Tuning the Optical Properties Through Hydrogen Bond-assisted H-aggregate Formation: The ODIN Case.

The current work focuses on the investigation of two functionalized naphthyridine derivatives, namely ODIN-EtPh and ODIN-But, to gain insights into the hydrogen bond-assisted H-aggregate formation and its impact on the optical properties of ODIN molecules. By employing a combination of X-ray and electron crystallography, absorption and emission spectroscopy, time resolved fluorescence and ultrafast pump-probe spectroscopy (visible and infrared) we unravel the correlation between the structure and light-matter response, with a particular emphasis on the influence of the polarity of the surrounding environment. Our experimental results and simulations confirm that in polar and good hydrogen-bond acceptor solvents (DMSO), the formation of dimers for ODIN derivatives is strongly inhibited. The presence of a phenyl group linked to the ureidic unit favors the folding of ODIN derivatives (forming an intramolecular hydrogen bond) leading to the stabilization of a charge-transfer excited state which almost completely quenches its fluorescence emission. In solvents with a poor aptitude for forming hydrogen bonds, the formation of dimers is favored and gives rise to H aggregates, with a consequent considerable reduction in the fluorescence emission. The urea-bound phenyl group furtherly stabilizes the dimers in chloroform.

Time-resolved infrared absorption spectroscopy applied to photoinduced reactions: how and why.

Time-resolved infrared (IR) spectroscopy is a widely used technique in the investigation of photoinduced reactions, given its capabilities of providing structural information about the presence of intermediates and the reaction mechanism. Despite the fact that it is used in several fields since the '80s, the communication between the different scientific communities (photochemists, photobiologists, etc.) has been to date quite limited. In some cases, this lack of communication happened-and still happens-even inside the same scientific community (for instance between specialists in ultrafast ps/fs IR and those in "fast" ns/µs/ms IR). Even more surprising is the difficulty of non-specialists to understand the potential of time-resolved IR spectroscopy, despite the fact that IR spectroscopy is normally taught to all chemistry and material science students, and to several biology and physics students. This tutorial review aims at helping to solve these issues, first by providing a comprehensive but reader-friendly overview of the different techniques, and second, by focusing on five "case studies" (from photobiology, gas-phase photocatalysis, photochemistry, semiconductors and metal-carbonyl complexes). We are confident that this approach can help the reader-whichever is its background-to understand the capabilities of time-resolved IR spectroscopy to study the mechanism of photoinduced reactions.

Free volume and dynamics in a lipid bilayer.

The lateral diffusion of lipids and of small molecules inside a membrane is strictly related to the arrangement of acyl chains and to their mobility. In this study, we use FTIR and time resolved 2D-IR spectroscopic techniques to characterize the structure and dynamics of the hydrophobic region of palmitoyl-oleylphosphatidylcholine/cholesterol vesicles dispersed in water/dimethylsulfoxide solutions. By means of a non-polar probe, hexacarbonyl tungsten, we monitor the distribution of free volumes inside the bilayer and the conformational dynamics of hydrophobic tails in relation to the different compositions of the membrane or the different compositions of the solvent. Despite the important structural changes induced by the presence of DMSO in the solvating medium, the picosecond dynamics of the membrane is preserved under the different conditions.

Understanding the influence of disorder on the exciton dynamics and energy transfer in Zn-phthalocyanine H-aggregates.

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl]phthalocyaninatozinc(ii) and its H-aggregates is studied in different solvents by means of ultrafast non-linear optical spectroscopy and computational modeling. In non-coordinating solvents, both stationary and time-resolved spectroscopies highlight the formation of extended molecular aggregates, whose dimension and spectral properties depends on the concentration. In all the explored experimental conditions, time-resolved transient absorption experiments show multi exponential decay of the signals. Additional insights into the excited state relaxation mechanisms of the system is obtained with 2D electronic spectroscopy, which is employed to compare the deactivation channels in the absence or presence of aggregates. In ethanol and diethylether, where only monomers are present, an ultrafast relaxation process among the two non-degenerate Q-states of the molecule is evidenced by the appearance of a cross peak in the 2D-maps. In chloroform or CCl4, where disordered H-aggregates are formed, an energy transfer channel among aggregates with different composition and size is observed, leading to the non-radiative decay towards the lower energy dark state of the aggregates. Efficient coupling between less and more aggregated species is highlighted in two-dimensional electronic spectra by the appearance of a cross peak. The kinetics and intensity of the latter depend on the concentration of the solution. Finally, the linear spectroscopic properties of the aggregate are reproduced using a simplified structural model of an extended aggregate, based on Frenkel Hamiltonian Calculations and on an estimate of the electronic couplings between each dimer composing the aggregate computed at DFT level.

