•Conformational change of myoglobin was explored in the presence of various ionic liquids in phosphate buffer by spectroscopic measurements.•1-Butyl-3-methyl cation can promote the unfolding of myoglobin.•Imidazolium cation with longer alkyl chain length has stronger denaturation ability for myoglobin.The conformational change of myoglobin (Mb) during guanidine hydrochloride (GuHCl)-induced protein unfolding in the presence of various ionic liquids (ILs) in phosphate buffer was investigated using both the Soret band absorption and the fluorescence of tryptophan measurements. The GuHCl-induced denaturation midpoints of Mb derived from the absorption and fluorescence spectra were almost similar in the presence of 150 mM ILs with the same cation 1-butyl-3-methylimidazolium (Bmim+) but different anions (BF4−, NO3−, Cl−, and Br−) in phosphate buffer. In addition, the denaturation midpoints of Mb in the presence of ILs were little lower than those in the absence of ILs in phosphate buffer. For the sake of clarity and comparison, we also measured the GuHCl-induced denaturation midpoints of Mb in the presence of 150 mM sodium salts with different anions (BF4−, NO3−, Cl−, and Br−) in phosphate buffer and found that their corresponding denaturation midpoints of Mb were almost similar to those observed in the absence of sodium salts in phosphate buffer. These experimental data indicate that Bmim+ cation can promote the unfolding of Mb. Further experiments revealed that the denaturation ability of ILs increases with increasing alkyl chain length of imidazolium cation of ILs and that hydroxyl-substituted imidazolium cation could also promote the unfolding of Mb.Download high-res image (125KB)Download full-size image

•Ultrafast fluorescence quenching dynamics of fluorescein, dibromofluorescein, eosin Y, and erythrosine B, were studied in 2.0 M TEOA solution.•The electron transfer rate from TEOA to each xanthene dye and the intersystem crossing rate of each selected xanthene dye were obtained.•TEOA mainly interacts with the singlet excited-state of fluorescein, dibromofluorescein, and eosin Y in 2.0 M TEOA solution.•TEOA can interact with both the singlet and triplet excited-states of erythrosine B in 2.0 M TEOA solution.Triethanolamine (TEOA) has been often used as a hole-scavenger in dye-sensitized semiconductor photocatalytic systems. However, the femtosecond time-resolved kinetics of the interaction between a sensitized dye and TEOA has not been reported in literatures. Herein, we selected four commonly used xanthene dyes, such as fluorescein, dibromofluorescein, eosin Y, and erythrosine B, and studied their ultrafast fluorescence quenching dynamics in the presence of TEOA in aqueous solution, respectively, by using both femtosecond transient absorption and time-resolved fluorescence measurements. We obtained the electron transfer rate from TEOA to each photoexcited xanthene dye in 2.0 M TEOA solution. We also obtained the intersystem crossing rate of each xanthene dye in aqueous solution with fluorescence quantum yield and lifetime measurements. Finally we found that TEOA mainly interacts with the singlet excited-state of fluorescein, dibromofluorescein, and eosin Y, and that TEOA can interact with both the singlet and triplet excited-states of erythrosine B in high concentration of TEOA aqueous solution.Download high-res image (99KB)Download full-size image

Carbon nitrides (CN) have been widely used in photocatalytic applications. However, the charge carrier kinetics of CN after light excitation remains unclear. Herein, we prepared a stable and transparent CN colloid in an aqueous tetraethylammonium hydroxide solution and investigated its carrier kinetics using both femtosecond transient absorption and picosecond time-resolved fluorescence spectroscopy. We found that a new and positive absorption band appears in the femtosecond transient absorption spectrum of the CN colloid, which could be attributed to the absorption of the photogenerated electron/hole pairs (or the electronic excited state) of the CN colloid after light excitation. Moreover, we found that the charge carrier kinetics obtained from the femtosecond transient absorption measurements is dramatically different from that obtained from the picosecond time-resolved fluorescence measurements, indicating that the photophysical process of the CN colloid after light excitation is complicated. With the results obtained from both the femtosecond transient absorption and picosecond time-resolved fluorescence measurements, we proposed a schematic to understand the photophysics and charge carrier kinetics of the CN colloid. We believe that the current study is also significant for researchers to understand the photophysics and charge carrier kinetics of bulk CN.

