Solvent‐Controlled Intermolecular Proton‐Transfer Follows an Irreversible Eigen‐Weller Model from fs to ns

Intermolecular Proton Transfer (PT) dynamics can be best studied by optical spectroscopy, which can cover the vast timescale spanned by the process. PT in a hydrogen bonding complex between a pyranine-based photoacid and a trialkyl-phosphine oxide is addressed. The photoreaction is traced with the help of femtosecond transient absorption and picosecond-resolved fluorescence. Characteristic kinetics and spectra of the intervening species are isolated by global analysis and spectral decomposition of time-resolved fluorescence. It is found that the shared proton shifts towards the phosphine site upon photoexcitation in acetonitrile. The process occurs on the sub-picosecond timescale, essentially, under solvent control. Despite the ultrafast rate, an equilibrium between the complex and the hydrogen-bonded ion pair (HBIP) is established. Further reaction steps are delayed to the nanosecond timescale, where formation of the excited deprotonated form is observed. The far-reaching consistency between the various methods supports an irreversible Eigen-Weller mechanism in the excited state

Solvent-Controlled Intermolecular Proton-Transfer Follows an Irreversible Eigen-Weller Model from fs to ns

Intermolecular Proton Transfer (PT) dynamics can be best studied by optical spectroscopy, which can cover the vast timescale spanned by the process. PT in a hydrogen bonding complex between a pyranine-based photoacid and a trialkyl-phosphine oxide is addressed. The photoreaction is traced with the help of femtosecond transient absorption and picosecond-resolved fluorescence. Characteristic kinetics and spectra of the intervening species are isolated by global analysis and spectral decomposition of time-resolved fluorescence. It is found that the shared proton shifts towards the phosphine site upon photoexcitation in acetonitrile. The process occurs on the sub-picosecond timescale, essentially, under solvent control. Despite the ultrafast rate, an equilibrium between the complex and the hydrogen-bonded ion pair (HBIP) is established. Further reaction steps are delayed to the nanosecond timescale, where formation of the excited deprotonated form is observed. The far-reaching consistency between the various methods supports an irreversible Eigen-Weller mechanism in the excited state

Femtosecond pump/supercontinuum-probe spectroscopy: Optimized setup and signal analysis for single-shot spectral referencing

A setup for pump/supercontinuum-probe spectroscopy is described which (i) is optimized to cancel fluctuations of the probe light by single-shot referencing, and (ii) extends the probe range into the near-uv (1000–270 nm). Reflective optics allow 50 μm spot size in the sample and upon entry into two separate spectrographs. The correlation γsame between sample and reference readings of probe light level at every pixel exceeds 0.99, compared to γconsec<0.92 reported for consecutive referencing. Statistical analysis provides the confidence interval of the induced optical density, ΔOD. For demonstration we first examine a dye (Hoechst 33258) bound in the minor groove of double-stranded DNA. A weak 1.1 ps spectral oscillation in the fluorescence region, assigned to DNA breathing, is shown to be significant. A second example concerns the weak vibrational structure around t=0 which reflects stimulated Raman processes. With 1% fluctuations of probe power, baseline noise for a transient absorption spectrum becomes 25 μOD rms in 1 s at 1 kHz, allowing to record resonance Raman spectra of flavine adenine dinucleotide in the S0 and S1 stateWe are grateful to the Deutsche Forschungsgemeinschaft for support (SFB 450 and Cluster of Excellence “Unifying Concepts in Catalysis”)S

Primary events in the blue light sensor plant cryptochrome: intraprotein electron and proton transfer revealed by femtosecond spectroscopy

Immeln D, Weigel A, Kottke T, Pérez Lustres JL. Primary events in the blue light sensor plant cryptochrome: intraprotein electron and proton transfer revealed by femtosecond spectroscopy. Journal of the American Chemical Society. 2012;134(30):12536-12546.Photoreceptors are chromoproteins that undergo fast conversion from dark to signaling states upon light absorption by the chromophore. The signaling state starts signal transduction in vivo and elicits a biological response. Therefore, photoreceptors are ideally suited for the analysis of protein activation by time-resolved spectroscopy. We focus on plant cryptochromes which are blue light sensors regulating the development and daily rhythm of plants. The signaling state of these flavoproteins is the neutral radical of the flavin chromophore. It forms on the microsecond timescale after light absorption by the oxidized state. We apply here femtosecond broadband transient absorption to early stages of signaling-state formation in an algal plant cryptochrome. Transient spectra show: i) sub-ps decay of flavin stimulated emission and ii) further decay of signal until 100 ps delay with nearly constant spectral shape.i) monitors electron transfer from a nearby tryptophan to the flavin and occurs with a time constant of τ(ET)=0.4 ps. ii) is analyzed by spectral decomposition and occurs with a characteristic time constant τ(1)=31 ps. We reason that hole transport through a tryptophan triad to the protein surface and partial deprotonation of tryptophan cation radical hide behind τ(1). These processes are probably governed by vibrational cooling. Spectral decomposition is used together with anisotropy to obtain the relative orientation of flavin and the final electron donor. This narrows the number of possible electron donors down to two tryptophans. Structural analysis suggests that a set of histidines surrounding the terminal tryptophan may act as proton acceptor and thereby stabilize the radical pair on a 100 ps timescale

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer

As narrow optical bandgap materials, semiconducting single-walled carbon nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced charge-transfer (PCT) reactions. However, the unique band structure and unusual exciton dynamics of SWCNTs add more possibilities to the classical PCT mechanism. In this work, we demonstrate PCT from photoexcited semiconducting (6,5) SWCNTs to a wide-bandgap wrapping poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(6,6′)-(2,2′-bipyridine)] (PFO–BPy) via femtosecond transient absorption spectroscopy. By monitoring the spectral dynamics of the SWCNT polaron, we show that charge transfer from photoexcited SWCNTs to PFO–BPy can be driven not only by the energetically favorable E$_{33}$ transition but also by the energetically unfavorable E$_{22}$ excitation under high pump fluence. This unusual PCT from narrow-bandgap SWCNTs toward a wide-bandgap polymer originates from the up-converted high-energy excitonic state (E$_{33}$ or higher) that is promoted by the Auger recombination of excitons and charge carriers in SWCNTs. These insights provide new pathways for charge separation in SWCNT-based photodetectors and photovoltaic cells