Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy

Abstract

The anti-Kasha process provides the possibility of using high-energy excited states to develop novel applications. Our previous research (Nature communications, 2020, 11, 793) has demonstrated a dual-emission anti-Kasha-active fluorophore for bioimaging application, which exhibits near-infrared emissions from the S 1 state and visible anti-Kasha emissions from the S 2 state. Here, we applied tunable blue-side femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy, assisted by quantum calculations, to reveal the anti-Kasha dual emission mechanism, in which the emergence of two fluorescing states is due to the retardation of internal conversion from the S 2 state to the S 1 state. It has been demonstrated that the facts of anti-Kasha high-energy emission are commonly attributed to a large energy gap between the two excited states, leading to a decrease in the internal conversion rate due to a poor Franck−Condon factor. In this study, analysis of the calculation and FSRS experimental results provide us further insight into the dual-emission anti-Kasha mechanism, where the observation of hydrogen out-of-plane Raman modes from FSRS suggested that, in addition to the energy-gap law, the initial photoinduced molecular conformational change plays a key role in influencing the rate of internal conversion.

Ultrafast Photophysics of Multiple-Resonance Ultrapure Blue Emitters

“…Species II exhibits slightly red-shifted and enhanced SAS due to the involvement of the SE band. Therefore, the I → II transition can be attributed to the rapid internal conversion (IC) from the initial populated S n to the lowest-lying fluorescent S 1 state (S n → S 1 ). − …”

Mapping the Complete Photocycle that Powers a Large Stokes Shift Red Fluorescent Protein

“…Since strong geometric constraints and a well‐defined H‐ bonding network inside the protein pocket limit the chromophore motions, the photoisomerization of an embedded chromophore likely undergoes some space‐conserving hula‐twist motions to minimize the energetic cost of isomerization through the excited‐ state potential energy surface (PES) [15] . Normally, hydrogen‐out‐of‐plane (HOOP) vibrations are highly sensitive to the restoring force exerted by the bridge C4=C5 bond during the chromophore twist (see Figure S15 for atomic numbering), and we identified a HOOP mode in the low‐frequency range [7h, 16] (Figure 5a, b) with an intensity increase at ∼860 cm −1 at pH/pD 7 (Figure 5c). Aided by DFT (for trans ‐I) and TD‐DFT (for trans ‐I*) calculations, this specific vibrational mode was attributed to the bridge C5−H6 HOOP along with P‐ring HOOP motions of trans ‐isomer (Figure S15).…”

Investigation of Ultrafast Configurational Photoisomerization of Bilirubin Using Femtosecond Stimulated Raman Spectroscopy

“…Here, the two weak Raman peaks at 910 and 866 cm –1 are assigned to the hydrogen out-of-plane (HOOP) wagging motion of vinyl C15–H and C5–H, respectively (Figures S14 and S15), indicating the rotation of the exocyclic C4C5 and C15C16 bonds. Notably, the CC-bond torsional motions are shown to couple to a new coordinate of the HOOP mode, the role of which in isomerization has been identified theoretically and experimentally. − The implication of HOOP in the chromophore’s isomerization has been confirmed by using FSRS in serval biological and chemical systems. ,− As shown in Figure e and Figure S16, the characteristics of the two HOOP modes are in striking contrast to those of the aforementioned Raman modes; that is, the transient amplitude is independent of viscosity and exhibits only a single-exponential decay with a time constant of ∼300 fs in both solvents, and the transient frequency red-shift is insensitive to viscosity (Figure f and Figure S16), which indicates a conformational change in a certain small defined volume by rotation of the C4C5 and C15C16 bonds.…”

“…In this study, we strategically combined tunable femtosecond stimulated Raman spectroscopy (FSRS), − transient absorption (TA), − and femtosecond fluorescence upconversion spectroscopy to comprehensively unravel the photoisomerization events of the ZZ-BR molecule in organic solvents. The detailed configurational photoisomerization reaction pathways for ZZ-BR were dissected to reveal an initial volume-conserving “hula twist” of the C5/C15 methine bridge, followed by distortion of intramolecular hydrogen bonds and rotation of the two dipyrrinone halves pivoted on the C10 methylene bridge of the ZZ-BR isomer to eventually generate the nonradiative intermediate state I** in a few picoseconds.…”