Photosensitive System Research Articles

Expression and Regulation of pde6h by Thyroid Hormone During Metamorphosis in Paralichthys olivaceus.

PDE6H is a cone cell-specific inhibitory subunit that plays a critical role in the adaptation of the photosensitive system to bright and dark phases of the light environment. Thyroid hormone (TH) is one of the most important factors that control development and metabolism in animals, composed mainly of triiodothyronine (T3), and thyroxine (T4). TH also plays a key role in the metamorphosis of the flounder (Paralichthys olivaceus), wherein exogenous TH can accelerate the behavioral changes of larvae from the pelagic to benthic type accompanying changes in the light environment from bright to dark. In this study, transcriptional analysis showed that pde6h is expressed in adult eye, that its expression peaks at the climax of metamorphosis, and that it can be significantly up-regulated to the highest level by exogenous T4 in the early stages of metamorphosis but is inhibited by thiourea (TU). The rescue experiment showed that metamorphic inhibition of larvae and expression inhibition of pde6h gene in TU groups can be rescued by removing TU. Further, dual-luciferase reporter assay indicated the putative regulatory effect of TH on pde6h expression, mediated directly on the gene promoter by the TRαA gene. Together, we speculated that TH may control physiological adaptation of the photosensitive system to light changes during metamorphosis by acting directly on pde6h. This study can help us further study the physiological function of pde6h during flounder metamorphosis in the future.

Protein assembled nano-vehicle entrapping photosensitizer molecules for efficient lung carcinoma therapy.

The efficiency of drug depends not only on its potency but also on its ability to reach the target sites in preference to non-target sites. In this regard, protein assembled nanocarrier is the most promising strategy for intracellular anti-cancer drug delivery. The key motive of this study is to fabricate biocompatible protein assembled nanocarrier conjugated photosensitizer system for stimuli-responsive treatment of lung carcinoma. Here, we have synthesized a unique nanohybrid of protein assembled gold nanoparticles (AuNPs), attaching a model photosensitizer, Protoporphyrin IX (PpIX) to the protein shell of the nanoparticles (NPs) imparting an ideal drug-carrier nature. Photo-induced alteration in hydrodynamic diameter suggests structural perturbation of the nanohybrid which in terms signifies on-demand drug delivery. The drug release profile has been further confirmed by using steady-state fluorescence experiments. AuNP-PpIX showed excellent anti-cancer efficiency upon green light irradiation on lung adenocarcinoma cell line (A549) through intracellular reactive oxygen species (ROS) generation. The cellular morphological changes upon PDT and internalization of nanohybrid were monitored using confocal laser scanning microscope. This anti-cancer effect of nanohybrid was associated with apoptotic pathway which was confirmed in the flow cytometric platform. The developed nanomedicine is expected to find relevance in clinical anti-cancer PDT models in the near future.

OPN4 belongs to the photosensitive system of the human skin.

The human skin has previously been described to be affected by light; however, the underlying mechanism remains unknown. OPN4 (melanopsin) expression was first identified in the skin of amphibians; however, whether it is also expressed and functioned in the human skin has not yet been identified. Here, we show that OPN4 was expressed in the human skin tissue and cultures of isolated keratinocytes, melanocytes and fibroblasts. Additionally, Ca2+ influx in vitro and ex vivo and phosphorylation of extracellular signal-regulated kinases 1/2 in human fibroblasts were observed by stimulation of blue light irradiation. Notably, our findings showed that this Ca2+ influx and phosphorylation of extracellular signal-regulated kinases 1/2 are promoted in an intensity-dependent manner, indicating that the light signal is converted to an intracellular signal via OPN4 in the human skin. Overall, in this study we showed that the human skin functions as a photoreceptor by demonstrating that in human skin, the photoreceptive protein was expressed, and photoreception was conducted via photoreceptive protein.

The Influence of Effect of Polysaccharides and Polyvinylpyrrolidone on the Photocatalytic Activity of Chlorin e6 in Tryptophan Oxidation

