Intramolecular Singlet Fission Coupled with Intermolecular Triplet Separation as a Strategy to Achieve High Triplet Yields in Fluorene-Based Small Molecules.

In this work, detailed experimental proof and in-depth analysis of the singlet fission (SF) mechanism, operative in fluorene-based small molecules, are carried out by employing advanced time-resolved spectroscopies with nanosecond and femtosecond resolution. The investigation of the effect of solution concentration and solvent viscosity together with temperature and excitation wavelength demonstrates INTRAmolecular formation of the correlated triplet pair followed by INTERmolecular independent triplet separation via a "super-diffusional" triplet-triplet transfer process. This unconventional INTRA- to INTERmolecular SF may be considered an "ideal" mechanism.

Unveiling the double triplet nature of the 2Ag state in conjugated stilbenoid compounds to achieve efficient singlet fission.

In this investigation, the excited-state evolution in a series of all-trans stilbenoid compounds, displaying a low-lying dark singlet state of 2Ag-like symmetry nearly degenerate with the bright 1Bu state, was unveiled by employing advanced ultrafast spectroscopies while probing the effect of solvent polarizability. Together with the dual emission, femtosecond transient absorption and broadband fluorescence up-conversion disclosed the double nature of the 2Ag-like state showing both singlet features, a lifetime typical of a singlet and the ability to emit, and a triplet character, exhibiting a triplet-like absorption spectrum. The ultrafast formation (in hundreds of femtoseconds) from the non-relaxed upper singlet state led to the identification of 2Ag as the correlated triplet pair of singlet fission.

Lighting-Up the Far-Red Fluorescence of RNA-Selective Dyes by Switching from Ortho to Para Position.

Fluorescence imaging is constantly searching for new far-red emitting probes whose turn-on response is selective upon the interaction with specific biological targets. Cationic push-pull dyes could indeed respond to these requirements due to their intramolecular charge transfer (ICT) character, by which their optical properties can be tuned, and their ability to interact strongly with nucleic acids. Starting from the intriguing results recently achieved with some push-pull dimethylamino-phenyl dyes, two isomers obtained by switching the cationic electron acceptor head (either a methylpyridinium or a methylquinolinium) from the ortho to the para position have been scrutinized for their ICT dynamics, their affinity towards DNA and RNA, and in vitro behavior.

Tuning the Photophysics of Two-Arm Bis[(dimethylamino)styryl]benzene Derivatives by Heterocyclic Substitution.

The identification of novel molecular systems with high fluorescence and significant non-linear optical (NLO) properties is a hot topic in the continuous search for new emissive probes. Here, the photobehavior of three two-arm bis[(dimethylamino)styryl]benzene derivatives, where the central benzene was replaced by pyridine, furan, or thiophene, was studied by stationary and time-resolved spectroscopic techniques with ns and fs resolution. The three molecules under investigation all showed positive fluorosolvatochromism, due to intramolecular charge-transfer (ICT) dynamics from the electron-donor dimethylamino groups, and significant fluorescence quantum yields, because of the population of a planar and emissive ICT state stabilized by intramolecular hydrogen-bond-like interactions.

Amphiphilicity-Controlled Localization of Red Emitting Bicationic Fluorophores in Tumor Cells Acting as Bio-Probes and Anticancer Drugs.

Small organic molecules arouse lively interest for their plethora of possible biological applications, such as anticancer therapy, for their ability to interact with nucleic acids, or bioimaging, thanks to their fluorescence emission. Here, a panchromatic series of styryl-azinium bicationic dyes, which have already proved to exhibit high water-solubility and significant red fluorescence in water, were investigated through spectrofluorimetric titrations to assess the extent of their association constants with DNA and RNA. Femtosecond-resolved transient absorption spectroscopy was also employed to characterize the changes in the photophysical properties of these fluorophores upon interaction with their biological targets.

Exploring a new class of singlet fission fluorene derivatives with high-energy triplets.

In this study, we report strong experimental evidence for singlet fission (SF) in a new class of fluorene-based molecules, exhibiting two-branched donor-acceptor structures. The time-resolved spectroscopic results disclose ultrafast formation of a double triplet state (occurring in few picoseconds) and efficient triplet exciton separation (up to 145% triplet yield). The solvent polarity effect and the role of intramolecular charge transfer (ICT) on the SF mechanism have been thoroughly investigated with several advanced spectroscopies.

Acid-base strength and acido(fluoro)chromism of three push-pull derivatives of 2,6-distyrylpyridine.

