Molecular Photochemistry
Molecular Photochemistry
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Advanced ultrafast methods allow study of unique photo-reactive molecular systems in solution which decades after the advent of femtosecond photochemistry have remained obscure. We seek to clarify the details of archetypal light activated molecular processes in solution making use of sub 10 fs laser pulses for measurement of transient absorption and impulsive vibrational spectroscopy. Recent examples are :
Advanced ultrafast methods allow study of unique photo-reactive molecular systems in solution which decades after the advent of femtosecond photochemistry have remained obscure. We seek to clarify the details of archetypal light activated molecular processes in solution making use of sub 10 fs laser pulses for measurement of transient absorption and impulsive vibrational spectroscopy. Recent examples are :
Excimer formation in liquid benzene
Excimer formation in liquid benzene
Excimer formation following UV excitation of the neat liquid was believed to start from a liquid structure very similar to that in solid benzene. The relative orientation of adjacent molecules in the solid prohibits the collapse to a sandwich structure required for stabilization of the excited dimer. Accordingly the gradual rise over tens of ps of excimer absorption centered at 500 nm was previously assigned to slow increase in excimer concentration due to a time consuming reorientation of nearest neighbors to the necessary parallel geometry. In our recent study we observed
Excimer formation following UV excitation of the neat liquid was believed to start from a liquid structure very similar to that in solid benzene. The relative orientation of adjacent molecules in the solid prohibits the collapse to a sandwich structure required for stabilization of the excited dimer. Accordingly the gradual rise over tens of ps of excimer absorption centered at 500 nm was previously assigned to slow increase in excimer concentration due to a time consuming reorientation of nearest neighbors to the necessary parallel geometry. In our recent study we observed
1) A prompt rise component in excimer transient absorption much before any reorientation or restructuring of the solvent can take place.
1) A prompt rise component in excimer transient absorption much before any reorientation or restructuring of the solvent can take place.
2) Three pulse experiments where the evolving excimers are probed by a secondary “dump” pulse proved that the subsequent 4 fold increase in excimer absorption over ~50 ps is due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers which form excimers in a delayed fashion.
2) Three pulse experiments where the evolving excimers are probed by a secondary “dump” pulse proved that the subsequent 4 fold increase in excimer absorption over ~50 ps is due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers which form excimers in a delayed fashion.
3) Theoretical analysis of these results was conducted with the group of A. I. Krylov from USC. Electronic structure calculations based on recent liquid benzene structural data these showed that nearly all excited monomers in solution have at least one neighboring molecule which is oriented such as to constitute a nascent embryonic excimer. They confirmed that slow refinement of these imperfect excimers leads to a significant increase in this transition moment as observed in the pump-dump-probe experiments.
3) Theoretical analysis of these results was conducted with the group of A. I. Krylov from USC. Electronic structure calculations based on recent liquid benzene structural data these showed that nearly all excited monomers in solution have at least one neighboring molecule which is oriented such as to constitute a nascent embryonic excimer. They confirmed that slow refinement of these imperfect excimers leads to a significant increase in this transition moment as observed in the pump-dump-probe experiments.
ESS Iyer, A Sadybekov, O Lioubashevski, AI Krylov, S Ruhman, JPCA, 2017, 121 (9), 1962-1975.
Top: Absorption spectrum of benzene excimer. Bottom: Kinetics of excimer formation probed at 530 nm (2.34 eV). The plot shows the rise of the excimer absorption near the peak. The inset shows the first picosecond of delay.
Photochemistry of aqueous permanganate
Photochemistry of aqueous permanganate
Permanganate, is an iconic oxidizing agent which undergoes photochemical decomposition to produce MnO2- + O2 upon excitation in the UV-Vis. Yet the underlying photochemical dynamics are largely unclear. Pioneering photochemical experiments proposed reaction scheme I: hot permanganate ions thermally decompose in the ground state following rapid internal conversion. A subsequent study promoted an alternative scheme II - dissociation from a photochemically generated long-lived peroxo intermediate.
