Hypochromic effect and electronic excitation energy transfer in Y_t(CH₂)_nadenine systems

Measurements of the UV absorption spectra of aqueous solutions of the bichromophore Y_t(CH₂)_nadenine systems (n = 2, 3, 5 or 6) and the corresponding solutions of 1-methyladenine were carried out. All absorption spectra were subjected to a gaussian component computer analysis and the values of hypochromicity were determined for the individual electronic transitions S₀ → S_n in the molecule under investigation. The strongest hypochromic effects were found in systems with n = 3. The relative fluorescence quantum yields of the systems were measured using an equimolar mixture of 1-methyl-Y_t and 9-methyladenine as a reference solution for two different wavenumbers of exciting light: ν₁ = 32 000 cm^-1 (where only 1-methyl-Y_t absorbs) and ν₂ = 38 000 cm^-1 (where both chromophores absorb). From these measurements, the efficiencies T of the energy transfer from adenine to the Y_t base were determined as 0.41, 0.81, 0.44 and 0.17 for systems with n = 2, 3, 5 or 6 respectively. The results obtained by measuring the hypochromism and the energy transfer are in good agreement.