The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-... more The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic ...
Fused polythiophenes attached to Fe(ii)NHC complexes induced a mixing of the t2g and π HOMO orbit... more Fused polythiophenes attached to Fe(ii)NHC complexes induced a mixing of the t2g and π HOMO orbitals promoting 80 nm red-shift of the MLCT transition, 3 fold increase of the molar extinction coefficient and 3MLCT excited state lifetime up to 18 ps.
Boosting iron(ii) complex excited-state lifetime by combining pyrazine and benzimidazolylidene NH... more Boosting iron(ii) complex excited-state lifetime by combining pyrazine and benzimidazolylidene NHC ligands.
Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of t... more Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal-ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO 2. In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.
We demonstrate the implementation of a broadband fluorescence up-conversion set-up with high sign... more We demonstrate the implementation of a broadband fluorescence up-conversion set-up with high signal-to-noise ratio and dynamic range allowing for the detection of weak luminescence from triplet states in Fe(II) NHC complexes. Based on the experimentally determined radiative rates and the emission spectra, these states have dominant MLCT character.
Ultrafast transient absorption spectroscopy reveals the effect of chemical substitutions on the p... more Ultrafast transient absorption spectroscopy reveals the effect of chemical substitutions on the photoreaction kinetics of biomimetic photoswitches displaying coherent dynamics. Ground state vibrational coherences are no longer observed when the excited state lifetime exceeds 300fs.
Modulations in the time gated ultrafast fluorescence of the protonated Schiff base of retinal in ... more Modulations in the time gated ultrafast fluorescence of the protonated Schiff base of retinal in various solvents are reported, which reflect the creation and evolution of vibrational wave packets due to the torsional mode of retinal of frequency $120 cm À1. The oscillations are damped in $400 fs. Their frequency is significantly decreased compared to the case of the protein (170 cm À1), and is insensitive to solvent properties such as the dielectric constant and the viscosity. This, along with previous results on mutants of bacteriorhodopsin, leads us to conclude that in the protein, the isomerization dynamics of retinal is governed by steric effects and by the inhomogeneous distribution of the electrostatic field due to amino-acid residues within the protein pocket. We also discuss the origin of the modulations and conclude that they are impulsively excited via the high frequency modes of retinal.
Physical chemistry chemical physics : PCCP, Jan 23, 2015
Anabaena Sensory Rhodopsin (ASR) stands out among the microbial retinal proteins in that, under l... more Anabaena Sensory Rhodopsin (ASR) stands out among the microbial retinal proteins in that, under light-adaptation (LA) conditions, it binds both the 13-cis isomer and the all-trans isomer of the protonated Schiff base of retinal (PSBR). In the dark-adapted (DA) state, more than 95% of the proteins bear all-trans PSBR, and the protein environment adopts a different equilibrium state. We report the excited state and photo-isomerization kinetics of ASR under different LA conditions. The full data set allows confirming that the photoisomerization of the 13C isomer occurs within 100 fs and indications of an excited and ground state wavepacket launched by the ultrafast non-adiabatic reaction are reported. Even though this recalls the record isomerization time and the coherent reaction scenario of 11-cis PSBR in rhodopsin, the photoisomerization quantum yield (QY) is much lower, actually the lowest value ever reported for retinal proteins (<15%). Noticeably, in ASR the excited state life...
The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-t... more The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that—despite the geometrical improvement—the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent ...
Physical chemistry chemical physics : PCCP, May 18, 2016
Herein we report the synthesis and time-resolved spectroscopic characterization of a homoleptic F... more Herein we report the synthesis and time-resolved spectroscopic characterization of a homoleptic Fe(ii) complex exhibiting a record (3)MLCT lifetime of 26 ps promoted by benzimidazolylidene-based ligands. Time dependent density functional molecular modeling of the triplet excited state manifold clearly reveals that, at equilibrium geometries, the lowest (3)MC state lies higher in energy than the lowest (3)MLCT one. This unprecedented energetic reversal in a series of iron complexes, with the stabilization of the charge-transfer state, opens up new perspectives towards iron-made excitonic and photonic devices, hampering the deactivation of the excitation via metal centered channels.
A 40-nm broad pulse centred at 320nm is produced from an amplified Yb-doped fiber laser operated ... more A 40-nm broad pulse centred at 320nm is produced from an amplified Yb-doped fiber laser operated at 50kHz, and used in a BOXCARS geometry setup for 2DES, with shot-to-shot monitoring of the relative optical phase stability.