The effect amphiphic polymer poly-N-vinylpyrrolidone (PVP) and polysaccharides (PSs), sodium alginate (SA) and hyaluronic acid (HA), have on the photosensitizing activity of chlorin e6 (Ce6) in the reaction of tryptophan oxidation is established. This reaction is considered a model in searching for the most efficient pharmaceutical formulations of photosensitizers for antimicrobial photodynamic therapy. It is shown that the initial photosensitizing activity of Ce6, characterized by effective rate constant keff of tryptophan oxidation, grows as binary (Ce6–PVP and Ce6–SA(HA)) and (to a greater extent) ternary systems (Ce6–PVP–SA and Ce6–PVP–SA(HA)) form in aqueous solutions. This rise in activity is found to be related to disaggregation of the associates of photosensitizer (PhS) molecules initially present in aqueous solutions, resulting from the formation of complex bonds between PhS molecules and fragments of polymers added to the solution. This conclusion is confirmed for the considered binary and ternary systems by the accumulated data of electronic absorption spectra and fluorescence spectra of chlorin e6 in the absence of polymers (PVP, SA, and HA) and after they are added to the reaction system, and by 1H NMR data. In light of the most recent findings (according to which each polymeric component (PVP, SA, and HA) affects the activity of PhS, while almost no direct interaction between PVP and PS is detected in the 1H NMR spectra), ternary photosensitizing systems form in aqueous media during the formation of intertwining PVP–PS polymeric chains upon exposure to local hydrodynamic flows, followed by complex bonding between PhS molecules and fragments of both polymers.

Inactivation of Escherichia coli and MS2 coliphage via singlet oxygen generated by homogeneous photosensitization

The inactivation kinetics of E. coli and MS2 coliphage by singlet oxygen (1O2) were investigated in a homogeneous photosensitization system using Rose Bengal (RB) and visible light illumination (the Vis/RB system). The inactivation of E. coli and MS2 in the Vis/RB system was monitored over time with variations of several parameters such as pH, light intensity, concentration of RB, and the presence of dissolved oxygen. In addition, the concentration of 1O2 generated by the Vis/RB system was quantified using furfuryl alcohol under each microbial inactivation conditions. Based on the obtained results, the degree of microbial inactivation was quantitatively correlated with 1O2 exposure using the (delayed) Chick-Watson model. The Ct (concentration-time product) values of 1O2 required for 2 log microbial inactivation were found to be 1.3×10−4 mg·min/L for E. coli and 1.9×10−5 mg·min/L for MS2, respectively. The inactivation of E. coli exhibited an initial lag phase until 0.5×10−4 mg·min/L of Ct.

Improving Photocatalysis for the Reduction of CO2 through Non-covalent Supramolecular Assembly.

We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TONCO = 100 ± 4) and quantum yield (ΦCO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TONCO = 28 ± 4 and ΦCO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO2 reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.

A Redox Stimulation-Activated Amphiphile for Enhanced Photodynamic Therapy.

The development of more efficient photosensitizers with minimal damage to surrounding normal tissues has been a valuable and challenging subject during photodynamic therapy (PDT). Herein, a stimuli-activated porphyrinic photosensitizer (PEG-TPP-DNB; PEG = poly(ethylene glycol); TPP = 5,10,15,20-tetraphenylporphyrin; DNB = 2,4-dinitrobenzene) with capabilities of fluorescence and, remarkably, singlet oxygen quenching was prepared successfully for photodynamic therapy with high efficiency and biosecurity. The amphiphilic PEG-TPP-DNB could be self-assembled into nanomicelles in aqueous media and dissociated in response to reductive thiol such as glutathione. Meanwhile, the fluorescence and singlet oxygen generation of porphyrinic photosensitizer would be activated to regenerate. Moreover, the intracellular uptake and localization effectively confirmed the redox-responsive and activated behavior of PEG-TPP-DNB micelles. The cytotoxicity in vitro revealed that the micelles had low dark toxicity and great phototoxicity, and in vivo bioimaging and antitumor evaluation further indicated that the micelles possessed selective tumor imaging and targeted PDT antitumor effect as well as low systemic toxicity. Overall, this tumor microenvironment-activated photosensitizer system may provide a useful strategy for precise photodynamic therapy.

New perylenebisimide decorated cyclotriphosphazene heavy atom free conjugate as singlet oxygen generator.

Perylenebisimide-cyclotriphosphazene based inorganic-organic system was synthesized by a multistep procedure. The substitution reaction of asymmetric perylenebisimide (PBI) derivative with the hexachloroyclotriphosphazene (trimer) resulted in the formation of fully PBI decorated cyclotriphosphazene (5). The identity of newly synthesized compound (5) was confirmed by using 31P, 1H and 13C NMR spectroscopies and mass spectrometry. The photophysical (UV- Vis absorption, fluorescence emission, fluorescence lifetime and fluorescence quantum yield) and photochemical (the singlet oxygen generation, and photostability) properties of this conjugate were investigated as novel heavy atom free triplet photosensitizer. The singlet oxygen quantum yield of the PBI-cyclotriphosphazene (5) was calculated to be 0.86 which is good for a heavy atom free triplet photosensitizer. These results will add to the development of cyclotriphosphazene based heavy atom free singlet oxygen triplet photosensitizer systems for applications in organic oxygenation reactions.

