The relaxation time of the free-exciton --> geometrically relaxed 2(1)A(g) state in polydiacetylene is determined as 65-80 fs, depending on the probe energy. The vibrational non-equilibrium in the relaxed 2(1)A(g) state is characterized by the modulations of instantaneous frequencies and amplitudes of C-C and C-C stretching modes with a 145-fs oscillation period. The frequency modulation is anti-phase between the two modes, and the amplitude and frequency are also anti-phase for both modes. These are explained by the coupling of the stretching modes through an in-plane bending mode with 230 cm(-1). This is the first real-time observation of dynamic vibrational mode coupling.