Dimitra Markovitsi replied: Your papers have indeed been very stimulating for our studies. Howeve... more Dimitra Markovitsi replied: Your papers have indeed been very stimulating for our studies. However, in our experiments, using much lower excitation intensities (<4 mW cm À2) than yours (>4 mW cm À2), we found that the one-photon ionization of mono-nucleosides and mono-nucleotides is lower than 3 Â 10 À4 , which corresponds to our detection limit. In contrast, when using similar excitation intensities to those reported in your 2002 paper we do observe ionization from the buffer. The fact that we observe one-photon ionization only for oligomers Faraday Discussions Discussions
Physical chemistry chemical physics : PCCP, Jan 3, 2016
The first combined theoretical and photovoltaic characterization of both homoleptic and heterolep... more The first combined theoretical and photovoltaic characterization of both homoleptic and heteroleptic Fe(ii)-carbene sensitized photoanodes in working dye sensitized solar cells (DSSCs) has been performed. Three new heteroleptic Fe(ii)-NHC dye sensitizers have been synthesized, characterized and tested. Despite an improved interfacial charge separation in comparison to the homoleptic compounds, the heteroleptic complexes did not show boosted photovoltaic performances. The ab initio quantitative analysis of the interfacial electron and hole transfers and the measured photovoltaic data clearly evidenced fast recombination reactions for heteroleptics, even associated with un unfavorable directional electron flow, and hence slower injection rates, in the case of homoleptics. Notably, quantum mechanics calculations revealed that deprotonation of the not anchored carboxylic function in the homoleptic complex can effectively accelerate the electron injection rate and completely suppress the...
Technical Digest. Summaries of Papers Presented at the International Quantum Electronics Conference. Conference Edition. 1998 Technical Digest Series, Vol.7 (IEEE Cat. No.98CH36236), 1998
Abstract Intersubband scattering is a process of great importance, especially for the understandi... more Abstract Intersubband scattering is a process of great importance, especially for the understanding of the possible performances of devices such as quantum cascade lasers. The involved time scales are relatively well understood in the case of intersubband separation larger than the energy of the optical phonons and correspond well to the computed interaction with LO phonons. In the case of intersubband separation smaller than the energy of the LO phonon, the results are quite widespread (from 1 ps to 1 ns). We ...
Abstract A dynamical theory of multiphoton transitions in semiconductors is developed using a den... more Abstract A dynamical theory of multiphoton transitions in semiconductors is developed using a density matrix approach. It is shown that, in dipole approximation, these transitions require a mixed parity of the basis states and that band-diagonal couplings of the electromagnetic field to Bloch electrons have to be included. The general theory is outlined using the simple, but solvable model of a diatomic tight-binding chain. Possible extensions and applications of the approach are discussed.
Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of ele... more Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing between the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carriercarrier interaction is responsible for the ...
The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-... more The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic ...
Fused polythiophenes attached to Fe(ii)NHC complexes induced a mixing of the t2g and π HOMO orbit... more Fused polythiophenes attached to Fe(ii)NHC complexes induced a mixing of the t2g and π HOMO orbitals promoting 80 nm red-shift of the MLCT transition, 3 fold increase of the molar extinction coefficient and 3MLCT excited state lifetime up to 18 ps.
Boosting iron(ii) complex excited-state lifetime by combining pyrazine and benzimidazolylidene NH... more Boosting iron(ii) complex excited-state lifetime by combining pyrazine and benzimidazolylidene NHC ligands.
Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of t... more Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal-ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO 2. In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.
We demonstrate the implementation of a broadband fluorescence up-conversion set-up with high sign... more We demonstrate the implementation of a broadband fluorescence up-conversion set-up with high signal-to-noise ratio and dynamic range allowing for the detection of weak luminescence from triplet states in Fe(II) NHC complexes. Based on the experimentally determined radiative rates and the emission spectra, these states have dominant MLCT character.
Ultrafast transient absorption spectroscopy reveals the effect of chemical substitutions on the p... more Ultrafast transient absorption spectroscopy reveals the effect of chemical substitutions on the photoreaction kinetics of biomimetic photoswitches displaying coherent dynamics. Ground state vibrational coherences are no longer observed when the excited state lifetime exceeds 300fs.
Modulations in the time gated ultrafast fluorescence of the protonated Schiff base of retinal in ... more Modulations in the time gated ultrafast fluorescence of the protonated Schiff base of retinal in various solvents are reported, which reflect the creation and evolution of vibrational wave packets due to the torsional mode of retinal of frequency $120 cm À1. The oscillations are damped in $400 fs. Their frequency is significantly decreased compared to the case of the protein (170 cm À1), and is insensitive to solvent properties such as the dielectric constant and the viscosity. This, along with previous results on mutants of bacteriorhodopsin, leads us to conclude that in the protein, the isomerization dynamics of retinal is governed by steric effects and by the inhomogeneous distribution of the electrostatic field due to amino-acid residues within the protein pocket. We also discuss the origin of the modulations and conclude that they are impulsively excited via the high frequency modes of retinal.
Physical chemistry chemical physics : PCCP, Jan 23, 2015
Anabaena Sensory Rhodopsin (ASR) stands out among the microbial retinal proteins in that, under l... more Anabaena Sensory Rhodopsin (ASR) stands out among the microbial retinal proteins in that, under light-adaptation (LA) conditions, it binds both the 13-cis isomer and the all-trans isomer of the protonated Schiff base of retinal (PSBR). In the dark-adapted (DA) state, more than 95% of the proteins bear all-trans PSBR, and the protein environment adopts a different equilibrium state. We report the excited state and photo-isomerization kinetics of ASR under different LA conditions. The full data set allows confirming that the photoisomerization of the 13C isomer occurs within 100 fs and indications of an excited and ground state wavepacket launched by the ultrafast non-adiabatic reaction are reported. Even though this recalls the record isomerization time and the coherent reaction scenario of 11-cis PSBR in rhodopsin, the photoisomerization quantum yield (QY) is much lower, actually the lowest value ever reported for retinal proteins (<15%). Noticeably, in ASR the excited state life...