The acidochromism and acid-base properties of 2,6-distyrylpyridine (2,6-DStP) derivatives bearing on the sides push/pull substituents (namely two dimethylamino, one nitro, and one methoxy and two nitro groups in the case of 2,6-bis[(E)-2-(4-dimetylaminophenyl)ethenyl]pyridine, 2-[(E)-2-(4-nitrophenyl)ethenyl],6-[(E)-2'-(4'-methoxyphenyl)ethenyl]pyridine and 2,6-bis[(E)-2-(4-nitrophenyl)ethenyl]pyridine, respectively) were investigated by stationary and time-resolved spectroscopies. The sensitivity of the absorption and emission spectrum to the medium acidity was found to enhance in the dimethylamino-derivative relative to the unsubstituted 2,6-DStP, also because of the second protonation by the N(CH) group. Spectrophotometric titrations, also processed by a global fitting approach, gave pKa values, for the protonation of the central pyridine, higher in the derivatives with electron-donor unities and lower in compounds bearing electron-acceptor groups.

The role of twisting in driving excited-state symmetry breaking and enhanced two-photon absorption in quadrupolar cationic pyridinium derivatives.

Two symmetric quadrupolar cationic push-pull compounds with a central electron-acceptor (N+-methylpyrydinium, A+) and different lateral electron-donors, (N,N-dimethylamino and N,N-diphenylamino, D) in a D-π-A+-π-D arrangement, were investigated together with their dipolar counterparts (D-π-A+) for their excited-state dynamics and NLO properties. As for the quadrupolar compounds, attention was focused on excited-state symmetry breaking (ESSB), which leads to a relaxed dipolar excited state. Both electron charge displacements and structural rearrangements were recognized in the excited-state dynamics of these molecules by resorting to femtosecond-resolved broadband fluorescence up-conversion experiments and advanced data analysis, used as a valuable alternative approach for fluorescent molecules compared to time-resolved IR spectroscopy, only suitable for compounds bearing IR markers.

Competition between fluorescence and triplet production ruled by nitro groups in one-arm and two-arm styrylbenzene heteroanalogues.

The competition between excited state deactivation processes in mono and double-arm push-pull systems bearing pyridine, furan or thiophene (electron donors) and nitro groups (electron acceptors) was investigated in several solvents through nanosecond and femtosecond transient absorption spectroscopy. Triplet population is the main deactivation pathway for the mono-arm compounds. The large triplet production is mainly ascribed to (n,π*) states almost isoenergetic to S, introduced by nitro groups, as predicted by TD-DFT calculations.

Effect of the size of polycyclic aryl groups on the competition between adiabatic/diabatic photoisomerization mechanisms of cis-styrylarenes.

The occurrence of adiabatic photoisomerization in the singlet manifold directly from Z* to E* has been found to be more common than expected. This mechanism has been experimentally evidenced through a detailed fluorimetric study for a large series of styrylarenes. Its weight on the overall cis-trans photoisomerization has been determined and found to increase when increasing the size of the polycyclic chromophore.

Photoinduced Intramolecular Charge Transfer and Hyperpolarizability Coefficient in Push-Pull Pyridinium Salts with Increasing Strength of the Acceptor Group.

The synthesis of three push-pull cationic dyes is reported here together with a photophysical study carried out by stationary and ultrafast spectroscopies. The hyperpolarizability (β) values of the three molecules have been estimated through a simple solvatochromic method based on conventional, low-cost equipment, which had been tested previously with success in our laboratory. The investigated pyridinium salts showing strong negative solvatochromism bear the same piperidine ring as a strong electron-donor group and the same thiophenes as electron-rich π-linkers, but differ in terms of the N-substituent on the electron-acceptor pyridinium unit, namely N-methyl in compound A, N-pyrimidin-2yl in B and N-2,4-dinitrophenyl in C.

New Styryl Phenanthroline Derivatives as Model D-π-A-π-D Materials for Non-Linear Optics.

Four novel push-pull systems combining a central phenanthroline acceptor moiety and two substituted benzene rings, as a part of the conjugated π-system between the donor and the acceptor moieties, have been synthetized through a straightforward and efficient one-step procedure. The chromophores display high fluorescence and a peculiar fluorosolvatochromic behaviour. Ultrafast investigation by means of state-of-the-art femtosecond-resolved transient absorption and fluorescence up-conversion spectroscopies allowed the role of intramolecular charge transfer (ICT) states to be evidenced, also revealing the crucial role played by both, the polarity and proticity of the medium on the excited state dynamics of the chromophores.