Permanganate, is an iconic oxidizing agent which undergoes photochemical decomposition to produce MnO2- + O2 upon excitation in the UV-Vis. Yet the underlying photochemical dynamics are largely unclear. Pioneering photochemical experiments proposed reaction scheme I: hot permanganate ions thermally decompose in the ground state following rapid internal conversion. A subsequent study promoted an alternative scheme II - dissociation from a photochemically generated long-lived peroxo intermediate.
We have recently followed photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Guided by electronic structure calculations conducted by A.I. Krylov and co-workers and observations from three pulse excitation experiments, we can now assign the majority of ultrafast changes in absorption to a cascade of non-radiative relaxation processes ending with the lowest Jahn-Teller component of the 3T1 triplet state . In conjunction with photodecomposition quantum yields obtained from the literature, these results demonstrate that aqueous permanganate photo-decomposition proceeds through a long-lived intermediate which is formed in parallel to the triplet in less than one ps upon UV absorption. The possibility that this is the postulated highly oxidative peroxo species, a fraction of which leads to the stable fragments, is discussed in the following publication:
We have recently followed photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Guided by electronic structure calculations conducted by A.I. Krylov and co-workers and observations from three pulse excitation experiments, we can now assign the majority of ultrafast changes in absorption to a cascade of non-radiative relaxation processes ending with the lowest Jahn-Teller component of the 3T1 triplet state . In conjunction with photodecomposition quantum yields obtained from the literature, these results demonstrate that aqueous permanganate photo-decomposition proceeds through a long-lived intermediate which is formed in parallel to the triplet in less than one ps upon UV absorption. The possibility that this is the postulated highly oxidative peroxo species, a fraction of which leads to the stable fragments, is discussed in the following publication:
O. S. Haggag, P. Malakar, P. Pokhilko, J. F. Stanton, A. I. Krylov, and S. Ruhman, “The elusive dynamics of aqueous permanganate photochemistry” Phys. Chem. Chem. Phys., 2020, 22, 10043-10055.
Excitonics in semicunducting materials
Excitonics in semicunducting materials
The fate of photon excitation energy in excess of the optical band gap in semiconductor nanocrystals (NC) is fundamentally interesting due to NCs unique level structure which starts with discreet atom-like and changes to quasi continuous states within a fraction of an eV of energy. NC exciton relaxation is thus unlike both these paradigms, and due to its briefness, requires ultrafast spectroscopy for its elucidation. Some recent examples of surprising discoveries in our lab are:
The fate of photon excitation energy in excess of the optical band gap in semiconductor nanocrystals (NC) is fundamentally interesting due to NCs unique level structure which starts with discreet atom-like and changes to quasi continuous states within a fraction of an eV of energy. NC exciton relaxation is thus unlike both these paradigms, and due to its briefness, requires ultrafast spectroscopy for its elucidation. Some recent examples of surprising discoveries in our lab are:
Spectator exciton effects on carrier relaxation in NCs
Spectator exciton effects on carrier relaxation in NCs
Hot multi excitons in nanocrystals are known to cool rapidly to the lowest available energy levels, as in bulk semiconductors. This conjecture has strangely not been directly tested. To do that we have recorded the effects of a single cold “spectator” exciton on the relaxation dynamics of a subsequently deposited hot counterpart. Results in CdSe/CdS nanodots show that a preexisting cold “spectator exciton” allows only half of the photoexcited electrons to relax directly to the band-edge. The rest are blocked in an excited quantum state due to conflicts in spin orientation. The latter fully relax in this sample only after ~25 ps, as the blocked electrons spins flip, prolonging the temporal window of opportunity for harvesting the retained energy more than 100 fold! Quantum chemical estimates of these rates in a model cluster prove they are well within bounds of feasibility. Common to all quantum confined nanocrystals, this process will delay cooling and impact the spectroscopic signatures of hot multi-excitons in all envisioned generation scenarios. As a demonstration in the graphic transient transmission spectra 1.2 ps after a low intensity above band edge pump pulse is compared for a sample without and with cold spectators. The 50% reduction in the bleaching signal at the 1S peak is obvious, and explained in the level schemes below the figure to the left.