Tailoring Photoisomerization Pathways in Donor-Acceptor Stenhouse Adducts: The Role of the Hydroxy Group.

Donor-acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activatable negative photochromes. They are closely related to (mero)cyanine dyes with the sole difference being a hydroxy group in the polyene chain. The presence or absence of the hydroxy group has far-reaching consequences for the photochemistry of the compound: cyanine dyes are widely used as fluorescent probes, whereas DASAs hold great promise for visible light-triggered photoswitching. Here we analyze the photophysical properties of a DASA lacking the hydroxy group. Ultrafast time-resolved pump-probe spectroscopy in both the visible and IR region show the occurrence of E-Z photoisomerization on a 20 ps time scale, similar to the photochemical behavior of DASAs, but on a slower time scale. In contrast to the parent DASA compounds, where the initial photoisomerization is constrained to a single position (next to the hydroxy group), 1H NMR in situ-irradiation studies at 213 K reveal that for nonhydroxy DASAs E-Z photoisomerization can take place at two different bonds, yielding two distinct isomers. These observations are supported by TD-DFT calculations, showing that in the excited state the hydroxy group (pre)selects the neighboring C2-C3 bond for isomerization. The TD-DFT analysis also explains the larger solvatochromic shift observed for the parent DASAs as compared to the nonhydroxy analogue, in terms of the dipole moment changes evoked upon excitation. Furthermore, computations provide helpful insights into the photoswitching energetics, indicating that without the hydroxy group the 4π-electrocyclization step is energetically forbidden. Our results establish the central role of the hydroxy group for DASA photoswitching and suggest that its introduction allows for tailoring photoisomerization pathways, presumably both through (steric) fixation via a hydrogen bond with the adjacent carbonyl group of the acceptor moiety, as well as through electronic effects on the polyene backbone. These insights are essential for the rational design of novel, improved DASA photoswitches and for a better understanding of the properties of both DASAs and cyanine dyes.

Solvent Effects on the Actinic Step of Donor-Acceptor Stenhouse Adduct Photoswitching.

Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that switch with visible light and are highly promising for applications ranging from smart materials to biological systems. However, the strong solvent dependence of the photoswitching kinetics limits their application. The nature of the photoswitching mechanism in different solvents is key for addressing the solvatochromism of DASAs, but as yet has remained elusive. Here, we employ spectroscopic analyses and TD-DFT calculations to reveal changing solvatochromic shifts and energies of the species involved in DASA photoswitching. Time-resolved visible pump-probe spectroscopy suggests that the primary photochemical step remains the same, irrespective of the polarity and protic nature of the solvent. Disentangling the different factors determining the solvent-dependence of DASA photoswitching, presented here, is crucial for the rational development of applications in a wide range of different media.

Shedding Light on the Photoisomerization Pathway of Donor-Acceptor Stenhouse Adducts.

Donor-acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump-probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z-E photoisomerization of C2-C3 is followed by a rotation around C3-C4 and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

Excitation Dynamics in Hetero-bichromophoric Calixarene Systems.

