Photochemistry of 5-aminoquinoline in protic and aprotic solvents

Photophysical properties of 5-aminoquinoline (5AQ) have been investigated in various non-polar and polar (protic and aprotic) solvents using steady state and time resolved fluorescence. In aprotic solvents, the spectral maxima depend on the polarity. However, in protic solvents both the fluorescence intensity as well decay time show decrease depending on the hydrogen bonding ability of the solvent. The results suggest that photochemistry 5AQ is quite sensitive towards the polarity as well as protic character of the solvent.     

Photochemische Reaktionen. 50. Mitteilung [1]. Zur Photochemie von gesättigten β-Ketosulfiden II. Synthese von 6-Hydroxy-2-thia-7-oxa-isotwistan

The results of the ultraviolet irradiation of the saturated β-ketosulfide 2 are discussed. The photochemistry of 2 is characterized by the occurrence of two primary photoprocesses. Their efficiences depend markedly on the excited transition, (charge-transfer)- or (n → π*)-excitation, respectively. In methanol solution (charge-transfer)-excitation leads almost exclusively to product 7 , due to (C_α – S)-fission, and (n → π*)-excitation to nearly equal amounts of 7 and of products 4 and 6 which result from α-cleavage. On solvent sensibilization in benzene products 4, 6, 7 and the still unidentified product 5 ² are formed. Compounds 4 and 6 have been described earlier [2]. The structure elucidation of 7 is reported in this paper. Acid-induced transformation of 7 yields the dihetero-isotwistane 15 .     

Solute-Solvent Interactions in Solutions of Solvatochromic Phenoxides: A Dynamics Simulation Study

Solute-solvent interactions in protic (water, methanol and 2-propanol) and aprotic (DMSO) solvents of four solvatochromic phenoxides, the 4- and 2-pyridiniophenoxides, Brooker’s merocyanine and the N-methyloxyquinolinium betaine, were investigated with the aid of molecular dynamics simulations. Although the size of the first solvation shell of the phenoxide oxygen of all betaines remains constant in the three protic solvents, it comprises increasingly fewer solvent molecules as the volume of the hydroxylic solvent increases. In DMSO, the donor phenoxide group of the 4-pyridiniophenoxide betaine is loosely solvated, leading to an internal charge-transfer with smaller transition energies than in protic media.     

Photochemische Reaktionen. 51. Mitteilung [1]. Zur Photochemie von gesättigten β-Ketosulfiden III. Strukturaufklärung eines weiteren Photoproduktes Zusammenfassende Beurteilung des photochemischen Reaktionsbildes

Ultraviolet irradiation of the saturated β-ketosulfide 2 [(charge-transfer)- or (n → π*)-excitation] in methanol or benzene solutions resulted in the formation of the products 4, 5a, 6 (due to α-cleavage) and 7 [due to (C_α–S)-fission]. Compounds 4, 6 and 7 have been described earlier [1] [2]. The structure elucidation of 5a is reported in this paper. The photochemistry of the β-ketosulfide 2 is summarized.     

Computational studies of the photophysics of hydrogen-bonded molecular systems

The role of electron- and proton-transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. Adopting indole, pyridine, and ammonia as molecular building blocks, we discuss generic mechanisms of the photophysics of isolated aromatic chromophores (indole), complexes of pi systems with solvent molecules (indole-ammonia, pyridine-ammonia), hydrogen-bonded aromatic pairs (indole-pyridine), and intramolecularly hydrogen-bonded pi systems (7-(2'-pyridyl)indole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections, and the properties of frontier orbitals. A common feature of the photochemistry of the various systems is the electron-driven proton-transfer (EDPT) mechanism: highly polar charge-transfer states of 1pipi*, 1npi*, or 1pisigma* character drive the proton transfer, which leads, in most cases, to a conical intersection of the S1 and S0 surfaces and thus ultrafast internal conversion. In intramolecularly hydrogen-bonded aromatic systems, out-of-plane torsion is additionally needed for barrierless access to the S1-S0 conical intersection. The EDPT process plays an essential role in diverse photophysical phenomena, such as fluorescence quenching in protic solvents, the function of organic photostabilizers, and the photostability of biological molecules.     

