Salah E. Morsi

Fluorescence and Photoreactivity in Crystalline Anthracene and Some Related Derivatives

Abstract Following fluorescence microscopical investigations a photochemical cycle is established to rationalise the behaviour of anthracene, anthracene 9,10 epiperoxide, anthraquinone, anthrone and di-p-anthracene when these compounds are exposed to u.v. irradiation under different environmental conditions. It is proposed that a green emitting intermediate, akin to an anthracenic excimer, is pivotal in this cycle and that the interconversions occur topotactically.

Luminescence and photoreactivity of 2,5-distyrylpyrazine crystals

The emission spectra of the photolabile polymerizable distyrylpyrazine (DSP) in its α-crystallographic modification have been studied in the form of both fresh and partially polymerized samples.A photochromism in emission is observed which is attributed to excitonic and monomeric emissions. At room temperature, fresh DSP single crystals of the α-modification give emission spectra with two maxima at 500 and 530 nm. During the photopolymerization a new emission peak develops at ca. 445 nm which is attributed to isolated monomer molecules being separated by the photoproduct from the exciton. The position of this developing peak compares with the emission from dilute DSP methanolic and chloroform solutions.The photopolymerization process obeys a zero-order kinetic law, and a kinetic study of the as-grown DSP single crystals reveals a negative energy of activation, Ea=–(38.15±1.6) kJ mol–1.

Calorimetric, spectroscopic and structural studies of the monomerization of crystalline 9-cyanoanthracene photodimer (9 CNAD)

The thermal monomerization of 9-cyanoanthracene photodimer (9 CNAD) crystals has been studied using differential scanning calorimetric (d.s.c.), luminescence and X-ray diffraction techniques. The early stages of monomerization (up to ≈ 3% decomposition) includes a homogeneous topochemical process with the monomer produced within the dimer host in the form of centrosymmetric pairs having different degrees of overlap. Such pairs give rise to different luminescence characteristics and the excimeric emission at 490 nm is confirmed as emanating from one of these pairs.The first exothermic peak which appears in the dynamic decomposition calorigrams of the more crystalline 9 CNAD samples at ≈ 425 K is due to the heterogeneous topotactic growth of monomer 9-cyanoanthracene (9 CNA) inside the dimer host crystal. This peak is absent for finely powdered material. At later stages an amorphous phase of 9 CNA forms and both crystalline and amorphous phases subsequently melt yielding separate endotherms at 452 and 453.5 K. Both the dynamic and isothermal decomposition calorigrams are found to be highly dependent on the nature of the 9 CNAD samples and contain information about at least one crystallization process.The unit cell parameters for the dimer have been evaluated together with those of the developing monomer phase. They differ in only the stack (c) axis.The dimer undergoes monomerization when irradiated by X-rays confirming our initial views about the involvement of a biradical intermediate during the course of monomerization.

Kinetics of the photopolymerization of 2,5-distyrylpyrazine in solution

The photoreactivity of 2,5-distyrylpyrazine (DSP) in methanolic solutions has been investigated using various spectroscopic and kinetic methods. The effect of the excitation wavelength has also been studied: light of wavelength λ= 403 nm induces a photo-oligomerization process whereas light of wavelength λ= 365 nm causes both photo-oligomerization and photopolymerization processes. These two processes are manifested as two stages in the reaction. The photo-oligomerization stage is characterized by higher values of quantum yield for chemical reaction (ϕ= 1.8 at 30 °C) and a higher energy of activation compared with the photopolymerization stage. The pattern of change in the absorption spectra of DSP solutions as a result of excitation with light of λ= 365 nm depends upon the exciting light intensity. A sharp isosbestic point at 287 nm is obtained when high light fluxes (ca. 7.57 × 10–7 ein min–1) are applied, whereas three isosbestic points are obtained at 350, 325 and 287 nm on applying low light fluxes (ca. 5 × 10–8 ein min–1) or by applying 403 nm light regardless of its intensity.The photo-oligomerization stage undergoes a power kinetic law and has an activation energy Ea= 38.4 ± 1.5 kJ mol–1, an activation enthalpy ΔH*= 36.9 ± 1.5 kJ mol–1 and an activation entropy ΔS*=–184.3 ± 0.3 J K–1 mol–1.The photopolymerization stage obeys second-order kinetics with activation parameters: Ea= 16.0 ± 1.0 kJ mol–1, ΔH*= 13.6 ± 1.0 kJ mol–1 and ΔS*=–240.2 ± 0.3 J K–1 mol–1. The energy of activation in both cases accounts for a diffusion-controlled process that brings the monomer molecules to a spatial configuration conductive for the photocycloaddition of the olefinic double bonds. The highly negative values of the entropies of activation in both stages indicate that the activated states are highly ordered compared with the ground states. A mechanism for the reaction has been postulated that is based on a two-centre attack by an excited DSP monomer molecule on two ground-state molecules during the photo-oligomerization stage. A stepwise mechanism prevails in the photopolymerization stage.Both the fluorescence and excitation spectra of DSP in methanolic solution decrease in intensity as the u.v. irradiation time increases, indicating that the photoproducts have no detectable fluorescence in this spectral region.