•The photophysics of C151 and C153 in non-hydroxyl and hydroxyl functionalized ILs were investigated.•[HOEmim][BF4] can quench the fluorescence of C153 while [Emim][BF4] does not.•The slowdown of the anisotropic rotation of C153 in [HOEmim][BF4] is observed.•There is a specific hydrogen bonding interaction between C153 and [HOEmim][BF4].Steady-state absorption and fluorescence as well as time-resolved fluorescence of coumarin 151 (C151) and coumarin 153 (C153) were measured in hydroxyl-functionalized ionic liquids ([HOEmim][BF4] and [HOEmim][N(CN)2]) and in nonhydroxyl-functionalized ionic liquids ([Emim][BF4] and [Emim][N(CN)2]). Both the steady-state fluorescence and time-resolved fluorescence observations reveal that hydroxyl-functionalized ionic liquid quenches the fluorescence of C153 while the nonhydroxyl-functionalized ionic liquid does not. We also measured the time-resolved fluorescence anisotropy of C151 and C153 in both [HOEmim][BF4] and [Emim][BF4]. It is found that the ratio of the rotational relaxation lifetime of C153 in [HOEmim][BF4] with respect to that in [Emim][BF4] is about 15% larger than that of C151 in [HOEmim][BF4] with respect to that in [Emim][BF4], indicating extra interaction between C153 and [HOEmim][BF4] exists except the effect of the viscosity of ionic liquid.

Dye sensitization is a promising strategy to extend the visible light absorption of carbon nitride (C3N4) and increase the photocatalytic hydrogen evolution efficiency of C3N4 under visible light irradiation. However, the interaction dynamics between C3N4 and a sensitized dye has not been reported in the literature. Herein, we selected four commonly used xanthene dyes such as fluorescein, dibromofluorescein, eosin Y, and erythrosine B and prepared their corresponding dye-sensitized-C3N4 composites. For the first time, we derived the electron transfer rate from the LUMO of each photoexcited xanthene dye to the conduction band of C3N4 using picoesecond time-resolved fluorescence measurements. We also obtained the reduction potentials of all selected xanthene dyes and C3N4 with cyclic voltammetry measurements. The cyclic voltammetry measurements gave a consistent result with the picosecond time-resolved fluorescence measurements. Besides, the possibility of the selected xanthene dye as an acceptor for the hole of the photoexcited C3N4 was also discussed. We believe this study is significant for the researcher to understanding the fundamental aspects in the xanthene dye-sensitized-C3N4 photocatalytic systems.Keywords: carbon nitride; dye-sensitized photocatalytic system; electron transfer; hole transfer; xanthene dye

The polarizability or hyperpolarizability of aqueous anions is closely related to important processes such as the surface tension change of water and the empirical Hofmeister effect. Herein, we measured the hyperpolarizabilities of six polyatomic anions (HPO42−, HSO4−, CO32−, CH3COO−, NO3− and SCN−) in their aqueous potassium salt solutions using femtosecond optical Kerr effect spectroscopy. We found that the hyperpolarizability of these six aqueous anions increased in the following order: HPO42− ∼ HSO4− < CO32− < CH3COO− < NO3− < SCN−, which correlated well with the Hofmeister series of aqueous anions in their decreasing ability to precipitate proteins. However, the CO32− anion is the exceptional anion. The current study is useful for disclosing these fundamental physicochemical properties in aqueous salt solutions.

The photophysical properties of carbon nitride (C3N4) are very basic and important in the understanding of its photocatalytic activity. Herein, we measured the UV–vis diffuse reflectance (UVDR) and fluorescence spectra of C3N4 prepared at different temperatures and studied their fluorescence decay kinetics under different wavelengths and different fluences of light excitation. We found, first, that the fluorescence lifetime of C3N4 under visible (465 nm) light excitation is shorter than that under UV (395 nm) light excitation; second, that the fluorescence lifetime of C3N4 under 465 nm light excitation decreases as increasing its processing temperature; and third, that the fluorescence lifetime of C3N4 under 395 nm light excitation decreases with the increase of the light excitation fluence. These findings revealed that the two distinct absorption bands in the UVDR spectrum of C3N4 arise from two different transitions of C3N4 and that the origin of the fluorescence emission of C3N4 arises from its singlet exciton. Besides, the photocatalytic H2 evolutions of C3N4 synthesized at different temperatures under visible light irradiation were also measured and discussed to correlate with the obtained photophysical properties of C3N4.

