Mechanism Of Photodimerization Research Articles

Water: An underappreciated reaction medium for photodimerizations

Organic solvents have been the sought-after medium to achieve light initiated transformations in laboratories and industries. Given the current emphasis on green chemistry and rising awareness of environmental pollution it may be necessary for us to utilize abundantly freely available, non-toxic and environmentally friendly water as the medium to perform photoreactions. Although water has attracted the attention of organic synthetic chemists, it is yet to receive the indispensable attention of photochemists. In this article we present examples of photocycloaddition reactions of four molecules namely coumarin, indene, cinnamic acid and acenaphthylene that are sparingly soluble in water. Photodimerization of these molecules in water is much faster and occurs at much lower concentrations than in organic solvents. Aggregation probably forced by hydrophobic association is suggested to be the cause for the increased reactivity even at lower concentrations. The dimer distribution in water is different from that in organic solvents. Further work is required to fully understand the mechanism of photodimerization in water. Although the poor solubility of organic molecules in water requires one to irradiate large volumes to collect enough useful amounts, availability of flow reactors and LEDs as light sources should help one overcome the challenges.

Cyclobutane Thymine Photodimerization Mechanism Revealed by Nonadiabatic Molecular Dynamics

The formation of cyclobutane thymine dimers is one of the most important DNA carcinogenic photolesions induced by ultraviolet irradiation. The long debated question whether thymine dimerization after direct light excitation involves singlet or triplet states is investigated here for the first time using nonadiabatic molecular dynamics simulations. We find that the precursor of this [2 + 2] cycloaddition reaction is the singlet doubly π2π*2 excited state, which is spectroscopically rather dark. Excitation to the bright 1ππ* or dark 1nπ* excited states does not lead to thymine dimer formation. In all cases, intersystem crossing to the triplet states is not observed during the simulated time, indicating that ultrafast dimerization occurs in the singlet manifold. The dynamics simulations also show that dimerization takes place only when conformational control happens in the doubly excited state.

Sunlight-exposure photodimerization behavior of some 7,8-ethylenedioxycoumarins: Experimental and theoretical evidence of photodimerization

Sunlight-exposure photodimerization properties of recently synthesized 7,8-ethylenedioxycoumarin (I) and 7,8-ethylenedioxy-4-methyl coumarin (II) were studied with respect to exposure time and pH in 5% v/v aqueous methanol by electronic absorption spectra. I was found to undergo a reversible [2+2] photodimerization upon exposure to sunlight only in basic solutions. In contrast, II was not found to undergo photodimerization upon exposure to sunlight either in acidic or basic solutions. The dimer formation constant was determined as pKDimer=10.650 (R2=0.9999). A photodimerization mechanism consistent with the experimental results was proposed, which is consistent with the experimental results. In addition, quantum chemical calculations showed agreement with the experimental evidence. The role of methyl group of II on the sunlight-exposure photochemical properties of ethylenedioxycoumarins was discussed.

Photochemical synthesis of novel 4,5,9,10-tetraaryl-tricyclo[6.2.0.03,6]decane-2,7-diones: a theoretical view of photodimerization

A sequence of photodimerization reactions of 1,5-diaryl-1,4-pentadien-3-ones have been carried out to produce 4,5,9,10-tetraaryl-tricyclo[6.2.0.03,6]decane-2,7-diones. The title compounds were characterized by means of infrared (IR), nuclear magnetic resonance (NMR) and mass spectrometric methods. The reaction proceeds via a diradical stepwise mechanism through the triplet excited state with asynchronous ring closure. Density functional theory calculations have been well established to probe the mechanism of photodimerization. The diradical mechanism for dimerization of chalcone proceeds via ring closure of the complex formed by two molecules.