The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-t... more The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes (C1 and C2), prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For C1, it is shown that—despite the geometrical improvement—the excited state deactivation is faster than for the parent pseudo-octahedral C0 complex. This unexpected result is due to the increased ligand flexibility in C1 that lowers the energetic barrier for the relaxation of 3MLCT into the 3MC state. For C2, the effect of the increased ligand field is not strong enough to close the prominent ...
Physical chemistry chemical physics : PCCP, May 18, 2016
Herein we report the synthesis and time-resolved spectroscopic characterization of a homoleptic F... more Herein we report the synthesis and time-resolved spectroscopic characterization of a homoleptic Fe(ii) complex exhibiting a record (3)MLCT lifetime of 26 ps promoted by benzimidazolylidene-based ligands. Time dependent density functional molecular modeling of the triplet excited state manifold clearly reveals that, at equilibrium geometries, the lowest (3)MC state lies higher in energy than the lowest (3)MLCT one. This unprecedented energetic reversal in a series of iron complexes, with the stabilization of the charge-transfer state, opens up new perspectives towards iron-made excitonic and photonic devices, hampering the deactivation of the excitation via metal centered channels.
A 40-nm broad pulse centred at 320nm is produced from an amplified Yb-doped fiber laser operated ... more A 40-nm broad pulse centred at 320nm is produced from an amplified Yb-doped fiber laser operated at 50kHz, and used in a BOXCARS geometry setup for 2DES, with shot-to-shot monitoring of the relative optical phase stability.
Dimitra Markovitsi replied: Your papers have indeed been very stimulating for our studies. Howeve... more Dimitra Markovitsi replied: Your papers have indeed been very stimulating for our studies. However, in our experiments, using much lower excitation intensities (<4 mW cm À2) than yours (>4 mW cm À2), we found that the one-photon ionization of mono-nucleosides and mono-nucleotides is lower than 3 Â 10 À4 , which corresponds to our detection limit. In contrast, when using similar excitation intensities to those reported in your 2002 paper we do observe ionization from the buffer. The fact that we observe one-photon ionization only for oligomers Faraday Discussions Discussions
Physical chemistry chemical physics : PCCP, Jan 3, 2016
The first combined theoretical and photovoltaic characterization of both homoleptic and heterolep... more The first combined theoretical and photovoltaic characterization of both homoleptic and heteroleptic Fe(ii)-carbene sensitized photoanodes in working dye sensitized solar cells (DSSCs) has been performed. Three new heteroleptic Fe(ii)-NHC dye sensitizers have been synthesized, characterized and tested. Despite an improved interfacial charge separation in comparison to the homoleptic compounds, the heteroleptic complexes did not show boosted photovoltaic performances. The ab initio quantitative analysis of the interfacial electron and hole transfers and the measured photovoltaic data clearly evidenced fast recombination reactions for heteroleptics, even associated with un unfavorable directional electron flow, and hence slower injection rates, in the case of homoleptics. Notably, quantum mechanics calculations revealed that deprotonation of the not anchored carboxylic function in the homoleptic complex can effectively accelerate the electron injection rate and completely suppress the...
Technical Digest. Summaries of Papers Presented at the International Quantum Electronics Conference. Conference Edition. 1998 Technical Digest Series, Vol.7 (IEEE Cat. No.98CH36236), 1998
Abstract Intersubband scattering is a process of great importance, especially for the understandi... more Abstract Intersubband scattering is a process of great importance, especially for the understanding of the possible performances of devices such as quantum cascade lasers. The involved time scales are relatively well understood in the case of intersubband separation larger than the energy of the optical phonons and correspond well to the computed interaction with LO phonons. In the case of intersubband separation smaller than the energy of the LO phonon, the results are quite widespread (from 1 ps to 1 ns). We ...
Abstract A dynamical theory of multiphoton transitions in semiconductors is developed using a den... more Abstract A dynamical theory of multiphoton transitions in semiconductors is developed using a density matrix approach. It is shown that, in dipole approximation, these transitions require a mixed parity of the basis states and that band-diagonal couplings of the electromagnetic field to Bloch electrons have to be included. The general theory is outlined using the simple, but solvable model of a diatomic tight-binding chain. Possible extensions and applications of the approach are discussed.
Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of ele... more Using femtosecond resonant luminescence, we have measured the intersubband scattering rate of electrons in wide GaAs quantum wells at very low excitation densities. Even when the spacing between the first two electron subbands is smaller than the LO phonon energy, we observe that intersubband scattering is a subpicosecond process, much faster than previously measured or anticipated. Our experimental results are in perfect agreement with Monte Carlo calculations, which show that carriercarrier interaction is responsible for the ...
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Papers by Stefan Haacke