In this work, the dynamics of electronic energy transfer (EET) in bichromophoric donor-acceptor systems, obtained by functionalizing a calix[4]arene scaffold with two dyes, was experimentally and theoretically characterized. The investigated compounds are highly versatile, due to the possibility of linking the dye molecules to the cone or partial cone structure of the calix[4]arene, which directs the two active units to the same or opposite side of the scaffold, respectively. The dynamics and efficiency of the EET process between the donor and acceptor units was investigated and discussed through a combined experimental and theoretical approach, involving ultrafast pump-probe spectroscopy and density functional theory based characterization of the energetic and spectroscopic properties of the system. Our results suggest that the external medium strongly determines the particular conformation adopted by the bichromophores, with a direct effect on the extent of excitonic coupling between the dyes and hence on the dynamics of the EET process itself.

Coacervation of α-elastin studied by ultrafast nonlinear infrared spectroscopy.

Elastin is the main protein to confer elasticity to biological tissues, through the formation of a hierarchical network of fibres. α-Elastin, a soluble form of the protein, is widely used in studies of the biosynthesis of human elastic tissue and exhibits coacervation in solution. This process involves the association of α-elastin molecules through a liquid-liquid phase transition, which is reversible unless the temperature is driven sufficiently high to induce the formation of insoluble aggregates. The thermodynamics of this process have attracted interest over many years and in the present work we used ultrafast nonlinear infrared spectroscopy of the amide I protein backbone vibration to resolve the secondary structural changes occurring during coacervation and probe the protein dynamics on a picosecond time scale. Four classes of carbonyl oscillators with distinct absorption peaks were revealed and, through narrowband excitation, vibrational and anisotropy decays could be distinguished. Analysis of the vibrational lifetimes and anisotropy decay times of these bands characterized the conformational changes and revealed the structural bases of the coacervation process.

Monitoring the intramolecular charge transfer process in the Z907 solar cell sensitizer: a transient Vis and IR spectroscopy and ab initio investigation.

We have analyzed the excited state dynamics of the heteroleptic [(NCS)2Ru(bpy-(COOH)2)(bpy-(C6H13)2)] Z907 solar cell sensitizer in solution and when adsorbed onto thin TiO2 films, by combining transient visible and infrared (IR) spectroscopies with ab initio Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations. Upon excitation with ultra-short pulses in ethanol and dimethyl-sulphoxide solutions, the visible spectra show the appearance of a positive signal around 650 nm, within the instrumental time resolution (<100 fs), which in ethanol undergoes a red-shift in about 20 ps. Measurements in the IR indicate that, upon excitation, both the CN and CO marker bands, associated with the NCS and COOH groups, downshift in frequency, in response to intramolecular ligand + metal (Ru-NCS) to ligand' (bpy-COOH2) charge transfer (LML'CT). Vibrational cooling is observed in both solvents; in ethanol it is overtaken by the hydrogen bond dynamics. On the basis of DFT/TDDFT calculations, explicitly modeling the interaction of the NCS and COOH groups with solvent (ethanol) molecules, we rationalize the observed IR and visible spectral evolution as arising from the change in the hydrogen-bond network, which accompanies the transition to the lowest-energy triplet state. This interpretation provides a consistent explanation of what is also observed in the transient visible spectra. Transient IR measurements repeated for molecules adsorbed on TiO2 and ZrO2 films, allow us to identify the structural changes signaling the dye triplet excited state formation and evidence multiexponential electron injection rates into the semiconductor TiO2 film.

Femtosecond transient infrared and stimulated Raman spectroscopy shed light on the relaxation mechanisms of photo-excited peridinin.

By means of one- and two-dimensional transient infrared spectroscopy and femtosecond stimulated Raman spectroscopy, we investigated the excited state dynamics of peridinin, a carbonyl carotenoid occurring in natural light harvesting complexes. The presence of singly and doubly excited states, as well as of an intramolecular charge transfer (ICT) state, makes the behavior of carbonyl carotenoids in the excited state very complex. In this work, we investigated by time resolved spectroscopy the relaxation of photo-excited peridinin in solvents of different polarities and as a function of the excitation wavelength. Our experimental results show that a characteristic pattern of one- and two-dimensional infrared bands in the C=C stretching region allows monitoring the relaxation pathway. In polar solvents, moderate distortions of the molecular geometry cause a variation of the single/double carbon bond character, so that the partially ionic ICT state is largely stabilized by the solvent reorganization. After vertical photoexcitation at 400 nm of the S2 state, the off-equilibrium population moves to the S1 state with ca. 175 fs time constant; from there, in less than 5 ps, the non-Franck Condon ICT state is reached, and finally, the ground state is recovered in 70 ps. That the relevant excited state dynamics takes place far from the Franck Condon region is demonstrated by its noticeable dependence on the excitation wavelength.

