R. Lapouyade

Photodimerization of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photocycloaddition and of the photochemical and thermal cleavage

One of the classics in photochemistry, the photodimerization of anthracenes can be considered as a paradigm of the photocycloaddition of non saturated hydrocarbons. The historical steps of the mechanistic studies are reviewed: based on fluorescence quenching, cyclization quantum yields measurement, the influence of dioxygen and solvents, they support a singlet state pathway; the dimerization rate constants are found to be generally high for reactions occurring within a few nanoseconds unless they are slowed down or inhibited by steric strain. In several cases, excimers have been demonstrated to be intermediates and it is shown that excimer fluorescence and cyclization are competitive processes. Another intermediate known as pericyclic minimum (or conical intersection) is postulated to form a sort of floppy cycloadduct where the reacting centres are at mutual distances shorter than in excimers and longer than in dimers. For intermolecular dimerizations, the triplet state is also reactive but through triplet–triplet annihilation in dilute solutions. Intramolecular photocycloadditions have also been carefully examined, for the role of multiple excimer formation, regioselectivity (9,10∶1′,4′ and 9,10∶1′,2′ cyclization) and solvent polarity. The triplet state reactivity is shown to lead to 4π + 2π or 4π + 4π cycloadducts, depending on geometric factors. In the latter case when intersystem crossing is favoured by the substituents, cyclization quantum yields as high as 0.65–0.72 have been observed. Photodissociation quantum yields are generally high and the reactions are partly adiabatic, leading to excimer and monomer fluorescence, but the major part follows another pathway not fully elucidated by flash photolysis. Thermodynamic and kinetic parameters for the thermal cleavage are given; they reveal a large gamut of stability for the photocycloadducts.

Multiple relaxation pathways in photoexcited dimethylaminonitro- and dimethylaminocyano-stilbenes

Abstract The photophysical behaviour of para-nitro-, para-cyano- and meta-nitro-4′-dimethylaminostilbenes is compared and explained. The increase of the fluorescence quantum yield with solvent polarity is the result of a negative solvatokinetic behaviour of the nonradiative channel leading to the ground state via the double-bond twisted species P* which is of low-polarity nature. The maximum in the fluorescence quantum yield at intermediate solvent polarities, observable for the nitro compounds, is due to a further nonradiative channel specifically linked with the nitro group and with positive solvatokinetic behaviour. This quenching channel becomes important for high-polarity solvents but is absent for compounds without the nitro group. TICT state formation (twisted nitro group) as a candidate for this channel is discussed.

Non-classical Fe II spin-crossover behaviour leading to an unprecedented extremely large apparent thermal hysteresis of 270 K: application for displays

[Fe(hyetrz) 3 ](anion) 2 ·3H 2 O [hyetrz=4-(2′-hydroxyethyl)-1,2,4-triazole, anion=3-nitrophenylsulfonate] is a novel linear polynuclear Fe II spin-crossover compound. The low-spin to high-spin transition accompanied by a pronounced thermochromic effect occurs at 370 K in a very abrupt way. Just before this temperature, the three non-coordinated water molecules are removed. The dehydrated high-spin form remains stable down to ca. 100 K, where it transforms into a new low-spin form, implying that this material shows an apparent thermal hysteresis width of 270 K. Applications of this material are discussed.

Synthesis and photophysical study of 4-(N-monoaza-15-crown-5) stilbenes forming TICT states and their complexation with cations

Three new 4-(N-monoaza-15-crown-5)stilbene derivatives (DS-Crown, DCS-Crown and PDS-Crown) were synthesized in order to study their spectroscopic properties in connection with the complexation of cations. The three fluoroionophores had almost identical photophysical properties as their counterparts, with a dimethylamino group instead of the macrocycle, but upon complexation by alkali and alkaline-earth metal ions, they showed hypso and hypochromic shifts of the absorption spectrum and a solvent dependence of the fluorescence

Multidimensional photochemistry in flexible dye systems

Abstract The photophysics of many flexible dye systems can be understood on the basis of adiabatic photoreactions to emissive or nonemissive product states. Nonemissive product states lead to fluorescence quenching and are often the reason for what is conventionally called “internal conversion”. For compounds with several flexible bonds, competing photoreaction pathways can exist. This is shown for the example of benzopyrylium dyes and for donor—acceptor—stillbenes. Chemical bridging of bonds and/or substituent changes can control the competition between the various channels. This opens a new view on the multidimensional nature of the excited-state hypersurfaces and gives access to control dye properties by molecular engineering.

Dual fluorescence in trans-4-dimethylamino-4'-cyanostilbene revealed by picosecond time-resolved spectroscopy: a possible new tict compound

Abstract The presence of two time-resolved emission bands is observed in trans-4-dimethylamino-4′-cyanostilbene dissolved in polar solvents using picosecond time-resolved spectroscopy. In a non-polar solvent, only the presence of a single emission band is observed at any temperature. A precursor-successor relationship occurs between the two species responsible for the two emission bands. It is concluded that the formation of a twisted intramolecular charge transfer (TICT) exicted state may explain these experimental results.