Carbon dots from roasted mackerel (scomberomorus niphonius) for free radical scavenging

Foodborne nanoparticles produced from food components during thermal processing have received attention due to their particular properties. Herein, the presence of fluorescent carbon dots (CDs) was reported from mackerel flesh roasted at 230 °C in an electronic oven, and the CD capability for free radical scavenging was investigated. The average size of the CDs was about 2.2 nm with a size distribution in the range of 0.9–3.5 nm as characterized with transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) showed that the mackerel CDs mainly contain C, N and O. Fourier transform infrared (FTIR) analysis showed the existence of hydroxyl, amino, and carboxyl groups on the mackerel CD surface. The fluorescence lifetime was about 9.1 ns. Two absorption peaks at 277 and 328 nm were observed in UV–vis spectra, which are assigned to the π-π* and n-π* charge-transfer, respectively. The mackerel CDs showed excellent photostability in various metal ion solutions and pH environment. In addition, these CDs exhibited strong dose-dependent scavenging capability for hydroxyl radicals (·OH) and methyl radicals (·CH3) produced from methylene blue (MB)/visible-light photosensitization system. The free radical scavenging properties make the mackerel CDs capable of protecting against oxidative stress after they are exposed to biosystem.

Functionalization of 2D Materials with Photosensitive Molecules: From Light‐Responsive Hybrid Systems to Multifunctional Devices

Abstract2D materials possess exceptional physical and chemical properties that render them appealing components for numerous potential applications in (opto)electronics, energy storage, sensing, and biomedicine. However, such unique properties are hardly tunable or modifiable. The functionalization of 2D crystals with molecules constitutes a powerful strategy to adjust and modulate their properties, by also imparting them new functions. In this framework, the combination of 2D materials with photosensitive molecules is a viable route for harnessing their light‐responsive nature. The latter takes full advantage of the extremely high sensitivity of 2D materials to subtle changes in the local environment and the capacity of photosensitive molecules to modify their intrinsic properties when exposed to electromagnetic fields. The hybrid molecule–2D materials can preserve the unique optical and electrical properties of 2D layers and can exhibit additional light‐tunable features. In this Progress Report, the protocols that can be pursued for the 2D material functionalization and switching mechanisms in photosensitive systems are reviewed, followed by an in‐depth discussion on their tunable optical properties and their exploitation when integrated in novel photoswitchable electronic devices. The opportunities and associated challenges to be tackled for the development of unprecedented and high‐performance light‐responsive devices are discussed.

Spectral Photoresponse of Biological Systems

The effects the spectrum of incident radiation can have on photobiological processes are studied experimentally and theoretically. The potential effective fluxes of visible light are calculated for energetic and regulatory processes. With these data at hand, we find a correlation between the spectral photoresponse and the energy characteristics of plant photoreceptor and photosynthesis systems. A qualitative model for a photosensitive organic system is formulated as a circuit diagram that includes semiconductor photocells. The thermodynamic and electrical parameters of the modeled system are described. We draw parallels between the functioning of phototransistors and photoreceptor complexes PHOT, CRY, Ph(660), and Ph(730). The control parameters of the energy-distribution process in a living system are estimated using mathematical modeling and an experimental model based on photocells.

Near‐Infrared Photoinduced Copper‐Catalyzed Azide‐Alkyne Click Chemistry with a Cyanine Comprising a Barbiturate Group

AbstractA new near‐infrared (NIR) photosensitization system for the activation of a copper‐catalyzed azide‐alkyne cycloaddition click reaction (CuAAC) is reported. Selected cyanines comprising either a cationic or zwitterionic structural pattern were tested for their photosensitization activity to generate Cu(I) by photoreduction of Cu(II) to trigger CuAAC under NIR light at ambient temperature. The photosensitization system described also facilitates chemical modification of UV‐sensitive materials by CuAAC. Upon exposure to light of wavelength 790 nm under anaerobic conditions, the cyanine with a zwitterionic structure comprising a barbiturate group; that is 5‐(6‐(2‐(3‐ethyl‐1,1‐dimethyl‐1H‐benzo[e]indol‐2(3H)‐ylidene)ethylidene)‐2‐(2‐(3‐ethyl‐1,1‐dimethyl‐1H‐benzo[e]‐indol‐3‐ium‐2‐yl)vinyl)cyclohex‐1‐en‐1‐yl)‐1,3‐dimethyl‐2,6‐dioxo‐1,2,3,6‐tetrahydropyrimidin‐4‐olate in combination with Cu(II)‐N,N,N′,N′′,N′′‐pentamethyldiethylene‐triamine complex, exhibited excellent activity with high yields. The possibility to synthesise different macromolecular structures such as telechelic polymers and block copolymers by NIR‐sensitized CuAAC is demonstrated. The structural and molecular characteristics of the intermediates and final products were evaluated by spectral and chromatographic analyses. This new photochemical reaction opens up new pathways for the macromolecular synthesis of complex macromolecular structures and applications requiring light as an activation tool.