Picosecond optical parametric generator and amplifier for large temperature-jump.

An optical parametric generator and amplifier producing 15 ps pulses at wavelengths tunable around 2 µm, with energies up to 15 mJ/pulse, has been realized and characterized. The output wavelength is chosen to match a vibrational combination band of water. By measuring the induced birefringence changes we prove that a single pulse is able to completely melt samples of ice in the 10⁻6 cm³ volume range, both at room pressure (263 K) and at high pressure (298 K, 1 GPa) in a sapphire anvil cell. This source opens the possibility of studying melting and freezing processes by spectroscopic probes in water or water solutions in a wide range of conditions as found in natural environments.

Ultrafast resonance energy transfer in the umbelliferone-alizarin bichromophore.

In this work we present the synthesis, time-resolved spectroscopic characterization and computational analysis of a bichromophore composed of two very well-known naturally occurring dyes: 7-hydroxycoumarin (umbelliferone) and 1,2-dihydroxyanthraquinone (alizarin). The umbelliferone donor (Dn) and alizarin acceptor (Ac) moieties are linked to a triazole ring viaσ bonds, providing a flexible structure. By measuring the fluorescence quantum yields and the ultrafast transient absorption spectra we demonstrate the high efficiency (∼85%) and the fast nature (∼1.5 ps) of the energy transfer in this compound. Quantum chemical calculations, within the density functional theory (DFT) approach, are used to characterize the electronic structure of the bichromophore (Bi) in the ground and excited states. We simulate the absorption and fluorescence spectra using the TD-DFT methods and the vertical gradient approach (VG), and include the solvent effects by adopting the conductor-like polarizable continuum model (CPCM). The calculated electronic structure suggests the occurrence of weak interactions between the electron densities of Dn and Ac in the excited state, indicating that the Förster-type transfer is the appropriate model for describing the energy transfer in this system. The average distance between Dn and Ac moieties calculated from the conformational analysis (12 Å) is in very good agreement with the value estimated from the Förster equation (∼11 Å). At the same time, the calculated rate constant for energy transfer, averaged over multiple conformations of the system (3.6 ps), is in reasonable agreement with the experimental value (1.6 ps) estimated by transient absorption spectroscopy. The agreement between experimental results and computational data leads us to conclude that the energy transfer in Bi is well described by the Förster mechanism.

Nanothermometer Based on Polychlorinated Trityl Radicals Showing Two-Photon Excitation and Emission in the Biological Transparency Window: Temperature Monitoring of Biological Tissues.

Nanothermometers are emerging probes as biomedical diagnostic tools. Especially appealing are nanoprobes using NIR light in the range of biological transparency window (BTW) since they have the advantages of a deeper penetration into biological tissues, better contrast, reduced phototoxicity and photobleaching. This article reports the preparation and characterization of organic nanoparticles (ONPs) doped with two polychlorinated trityl radicals (TTM and PTM), as well as studies of their electronic and optical properties. Such ONPs having inside isolated radical molecules and dimeric excimers, can be two-photon excited showing optimal properties for temperature sensing. Remarkably, in TTM-based ONPs the emission intensity of the isolated radical species is unaltered increasing temperature, while the excimer emission intensity decreases strongly being thereby able to monitor temperature changes with an excellent thermal absolute sensitivity of 0.6-3.7% K-1 in the temperature range of 278-328 K. The temperature dependence of the excimeric bands of ONPs are theoretically simulated by using electronic structure calculations and a vibronic Hamiltonian model. Finally, TTM-doped ONPs as ratiometric NIR-nanothermometers are tested with two-photon excitationwith enucleated pig eye sclera, as a real tissue model, obtaining a similar temperature sensitivity as in aqueous suspensions, demonstrating their potential as NIR nanothermometers for bio applications.