Kinetics of solid state thermal monomerization of 9-cyanoanthracene photodimer (9CNAD) and photodimerization of 9-cyanoanthracene (9CNA)

The kinetics of the thermal monomerization of 9-cyanoanthracene dimer (9CNAD) has been followed both dynamically and isothermally by differential scanning calorimetry (DSC) and isothermally by spectrophotometry. The reaction is found to be similar to many other solid state decomposition reactions and exhibits an induction period corresponding to surface nucleation, an acceleratory region reflecting the formation of the product at the surface of nucleii and a deceleratory region where the reaction centres interact. These DSC experiments yield activation parameters which are different from previously published data. The more reliable isothermal study by spectrophotometry confirms that the reaction occurs in three stages; an induction period, a zero order process with an activation enthalpy of 98 kJ mol–1 followed by a first-order process with an activation enthalpy of 68 kJ mol–1. The involvement of an intermediate radical is postulated.By following the build up of luminescence which develops as a result of the photodimerization of 9-cyanoanthracene single crystals with time and temperature it has been possible to obtain an activation enthalpy for this reaction. The value of 22 kJ mol–1 is identified with the energy required to bring a pair of 9-cyanoanthracene molecules into a favoured “incipient dimer” orientation to form a chemically bonded species and relates to a diffusion controlled process.

Luminescence of 9-cyanoanthracene. Identification of new excimeric species

From studies of the emission spectra of 9-cyanoanthracene (9 CNA) solutions, glasses, polycrystalline films and single crystals at 298 and 77 K a centrosymmetric excimeric species with emission maximum at ≈ 490 nm has been observed. The presence of “incipient-dimer” pairs giving rise to such emission has been confirmed by investigations of the photolytic and thermal cleavage of the photodimer in 9-cyanoanthracene single crystals and polycrystalline matrices. Luminescence decay time measurements substantiate our views. The emission spectrum of 9 CNA single crystals is asymmetric, with a maximum at 490 nm at room temperature, but upon cooling to 77 K the peak becomes symmetrical and occurs at lower energy. The energy difference of ≈ 2000 cm–1 is appreciably higher than the value predicted on the basis of the “frequency effect” and often used to explain the temperature shift of an excimeric emission in both “pair” and “stack” structures. At the higher temperatures in 9 CNA single crystals at least two excimeric forms exist in thermal equilibrium but at the lower temperatures emission occurs from the mirror-symmetric form.

Fluorescence and reactivity of p-aminosalicylic acid: an example of proton transfer in the solid state

The emission spectrum of p-aminosalicylic acid in ethanol solutions exhibits maxima in the visible (445 nm) and u.v. (ca. 350 nm) spectral regions. These maxima are attributed to the protonated and unprotonated forms of this molecule. Protonation occurs in the excited state as a result of intramolecular transfer of the phenolic proton to the oxygen of the carbonyl group and involves an ordered transition state. A similar process is identified in the solid state. By following the fluorescence emission attributed to this protonated species as a function of time and temperature we have evaluated the activation parameters for the deprotonation occurring in the excited state. Decarboxylation of p-aminosalicylic acid together with the dehydration and decarboxylation of the sodium salt have also been investigated. The mechanism of the solid state decomposition of these materials involves the initial step of intramolecular proton transfer from the phenolic hydroxy to the carbonyl oxygen with the creation of a disordered transition state leading to the final products.

Role of structural imperfections in the decomposition of dibenzoyl peroxide

By employing optical microscopic and goniometric techniques the morphology of solution grown dibenzoyl peroxide has been characterised and the slip systems responsible for emergent dislocations at growth and cleavage surfaces identified. Topographical changes taking place at thermally and photochemically decomposed single crystal surfaces have been correlated with the abnormal structure and stereochemistry of molecules in the vicinity of defective regions.The kinetics of the isothermal decomposition of single-crystal dibenzoyl peroxide have been studied by micro-gravimetry in high vacua. In all experiments the mass continuously decreases with time in a deceleratory manner, after first traversing a minor acceleratory stage preceded by a very short induction time. The initial nucleation process obeys the Avrami–Erofeeve equation and the bulk reaction, occurring with a deceleratory rate, satisfies a contracting envelope model. Values of the activation enthalpies were found to be 45 kJ mol–1 and 72.0 kJ mol–1 for the nucleation stage and the main reaction respectively, for both as-grown and deliberately deformed crystals. The value of 45 kJ mol–1 for the nucleation process is attributed to the energy required to break the O—O bond of molecules situated at defective regions, and the value of 72 kJ mol–1 for the main reaction assigned to the induced decomposition of peroxide molecules in perfect regions of the crystal as a result of the attack of phenyl radicals generated in the nucleation process. Phenyl benzoate, the main product of the reaction, is believed to form in a topochemical manner within the dibenzoyl peroxide matrix, the b axis of both structures appear to coincide.