The effect of oligomer length on the vibrational mode coupling and energy relaxation mechanisms of AT-rich DNA oligomers in double- and single-stranded conformations has been investigated using two-dimensional infrared spectroscopy. Vibrational coupling of modes of the DNA bases to the symmetric stretching vibration of the backbone phosphate group was observed for oligomers long enough to form duplex-DNA structures. The coupling was lost upon melting of the duplex. No significant effect of oligomer length or DNA secondary structure was found on either the timescale for vibrational relaxation of the base modes or the mechanism, which was consistent with a cascade process from base modes to intermediate modes, some of which are located on the deoxyribose group, and subsequently to the phosphate backbone. The study shows that vibrational coupling between base and backbone requires formation of the double-helix structure while vibrational energy management is an inherent property of the nucleotide.