Amyloidogenic and non-amyloidogenic molten globule conformation of ß-lactoglobulin in self-crowded regime.

Molecular insights on the ß-lactoglobulin thermal unfolding and aggregation are derived from FTIR and UV Resonance Raman (UVRR) investigations. We propose an in situ and in real-time approach that thanks to the identification of specific spectroscopic markers can distinguish the two different unfolding pathways pursued by ß-lactoglobulin during the conformational transition from the folded to the molten globule state, as triggered by the pH conditions. For both the investigated pH values (1.4 and 7.5) the greatest conformational variation of ß-lactoglobulin occurs at 80 °C and a high degree of structural reversibility after cooling is observed. In acidic condition ß-lactoglobulin exposes to the solvent its hydrophobic moieties in a much higher extent than in neutral solution, resulting on a highly open conformation. Moving from the diluted to the self-crowded regime, the solution pH and consequently the different molten globule conformation select the amyloid or non-amyloid aggregation pathway. At acidic condition the amyloid aggregates form during the heating cycle leading to the formation of transparent hydrogel. On the contrary, in neutral condition the amyloid aggregates never form. Information on the secondary structure conformational change of ß-lactoglobulin and the formation of amyloid aggregates are obtained by FTIR spectroscopy and are related to the information of the structural changes localized around the aromatic amino acid sites by UVRR technique. Our results highlight a strong involvement of the chain portions where tryptophan is located on the formation of amyloid aggregates.

From symmetry breaking to symmetry swapping: is Kasha's rule violated in multibranched phenyleneethynylenes?

The phenomenon of excited-state symmetry breaking is often observed in multipolar molecular systems, significantly affecting their photophysical and charge separation behavior. As a result of this phenomenon, the electronic excitation is partially localized in one of the molecular branches. However, the intrinsic structural and electronic factors that regulate excited-state symmetry breaking in multibranched systems have hardly been investigated. Herein, we explore these aspects by adopting a joint experimental and theoretical investigation for a class of phenyleneethynylenes, one of the most widely used molecular building blocks for optoelectronic applications. The large Stokes shifts observed for highly symmetric phenyleneethynylenes are explained by the presence of low-lying dark states, as also established by two-photon absorption measurements and TDDFT calculations. In spite of the presence of low-lying dark states, these systems show an intense fluorescence in striking contrast to Kasha's rule. This intriguing behavior is explained in terms of a novel phenomenon, dubbed "symmetry swapping" that describes the inversion of the energy order of excited states, i.e., the swapping of excited states occurring as a consequence of symmetry breaking. Thus, symmetry swapping explains quite naturally the observation of an intense fluorescence emission in molecular systems whose lowest vertical excited state is a dark state. In short, symmetry swapping is observed in highly symmetric molecules having multiple degenerate or quasi-degenerate excited states that are prone to symmetry breaking.

Solid state solvation: a fresh view.

The design of efficient organic electronic devices, including OLEDs, OPVs, luminescent solar concentrators, etc., relies on the optimization of relevant materials, often constituted by an active (functional) dye embedded in a matrix. Understanding solid state solvation (SSS), i.e. how the properties of the active dye are affected by the matrix, is therefore an issue of fundamental and technological relevance. Here an extensive experimental and theoretical investigation is presented shedding light on this, somewhat controversial, topic. The spectral properties of the dye at equilibrium, i.e. absorption and Raman spectra, are not affected by the matrix dynamics. Reliable estimates of the matrix polarity are then obtained from an analysis of the micro-Raman spectra of polar dyes. Specifically, to establish a reliable polarity scale, the spectra of DCM or NR dispersed in amorphous matrices are compared with the spectra of the same dyes in liquid solvents with known polarity. On the other hand, steady-state emission spectra obtained in solid matrices depend in a highly non-trivial way on the matrix polarity and its dynamics. An extensive experimental and theoretical analysis of the time-resolved emission spectra of NR in a very large time window (15 fs-15 ns) allows us to validate this dye as a good probe of the dielectric dynamics of the surrounding medium. We provide a first assessment of the relaxation dynamics of two matrices (mCBPCN and DPEPO) of interest for OLED application, unambiguously demonstrating that the matrix readjusts for at least 15 ns after the dye photoexcitation.

