We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light-responsive photocatalysts for the photoconversion of CO2 into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO2 photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO2 photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO2 photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compared with AuNPs.

Rebinding dynamics of CO to carboxymethyl cytochrome c (Ccytc), a chemically modified cytochrome c to bind ligands in its ferrous form, in D(2)O solution at 283 K after photodeligation, was investigated using femtosecond vibrational spectroscopy. The stretching mode of CO bound to the protein shows four stretching bands near 1962 cm(-1). Time-resolved spectra of the bound CO revealed a slight band-position-dependent rebinding kinetics, suggesting that the geminate rebinding of CO depends on the conformation of the protein. The overall rebinding kinetics of CO to Ccytc was more than 1000 times faster than that to myoglobin (Mb), a ligand-binding protein, and is also faster than a model heme, microperoxidase-8 in viscous solvent. The efficient rebinding of CO to Ccytc was attributed to the longer retention of the dissociated CO near the active binding site by the organized protein matrix of Ccytc. The spectra of the dissociated CO reveal a fast-growing band in the picosecond time scale that is assigned to CO in D(2)O solvent. The ultrafast CO escape to bulk solution is consistent with its 3D structure showing a sizable opening in the active site. It appears that most of the dissociated CO rebinds within 1 ns, except for those that escape to the bulk solution through the opening. The CO rebinding in Ccytc indicates that the primary heme pocket in Mb, located near the active site and holding the dissociated ligand for longer than tens of nanoseconds, has a specific structure to suppress CO rebinding.

Femtosecond vibrational spectroscopy was used to investigate the photoexcitation dynamics of NO-bound ferric myoglobin (Mb(III)NO) in D2O solution at 294 K after excitation with a 575 nm pulse. The stretching mode of NO in Mb(III)NO consists of a major band at 1922 cm(-1) (97.7%) and a minor band at 1902 cm(-1) (2.3%), suggesting that Mb(III)NO in room temperature solution has two conformational substates. The time-resolved spectra show small but significant new absorption features at the lower-energy side of the main band (1920-1800 cm(-1)). One new absorption feature in the region of 1920-1880 cm(-1) exhibits the (15)NO isotope shift (37 cm(-1)) the same as that of the NO band in the ground electronic state of Mb(III)NO. This absorption shifts toward higher energy and narrows with a time constant of 2.4 ps, indicating that it evolves with rapid electronic and thermal relaxation of the photoexcited Mb(III)NO without photodeligation of the NO from the heme. Absorption features assigned to proteins undergoing thermal relaxation without NO deligation add up to 14 ± 1% of the total bleach, implying that the photolysis quantum yield of Mb(III)NO with a Q-band excitation is ≤0.86 ± 0.01. The remaining absorption bands peaked near 1867, 1845, and 1815 cm(-1), each showing the (15)NO isotope shift the same as that of the free NO radical (33 cm(-1)), were assigned to the vibrational band of the photodeligated NO, the NO band of Mb(III)NO in an intermediate electronic state with low-spin Fe(III)-NO(radical) character (denoted as the R state), and the NO band of the vibrationally excited NO in the R state, respectively. A kinetics model successfully reproducing the time-dependent intensity changes of the transient bands suggests that every rebound NO forms the R state that eventually relaxes into the ground electronic state nonexponentially. Most of the photodissociated NO undergoes fast geminate recombination (GR), and the rebinding kinetics depends on the conformation of the protein. GR of NO to Mb(III) in the major conformation shows highly nonexponential kinetics described by a stretched exponential function, exp(-(t/290 ps)(0.44). The NO rebinding to Mb(III) in the minor conformation is exponential, exp(-t/1.8 ns), suggesting that the distal histidine, the interaction of which dictates the conformation of Mb(III)NO, participates in mediating the binding of NO to Mb(III). In Mb(III)NO, the elusive low-spin Fe(III)-NO(radical) state, proposed in electronic structure calculations, indeed exists at >12 kJ/mol above the ground state and takes part in the bond formation of Fe(III)-NO, suggesting that it plays a significant role in the function of NO-bound ferric protein. Time-resolved vibrational spectra with high sensitivity reveal rich photophysical and photochemical processes of photoexcited Mb(III)NO.

Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of [26]- and [28]hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of [26]bis(rhodium) hexaphyrin (R26H), those of [28]bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.

Using femtosecond vibrational spectroscopy, we investigated the rebinding dynamics of NO to cytochrome c (Cytc) and a model heme, microperoxidase-8 (Mp), after photodeligation of CytcNO in D(2)O solution and MpNO in an 81% glycerol/water (v/v) mixture at room temperature. Whereas the stretching mode of the NO band in MpNO was described by a Gaussian centered at 1653 cm(-1) with a full width at half-maximum (fwhm) of 41 cm(-1), that in CytcNO revealed an asymmetric structured band that peaked at 1619 cm(-1) with an fwhm of about 27 cm(-1). The structured NO band in CytcNO was well described by the sum of three Gaussians, and its shape did not evolve with time but its amplitude decayed exponentially with a time constant of 7 ± 1 ps. The transient NO band in MpNO also decayed exponentially with a time constant of 8 ± 1 ps. Rebinding of NO to Cytc was slightly faster than that of NO to Mp and was almost complete by 30 ps, which was much faster than the rebinding of NO to myoglobin (Mb). When the deligated NO was constrained near the Fe atom either by a viscous solvent or by the protein matrix, it rebound to heme Fe much faster than CO, suggesting that NO has a higher propensity for binding to heme Fe and the high reactivity governed the rebinding kinetics. Moreover, the faster ligand rebinding in Cytc than in Mb suggests that Cytc does not have a primary docking site (PDS)-like structure found in Mb that suppresses rebinding by restraining ligand motion and the PDS can also hold the deligated NO in a manner that impedes NO rebinding; however, due to higher NO reactivity with heme Fe, the impediment is not as efficient as for CO.