Photoinduced ICT vs. excited rotamer intercoversion in two quadrupolar polyaromatic N-methylpyridinium cations.

The excited state dynamics of two quadrupolar polyaromatic N-methylpyridinium cations have been fully investigated in order to acquire detailed information on their photo-induced behavior. The two molecules are symmetric push-pull compounds having a D-π-A-π-D motif, with the same electron-acceptor central unit (A = N-methylpyridinium) and two distinctive electron-donor polyaromatic side groups (D), namely naphthyl and pyrenyl substituents. Both molecules undergo charge transfer during the absorption, as revealed by a significant solvatochromism exhibited with solvent polarity, but the fate of their excited state was found to be markedly different.

Optical Communication among Oscillatory Reactions and Photo-Excitable Systems: UV and Visible Radiation Can Synchronize Artificial Neuron Models.

Neuromorphic engineering promises to have a revolutionary impact in our societies. A strategy to develop artificial neurons (ANs) is to use oscillatory and excitable chemical systems. Herein, we use UV and visible radiation as both excitatory and inhibitory signals for the communication among oscillatory reactions, such as the Belousov-Zhabotinsky and the chemiluminescent Orban transformations, and photo-excitable photochromic and fluorescent species.

A cationic naphthyl derivative defies the non-equilibrated excited rotamers principle.

The ground and excited state properties of 1-methyl-2-[(E)-2-(2-naphthyl) vinyl]pyridinium iodide have been investigated in solvents of different polarities and viscosities using stationary and ultrafast time resolved spectroscopic techniques supported by density functional theory (DFT) calculations. The investigated compound shows an important negative solvatochromism, which serves as evidence of a certain push-pull character exhibited upon photoexcitation, but the most remarkable feature is an extremely large absorption spectrum, as opposed to the narrower emission band. Interestingly, both experiments and calculations have revealed a conformational disorder in the ground state between four quasi-isoenergetic rotamers which contribute to the broad absorption spectrum.

A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D-π-D anthracene derivative.

The peculiar photobehaviour of a symmetrical arylenevinylene anthracene derivative bearing mild electron donors (alkoxy groups) at the sides of its structure has been fully comprehended through this study. An investigation into the effect of solvent polarity and temperature on the stationary fluorescence spectrum allowed a clear dual emission to be revealed. A further valuable insight was obtained, thanks to the employment of ultrafast spectroscopies.

Twisting in the excited state of an N-methylpyridinium fluorescent dye modulated by nano-heterogeneous micellar systems.

A push-pull N-methylpyridinium fluorescent dye with a pyrenyl group as the electron-donor portion was investigated within the nano-heterogeneous media provided by some micellar systems. The molecule was studied by stationary and time-resolved spectroscopic techniques in spherical micellar solutions and viscoelastic hydrogels, in order to throw light on the role played by twisting in its excited state deactivation. As proven by femtosecond fluorescence up-conversion and transient absorption experiments, the excited state dynamics of the molecule is ruled by charge transfer and twisting processes, which, from the locally excited (LE) state initially populated upon excitation, progressively lead to twisted (TICT) and planar (PICT) intramolecular charge transfer states.

Unexpected multiple activated steps in the excited state decay of some bis(phenylethynyl)-fluorenes and -anthracenes.

The temperature effect on the photophysical parameters of four acetylene-derivatives [bis(phenylethynyl)-anthracenes and -fluorenes with substituents of different electron acceptor efficiencies] has been investigated by absorption and emission spectroscopy, using stationary and pulsed (ns/fs resolution) techniques. The nature of the central nucleus (anthracene or fluorene) and the peripheral electron-withdrawing group (nitro or formyl) strongly affect the deactivation of the excited states of these push-pull molecules. In some cases the study evidenced an interesting role of two activated steps in the deactivation of the excited singlet state, namely an activated inter-system crossing to an upper triplet state of n,π* nature (previously hypothesized on the basis of TD-DFT calculations) and a sort of activated internal conversion, discussed also on the basis of maximum entropy method analysis of the fluorescence decay data.

Efficient Excited-State Symmetry Breaking in a Cationic Quadrupolar System Bearing Diphenylamino Donors.

We report a joint experimental and theoretical investigation of a quadrupolar D-π-A(+) -π-D system, the electron donors being diphenylamino groups and the electron acceptor being a methylpyridinium, in comparison with the dipolar D-π-A(+) system. The emission spectra of the two compounds overlap in all the investigated solvents. This finding could be rationalized by TD-DFT calculations: the LUMO-HOMO molecular orbitals involved in the emission transition are localized on the same branch of the quadrupolar structure that becomes the fluorescent portion, corresponding to that of the single-arm compound.