Transient infrared spectroscopy: a new approach to investigate valence tautomerism.

In this work we present, to our knowledge for the first time, the results of a transient infrared spectroscopic study of the photoinduced valence tautomerism process in cobalt-dioxolene complexes with sub-picosecond time resolution. The molecular systems investigated were [Co(tpa)(diox)]PF(6) (1) and [Co(Me(3)tpa)(diox)]PF(6) (2), where diox = 3,5-di-tert-butyl-1,2-dioxolene; tpa = tris(2-pyridylmethyl)amine and Me(3)tpa its 6-methylated analogue. Complex (1) is present in solution as ls-Co(III)(catecholate) (1-CAT), while (2) as hs-Co(II)(semiquinonate) (2-SQ). DFT calculation of the harmonic frequencies for (1) and (2) allowed us to identify the vibrational markers of catecholate and semiquinonate redox isomers. Irradiation with 405 and 810 nm pulses (~35 fs) of (1-CAT) induces the formation of an intermediate excited species from which the ground state population is recovered with a time constant of 1.5 ± 0.3 ns. Comparing the 1 ns transient infrared spectrum with the experimental difference spectrum FTIR(2-SQ)-FTIR(1-CAT) and with the calculated difference spectrum IR(c)(1-SQ)-IR(c)(1-CAT) we are able to unequivocally identify the long lived species as the semiquinonate redox isomer of (1). On the other hand, no evidence of photoconversion is observed upon irradiation of (2) with 405 nm. Temporal evolution of transient spectra was analyzed with the combined approach consisting of singular values decomposition and global fitting (global analysis). After 405 and 810 nm excitation of (1-CAT), the semiquinonate excited species is formed on an ultrafast time scale (<200 fs) and cools down within the first 50 ps. Excitation of (2-SQ) with 405 nm wavelength produces a short lived excited state in which the semiquinonate nature of dioxolene is preserved and the ground state recovery is completed within 30 ps.

Biomimetic design of bioartificial scaffolds for the in vitro modelling of human cardiac fibrosis.

In vitro models of pathological cardiac tissue have attracted interest as predictive platforms for preclinical validation of therapies. However, models reproducing specific pathological features, such as cardiac fibrosis size (i.e., thickness and width) and stage of development are missing. This research was aimed at engineering 2D and 3D models of early-stage post-infarct fibrotic tissue (i.e., characterized by non-aligned tissue organization) on bioartificial scaffolds with biomimetic composition, design, and surface stiffness. 2D scaffolds with random nanofibrous structure and 3D scaffolds with 150 µm square-meshed architecture were fabricated from polycaprolactone, surface-grafted with gelatin by mussel-inspired approach and coated with cardiac extracellular matrix (ECM) by 3 weeks culture of human cardiac fibroblasts. Scaffold physicochemical properties were thoroughly investigated. AFM analysis of scaffolds in wet state, before cell culture, confirmed their close surface stiffness to human cardiac fibrotic tissue. Following 3 weeks culture, biomimetic biophysical and biochemical scaffold properties triggered the activation of myofibroblast phenotype. Upon decellularization, immunostaining, SEM and two-photon excitation fluorescence microscopy showed homogeneous decoration of both 2D and 3D scaffolds with cardiac ECM. The versatility of the approach was demonstrated by culturing ventricular or atrial cardiac fibroblasts on scaffolds, thus suggesting the possibility to use the same scaffold platforms to model both ventricular and atrial cardiac fibrosis. In the future, herein developed in vitro models of cardiac fibrotic tissue, reproducing specific pathological features, will be exploited for a fine preclinical tuning of therapies.