Фотоперетворення в плівках целюлозо-циннамату при опроміненні поляризованим ультрафіолетовим світлом

Light-induced changes in photosensitive organic polymer films based on cellulose cinnamate have been studied. Film specimens illuminated with polarized UV light are found to become anisotropic. The dependence of the birefringence in the polymer film on the illumination time is measured, and the difference between the refractive indices for the extraordinary and ordinary waves is found to have a maximum. The constant and dynamic anisotropy components are found to exist in the films under illumination, and they are driven by different recording mechanisms. The relaxation process in the films after their UV illumination has been switched-off can be described by an exponential dependence with the characteristic times typical of the cis-trans isomerization. The mechanism of photodimerization is found to be responsible for the existence of a constant anisotropy component.

Generic, Anthracene‐Based Hydrogel Crosslinkers for Photo‐controllable Drug Delivery

Light-responsive polymers with controllable, reversible crosslink mechanisms have the potential to create unique biomaterials with stimulus-controlled swelling, degradation and diffusion properties useful in tissue engineering and drug delivery applications. Generic photodimerizing polyethylene glycol-anthracene macromolecules that may be grafted to various polymers to effectively control their crosslinking via a photodimerization mechanism have been developed. These generic crosslinkers were shown to effectively introduce photoresponsive properties into hyaluronate and alginate as model hydrophilic polymers. In vitro testing using human corneal epithelial cells was used to demonstrate cytocompatibility of the resulting photogels. The effective crosslinking density of the photogels could be increased resulting in a decrease in the release rate of small and large molecules from the photogels following exposure to 365 nm light. This tuneable crosslinking has the potential to manipulate the delivery rates of therapeutics resulting in control over treatment profiles and may lend itself to various applications, which may benefit from light induced changes in crosslinking.

Photodimers of N-alkyl-3,4-dimethylmaleimides—Product ratios and reaction mechanism

The photodimerization of three N-alkyl-3,4-dimethylmaleimides (DMI) – frequently used to form crosslinks in polymers – was examined. N-Isobutyl-3,4-dimethylmaleimide, N-(2-acetamido)ethyl-3,4-dimethylmaleimide, N-(3-propionic acid)-3,4-dimethylmaleimide were studied by steady-state and time-resolved photolysis in various solvents. Unexpectedly, under all conditions a single-bonded dimer distinct from the well-known cyclobutane fused cyclic dimer was identified. Depending on the solvent the single-bonded dimer amounts to 5–50% of the product yield, allowing for considerable selectivity. Dimerization quantum yields, Φ dim, range from 0.01 to 0.5. Results from time-resolved experiments establish the involvement of the triplet state in the photodimerization mechanism, the quantum yield of singlet molecular oxygen formation accounts for the Φ dim values. Electron transfer and hydrogen abstraction reactions of DMI in polar and protic solvents, in contrast to N-alkylmaleimides, are inefficient processes.

Aspects of the Photodimerization Mechanism of 2,4-Dichlorocinnamic Acid Studied by Kinetic Photocrystallography

Photoinduced structural variations in single crystals of 2,4-dichloro-trans-cinnamic acid (C9H6Cl2O2, DiClCA) have been investigated using X-ray diffraction (photocrystallography) and optical spectroscopic methods. During UV irradiation, which initiates the irreversible dimerization reaction, a loss of the long-range order of the reactant single crystal was found, i.e., that the dimerization is a heterogeneous one. This unexpected result emphasizes the still-existing problem of predicting changes or of remaining periodicity during chemical reactions in the solid state. On the basis of the experimental results, we propose a qualitative kinetic reaction scheme for DiClCA heterogeneous dimerization reaction.

