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. The presence of nitro-groups in the 1,4-distyrylbenzene skeleton causes the usual strong decrease of fluorescence in favour of intersystem crossing to a reactive triplet state. However, the favoured formation of the ICT state in polar solvents induces an unexpected important increase of the fluorescence quantum yield (three/two order of magnitude for the nitro and dinitro derivatives, respectively). The ultrafast spectral transients helped to understand the solvent effects measured by stationary techniques and gave information on the dynamics of the locally excited singlet state ((1)LE*) and the (1)ICT* state, fast produced in polar solvents. Evidence of dual fluorescence in a limited range of solvent polarity, particularly for compound 1, is also reported. The role of an upper triplet state in a non-polar solvent is discussed also based on quantum-mechanical calculations (TD-DFT method) and temperature effects on the photophysical parameters.
We report here experimental evidence of dual emission in a cationic push-pull system (bearing a methyl pyridinium group as an electron acceptor and a diphenylamino group as an electron donor), which ...shows negative solvatochromism. An intriguing blue shift and enlargement of the fluorescence band upon increasing the solvent polarity have suggested a possible contribution of an upper excited state to the stationary emission. Ultrafast transient absorption has indeed revealed the presence of an intermediate transient species in some solvents. The investigation of the fluorescence properties at low temperatures and in the rigid matrix has given a clear indication of this additional emission at shorter wavelengths. Femtosecond up-conversion measurements have shown interesting rise-decay dynamics in the kinetics and two well distinguished emission bands characterized by different deactivations. A single isoemissive point in the time-resolved area-normalized spectra has unambiguously pointed out the presence of two consecutive emissive species: the locally excited and the intramolecular charge transfer excited states.
Strong fluorosolvatochromism exhibited by compounds with Acceptor-π-Het and Acceptor-π-Het-π-Acceptor structures in agreement with the substantial photoinduced intramolecular charge transfer (from ...the heteroaromatics to the nitro groups) allowed hyperpolarizability coefficients of significant value to be estimated through the solvatochromic method (βCT up to 1030 × 10−30 esu-1 cm5, larger for the thiophen-derivatives and for the two-arms compounds).
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•Synthesis and spectral behaviour of mono- and two-branched nitro-derivatives bearing pyridine, furan or thiophene rings.•Strong fluorosolvatochromism and photoinduced ICT, from the heteroaromatics to the nitro groups, undisclosed by DFT calculations.•High hyperpolarizability coefficient βCT estimated through a solvatochromic method.•The new Acceptor-π-Core π-Acceptor structural motif of interest for new efficient NLO materials.
We report here the spectral characterization and hyperpolazizatin coefficinet of all-trans isomers of two-branched push-pull systems of interest as new efficient organic materials for Non Linear Optics, compared with the corresponding mono-branched analogues. These molecules are characterized by Acceptor-π-Het-π-Acceptor and Acceptor-π-Het structures, respectively, where the strong electron acceptor is the nitro group and the electron rich portion is a heteroaromatic ring (pyridine, furan and thiophene). These compounds exhibit strong fluorosolvatochromism, in agreement with the substantial photoinduced intramolecular charge transfer (from the heteroaromatics to the nitro groups) undisclosed by quantum mechanical calculations. The hyperpolarizability of these molecules, here estimated by the solvatochromic method, reaches significant values in all cases: enhanced for the two-branched systems with respect to their mono-branched analogues and higher for the furan/thiophene derivatives with respect to the pyridine ones. Surprisingly, hyperpolarizabilities of the symmetrical Acceptor-π-Het-π-Acceptor structures are found to be similar or higher than those of the asymmetrical Acceptor-π-Het-π-Donor counterparts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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. The careful analysis of the data gathered from femtosecond-resolved fluorescence up-conversion and transient absorption experiments, supported by DFT quantum mechanical calculations and temperature-dependent stationary measurements, shows the leading role of intramolecular charge transfer, assisted by symmetry breaking, in the pyrenyl derivative and that of rotamer interconversion in the naphthtyl one. Both excited state processes are controlled by the viscosity rather than polarity of the solvent, and they occur during inertial solvation in low-viscous media and lengthening up to tens of picoseconds in highly viscous solvents.
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. The comparison between the spectral shapes of those transients experiencing an aqueous surrounding and those embedded in the hydrophobic domains allowed the fully relaxed excited state to be assigned to a twisted intramolecular charge transfer (TICT) state. The latter cannot be formed in the rigid hydrogel domains where the excited state charge separation is thus prevented and the stationary fluorescence comes from a scarcely polar locally excited (LE) state.
By confining two push-pull
N
-methylpyridinium derivatives within the rigid domains of surfactant hydrogels, the twisted nature of their intramolecular charge transfer state (formed in water upon excitation) was unravelled.
