Three-State Fluorescence of a 2-Functionalized Pyrene-Based RNA Label

The pyrene-based RNA-fluorescence label 2-(2-pyrenylethynyl) adenosine (2PyA) shows triexponential fluorescence, which depends strongly on the excitation wavelength. Most strikingly, a structured, long-lived fluorescence is observed in solution at room temperature after excitation into the S₂ state, which is shifted hypsochromically by 30 nm compared to excitation into the S₁ state. This very unusual behavior is investigated in detail with steady-state and time-resolved emission spectroscopy, ultrafast transient absorption spectroscopy, and quantum chemical calculations with both wave functions (CC2-level) and density-functional theory (DFT). 2PyA is found to emit simultaneously from two different intramolecular charge transfer states (mesomeric and twisted, MICT and TICT) which are populated most efficiently via the S₁ state and a pyrene-like locally excited (LE) state. Rotational momentum derived from excess excitation energy is required to populate twisted LE configurations. Therefore, the LE state is most efficiently accessible via excitation to the S₂. The stabilization of the different substates is related to two distinct reaction coordinates: the adenine-pyrene distance and the adenine-pyrene tilt angle, respectively.