Photoinduced Electron Transfer between Acenaphthylene and Tetracyanoethylene: Effect of Irradiation Mode on Reactivity of the Charge-Transfer Complex and the Resulted Radical Ion Pair in Solution and Crystalline State

The mechanism of photodimerization of acenaphthylene (ACN) and of reactions with tetracyanoethylene (TCNE) by electron transfer (ET) has been investigated in solution and solid state to elucidate the role of the radical cation of ACN (ACN+•) in formation of the cisoid-dimer (cisoid-1) and the transoid-dimer (transoid-1) of ACN and addition products to TCNE. Selective excitation of the 1:1 charge-transfer (CT) complex between ACN and TCNE with light of >500 nm did not result in any reaction in acetonitrile (AN) or 1,2-dichloroethane (DCE). On the other hand, direct irradiation of ACN with light of >400 nm in solution in the presence of TCNE gave cisoid-1 and transoid-1 as the major products together with a [2 + 2]-adduct (2) and two isomeric [2 + 2 + 2]-adducts (3 and 4) of ACN and TCNE as minor products. Distinction of photochemical reactivity between selective CT excitation and direct excitation of ACN can be attributed to faster backward electron transfer (BET) from the contact radical ion pair (CIP) on CT excitation than from the solvent-separated radical ion pair (SSIP) on direct excitation of ACN due to very low energy for BET, as low as 1.34 V. Effect of [TCNE] on quantum yield for the dimerization of ACN and on the cisoid/transoid ratio of the resulted 1 rationalizes the mechanism involving the singlet and triplet SSIP; the former tends to undergo BET, but the latter undergoes dissociation to ACN+•, followed by formation of dimeric radical cation of ACN, ACN2+•, finally leading to 1. A possible mechanism for formation of 3 and 4 is discussed on the basis of concentration dependence of ACN. Contrary to photochemical inertness of the CT complex in solutions, CT excitation of the 1:1 crystal of ACN and TCNE (ACN·TCNE) gave 2 as the sole product. The selective formation of 2 indicates that fixation of the two alkenic CC double bonds in ACN·TCNE separated by 3−4 Å in both the excited CT state and the resulted CIP retards the deactivation and BET but enables them to undergo cycloaddition.

Mechanism of Photodimerization of Acenaphthylene

The mechanism of photodimerization of acenaphthylene (ACN) has been investigated in order to elucidate the roles of the singlet and the triplet excited states of ACN in the formation of the Z- and E-dimers in several solvents. The quantum yields and the ratio of the produced Z- to E-dimer were determined under irradiation of ACN at 435.8 nm in several solvents (1,2-dichloroethane, acetonitrile, cyclohexane, benzene, methanol, and DMF) over a wide concentration range (2.0 × 10-4−2.0 M) in the absence of additives and in the presence of 1,2-dibromoethane, a heavy-atom-containing solvent; Eosin-Y, a triplet sensitizer (irradiated at 546.1 nm); and ferrocene, a triplet quencher. On irradiation of ACN in dilute solution, the initially generated S1 state crosses over to the T1 state, though with low quantum yield, before reaction with the ground state ACN can occur due to the very short lifetime of the S1 state, and the resultant T1 state reacts with ground state ACN to give a mixture of the Z- and E-dimers in a ratio that depends on the solvent employed. In more concentrated solution, the S1 state reacts with ACN before intersystem crossing to the T1 state can occur, affording exclusively the Z-dimer.

THEORETICAL INVESTIGATION OF EXCIMER AND EXCIPLEX STATES OF URACIL AND HALOGEN DERIVATIVES: EFFECT OF NONPARALLELISM OF BASES

Theoretical modeling of initial steps of the photodimerization mechanism of uracil, 5-methyl-and 5-halogen derivatives was performed. The interaction energy of bases in stacked dimers in the ground and lowest excited states was calculated as a function of the distance between the base planes and of the rotation angles within the perturbation theory for the extended Hückel treatment. The existence of excimer and exciplex region on the potential surface of the excited state was revealed. The excimer (exciplex) geometry has the planes nonparallel with more close contact of the C5-C6 bonds as compared to the ground state of dimers. The results provide new information useful for understanding the photodimerization mechanism of bases and testifies that the singlet excimer state can be a precursor of the photodimerization reaction.