Exciplex quenching of photo-excitd copper complexes

Exciplexes are less common in transition metal photochemistry than in organic photochemistry. However, Lewis bases and coordinating anions quench metal-to-ligand charge-transfer excited states of Cu(I) systems, and the quenching is ascribed to exciplex formation. A variety of structural arguments and steric effects lend strong support to the model. These results are related to the broader context of transition metal systems, and future areas of potential interest are briefly discussed.     

Ultrafast intramolecular charge transfer of formyl perylene observed using femtosecond transient absorption spectroscopy

The excited-state photophysics of formylperylene (FPe) have been investigated in a series of nonpolar, polar aprotic, and polar protic solvents. A variety of experimental and theoretical methods were employed including femtosecond transient absorption (fs-TA) spectroscopy with 130 fs temporal resolution. We report that the ultrafast intramolecular charge transfer from the perylene unit to the formyl (CHO) group can be facilitated drastically by hydrogen-bonding interactions between the carbonyl group oxygen of FPe and hydrogen-donating solvents in the electronically excited state. The excited-state absorption of FPe in methanol (MeOH) is close to the reported perylene radical cation produced by bimolecular quenching by an electron acceptor. This is a strong indication for a substantial charge transfer in the S(1) state in protic solvents. The larger increase of the dipole moment change in the protic solvents than that in aprotic ones strongly supports this observation. Relaxation mechanisms including vibrational cooling and solvation coupled to the charge-transfer state are also discussed.     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIV. Solvent Effect on the Heterolysis Rate of 1-Chloro-1-methylcyclohexane. Correlation Analysis of Solvation Effects in Heterolysis of 1-Chloro-1-methylcyclohexane and 1-Chloro-1-methylcyclopentane

The kinetics of heterolysis of 1-chloro-1-methylcyclohexane in 9 protic and 25 aprotic solvents at 25°C were studied by the verdazyl method. The kinetic equation is v = k[RCl] (E1 mechanism). The heterolysis rate of 1-chloro-1-methylcyclohexane in protic solvents is two orders of magnitude lower than that of 1-chloro-1-methylcyclopentane, whereas in low-polarity and nonpolar aprotic solvents the rates are close. A correlation analysis was made to reveal the solvation effects in heterolysis of both chlorides in a set of 9 protic and 25 aprotic solvents, and separately in protic and aprotic solvents.     

Studies directed toward the synthesis of conformationally restricted neurotransmitter analogs: The addition of N-hydroxyimides to ethyl propiolate

N-Hydroxyimides were found to add readily to ethyl propiolate to yield the imidooxyacrylates in both protic and aprotic solvents. The trans isomer only was formed in aprotic solvents while both isomers were formed in protic solvents.     

Femtosecond pump-probe studies of nitrosyl chloride photochemistry in solution

We present a femtosecond pump-probe study of the primary events of nitrosyl chloride (ClNO) photochemistry in solution. Following 266 nm photolysis, the resulting evolution in optical density is measured for ClNO dissolved in acetonitrile, chloroform, and dichloromethane. The results demonstrate that photolysis results in the production of a photoproduct that has an absorption band maximum at 295 nm in acetonitrile and 330 nm in chloroform and dichloromethane. To determine the extent of Cl production, comparative photochemical studies of methyl hypochlorite (MeOCl) and ClNO are performed. Photolysis of MeOCl in solution results in the production of the Cl:solvent charge-transfer complex; therefore, a comparison of the spectral evolution observed following MeOCl and ClNO photolysis under identical photolysis conditions is performed to determine the extent of Cl production following ClNO photolysis. We find that similar to the gas-phase photochemistry, Cl and NO formation is the dominant photochemical channel in acetonitrile. However, the photochemistry in chloroform and dichloromethane is more complex, with a second product formed in addition to Cl and NO. It is proposed that in these solvents photoisomerization also occurs, resulting in the production of ClON. The results presented here represent the first detailed examination of the solution phase photochemistry of ClNO.     