Hot multi excitons in nanocrystals are known to cool rapidly to the lowest available energy levels, as in bulk semiconductors. This conjecture has strangely not been directly tested. To do that we have recorded the effects of a single cold “spectator” exciton on the relaxation dynamics of a subsequently deposited hot counterpart. Results in CdSe/CdS nanodots show that a preexisting cold “spectator exciton” allows only half of the photoexcited electrons to relax directly to the band-edge. The rest are blocked in an excited quantum state due to conflicts in spin orientation. The latter fully relax in this sample only after ~25 ps, as the blocked electrons spins flip, prolonging the temporal window of opportunity for harvesting the retained energy more than 100 fold! Quantum chemical estimates of these rates in a model cluster prove they are well within bounds of feasibility. Common to all quantum confined nanocrystals, this process will delay cooling and impact the spectroscopic signatures of hot multi-excitons in all envisioned generation scenarios. As a demonstration in the graphic transient transmission spectra 1.2 ps after a low intensity above band edge pump pulse is compared for a sample without and with cold spectators. The 50% reduction in the bleaching signal at the 1S peak is obvious, and explained in the level schemes below the figure to the left.
Ghosh, T.; Fabian, M. D.; Dehnel, J.; Lifshitz, E.; Baer, R.; Ruhman, S., J. Phys. Chem. Lett. 2019, 10, 2341-2348.
Free carrier emergence in MAPbI3 films
Free carrier emergence in MAPbI3 films
To uncover early stages of carrier dynamics and electron-optic phonon coupling in lead halide perovskite films, high signal to noise pump-probe measurements with sub-10 fs pulses were performed. Immediately after photoexcitation, localized hot excitons are generated which exhibit a sharp red shift of the excitonic peak in the TA spectra. This is the first time such a short lived precursor to separated hot carriers in bulk semiconductor materials has been documented. These localized hot excitons then dissociate to free carriers with a time scale ~20 fs followed by thermalization which is complete within 100 fs as evidenced in band integrals and global kinetic analysis. Later, hot carriers cool to lattice temperature through scattering with optic phonons within 0.5 ps. The charge carriers also couple to the optic phonons which result in spectral modulations surrounding the band edge in both the materials. Fourier analysis of these modulations shows active phonons with frequency ~100 cm-1 related to stretching modes of Pb-halide moieties and in the 250-350 cm-1 range previously assigned to torsional motion of the methyl-ammonium cation. The amplitudes of these modulations allowed estimates for the underlying electron phonon coupling strengths showing all to be well within the weak regime and compatible for large polaron formation in these materials, and thus probably not responsible to the moderate carrier mobilities in these materials.
To uncover early stages of carrier dynamics and electron-optic phonon coupling in lead halide perovskite films, high signal to noise pump-probe measurements with sub-10 fs pulses were performed. Immediately after photoexcitation, localized hot excitons are generated which exhibit a sharp red shift of the excitonic peak in the TA spectra. This is the first time such a short lived precursor to separated hot carriers in bulk semiconductor materials has been documented. These localized hot excitons then dissociate to free carriers with a time scale ~20 fs followed by thermalization which is complete within 100 fs as evidenced in band integrals and global kinetic analysis. Later, hot carriers cool to lattice temperature through scattering with optic phonons within 0.5 ps. The charge carriers also couple to the optic phonons which result in spectral modulations surrounding the band edge in both the materials. Fourier analysis of these modulations shows active phonons with frequency ~100 cm-1 related to stretching modes of Pb-halide moieties and in the 250-350 cm-1 range previously assigned to torsional motion of the methyl-ammonium cation. The amplitudes of these modulations allowed estimates for the underlying electron phonon coupling strengths showing all to be well within the weak regime and compatible for large polaron formation in these materials, and thus probably not responsible to the moderate carrier mobilities in these materials.