We studied the ultrafast fluorescence quenching dynamics of Atto655 in the presence of N-acetyltyrosine (AcTyr) and N-acetyltryptophan (AcTrp) in aqueous solution with femtosecond transient absorption spectroscopy. We found that the charge-transfer rate between Atto655 and AcTyr is about 240 times smaller than that between Atto655 and AcTrp. The pH value and D2O dependences of the excited-state decay kinetics of Atto655 in the presence of AcTyr and AcTrp reveal that the quenching of Atto655 fluorescence by AcTyr in aqueous solution is via a proton-coupled electron-transfer (PCET) process and that the quenching of Atto655 fluorescence by AcTrp in aqueous solution is via an electron-transfer process. With the version of the semiclassical Marcus ET theory, we derived that the electronic coupling constant for the PCET reaction between Atto655 and AcTyr in aqueous solution is 8.3 cm–1, indicating that the PCET reaction between Atto655 and AcTyr in aqueous solution is nonadiabatic.

Methanol has appeared in many publications reporting reverse micelles (RMs) formation, but it remains debate whether it can be effectively encapsulated by surfactant AOT to form RMs. In this work, we investigated the formation of methanol-containing AOT RMs in n-heptane solvents by using IR125 as a probe. We studied the photophysics of IR125 in methanol/AOT/n-heptane RMs at different loading methanol content (w′0w′0: w′0=[methanol]/[AOT]w′0=[methanol]/[AOT]) by means of steady-state absorption and fluorescence spectroscopies as well as time-resolved fluorescence spectroscopy. We obtained the absorption and fluorescence maxima, fluorescence lifetimes, and reorientation times of IR125 in methanol/AOT/n-heptane RMs at different w′0w′0. We found that the methanol can be effectively encapsulated by AOT in n-heptane solvents to form stable methanol/AOT/n-heptane RMs and the size of methanol/AOT/n-heptane RMs maintains constant with the increase of w′0w′0.Graphical abstractHighlights► The formation of CH3OH/AOT/n-heptane RMs was explored using IR125 as a probe. ► The photophysics of IR125 in CH3OH/AOT/n-heptane RMs at different w′0w′0 was studied. ► The lifetime of IR125 in CH3OH/AOT/n-heptane RMs decreases upon increasing w′0w′0. ► The size of CH3OH/AOT/n-heptane RMs maintains constant as a function of w′0w′0.

•PET between coumarins and DMA are explored in RTIL with different alkyl length.•The quenching efficiency of C152 by DMA increases upon increasing alkyl length.•The static quenching component of C152 by DMA in RTIL is dominated.•The fluorescence lifetime of C152 in RTIL increases upon increasing alkyl length.•The fluorophore's lifetime plays an important role in its bimolecular PET process.Photoinduced electron transfer (PET) reactions in room temperature ionic liquid (RTIL) have been paid great interests due to their potential applications in the field of solar energy conversion. In this work, we selected four commonly used coumarin dyes and studied the bimolecular PET reaction between respective coumarin dye and N,N-dimethylaniline (DMA) in [Emim][BF4], [Bmim][BF4], [Hmim][BF4] and [Dmim][BF4] by means of both the steady-state and time-resolved fluorescence spectroscopies. We found that the fluorescence quenching efficiency of Coumarin 152 (C152) by DMA increases with the increase of the cation's alkyl chain length of RTIL and that the fluorescence quenching efficiencies of Coumarin 102 (C102), Coumarin 151 (C151) and Coumarin 153 (C153) by DMA decrease with the increase of the cation's alkyl chain length of RTIL. We also found that the fluorescence lifetime of C152 increases with the increase of the cation's alkyl chain length of RTIL and that the fluorescence lifetimes of C102, C151 and C153 maintain constant with the increase of the cation's alkyl chain length of RTIL. In addition, we also found that the static quenching components in the fluorescence quenching of C152 by DMA in RTIL are dominated. These results demonstrate that the fluorescence lifetimes of selected fluorophores play important roles in their bimolecular PET fluorescence quenching processes in viscous media.Download full-size image

The rebinding kinetics of an amino acid ligand to ferrous microperoxidase-11 (MP11) after photolysis of aggregated ferrous MP11 was measured in aqueous solution with femtosecond transient visible absorption spectroscopy. The kinetics of CO rebinding to ferrous MP11 after photolysis of MP11CO was also measured in aqueous solution with femtosecond transient visible absorption spectroscopy. From these measurements, we found that either Val-11 or Lys-13 rebinds to ferrous MP11 exponentially with an 8 picosecond time constant in aggregated ferrous MP11 solution and that CO rebinds to ferrous MP11 nonexponentially with subnanosecond time scale in MP11CO solution. The kinetics of both the amino acid and CO rebinding to ferrous MP11 in MP11 system mimics that in carbon monoxide oxidation activator protein (CooA) or carboxymethyl cytochrome c (CmCytC) system. We also measured the kinetics of CO rebinding to ferrous MP11 in aqueous solution at different MP11CO concentrations and found that MP11CO concentration has an obvious effect on the kinetics of CO rebinding to ferrous MP11, where both the germinate yield and rate of CO rebinding to ferrous MP11 increase with the increase of MP11CO concentration. These findings suggested that the picosecond amino acid ligand rebinding process could disturb the proximal heme-ligand structure that possibly leads to the subnanosecond CO rebinding kinetics in MP11CO, CooACO and CmCytCCO systems.

