Unusual spectral shifts in the prototropic reactions of 2,7-diaminofluorene

Abstract

R. MANOHARAN and SNEH K. DOGRA. Can. J. Chem. 65, 2013Chem. 65, (1987. The red shift observed in the absorption spectrum of 2,7-diaminofluorene (DAF) in ether and acetonitrile is either due to the proton-donating capacity of the solute or due to its dispersive interactions with the solvent and a blue shift in methanol and water is due to the proton-accepting nature of DAF. DAF acts as a proton donor in S1 state in all solvents. The red shift in the fluorescence spectrum of the monocation of DAF relative to that of DAF is not because of the -NH2 group becoming more basic but because of the large solvent relaxating in aqueous medium. On the other hand, the monocation of DAF in cyclohexane follows the normal blue shift in the fluorescence spectrum. pK, values for the various prototropic reactions in So and SI states are determined and discussed.R. MANOHARAN et SNEH K. DOGRA. Can. J. Chem. 65, 2013Chem. 65, (1987. Lorsqu'on determine les spectres d'absorption du diamino-2,7 fluorkne (DAF) dans l'kther et dans l'acktonitrile, on observe un deplacement vers le rouge; ce resultat est dC1 soit a la capacite du solutC a donner des protons ou aux interactions dispersives du solvant. Par ailleurs, on observe un deplacement vers le bleu lorsque les spectres sont determines dans le mCthanol et dans l'eau; ces resultats sont dus a la nature du DAF qui a tendance a accepter des protons. Dans tous les solvants, le DAF agit comme donneur de proton dans 1'Ctat S, . Si on compare le spectre de fluorescence du monocation du DAF avec celui du DAF, on observe un dCplacement vers le rouge qui n'est pas dC1 au fait que le groupement -NH2 devient plus basique mais plutbt a la relaxation importante du solvant dans le milieu aqueux. Par ailleurs, le spectre de fluorescence du monocation du DAF dans le cyclohexane subit le dkplacement normal vers le bleu. On a determink et on discute des valeurs de pK, pour diverses reactions prototropiques dans les Ctats So et S,.[Traduit par la revue]During electronic transitions, the charge density changes at each atom of the molecule. Depending upon the nature of the transition as well as the nature of the molecule itself. the molecule may become less polar or more polar in the excited state as compared to the ground state. Due to this, the solvent molecules rearrange themselves in the S1 state, which is different from the ground state. Since the rearrangement of the solvent molecules involves bond breaking or bond formation, the time taken for this phenomenon can be compared with the period of vibrational motion and this is much smaller than the fluorescence lifetime. In other words, the fluorescence always occurs from the thermally equilibrated excited state even though the molecule, to begin with, is in the Franck-Condon state. The equilibrated state is of lower energy than the Franck-Condon state provided the polarity of the molecule changes (increases or decreases) and the solvent is polar (1, 2). Sometimes the stabilisation of the states, called solvent relaxation, is so large that either rev...

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