Cinnamates. III. The light sensitive polymers: Cinnamates with internal charge-transfer spectroscopic properties

In this paper, we outline the photochemical and spectroscopic properties of a new light-sensitive polymer obtained as a result of polystyrene modification by attachment of a pendant 4-dimethylamino-cinnamate ester group. This specific cinnamate displays typical charge-transfer character in the absorption and emission spectra. The photochemical and photophysical properties of photopolymer are compared with the monomeric model photochemistry and photophysics.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXI. Solvent Effect on the Rate of 1-Methyl-1-chlorocyclopentane Heterolysis. Correlation Analysis of Solvation Effects

Heterolysis of 1-methyl-1-chlorocyclopentane in protic and aprotic solvents occurs by the E1 mechanism. The reaction rate in aprotic solvents or in a set of protic and aprotic solvents is satisfactorily described by the parameters of the polarity and electrophilicity or ionizing power of the solvents. In protic solvents, the reaction rate grows with increasing polarity or ionizing power of the solvent and decreases with increasing nucleophilicity.     

UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry

The electronic structure and photochemistry of copper formate clusters, Cu^I₂(HCO₂)₃⁻ and Cu^II_n(HCO₂)_2n+1⁻, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with Cu^I and Cu^II copper ions is observed. For the Cu^I species, transitions between copper d and s/p orbitals are recorded. For stoichiometric Cu^II formate clusters, the spectra are dominated by copper d–d transitions and charge-transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low-lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In Cu^II(HCO₂)₃⁻, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge-transfer excitations, specific excited-state reaction channels are observed in addition, such as formyloxyl radical loss. In Cu^I₂(HCO₂)₃⁻, the system relaxes to a local minimum on an excited-state potential-energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a Cu^II(HCO₂)₃Cu⁰⁻ biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.     

Rh-Catalyzed asymmetric hydrogenation of (Z)-ethyl 3-acetamido-2-butenoate in supercritical carbon dioxide: the effect of cosolvents

Rh-Catalyzed asymmetric hydrogenation (up to 85% ee) of (Z)-ethyl 3-acetamido-2-butenoate was carried out for the first time in supercritical CO₂ using protic cosolvents. The activity of the Rh catalyst bearing the amidophosphite ligand increases with an increase in acidity of the protic cosolvents and with hydrogen pressure.     

Donor-and-acceptor substituted truxenes as multifunctional fluorescent probes

A series of dimesitylboryl acceptor (mesityl=2,4,6-trimethylphenyl) and/or diphenylamino donor (-N(Ph)2)-substituted truxene derivatives, classified as D-or-A substituted compounds and D-and-A substituted charge-transfer compounds, have been synthesized. Two D-and-A substituted truxene compounds, namely, 2-dimesitylboryl-7,12-di(N,N-diphenylamino)-5,5',10,10',15,15'-hexaethyltruxene (BN2) and 2,7-di(dimesitylboryl)-12-(N,N-diphenylamino)-5,5',10,10',15,15'-hexaethyltruxene (B2N), exhibit extraordinarily large solvatochromism ranging from 420 nm (in hexane) to 580 nm (in acetonitrile) in aprotic solvents, which can be used to probe the polarity of the solution environment. Due to proton-donor interactions, the solvatochromic red shift of BN2 and B2N in protic solvents has been significantly decreased, and this effect can be used to identify local protic and aprotic environment. Furthermore, because of the interaction between F- and acceptor, BN2 and B2N show sharp spectral response to fluoride ion concentration. The simultaneous "turn-off" at 500 nm and "turn-on" at 380 nm of the fluorescence signal have provided a good example of a fluorescent ratiometric method, which can greatly enhance the sensitivity of the fluoride ion probe. Underlying these interesting spectral phenomena and multifunctional probe properties is the charge-transfer strategy of grafting donor and acceptor moieties, as A-pi-D2 or A2-pi-D style, to the triangular truxene.     