New light-sensitive materials based on TiO2 and merocianin polymetine dye

The influence of the structure of a number of merocyanine dyes (D), which differ in the length of the polyethylene chain on one vinyl group, on their spectral, electrochemical and energy characteristics is established. Cyclic voltammetry determines their oxidation and reduction potentials. The values ​​of the HOMO and LUMO energy levels of the studied dyes from spectral and electrochemical data were calculated. It is shown that the obtained values ​​of the potentials of the energy levels of the dyes differ little (the energy difference ranges from 0,22–0,53 eV). The possibility of using merocyanins as sensitizers of titanium(IV) oxide and the creation of photosensitive heterostructures has been made. In order to investigate the possibility of using the merocyanin dyes under study as TiO2 sensitizers, the energy of the «semiconductor – dye – reagents» systems was considered. For this purpose, we compared the redox characteristics of merocyanine dyes with the electrophysical parameters of titanium(IV) oxide, as well as the redox characteristics of the reaction participants. It is established that in such systems the processes of photon transfer of the electron from the dye to the semiconductor are necessary for sensitization, and the regeneration of the original form of the sensitizers by the action of the reducing agent on the cation radicals formed is thermodynamically allowed. The conclusion about the possibility of using the studied merocyanines as effective sensitizers was confirmed by the results of the study of the photocatalytic activity of D/TiO2 heterostructures in the reaction of methylene blue formaldehyde recovery under different irradiation conditions and at different dye concentrations. The explanation for the found laws is offered. Scientific approaches to the production of new photosensitive systems with an expanded range of photosensitivity and a given level of photocatalytic activity have been developed.

Recent advances in photo-crosslinkable hydrogels for biomedical applications.

Photo-crosslinkable hydrogels have recently attracted significant scientific interest. Their properties can be manipulated in a spatiotemporal manner through exposure to light to achieve the desirable functionality for various biomedical applications. This review article discusses the recent advances of the most common photo-crosslinkable hydrogels, including poly(ethylene glycol) diacrylate, gelatin methacryloyl and methacrylated hyaluronic acid, for various biomedical applications. We first highlight the advantages of photopolymerization and discuss diverse photosensitive systems used for the synthesis of photo-crosslinkable hydrogels. We then introduce their synthesis methods and review their latest state of development in biomedical applications, including tissue engineering and regenerative medicine, drug delivery, cancer therapies and biosensing. Lastly, the existing challenges and future perspectives of engineering photo-crosslinkable hydrogels for biomedical applications are briefly discussed.

Role of phosphate in ruthenium-complex-sensitized TiO2 system for hydrogen production: Mechanism and kinetics

Most Ru(II) complex-sensitized TiO2 systems for hydrogen (H2) production suffer from instability of the photosensitized system because the anchoring groups of Ru(II) dyes, which are required for their adsorption on TiO2, are intrinsically vulnerable to chemical and photochemical cleavage. In this study, a new method that enables the use of a Ru(II) dye without any anchoring groups (Ru(bpy)32+) was developed and investigated. The stable photocatalytic efficiency in repeated H2 production cycles under visible-light irradiation indicates that the Ru(II) dye without anchoring groups is highly stable during dye-sensitized H2 production. The dye-sensitized H2 production in the Ru(bpy)32+-sensitized TiO2 system comprising Ru(bpy)32+ as a photosensitizer, platinized TiO2 (Pt-TiO2) as a cocatalyst-electron mediator, and ethylenediaminetetraacetic acid as an electron donor was negligible. However, the addition of phosphate (PO43–) to the Ru(bpy)32+-sensitized TiO2 system enabled the production of H2 via dye sensitization in the absence of any anchoring groups on the dye. The adsorption of PO43– changed the surface charge of Pt-TiO2 from positive to negative under acidic conditions, thereby inducing adsorption of cationic Ru(bpy)32+ on the surface of Pt-TiO2 and facilitating electron transfer from excited Ru(bpy)32+ to the conduction band of TiO2. The PO43– adsorption-induced change in the surface charge and the subsequent adsorption of Ru(bpy)32+ on the surface of PO43–-adsorbed Pt-TiO2 were confirmed by zeta potential measurements and Fourier transform infrared spectroscopy, respectively. In contrast with H2 production, the presence of PO43– had little effect on the kinetics of anionic chromate (CrO42–) reduction in the Ru(bpy)32+-sensitized TiO2 system. This result indicates that electron transfer from Pt to the electron acceptor on PO43–-adsorbed Pt-TiO2 is highly dependent on the charge character of the electron acceptor (i.e., electron transfer to the cationic electron acceptor is more favored). The negative charge on the surface of Pt-TiO2 induced by the adsorption of PO43– attracts the positively charged protons to the surface, which kinetically enhanced electron transfer from Pt to the protons. The (photo)electrochemical data demonstrate that PO43– adsorbed on Pt-TiO2 facilitates the interfacial electron transfer processes by enhancing the adsorption of Ru(bpy)32+ and attracting protons to the surface. The positive effect of PO43– on H2 production increased with increasing PO43– concentration and decreasing pH, where the conditions are more favorable for PO43– and proton adsorption on the surface of Pt-TiO2. Among the five anions evaluated in this study (PO43–, AsO43–, F–, NO3–, and SO42–), PO43– was most efficient and facilitated stable H2 production.