T. Ghosh, S. Aharon, L. Etgar and S. Ruhman. J. Am. Chem. Soc. 2017, 139, 18262-18270
Photobiology of Retinal proteins
Photobiology of Retinal proteins
The search for the determinants of rate and efficiency of photoisomerization in microbial retinal proteins has been revolutionized by the ever growing library new family members, with the aim of their use in optogenetics providing added impetus to this quest. At the same time early conjectures of how wavelength tuning and other characteristics impact these measures appear to be unfounded, renewing the importance of further investigation. A recent example is:
The search for the determinants of rate and efficiency of photoisomerization in microbial retinal proteins has been revolutionized by the ever growing library new family members, with the aim of their use in optogenetics providing added impetus to this quest. At the same time early conjectures of how wavelength tuning and other characteristics impact these measures appear to be unfounded, renewing the importance of further investigation. A recent example is:
T dependence of photoisomerization in thermophilic rhodopsin, and comparison with bacteriorhodopsin.
T dependence of photoisomerization in thermophilic rhodopsin, and comparison with bacteriorhodopsin.
In the ongoing quest of discovering new microbial retinal proteins, a proton pump indigenous to the extreme thermophilic bacterium Thermus thermophilus JL-18 (TR) was identified. As a means of characterizing the specific capabilities of this protein to function at elevated temperatures, its photoisomerization dynamics were recorded for the first time as a function of temperature in the range 20-70 0C. As shown at the right, TR is found to have the fastest isomerization rate of any microbial proton pump, isomerizing 30% faster than BR. TR excited state also exhibits unusually mono-exponential internal conversion kinetics. The isomerization process is estimated to proceed with high quantum efficiency of at least 0.6, indicating that this short lifetime is not a consequence of reduced isomerization yield. No temperature effects were observed on the primary photo-induced events of TR or BR from room temperature to the verge of thermal denaturation. This observation rules out non exponential isomerization in BR to be ascribed to structural inhomogeneity in the irradiated sample. Coherent torsional wave-packets launched by impulsive photoexcitation of TR have a marked similarity to its structural analogue Gloeobacter Rhodopsin. However, these are significantly different from those observed for other proton pumps like BR or pHR. These observations show that these modulations, now recorded in several microbial rhodopsins as well as in the retinal chromophore in solution, serve as sensitive probes of the protein binding pocket and its interaction with the prosthetic group.
In the ongoing quest of discovering new microbial retinal proteins, a proton pump indigenous to the extreme thermophilic bacterium Thermus thermophilus JL-18 (TR) was identified. As a means of characterizing the specific capabilities of this protein to function at elevated temperatures, its photoisomerization dynamics were recorded for the first time as a function of temperature in the range 20-70 0C. As shown at the right, TR is found to have the fastest isomerization rate of any microbial proton pump, isomerizing 30% faster than BR. TR excited state also exhibits unusually mono-exponential internal conversion kinetics. The isomerization process is estimated to proceed with high quantum efficiency of at least 0.6, indicating that this short lifetime is not a consequence of reduced isomerization yield. No temperature effects were observed on the primary photo-induced events of TR or BR from room temperature to the verge of thermal denaturation. This observation rules out non exponential isomerization in BR to be ascribed to structural inhomogeneity in the irradiated sample. Coherent torsional wave-packets launched by impulsive photoexcitation of TR have a marked similarity to its structural analogue Gloeobacter Rhodopsin. However, these are significantly different from those observed for other proton pumps like BR or pHR. These observations show that these modulations, now recorded in several microbial rhodopsins as well as in the retinal chromophore in solution, serve as sensitive probes of the protein binding pocket and its interaction with the prosthetic group.
E. S. S. Iyer, R. Misra, A. Maity, O. Liubashevski, Y. Sudo, M. Sheves, S. Ruhman, J. Am. Chem. Soc., 2016, 138 (38), pp 12401–12407
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