A Comprehensive Approach to Exciton Delocalization and Energy Transfer.

Electrostatic intermolecular interactions lie at the heart of both the Förster model for resonance energy transfer (RET) and the exciton model for energy delocalization. In the Förster theory of RET, the excitation energy incoherently flows from the energy donor to a weakly coupled energy acceptor. The exciton model describes instead the energy delocalization in aggregates of identical (or nearly so) molecules. Here, we introduce a model that brings together molecular aggregates and RET. We will consider a couple of molecules, each described in terms of two diabatic electronic states, coupled to an effective molecular vibration. Electrostatic intermolecular interactions drive energy fluxes between the molecules, that, depending on model parameters, can be described as RET or energy delocalization. At variance with the standard Förster model for RET and of the exciton model for aggregates, our approach applies both in the weak and in the strong coupling regimes and fully accounts for the quantum nature of molecular vibrations in a nonadiabatic approach. Coupling the system to a thermal bath, we follow RET and energy delocalization in real time and simulate time-resolved emission spectra.

The fate of molecular excited states: modeling donor-acceptor dyes.

We investigate the relaxation of a coherently excited molecule in the Redfield approximation. The molecular model, parametrized to describe donor-acceptor dyes that represent a large family of molecules of interest for several applications, accounts for two diabatic electronic states non-adiabatically coupled to a few vibrational coordinates. The proposed approach successfully describes the fast vibrational relaxation, followed by a much slower relaxation towards the ground state, a physically relevant result that is robust vs. the specific model adopted for the system-bath coupling and the specific (reasonable) choice of the bath spectral density. We demonstrate that, when dealing with more than a single vibration, it is important that each vibration is separately coupled to an independent bath so as to avoid the cross-talking of the modes through their coupling to the same bath. Provided that the overall strength of the electron-vibration coupling is maintained constant, the number of molecular vibrations introduced in the model does not affect the system dynamics, supporting the use of effective and easy models for donor-acceptor dyes accounting for a single coupled vibration.

Dynamical disorder and resonance energy transfer: a novel quantum-classical approach.

Resonance energy transfer (RET), at the heart of photosynthesis, supports life on earth, but also guarantees the operation of several technological devices, like organic light-emitting diodes and solar cells. Medium properties and dynamics largely affect RET efficiency, but reliable models addressing how molecular electron-vibration motion and solvent dynamics jointly affect RET are still missing. Here we propose a novel quantum-classical approach to describe RET in a non-adiabatic molecular system embedded in a dynamic polar environment. The approach, validated against optical properties of a dye in solution, is then applied to a RET-pair, demonstrating that dynamic disorder, as induced by a liquid polar solvent, boosts RET efficiency.

Optical spectra of molecular aggregates and crystals: testing approximation schemes.

The interplay between exciton delocalization and molecular vibrations profoundly affects optical spectra of molecular aggregates and crystals. The exciton motion occurs on a similar timescale as molecular vibrations, leading to a complex and intrinsically non-adiabatic problem that has been handled over the years introducing several approximation schemes. Here we discuss systems where intermolecular distances are large enough so that only electrostatic intermolecular interactions enter into play and can be treated in the dipolar approximation. Moreover, we only account for interactions between transition dipole moments, as relevant to symmetric molecules, with negligible permanent (multi)polar moments in the ground and low-lying excited states. Translational symmetry is fully exploited to obtain numerically exact solutions of the relevant Hamiltonian for systems of comparatively large size. This offers a unique opportunity to assess the reliability of different approximation schemes. The so-called Heitler-London approximation, only accounting for the effects of intermolecular interactions among degenerate electronic states, leads to the celebrated exciton model, widely adopted to describe optical spectra of molecular aggregates and crystals. We demonstrate that, mainly due to a cancellation of errors, the exciton model approximates well the position of exciton bands and reasonably well the bandshapes, but it fails to predict spectral intensities, leading to underestimated intensities in J-aggregates and overestimated intensities in H-aggregates. This general result is validated against an exact sum-rule. Finally, we address the validity of several approximation schemes adopted to reduce the dimension of the vibrational basis.