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. Fluorescence up-conversion measurements and the Time Resolved Area Normalised Spectra analysis provided a clear-cut proof of the presence of two distinct fluorescent states ((1)A* and (1)B*), with (1)A* being responsible for the steady-state emission in highly polar and viscous media. Femtosecond transient absorption spectra were acquired in several organic solvents of different polarity and viscosity. Interestingly, the lifetime of (1)A* was found to be dependent on solvent viscosity whereas the lifetime of (1)B* showed a trend which matches the change in solvent polarity. Indeed, the Density functional theory calculations predicted a structural rearrangement in the fully relaxed lowest excited singlet state. The (1)A* → (1)B* transition is thus likely accompanied by large amplitude motions of the molecular structure, with the (1)B* state also exhibiting a small intramolecular charge transfer character. The investigated flexible quadrupolar D-π-D system arouses therefore great interest as a novel material for applications in organic electronics and photonics.
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 S
1
on the ground of quantum-mechanical calculations. A low polar and planar Locally Excited (LE) state together with a highly polar and Twisted Intramolecular Charge Transfer (TICT) state is identified to be responsible for the dual emission of the trimethoxyphenyl compound. Interestingly, the more complex photobehaviour of the pyrenyl derivative was explained considering the contribution to the fluorescence coming not only from the LE and TICT states but also from a nearly Planar Intramolecular Charge Transfer (PICT) state, with both the TICT and the PICT generated from LE by progressive torsion around the quasi-single bond between the methylpyridinium and the ethene bridge. These findings point to an interconversion between rotamers for the pyrene compound taking place in its excited state against the Non-equilibrated Excited Rotamers (NEER) principle.
An interconversion between rotamers in S
1
was evidenced for a cationic push-pull pyrenyl derivative through a joint femtosecond fluorescence up-conversion and density functional theoretical study.
This paper reports for the first time a detailed spectroscopic investigation into the ground- and excited-state properties of α-amino-orcein (α-AO), one of the main components of the orcein dye, in ...solvents of different proticity and water at different pHs. In order to gain insight into the nature of the involved transitions and excited state deactivation pathways, the study was carried out by means of UV–Visible steady state and ultrafast spectroscopic techniques with the support of quantum mechanical calculations (DFT and TDDFT).
The results highlight that the photophysical and photodynamic behaviour of α-AO are highly sensitive to the solvent proticity and pH. In particular, protic environment induces a red shift (55 nm) of the absorption spectrum together with a relevant decrease of the fluorescence quantum yield (from 0.19 in acetonitrile to 6.6 × 10−3 in methanol) and radiative rate constant (two orders of magnitude). A notable red shift is also caused by increasing the pH leading the molecule from monocationic to neutral and then monoanionic form through two deprotonation steps (pKa = 3.539 ± 0.006 and 11.180 ± 0.006). Following deprotonation, the molecule assumes spectral and photophysical properties very similar to those retrieved in protic media. The observed behaviour has been rationalized through the occurrence of hydrogen bonding, likely involving to a greater extent the carbonyl oxygen of α-AO and the protic solvent, that favours the charge delocalization on the whole chromophore as well as fast non-radiative excited state deactivation. The ultrafast spectroscopic investigation revealed in fact the presence, in protic solvent, of a short living component (tens of picoseconds), assignable to solvent complexed S1 state, alongside the long living component (few nanoseconds) observed in aprotic media and attributed to the solvent free S1 state.
The results achieved in this study for α-AO provides an important contribution to the interpretation of absorption and fluorescence features of orcein dye mixture in more complex systems (protein based substrates within the many aspects of the cultural heritage and biomedical field) where hydrogen bonds are expected to play a crucial role in mediating the interaction with the environment.
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•Electronic properties of α-AO are highly sensitive to solvent proticity and pH.•α-AO shows high fluorescence quantum yield in aprotic environment.•Evidence of specific solvent interaction in protic media via hydrogen bonding•Spectral features of α-AO in MeOH and MeCN very similar to that of natural orcein
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The ground and excited state properties of 1-methyl-2-(E)-2-(2-naphthyl) vinylpyridinium 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. These equilibria are shifted towards one prevailing species in the excited state, pointing out an unexpected and efficient rotamer interconversion during the S
lifetime, manifestly against the non-equilibrated excited rotamers principle. The rotamer interconversion has been found to be very fast and only hindered in a rigid matrix at low temperatures.
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. Nicely, an efficient ISC was found for the fluorene-derivatives where small energy gaps between S1 (π,π*) and Tn (n,π*) states had been calculated while no activated ISC was evidenced in the case of anthryl-derivatives where higher S1-Tn energy gaps are expected. A peculiar temperature effect for a fluorene-derivative was pointed out and also explained on the basis of quantum-mechanical calculations at the DFT level taking into account the solvation effects by means of the conductor-like polarizable continuum model CPCM. The presence of dual emission, at first evidenced by a shoulder in the emission spectrum of the fluorene-derivative featuring a peripheral formyl group in dichloromethane at low temperatures, was nicely confirmed by femtosecond up-conversion measurements at room temperature.
