Proton transfer reaction of 4-methyl-2,6-dicarbomethoxyphenol in polar aprotic solvents at room temperature and 77 K
Introduction
Proton transfer reaction in o-hydroxybenzaldehyde (OHBA), 4-methyl-2,6-diformylphenol (MFOH), 4-methyl-2,6-diacetylphenol (MAOH), 4-methyl-2,6-diamidophenol (MDOH) and 3-methyl-6-hydroxy-m-phthalic acid (HmPA) has been studied in detail [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Nagaoka et al. [7], [8] studied the spectral features and dynamics of some proton transfer processes in the S0 states as well as in different excited electronic states of OHBA and its derivatives. They proposed that benzaldehyde type of molecules without intramolecular hydrogen bonding (open conformer) are likely to be produced after irradiation at 77 K. They suggested that occurrence of phosphorescence is from the open conformer. MFOH, MAOH, MDOH and HmPA are chromophores structurally similar to OHBA and its derivatives. In our earlier work we have discussed the structures and dynamic processes of the excited states of such compounds in different nonpolar and polar solvents. We have detected intramolecularly hydrogen bonded closed conformer in the ground state and excited state intramolecular proton transfer (ESIPT) in nonpolar solvents in the case of all such compounds studied. In polar solvents we have detected both the closed conformer and anion in the case of MFOH and MDOH in the ground state. However, in the case of MAOH only the closed conformer is observed in the ground state. In our earlier work, we have detected phosphorescence spectra in the case of MFOH [1], [4] and MDOH [5], [11]. However, in the case of MAOH, we are unable to detect any phosphorescence spectra after irradiation [2], [10]. It is proposed that due to the presence of bulky alkyl groups attached to the carbonyl groups, formyl groups in MAOH are unable to rotate and hence, phosphorescence was not observed. The decay rates of MDOH and MFOH are relatively slow compared to those of MAOH [2], [10], OHBA [7], [8] and methylsalicylate (MS) [12], [13]. It is suggested that this difference in transfer rates is due to difference in the acidic character of the S1 states [11].
Nagaoka et al. [7], [8], [14] have shown that, for OHBA and o-hydroxyacetophenone (OHAP), the main species in protic solvents is the hydrogen bonded open conformer. In nonpolar solvents, on the other hand these compounds show excited state intramolecular proton transfer (ESIPT) due to the formation of enol tautomer. Acuna et al. [15], [16] have proposed that the single broad fluorescence band observed in the case of MS and OHBA, is due to a zwitterionic structure. In a similar study Smith and Kaufmann [17] pointed out that the formation of a zwitterion is due to the change in acidic properties of MS. Acid–base type interactions are dependent on the strength of the acid and base and the nature of the solvent used. Added base may produce different species from that observed in pure solvents.
In this present work we have examined the spectral properties of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) in some polar aprotic solvents at room temperature and 77 K. We report here the results of both the steady state and time resolved kinetic measurements. We have compared the results of CMOH with those of MFOH obtained in polar aprotic solvents. Some experiments are also carried out to examine the effect of added acid and base.
Section snippets
Experimental
Like MFOH and MAOH, CMOH was prepared in the department of inorganic chemistry of this institute as described earlier [18]. The compound was recrystallised from methanol and dried before use. Spectroscopic grade solvents were received from Sigma and Aldrich Chemical. The base, analytical grade triethylamine (TEA) was used as received from E. Marck. All solvents were checked for purity by both steady state fluorescence and by the nanosecond apparatus. The relative quantum yields were measured
Room-temperature spectra
The absorption spectra of both CMOH show a single band at 322 nm in all the aprotic solvents studied here except in DMF and DMSO. On the other hand absorption spectra of MFOH show a single band at 350 nm in 3-methylpentane (3MP) and acetonitrile (ACN). Moreover, in the case of MFOH another absorption band appeared at 480 and 460 nm in DMSO and DMF, respectively, in addition to 350 nm band. Like MFOH, the absorption spectra of CMOH can be assigned as due to the intramolecularly hydrogen bonded
Conclusion
Among MFOH, MAOH and CMOH we are able to detect the emission of the closed conformer (another rotamer) only in the case of CMOH. The ESIPT band of CMOH is observed only in 3MP and ACN among all the solvents studied here. Both the emissions of anion and intermolecularly bonded complex can be observed simply by changing the excitation wavelength. In the ground state solute–solvent interaction results in a number of isomers in polar aprotic solvents. In the excited state both anion and open
Acknowledgements
The authors express their thanks to Prof. S.P. Bhattacharyya of IACS for his constructive suggestions on theoretical work. The Department of Spectroscopy of IACS is gratefully acknowledged for the low-temperature experiments.
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