Vicente G. Toscano

Photophysical characterization of a 1,4,5,8-naphthalenediimide derivative

Abstract The photophysical properties of N , N -dibutyl-1,4,5,8-naphthalenediimide ( 1 , Scheme 1) were studied using absorption and fluorescence spectroscopy. The absorption and emission spectra of 1 are mirror images and show resolved vibrational structure indicating that 1 is a rigid molecule which does not relax substantially from the initially formed Franck-Condon state. The fluorescence of 1 is rather weak (quantum yield, approximately 0.002), supposedly due to fast intersystem crossing to a close-lying triplet level. Diimide 1 aggregates in acetonitrile and in aqueous medium, but aggregation is prevented by the presence of α-cyclodextrin (α-CD) due to complex formation between α-CD and the butyl substituents of 1 . Diimide 1 displays unusual spectral characteristics in aromatic hydrocarbon solvents, which can be attributed to ground state complex formation between 1 and the aromatic compound. The addition of benzene, toluene and p -xylene to an acetonitrile solution of 1 was followed by fluorescence spectroscopy, and the data obtained were treated as simple 1 : 1 equilibria. Association constants were calculated for the complexes between 1 and the aromatic compounds. The magnitude of the constants suggests that these ground state complexes are basically of the π-stacking type, with no charge transfer character. In the excited state, however, the complexes show polar character, suggesting that electron transfer occurs following excitation.

Intramolecular excimer formation from 1,8-N-alkyldinaphthalimides

Abstract Conjugation of π electrons between imide groups vicinal to arene moieties can drive the photophysical and photochemical properties to a mixed transition of close lying states having n , π ∗ and π,π ∗ characters (Almeida et al., J. Photochem. Photobiol. A: Chem., 58 (1991) 289). Furthermore, the extent of mixing between these states is a function of the molecular geometry (Barros et al., J. Photochem. Photobiol. A: Chem., 76 (1993) 55). New imides were prepared (1,8-N-propyldinaphthalimide (C3) and 1,8-N-butyldinaphthalimide (C4) to investigate the n , π ∗ and π,π ∗ conjugative effects through space. The steady state and time-resolved fluorescence emission of C3 and C4 in solvents of high polarity shows the presence of a new emitting species at longer wavelength. This emission arises from a rotation in the alkyl chain, resulting in an intramolecular energy transfer between the two chromophores during the excited state lifetime of the extended form.

Photophysical properties of N-alkylnaphthalimides and analogs

Abstract Photophysical studies of 1.8 ( I ) and 2.3 ( II ) N-butylnaphthalimides were realized. UV/Vis absorption and fluorescence properties for I and II as a function of solvent polarity and proticity show a higher π orbital interaction between the imide and naphthalene rings for I . A comparison with the spectral data for their respectives anhydrides, amides and carboxylates corroborates this “pattern”. The distinct electronic energy configuration which arises from 1.8 and 2.3 containing conjugated π electrons on the naphthalene ring, is so dramatic that it governs processes spanning from nanoseconds to seconds.

Solvent-induced changes in the photophysical properties of N-alkylphthalimides

Abstract The photophysical properties of N -alkylphthalimides (alkyl being methyl, ethyl, propyl and butyl) are investigated in various solvents. Inversion of excited singlet states of different configurations, resulting in an increase in fluorescence yield, is promoted by hydrogen bonding with the solvent. Although these compounds are non-emissive in aprotic solvents, their fluorescence intensity increases markedly on increase in the medium proticity. Very acidic conditions (concentrated HClO 4 ) result in protonation of the ground state and a decreased fluorescence yield.

Solvent-induced changes in the photophysical properties of N-alkylphthalimides. II, Temperature and acidity effects

Abstract The fluorescence quantum yields of N -propylphthalimide were determined in methyl, ethyl, isopropyl and butyl alcohols, water and aqueous solutions of HClO 4 . An increase in the protic nature and polarity of the solvent resulted in higher fluorescence yields. Temperature effects were similar in water and ethanol. Solvent and temperature effects are interpreted in terms of electronic energy distribution, the increase in fluorescence quantum yield being ascribed to the mixing of the ππ* and nπ* states. Prototropic equilibrium is shown to be present only in the ground state.