Solvent effects in the absorption spectra of the Ninhydrin chromophore

Visible spectra of the Ninhydrin chromophore, Ruhemann's purple, were studied in dimethyl sulfoxide (DMSO), formamide, N, N-dimethylformamide (DMF), and pyridine, as well as in mixed aqueous-nonaqueous solvent media. Large differences in both the position of absorption maxima and extinction coefficients for the two bands in the visible spectra in the various solvent media were observed. Both the absorption maxima and the extinction coefficients of the Ninhydrin chromophore were a linear function of the composition of dimethyl sulfoxide-H₂O solvent media. The experimental evidence allows predictions of values for the two absorption maxima of Ruhemann's purple as a function of the nature of the solvent medium. In nonaqueous aprotic solvents (i.e., DMSO and DMF), the maxima should be near 605 and 420 mμ; in nonaqueous aprotic solvents capable of undergoing charge-transfer interactions (i.e., pyridine), near 550 and 420 mμ; and in nonaqueous protic solvents (i.e., formamide), near 575 and 410 mμ. The maxima in aprotic media will be displaced to about 575 mμ (higher wavelength band) and 410 mμ (lower wavelength band) on dilution with protic solvents.     

185-nm Photochemistry of Olefins,Strained Hydrocarbons,and Azoalkanes in Solution

The possibility to excite directly at 185 nm chromophores that absorb in the vacuum-UV has stimulated increased activity during the last decade in this field of photochemistry. Whereas photochemical reactions at λ<200 nm have been thoroughly investigated in the gas phase, only recently have intensive studies on the 185-nm photochemistry of organic compounds in solution provided new insights. Despite the high excitation energies, selective photoreactions are promoted in the short-lived singlet excited states (Rydberg photochemistry). In contrast to conventional photochemistry (λ > 220 nm), 185-nm irradiation preferentially results in intramolecular rearrangement, fragmentation, and isomerization reactions. Intermolecular radical couplings and abstractions as well as dimerizations (π, π^*-excitation) compete minimally. Besides the straightforward denitrogenation of photoresistant (“reluctant”) azoalkanes, important applications of the short-wavelength photolysis are also found in technology (photolithography) and medicine (193-nm laser). Broadening the scope of the synthetic potential of the 185-nm photochemistry, which so far has been limited to direct cis/trans isomerizations, presents a challenge for the chemist.     

The Irreversible Photochemistry of Bianthrone

A 4a,4b-dihydrophenanthrene-type cyclic photoisomer, the C isomer, is the major primary photoproduct of bianthrone in protic and aprotic polar solvents, and undergoes solvent-dependent secondary reactions, including the formation of dihydrohelianthrone in protic solvents. The C isomer was shown to be formed through the singlet excited state while the B isomer is formed via the triplet manifold.     

Near ultraviolet spectroscopic studies of 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) and 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil)

The near u.v. spectra of 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil) and 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) can be viewed as perturbed pyrimidine spectra. The u.v. properties of pyrimidine and a series of aminopyrimidines, specifically 2,4,6-triaminopyrimidine, are examined to obtain u.v. spectral assignments for desoxyminoxidil and minoxidil. Minoxidil and its desoxy counterpart have C_s symmetry, and all π → π* absorptions are allowed ¹A′ ← ¹A′ transitions. The two lowest energy π →- π* absorptions observed in minoxidil (262 nm, 292 nm) are tentatively assigned as very mild oxygen → pyrimidine ring charge-transfer transitions. Intensity decreases in protic solvents, and the results of simple Hückel molecular orbital calculations indicate that the 292 nm transition has more charge-transfer character than the 262 nm absorption. The protonated species of desoxyminoxidil and minoxidil have very similar u.v. spectra. This is due to the lack of oxygen-related charge transfer in protonated minoxidil, and the high probability that the positive charge resides in similar environments in the minoxidil and desoxyminoxidil molecular frameworks.