Photodimerization Mechanism of m-Methoxystyrene via Its Cation Radical. Open Type Dimer Cation Radical as Its Intermediate

Abstract Two isomeric tetralin type dimers were obtained by UV excitation of m-methoxystyrene in the presence of an electron acceptor. Stepwise electrophilic dimerization mechanism through the dimer cation radical was confirmed. The scheme was analyzed with the aid of MO calculation.

From anthracene photodimerization to jaw photochromic materials and photocrowns

The inter and intramolecular photochemical reactions of anthracenes in the absence of oxygen is discussed. The intermolecular photodimerization of 9-substituted anthracenes in fluid solution usually leads to head-to-tail photodimers; it will be shown that this trend can be obviated by non bonding attractive interactions between substituents or by mixed photodimerization. The mechanistic aspects will be examined, next, in connection with the problem of excimer intermediacy. Bis-9-anthracenes can form photocyclomers and, in certain cases, intramolecular excimers at room temperature. Those which display interesting photochromic properties, were called jaw compounds; some of them induce an unusual anthracene ring cyclomerization. A study of photophysical and photochemical processes by steady state and transient kinetic analysis of ,c)-bis-9-anthryl-n-alkanes (ethane to decane) and cC ,C) -bis-9-anthryl-polyoxaalkanes allows a deeper insight into the mechanism of photodimerization. By irradiation, the latter derivatives can generate photocrowns; this is the first photochemical synthesis of crown-ethers.

Electronic structure, spectra, and mechanism of photodimerization of pyrimidine bases

AbstractVarious electronic indices calculated by the CNDO/s‐CI method for uracil, 5‐fluorouracil, thymine, and cytosine and the Woodward‐Hoffmann rule applied to the photodimerization of these molecules lead us to the following conclusions: (i) decrease of the C5C6 bond order consequent to excitation is generally correlated with the photodimerization reaction for all the biological pyrimidines; (ii) excited pyrimidine molecules may be nonplanar and the excitation would be delocalized over both the molecules which photodimerize; (iii) electrostatic interactions may play a significant role in the initial stages of the photodimerization reaction; (iv) singlet precursors to the stable triplet photodimers may exist; and (v) electron density does not, in general, increase in the C5C6 bond in the excited state as compared to the ground state, contradicting the proposals of some earlier workers. Hybrid population densities and σ‐bond orders have been calculated for the first time for the molecules. Fluorine in 5‐fluorouracil perturbs the hybrid populations as compared to those of uracil mainly in its vicinity.

The electron‐beam induced monomerization of di‐p‐anthracene

SUMMARYThe electron beam stability of di‐para‐anthracene has been investigated with regard to the study, in situ, of the mechanism of photodimerization of anthracene and related materials. It is found that the electron beam converts di‐para‐anthracene to anthracene, the process taking place in a single crystal to single crystal manner. Electron doses greater than 6.0 × 10−5 C mm−2 result in a diffraction pattern corresponding to the anthracene lattice.

Mechanism of photodimerization in single crystals of anthracene

The anthracene photodimer, di-p-anthracene, crystallizes in at least six orientations within the anthracene single crystal. The orientations are classified, according to mosaic spread, into two sets of three orientations each. The relative spatial arrangements of the three orientations in each set are identical and each form a simulated repeated twin. The photodimerization begins as a surface reaction. The particular mechanism depends upon the geometry of the surface where the dimerization begins. Two mechanisms are described in this paper. One, associated with the (001) face of anthracene and low mosaic spread reflections, is the uniting of symmetry-related monomer units and the other, associated with the (20{\bar 1}) face and the high mosaic spread reflections, is the uniting of translation-related monomer units. More molecular movement is needed for the uniting of the translation-related monomer units as inferred from the greater mosaic spread of their reflections.

On the Mechanism of Photodimerization of 9-Cyanoanthracene Monocrystal

A study of photodimerization of 9-cyanoanthracene single crystals has been performed by following the growth of 4-sided photoetched pits as a function of time, light intensity, and temperature. Gro...