Influence of methoxy substitution on flavonoid photophysics: a steady state and laser flash photolysis study

Abstract A comparative study of the photophysical properties of three flavonoids (flavone (F), 7-methoxyflavone (7MF) and 3-methoxyflavone (3MF)), which may serve as models for naturally occurring flavonoids, shows a distinct behaviour of the 3-methoxy derivative. Steady state and transient spectroscopic investigations reveal an almost quantitative formation of the triplet states of F and 7MF, which can be observed as transient absorptions and show electron donor and acceptor properties. In contrast, 3MF shows reduced triplet formation, and the transient observed is identified as a 1,4-biradical, formed by intramolecular hydrogen abstraction, which behaves as an electron donor.

Photodegradation of poly(vinyl acetate)

Abstract Poly(vinyl acetate) (PVAc) films with different molecular masses were exposed to ultraviolet radiation. The infra-red spectra of all the samples showed a continuous attenuation of the intensity of the carbonyl stretching bands (1740 cm −1 ) and of the symmetrical methyl bending vibration bands (1375 cm −1 ), which suggests a steady loss of acetate side groups during irradiation. The electronic spectra showed an increase in the uv absorption at about 272 nm, suggesting the occurrence of polyene sequences in the polymer chains. In the low molecular mass sample there was almost no insoluble gel fraction formed, even after 20 h irradiation; in the high molecular mass polymer there was already a 50% insoluble fraction after 5 h irradiation, indicating a high degree of crosslinking, although the average viscometric molecular mass of the soluble fraction was only about one sixth of the initial value.

Fluorescence quenching of anthracenes by N,N-dimethylaniline: a comparison with quenching by triphenylphosphine

Abstract Rate constants kq for fluorescence quenching by N,N-dimethylaniline (DMA) of eleven anthracenes (Ai) substituted at the 9 or 9, 10 positions have been measured in benzene and acetonitrile. The results are compared with kq obtained for the anthracenes with triphenylphosphine (P) as quencher. Emissions in benzene were observed for the exciplexes between DMA and A4 - A11. The data are correlated with physical constants for Ai, DMA, P and solvent according to the charge-transfer model of Weller and the excitation-resonance model of Hammond. It is shown that neither theory can explain all of the data. It is suggested that the Ai-DMA and Ai-P exciplexes may be of different geometries.

Kinetic studies on the peroxyoxalate chemiluminescent reaction: imidazole as a nucleophilic catalyst

The peroxyoxalate system undergoes one of the most efficient chemiluminescence reactions and is the only one supposed to involve an intermolecular chemically induced electron exchange luminescence (CIEEL) mechanism, with proven high efficiency.In this work we report kinetic results on the reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide, catalysed by imidazole (IMI), in the presence of chemiluminescence activators. The kinetics were followed by measurement of the intensity of light emission and the 2,4,6- trichlorophenol (TCP) release, observed as an absorption change. From the dependence of the observed rate constants on the concentrations of TCPO, imidazole, hydrogen peroxide and activator, we attribute rate constants to three elementary steps in the proposed simplified mechanistic scheme. The initial step consists of attack of the nucleophilic imidazole on TCPO. A bimolecular (k1(2)= 1.4 ± 0.1 dm3 mol–1 s–1) and a trimolecular [k1(3)=(9.78 ± 0.08) 102 dm6 mol–2 s–1] rate constant can be attributed to this step. The imidazolide subsequently suffers imidazole-catalysed peroxide attack, leading to a peracid derivative; trimolecular rate constants k2(3)=(1.86 ± 0.06)104 dm6 mol–2s–1 and k2(3)′=(8.7 ± 0.2) 103 dm6 mol–2 s–1 can be obtained for this step from the peroxide and the imidazole dependence, respectively. The cyclization of the peracid affords the reactive intermediate, which we believe to be 1,2-dioxetanedione; a rough estimate of k3∼ 0.2 s–1(at [IMI]= 1.0 mmol dm–3) is obtained for this step. Finally, the interaction of the activator with the reactive intermediate, probably involving the CIEEL sequence and leading ultimately to light emission, is extremely fast and cannot be observed kinetically.

Fluorescence quenching of substituted anthracenes by tris(pentafluorophenyl)phosphine

Abstract Rate constants k q for fluorescence quenching of eleven 9 and 9,10 substituted anthracences (A i ) by tris(pentafluorophenyl) phosphine (FP) are reported. Correlations of k q with the electronic properties of A i and FP reveal that A + FP − charge-transfer interactions are important in the quenching process for several of the A i . No one quenching mechanism is able to explain all of the data. The quenching mechanism of anthracene, 9-methyl-anthracene and 9,10-dimethylanthracene in benzene appears to lack a pathway available to anthracenes whose substituents contain a lone pair of electrons.