Fluorescence Spectral Study of 9-Acridinecarboxylic Acid and Its Methyl Ester. Understanding the Unusual Fluorescence Behavior of 9-Anthroic Acid

Abstract

The absorption and fluorescence spectral characteristics of 9-acridinecarboxylic acid (9-ACA) and 9-(methoxycarbonyl)acridine (9-MCA) were studied in a series of organic solvents and in aqueous solutions. Fluorescence quantum yields (Φf) and lifetimes (τf) of the compounds were measured in these solvents. Unlike 9-anthroic acid (9-AA), as reported in the literature, no large Stokes-shifted fluorescence emission band was observed for 9-ACA and 9-MCA in neutral organic solvents or water. The absence of large Stokes-shifted emission in the case of 9-ACA and 9-MCA suggests the existence of a charge-transfer emitting state in 9-AA in which the carboxyl group is nearly coplanar with the aromatic ring. The Φf values for both compounds increase as a function of hydrogen-bonding capacity of the solvents. In near neutral to slightly acidic solutions, 9-ACA exists mainly in the zwitterionic form. Both 9-ACA and 9-MCA form monoprotonated species in moderately concentrated acid solutions. The acidium cation of 9-AA formed in the excited state in moderately concentrated acid solution reorganizes to produce a carbocation centered at the carbon atom of the carboxyl group. However, there was no indication of the formation of such acidium cations in the case of 9-ACA and 9-MCA even in concentrated perchloric acid medium. The pK as of various prototropic equilibria involved in the ground electronic state of the compounds were estimated. Semiempirical AM1 calculations were performed to obtain the energies of the various configurations of 9-AA and 9-ACA in the ground (S0) as well as in the lowest excited singlet (S1) electronic state. The results suggest that the COOH group is oriented at an angle of ∼55° with respect to the aromatic ring in the S0 state in both the molecules. However, in the S1 state, it approaches coplanarity with the aromatic ring. The calculated bond lengths, charge densities, and dipole moments suggest that the resonance charge transfer from the aromatic ring to the COOH group increases in the S1 state of 9-AA. However, despite the decrease of twist angle of the COOH group, no significant charge transfer was observed in 9-ACA. The charge density data indicate that the ring nitrogen and the carbonyl oxygen of the COOH group become more basic upon electronic excitation.

Thermal curing of glass‐epoxy prepregs by luminescence spectroscopy

“…The photophysical behavior of the 9‐AA probe was strongly dependent on the medium polarity and concentration 39, 40. The carboxylic group of 9‐AA in the monomeric form had a perpendicular plane in relation to the anthracene ring in the ionized form in both electronic states, ground and excited; this resulted in an anthracene‐like emission 39–43. The 9‐AA broad emission band was attributed to the 9‐AA protonated form,4, 41, 44 which presented a rotation of the carboxyl group in the excited state into a position approaching coplanarity with the anthracene ring.…”

Efeito higrotérmico em prepregs de fibra de vidro/epóxi por espectroscopia de luminescência

“…Pode ser observado que o espectro de emissão do prepreg F-161 Referência, conforme mostra a curva a da Figura 1, apresenta um ombro em 421 nm sobreposto a uma banda não-estruturada com o máximo de emissão em 447 nm, que pode ser atribuída à existência das formas em equilíbrio, ionizada e protonada do 9-AA no meio [21][22][23] . sistência mecânica à tração, resultantes da microdeformação das fibras e ruptura prematura.…”

“…A fonte de excitação utilizada foi uma lâmpada de Xenônio (Xe900 -450W -Osram Lamp) e monocromadores de excitação e de emissão do tipo Czerny-Turner providos de dupla grade holográfica de difração e um fotomultiplicador no modo de apresentam mudanças espectrais significativas em relação ao espectro de emissão da amostra "Referência", que, como discutido anteriormente, pode ser atribuída à existência da forma ionizada e protonada do 9-AA no meio [21][22][23] . O espectro de fluorescência do 9-AA da amostra submetida à umidade relativa por 15 dias, conforme mostra a curva c, apresenta uma banda de emissão em 454 nm, atribuída à forma protonada do 9-AA.…”

“…O grupo carboxilato do 9-AA assume uma configuração espacial perpendicular ao plano do anel antracênico na forma ionizada, tanto no estado eletrônico fundamental como no estado excitado, mantendo a mesma geometria [21,[24][25][26] . A banda não-estruturada do 9-AA pode ser atribuída à emissão da forma protonada do 9-AA [21][22][23] , uma vez que o grupo carboxilato adquire uma posição coplanar ao anel antracênico, fazendo com que ocorra uma mudança significativa da geometria da molécula no estado eletrônico excitado. Pode-se supor que quando aumenta a concentração da sonda luminescente, ocorre um deslocamento do equilí-brio, conforme mostra a Figura 2 no sentido de formação da forma protonada do 9-AA.…”

The use of 9-anthroic acid and new amide derivatives to monitorize curing of epoxy resins

“…9-Anthroic acid (AA) and its ester derivatives [16][17][18][19][20][21][22], are common fluorescenc dyes. In this work, we study amide derivatives of anthroic acid.…”