Photophysics of 4-dimethylamino 4′-cyanostilbene and model compounds: dual excited states revealed by sub-picosecond transient absorption and Kerr ellipsometry

4-Dimethylamino 4′-cyanostilbene (DCS) and two selectively bridged compounds are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentrations, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied compounds. In polar solvents, the bridged derivative where twisting of the anilino moiety is prevented but double bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS compound and the related bridged derivative where the anilino moiety is still able to twist, a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the DE state (delocalized excited state), the ICT state (internal charge transfer, highly polar, nearly planar configuration, formed quasi-instantaneously from the DE state by electronic reorganization), the CRICT state (conformational relaxed ICT state, highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the “phantom” state P∗ on the trans⇄cis isomerization pathway (twisted double bond, low polar, non-fluorescent). In this study, we observed the formation of the CRICT excited state on a time scale ranging from 2 to 20 ps depending on the solvent characteristics (polarity, viscosity and hydrogen bonding ability).

Non-classical FeII spin-crossover behaviour in polymeric iron(II) compounds of formula [Fe(NH2trz)3]X2xH2O (NH2trz=4-amino-1,2,4-triazole; X=derivatives of naphthalene sulfonate)

A new series of iron(ii) spin-crossover materials of formula [Fe(NH2trz)3 ]X2xH2O [X=1-naphthalene sulfonate (1ns), 2-naphthalene sulfonate (2ns), 4-hydroxy-1-naphthalene sulfonate (4OH-1ns), 4-amino-1-naphthalene sulfonate (4NH2-1ns) and 6-hydroxy-2-naphthalene sulfonate (6OH-2ns)] are reported. The structure of these compounds consists of linear chains in which the Fe(ii) ions are linked by triple N1 ,N2-1,2,4-triazole bridges. All compounds show non-classical spin-crossover behaviour. Optical and magnetic measurements recorded upon heating show an abrupt low-spin to high-spin transition accompanied by a pronounced thermochromic effect occuring between 330 and 340 K depending on the anion. Thermogravimetric analyses show that the transition is induced by the removal of the two lattice water molecules, which initially stabilized the low-spin state. The dynamical character of this transition has been monitored by extended 57Fe Mossbauer spectroscopic studies on [Fe(NH2trz)3 ](2ns)2xH2O. Upon cooling, the dehydrated modifications show classical spin-crossover behaviour with hysteresis at much lower temperatures, ranging from 229 to 297 K depending on the anion. [Fe(NH2trz)3 ](2ns)2 represents one of the first iron(ii) spin-crossover materials showing a spin transition in the close vicinity of room temperature (290 K) accompanied by hysteresis (14 K).

Anomalous behavior of selected methyl-substituted polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

Abstract In the reversed-phase liquid chromatographic separation of polycyclic aromatic hydrocarbons (PAHs) on C18 phases, methyl-substituted PAHs are expected to elute after the unsubstituted parent PAH based on the increase in hydrocarbonaceous contact area of the methyl-substituted PAHs. However, we have observed that several methyl-substituted PAHs elute prior to the parent compound, e.g., 1-methylperylene, 1-methylpicene and 13-methylpicene. To investigate this anomalous retention behavior, the retention characteristics of all methyl-substituted isomers of chrysene, picene and perylene were compared on a series of sixteen commercially prepared C18 phases. The anomalous retention behavior was observed only on polymeric C18 phases (i.e., those prepared using trifunctional silanes) whereas the methyl-substituted PAHs elute after the parent PAH, as would be expected, on monomeric C18 phases (i.e., those prepared using monofunctional silanes). The early elution of some of these methyl-substituted isomers is related to the non-planarity of these PAHs due to the presence of the methyl group in the so-called “bay-region” of the PAH structure. The non-planarity of these methyl-PAHs can be characterized by the dihedral angle of distortion between the aromatic rings.

Metal-organic compounds: a new approach for drug discovery. N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide copper(II) complex as an inhibitor of human immunodeficiency virus 1 protease.

The use of metal-organic complexes is a potentially fruitful approach for the development of novel enzyme inhibitors. They hold the attractive promise of forming stronger attachments with the target by combining the co-ordination ability of metals with the unique stereoelectronic properties of the ligand. We demonstrated that this approach can be successfully used to inhibit the protease of the human immunodeficiency virus (type 1). Several ligands bearing substituents designed to interact with the catalytic site of the enzyme when complexed to Cu(2+) were synthesised. The inhibition pattern of the resulting copper(II) complexes was analysed. We showed that the copper(II) complex of N1-(4-methyl-2-pyridyl)-2,3,6-trimethoxybenzamide (C1) interacts with the active site of the enzyme leading to competitive inhibition. On the other hand, N2-pyridine-amide ligands and oxazinane carboxamide ligand were found to be poor chelators of the cupric ion under the enzymatic assay conditions. In these cases, the observed inhibition was attributed to released cupric ions which react with cysteine residues on the surface of the protease. While unchelated metal cations are not likely to be useful agents, metal chelates such as C1 should be considered as promising lead compounds for the development of targeted drugs.