An ultrafast spectroscopic and quantum mechanical investigation of multiple emissions in push-pull pyridinium derivatives bearing different electron donors.

A joint experimental and theoretical approach, involving state-of-the-art femtosecond fluorescence up-conversion measurements and quantum mechanical computations including vibronic effects, was employed to get a deep insight into the excited state dynamics of two cationic dipolar chromophores (Donor-π-Acceptor(+)) where the electron deficient portion is a N-methyl pyridinium and the electron donor a trimethoxyphenyl or a pyrene, respectively. The ultrafast spectroscopic investigation, and the time resolved area normalised emission spectra in particular, revealed a peculiar multiple emissive behaviour and allowed the distinct emitting states to be remarkably distinguished from solvation dynamics, occurring in water in a similar timescale. The two and three emissions experimentally detected for the trimethoxyphenyl and pyrene derivatives, respectively, were associated with specific local emissive minima in the potential energy surface of S1 on the ground of quantum-mechanical calculations.

Inclusion of push-pull N-methylpyridinium salts within surfactant hydrogels: is their excited state intramolecular charge transfer mediated by twisting?

In order to get a deep insight into the environment-dependent photophysics of push-pull pyridinium derivatives, two N-methylpyridinium salts were dissolved within surfactant hydrogels. Surfactant viscoelastic solutions can potentially block or at least limit the torsion of these fluorescent dyes, uncovering the nature of the excited states involved in their deactivation. The excited state dynamics of the two molecules in hydrogels was investigated by means of femtosecond transient absorption spectroscopy, revealing the distribution of the dyes between the hydrophobic domains and the water pools making up the microscopic structure of the surfactant hydrogels.

Spectroscopic and Photophysical Characterization of Acetylenic Fluorophores: The Role of the Proximity Effect on Increasing Internal Conversion.

The spectroscopic and photophysical behavior of a series of six extended arylacetylenes and one 1,3-diyne derivative was studied in solvents of different polarity to gain insight into the relationships between molecular architecture and optical/photophysical properties. The radiative decay channel was revealed to be the most important one for all the compounds, particularly in nonpolar solvents. A notable fluoro-solvatochromism was observed for the 1,3-diyne derivative in line with the large increase of its dipole moment under excitation.

Unusual high fluorescence of two nitro-distyrylbenzene-like compounds induced by CT processes affecting the fluorescence/intersystem-crossing competition.

Two nitro-substituted 1,4-distyrylbenzene-like compounds have been investigated using stationary and time-resolved (ns/fs) spectrometric techniques as a function of solvent polarity. In the two compounds the central benzene ring is substituted with a p-nitrostyryl group at one side while, at the other side, compound 1 (asymmetric) bears a pyrid-4-ylethenyl group and compound 2 (symmetric) another p-nitrostyryl group. The solvent dependent intramolecular charge transfer (ICT) in the singlet manifold was found to strongly affect the competition among fluorescence, intersystem crossing and trans-cis photoisomerization.

Inclusion of two push-pull N-methylpyridinium salts in anionic surfactant solutions: a comprehensive photophysical investigation.

Two N-methylpyridinium salts with push-pull properties have been investigated in the aqueous solution of anionic micelles of sodium dodecyl sulfate (SDS) and potassium p-(octyloxy)benzenesulfonate (pOoBSK) surfactants. These molecules are known to be extremely sensitive to the local environment, with their absorption spectrum being subjected to a net negative solvatochromism. These compounds are also characterized by an excited state deactivation strictly dependent on the physical properties of the chemical surrounding, with the formation of intramolecular charge-transfer (ICT) states accordingly stabilized.

Presence of two emissive minima in the lowest excited state of a push-pull cationic dye unequivocally proved by femtosecond up-conversion spectroscopy and vibronic quantum-mechanical computations.

The long-standing controversy about the presence of two different emissive minima in the lowest excited state of the cationic push-pull dye o-(p-dimethylamino-styryl)-methylpyridinium (DASPMI) was definitively proved through the observation of dual emission, evidenced by both experimental (femtosecond up-conversion measurements) and theoretical (density functional theory calculations) approaches. From the fluorescence up-conversion data of DASPMI in water, the time resolved area normalized spectra (TRANES) were calculated, showing one isoemissive point and therefore revealing the presence of two distinct emissive minima of the excited state potential energy hypersurface with lifetimes of 0.51 and 4.