The heterogeneity on photoinduced electron transfer (PET) kinetics between a labeled fluorophore and an amino acid residue has been extensively studied in biopolymers. However in aqueous solutions, the heterogeneity on PET kinetics between a fluorophore and a quencher has rarely been reported. Herein, we selected four commonly used fluorophores, such as tetramethylrhodamine (TMR), Rhodamine B (RhB), Alexa fluor 546 (Alexa546), and Atto655, and studied their respective PET kinetics in 50 mM tryptophan solutions with femtosecond transient absorption spectroscopy to explore the structural heterogeneity in their corresponding collision complexes. We measured the decay of the first excited electronic state of respective fluorophore with and without 50 mM tryptophan in aqueous solutions, and derived the charge separation rate in their corresponding collision complexes. We found that the PET process of all selected fluorophores in 50 mM tryptophan solutions has two charge separation rates, which indicates that the relevant states in the collision complex between respective fluorophore and tryptophan have strong structural heterogeneity. These femtosecond PET measurements are in agreement with Vaiana’s molecular dynamics simulation (J. Am. Chem. Soc.2003, 125, 14564). In addition, with the obtained PET kinetic parameters, we derived the relative brightness of the collision complex between respective fluorophore and tryptophan, which are important parameters for the PET based fluorescence correlation spectroscopy study involving these fluorophores in biopolymers.

Many fluorescent chromophores have been employed to investigate the nature and dynamics of the water confined in reverse micelles (RMs). However, some questions remain as to the location of a probe in a RM and the diameter of the RM at which the physical characteristic of the water inside RMs becomes similar to that of bulk water. In this work, we systematically studied the photophysics of IR125 and C152 in AOT RMs at different w0 by means of static absorption and fluorescence spectroscopy as well as time-resolved fluorescence spectroscopy. We obtained the absorption maxima, fluorescence emission maxima, fluorescence lifetime, and reorientation time of IR125 and C152 in AOT RMs at corresponding w0. We found that all obtained photophysical parameters of IR125 and C152 in AOT RMs as a function of w0 have a distinct changeover point around w0 = 8, indicating that there is a dramatic change in the nature of the water confined in AOT RMs around w0 = 8. The observed changeover point around w0 = 8 is well in agreement with the Satpati's report (ChemPhysChem, 2009, 10, 2966). In addition, we observed that the measured reorientation time of IR125 in AOT RMs increases with the increase of w0, which is opposite to the trend of change in the measured reorientation time of C152 in AOT RMs with the increase of w0. We found that IR125 prefers to reside in the water pool of AOT RMs and that C152 prefers to reside in the outer side of the interfacial region or the nonpolar n-heptane phase of AOT RMs. Furthermore, we found that the time-resolved fluorescence anisotropy of IR125 in smaller w0 AOT RMs primarily measures the reorientation of RMs and the time-resolved fluorescence anisotropy of IR125 in larger w0 AOT RMs measures the reorientation of IR125 in the water pool confined in RMs. This work demonstrated that IR125 is an excellent probe to study the nature and dynamics of the water confined in AOT RMs.

Room-temperature Ionic Liquids (ILs) have numerous unique properties that differ from those of conventional molecular solvents. Although the unique properties of ILs have been suggested to origin from their microscopic interionic interaction, detailed dynamics of interionic interaction of ILs has not been fully understood. Here, with the Femtosecond Optical Heterodyne-Detected Raman Induced Kerr Effect Spectroscopy (fs-OHD-RIKES), we measured the ultrafast dynamics of the interionic interaction of three typical imidazolium based ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), and 1-decyl-3-methylimidazolium tetrafluoroborate ([dmim][BF4]). We observed several periods of subpicosecond oscillation in their fs-OHD-RIKES signals. Through decomposing their fs-OHD-RIKES signals into four Brownian oscillators in time domain, we explored the cation and anion substitution effects on the ultrafast dynamics of interionic interaction of ILs. We found that the cation substitution affected all the low frequency motions we observed, while the anion substitution only affected the two higher low frequency motions.

