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Steady-state and time-resolved emission techniques were employed to study the photophysical properties of 5-(thien-2-yl)-2'-deoxyuridine (dUTh), an isomorphic fluorescent nucleoside analog. We found that the emission lifetime of dUTh is dependent upon the solvent viscosity and obeys the F\"orster-Hoffman relation over a wide range of temperatures in 1-propanol, a glass-forming liquid. Upon incorporation into oligodeoxynucleotides, the average emission lifetime significantly increases, and the decay is non-exponential. We use a non-radiative decay model that takes into account the twist angle of the thiophene ring to fit the time-resolved emission decay curves., Comment: 22 pages, 10 figures, 8 tables