium and terbium complexes though theoretically the electronic structure of samarium should be as suitable as the one of terbium. This discrepancy can be accounted for by the high susceptibility of samarium to non-radiative deactivation processes. The aim of this study was to strategically circumvent this difficulty by the use of a ligand scaffold which has already proven to efficiently suppress these processes, namely the cryptates. The prepared partly deuterated samarium and europium complexes exhibit distinct circularly polarised luminescence with dissymmetry factors up to glum=+0.13 (SmIII) or glum=−0.19 (EuIII)

Chiroptical methods in a wide wavelength range for obtaining Ln3+complexes with circularly polarized luminescence of practical interest

We studied enantiopure chiral trivalent lanthanide (Ln3+= La3+, Sm3+, Eu3+, Gd3+, Tm3+, and Yb3+) complexes with two fluorinated achiral tris(β-diketonate) ligands (HFA = hexafluoroacetylacetonate and TTA = 2-thenoyltrifluoroacetonate), incorporating a chiral bis(oxazolinyl)pyridine (PyBox) unit as a neutral ancillary ligand, by the combined use of optical and chiroptical methods, ranging from UV to IR both in absorption and circular dichroism (CD), and including circularly polarized luminescence (CPL). Ultimately, all the spectroscopic information is integrated into a total and a chiroptical super-spectrum, which allows one to characterize a multidimensional chemical space, spanned by the different Ln3+ions, the acidity and steric demand of the diketone and the chirality of the PyBox ligand. In all cases, the Ln3+ions endow the systems with peculiar chiroptical properties, either allied to f-f transitions or induced by the metal onto the ligand. In more detail, we found that Sm3+complexes display interesting CPL features, which partly superimpose and partly integrate the more common Eu3+properties. Especially, in the context of security tags, the pair Sm/Eu may be a winning choice for chiroptical barcoding

Remarkable near-infrared chiroptical properties of chiral Yb, Tm and Er complexes

We carried out a study of absorption (CD) and emission (CPL) chiroptical properties in the NIR region of two sets of Yb, Tm and Er complexes. The two complexes include a D3 symmetric, [TMG-H+]3Ln(BINOLate)3 (Ln = Yb, Tm, Er; TMG = 1,1,3,3-tetramethylguanidine; BINOLate = 1,1'-bi-2-naphtholate), and a tetrakis, C4 symmetric, CsLn(hfbc)4 (Ln = Yb, Tm, Er; hfbc = 3-heptafluorobutylyrylcamphorate). The lanthanides studied gave access to three discrete energy domains, Yb (900-1040 nm), Tm (1180-1240 nm) and Er (1430-1600 nm) in which the chiroptical activity was assessed using gabs (and glum for Yb complexes). Exceptionally high discrimination between left and right circularly polarised light was observed, with values up to almost the theoretical maximum (±2)

Diagnostic accuracy of sonohysterography vs hysteroscopy in benign uterine endocavitary findings

OBJECTIVE: To compare diagnostic accuracy of sonohysterography vs hysteroscopy in patients with benign uterine endocavitary findings. PATIENTS AND METHODS: This retrospective study evaluated 202 patients submitted to sonohysterography after transvaginal ultrasound examination suspicious for uterine endocavitary findings. Cytological sample was taken and analyzed from the fluid used to distend the uterine cavity. Of 202 patients enrolled for this study, 86 patients underwent gynaecological surgery, of whom 77 were treated with operative hysteroscopy and 9 with other gynaecological surgical techniques. Statistical analysis was performed to evaluate diagnostic agreement between sonohysterography vs hysteroscopy and cytology vs histology. RESULTS: Diagnostic concordance between sonohysterography and hysteroscopy was significant (k value 0.87). The correlation between cytological and histological findings had a moderate level of concordance (k value 0.49).CONCLUSIONS: Sonohysterography provides a diagnostic accuracy as well as hysteroscopy, therefore, it could be considered an alternative procedure in the diagnosis of benign uterine endocavitary findings

A circular dichroism study of the protective role of polyphosphoesters polymer chains in polyphosphoester‐myoglobin conjugates

Protein-polymer conjugates are a blooming class of hybrid systems with high biomedical potential. Despite a plethora of papers on their biomedical properties, the physical–chemical characterization of many protein-polymer conjugates is missing. Here, we evaluated the thermal stability of a set of fully-degradable polyphosphoester-protein conjugates by variable temperature circular dichroism, a common but powerful technique. We extensively describe their thermodynamic stability in different environments (in physiological buffer or in presence of chemical denaturants, e.g., acid or urea), highlighting the protective role of the polymer in preserving the protein from denaturation. For the first time, we propose a simple but effective protocol to achieve useful information on these systems in vitro, useful to screen new samples in their early stages

Modular chiral Eu(iii) complexes for efficient circularly polarized OLEDs

Achieving both high dissymmetry factors and strong emission in circularly polarized (CP) luminescent materials and, at the same time, compatibility with manufacturing processes for organic electronic devices, is a crucial issue for reliable applications of CP emitters in many fields, such as chiral electronics and optoelectronics. In this communication, we show that the independent choice of the sensitizing and the chirality inducing ligands allows europium(III) complexes to meet the multiple requirements for solution processed efficient CP electroluminescent devices