Photoinduced electron transfer based fluorescence correlation spectroscopy (PET-FCS) is a powerful tool to study biomolecular processes. However, some questions remain as to how to correctly interpret the PET-FCS data. In this work, we studied the PET process between tetramethylrhodamine and guanosine by means of femtosecond transient absorption spectroscopy. We derived that the charge separation rate is 4.1 × 109 s–1 and the charge recombination rate is 5.2 × 1010 s–1 for the current system, supporting the three-state model and the interpretation on PET-FCS experiments given by Qu et al. (J. Phys Chem. B, 2010, 114, 8235). At the limit that both the charge separation and recombination rates are much faster than the process that PET-FCS reveals, the three-state model can be simplified to an equivalent two-state model with a dark state whose brightness is nonzero. We propose ways to obtain the brightness of the dark state with additional experiments, which is necessary for a PET-FCS study.

Atto655 has been widely used as an excellent probing dye through photoinduced electron transfer (PET) for biochemical processes in oligonucleotides or polypeptides. However, its photophysical properties in the presence of the quenchers guanosine and tryptophan have not been carefully studied. In this work, we investigated the dynamics of PET between Atto655 and the two quenchers in aqueous solution with femtosecond transient absorption experiments. We derived that the charge separation rate is 8.1 × 109 s−1 and the charge recombination rate is 7.7 × 1010 s−1 for the collision complex between Atto655 and guanosine and that the corresponding values for the collision complex between Atto655 and tryptophan are 4.0 × 1011 and 5.0 × 1012 s−1, respectively. These experimental results are quite consistent with the prediction of Marcus-type theory for electron transfer. The implications of this work for the data analysis of PET-based fluorescence correlation spectroscopy are discussed.

Carbon nitrides (CN) have been widely used in photocatalytic applications. However, the charge carrier kinetics of CN after light excitation remains unclear. Herein, we prepared a stable and transparent CN colloid in an aqueous tetraethylammonium hydroxide solution and investigated its carrier kinetics using both femtosecond transient absorption and picosecond time-resolved fluorescence spectroscopy. We found that a new and positive absorption band appears in the femtosecond transient absorption spectrum of the CN colloid, which could be attributed to the absorption of the photogenerated electron/hole pairs (or the electronic excited state) of the CN colloid after light excitation. Moreover, we found that the charge carrier kinetics obtained from the femtosecond transient absorption measurements is dramatically different from that obtained from the picosecond time-resolved fluorescence measurements, indicating that the photophysical process of the CN colloid after light excitation is complicated. With the results obtained from both the femtosecond transient absorption and picosecond time-resolved fluorescence measurements, we proposed a schematic to understand the photophysics and charge carrier kinetics of the CN colloid. We believe that the current study is also significant for researchers to understand the photophysics and charge carrier kinetics of bulk CN.

Many fluorescent chromophores have been employed to investigate the nature and dynamics of the water confined in reverse micelles (RMs). However, some questions remain as to the location of a probe in a RM and the diameter of the RM at which the physical characteristic of the water inside RMs becomes similar to that of bulk water. In this work, we systematically studied the photophysics of IR125 and C152 in AOT RMs at different w0 by means of static absorption and fluorescence spectroscopy as well as time-resolved fluorescence spectroscopy. We obtained the absorption maxima, fluorescence emission maxima, fluorescence lifetime, and reorientation time of IR125 and C152 in AOT RMs at corresponding w0. We found that all obtained photophysical parameters of IR125 and C152 in AOT RMs as a function of w0 have a distinct changeover point around w0 = 8, indicating that there is a dramatic change in the nature of the water confined in AOT RMs around w0 = 8. The observed changeover point around w0 = 8 is well in agreement with the Satpati's report (ChemPhysChem, 2009, 10, 2966). In addition, we observed that the measured reorientation time of IR125 in AOT RMs increases with the increase of w0, which is opposite to the trend of change in the measured reorientation time of C152 in AOT RMs with the increase of w0. We found that IR125 prefers to reside in the water pool of AOT RMs and that C152 prefers to reside in the outer side of the interfacial region or the nonpolar n-heptane phase of AOT RMs. Furthermore, we found that the time-resolved fluorescence anisotropy of IR125 in smaller w0 AOT RMs primarily measures the reorientation of RMs and the time-resolved fluorescence anisotropy of IR125 in larger w0 AOT RMs measures the reorientation of IR125 in the water pool confined in RMs. This work demonstrated that IR125 is an excellent probe to study the nature and dynamics of the water confined in AOT RMs.