Photophysics of pyrene-labelled compounds of biophysical interest

Abstract

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py–NH–CO–C₂H₅, Py–NH–CO–Leu–Boc, Py–CH₂–NH–CO–C₂H₅, Py–CH₂–NH–CO–Leu–Boc, Py–CO–NH–C₃H₇, Py–CO–NH–Leu–OMe, Py–CH₂–CO–NH–C₃H₇, Py–CH₂–CO–NH–Leu–OMe and Py–C₃H₆–CO–NH–Leu–OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, k_r, and non-radiative decay constants of their S₁ states and the values of these constants correlated with the structures of the tethers. Non-radiative S₁ decay rates (mainly intersystem crossing to T₁) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between ε_max of the S₁–S₀ transition and either k_r or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the ‘normal’ radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S₁ states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S₁–T₁ electronic energy spacings of all the derivatives exceed the O₂ (¹Δ_g–³Σ⁻_g) electronic excitation energy of ca. 1 eV.