Influence of Electron-Donating and Electron-Withdrawing Groups on One- and Two-Photon Absorption Properties on Modified Adenine and Guanine Nucleobases

Fluorescent nucleobase analogues are valuable probes for nucleic acid structure and dynamics, yet rational design remains challenging. Here, we investigate one- and two-photon absorption (1PA and 2PA) properties of 66 modified adenine and guanine derivatives using linear response for 1PA and quadratic response theory combined with the two-state model (2SM) for 2PA, employing RI-CC2 and time-dependent density functional theory (CAM-B3LYP and MN15) approaches. Each system includes either electron-donating groups (EDGs: CH 3, NH 2, OCH 3, OH) or electron-withdrawing groups (EWGs: CF 3, NO 2, COOH, CN) attached at different positions on a phenyl ring appended to the parent adenine or guanine nucleobase. Phenylation enhances the brightness of the parent nucleobases and red-shifts the excitation wavelengths, λ 1PA, by up to 47 nm (i.e., 94 nm for λ 2PA ), while EDG/EWG substitution induces further shifts of up to 113 nm in λ 1PA (i.e., 226 nm for λ 2PA ), thereby extending 2PA activity into the near-infrared region. The largest 2PA responses are associated with substantial permanent dipole changes upon excitation (|Δμ| = (μ 11, x – μ 00, x ) 2 + (μ 11, y – μ 00, y ) 2 + (μ 11, z – μ 00, z ) 2 1/2 ≈ 13 Debye) and near-colinear alignment between the change in the permanent dipole moment (Δ μ ) and the transition dipole moment (μ 01 ), cos 2 θ ≈ 1, as captured by the 2SM. While EDG substitution enhances the photophysical response relative to the parent nucleobases, EWG substitution leads to the most pronounced improvements.