The UV-visible absorption of halogenated anthracenyl ketones is dominated by the absorption characteristic of the anthracene moiety due to pi to pi* transitions. In certain media e.g. methanol, this absorption also exhibits the vibronic fine structure characteristic of anthracene. These vibronic bands represent the aromatic ring breathing vibrational modes. The lambda max of aromatic pi-pi* absorption is well known to red shift as solvent polarity increases. However, for anthracenyl ketones the lambda max blue shifts when solvent polarity increases and the solvent is a hydrogen bonding solvent such as methanol. This shift is as much as 18 nm (chloroform to methanol) for 10-chloro-9-anthraldehyde. This molecule can hydrogen bond with methanol via the unshared electron pairs on the carbonyl oxygen. This intermolecular hydrogen bonding is the apparent reason for the observed blue shift. Anthracene derivatives with nitrogen atoms capable of hydrogen bonding have recently been reported to blue shift by a few nanometers in alcoholic solvents presumably also due to hydrogen bonding (J. Phys. Chem., 2004, 108, 7843-7852). In this study, selected parameters of the vibronic fine structure of the anthracenyl moiety of halogenated aromatic ketones were measured in a variety of hydrogen bonding media. Other molecules studied included 9-trifluoromethylanthryl ketone and 9-anthraldehyde. Parameters measured included lambda max (0-0 transition), band width, band separation, band shape and molar extinction coefficient. These parameters were then compared to determine which parameters would be most sensitive to monitor hydrogen bonding via the carbonyl.
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