[Diagnostic approach to popliteal venous aneurysm]. / L'approccio diagnostico all'aneurisma venoso popliteo.

Venous aneurysms are rare vascular abnormalities and are often described in patients presenting with pulmonary embolism. The occasional real time ultrasound examination of an aneurysm of venous system in the popliteal fossa has focalized our interest in the diagnostic approach of this entity. In our experience and in the literature the physical examination is usually negative while the ultrasound gives high quality imaging combined to a Doppler evaluation, so this kind of diagnostic tool is desirable as a first approach. Both morphology and flow hemodynamics can be detected in a non invasive way. About wall aspect and surrounding tissue is underlined the MRT and CT scanning, the thrombotic extension is well inspected. The venography is the definitive study and should always be performed because it is the most reliable examination and specific until the sac of aneurysm is filled from the contrast. However this diagnostic modality is useful in the patient undergoing surgical repair.

[The popliteal artery entrapment syndrome. Clinical experience]. / Sindrome da intrappolamento dell'arteria poplitea. Esperienza clinica.

The authors report their experience in treating three cases of popliteal artery entrapment. The analysis is based on the onset and the gap of time between the symptoms and the diagnostic certainty referred to the age of the patients. The diagnostic approach and follow-up is discussed including magnetic resonance of popliteal fossa. The surgical approach was constantly a shaped dorsal incision, and the reconstruction was performed in two cases with carbon coated dacron grafts of 6 mm diameter for the inadequacy of veins available. Any true aneurysm formation was identified. All grafts are patent on follow-up and the viability of the popliteal artery that underwent to simply myotomy is confirmed too by doppler ultrasound. Any recurrence of symptoms is recognized, in only one case was observed a concomitant entrapment in the other leg, not treated now is underlined. The advantage available for the patient in a physiokinetic therapy program, forerunner of a complete functional restoring, especially in athletic patients.

[Composite bypass using endarterectomized autologous superficial femoral artery. A valid concept in the extension of lower limb salvage surgery]. / By-pass composito con arteria femorale superficiale autologa endoarterectomizzata. Un valido concetto nell'estensione della chirurgia di salvataggio dell'arto inferiore.

Saphenous vein is nowadays the material of choice performing on femoro-distal revascularisation; when this is not available, it is important to use a material which gives the closest approximation of an ideal conduit and for same time an easy handling during the execution of the anastomosis. Although vein's degenerative alterations are very rare, it has now been shown that there is widespread destruction of the endothelium among infrainguinal vein grafting, producing a relatively thrombogenic surface. These factors may contribute to the initial failure rate of these bypasses. For this reason we suggest to employ a segment of thromboendarterectomized SFA (superficial femoral artery) as a distal part of a composite bypass. Twenty-four composite bypasses were performed using three different methods over a total of 123 femoro-distal revascularizations. Eighteen months follow-up showed more than 50% and more than 75% patency rate comparing type B (graft or thromboendarterectomized SFA + autogenous saphenous vein) and type C (thromboendarterectomyied SFA + graft). We believe, waiting for a wider follow-up, that this technique could be a valid alternative to a femoro-distal revascularization when saphenous vein is not available.

[Our experience in the use of a new objective pain measuring system in rheumarthropatic subjects treated with transcutaneous electroanalgesia and ultrasound]. / Nostre esperienze sull'utilizzo di un nuovo sistema obiettivo di misurazione del dolore in soggetti reumoartropatici trattati con elettroanalgesia transcutanea (T.E.N.S.) ed ultrasuoni.

The results were assessed in a group of 38 patients with degenerative skeletal pathologies and insertional tendinopathies who were treated with transcutaneous electroanalgesia and ultrasound therapy. For this purpose a new type of painmeter was used and proved appropriate for the more rational and objective quantification of pain at precise trigger points. The case series also revealed that the best clinical response came from T.E.N.S. treatment followed by some 20 days later by a cycle of ultrasound therapy.