Photosensitive Polyimide having low loss tangent for RF application

Abstract Progress of 5G telecommunication and mm radar for autopilot, high frequency operation is required. Insulator materials having low loss at high frequency is desired for the applications. We designed the low dielectric constant, and low dielectric loss materials examined molecular structure of the polyimide and found that permittivity 2.6 at 20GHz, dielectric loss 0.002. Furthermore, in consideration of mechanical properties such as the toughness and adhesion to copper from a point of practical use. Dielectric properties largely turned worse when giving photosensitivity. To overcome the poor dielectric properties, we designed the photosensitive system. After all, we successfully obtained 3.5 of dielectric constant and 0.004 of dielectric loss, and 100% of elongation at break. In addition, we offered a B stage sheet as well as varnish. These materials are applicable to re-distribution layer of FO-WLP, Interposer and other RF applications for microelectronics.

Functionalizing the Mesoporous Silica Shell of Upconversion Nanoparticles To Enhance Bacterial Targeting and Killing via Photosensitizer-Induced Antimicrobial Photodynamic Therapy.

Core-shell nanoparticles operating by infrared-to-visible energy upconversion (UCNPs) have been proposed as theranostic carriers for photosensitizers to increase deep-tissue penetration of photodynamic therapy against tumors and bacterial infections. Herein we present a series of core-shell mesoporous silica-coated NaYF4:Yb:Er UCNPs (mSiO2@UCNP) with different surface functionalizations to enhance bacterial targeting and loaded with the hydrophobic photosensitizer SiPc (silicon 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine dihydroxide) to boost the bactericidal effect against Gram-positive and Gram-negative bacteria upon near-infrared irradiation. Förster resonance energy transfer (FRET) from the UCNP core to loaded SiPc was facilitated, while its efficiency depended on UCNP shell functionalization, which influences the SiPc penetration depth into the mesoporous silica, constituting a convenient tool to modify FRET intensity. Functionalized UCNPs displayed dark toxicity toward Gram-negative E. coli of up to 5 orders of magnitude, while Gram-positive S. aureus viability was not decreased in the dark, offering practical means for discriminating between the two bacterial strains. Directly exciting SiPc on the UNCP led to complete eradication of E. coli and a drastic decrease of colony-forming units of S. aureus of up to 7 orders of magnitude. With this study, we demonstrate strategies to potentiate antimicrobial photodynamic therapy on nanoparticular structures that can lead to next-generation photosensitizing systems based on UCNPs to help encounter and eradicate resistant bacteria, as well as for theranostics and future in vivo applications.

Diiodo-Bodipy-Encapsulated Nanoscale Metal–Organic Framework for pH-Driven Selective and Mitochondria Targeted Photodynamic Therapy

We report herein a new ZIF-90-based PDT agent which was synthesized by in situ assembly of imidazole-2-carboxaldehyde (IcaH), Zn(NO3)2, and heavy atom iodine-attached Bodipy. The obtained 2I-BodipyPhNO2@ZIF-90 (1) host-guest photosensitive system featured low cytotoxicity, good biocompatibility, pH-driven selective cancer cell uptake and release, mitochondria targeting, and highly efficient pH-triggered 1O2 generation. Therefore, it can be used as a high-performing PDT agent to selectively kill tumor cells. In comparison to free 2I-BodipyPhNO2, 1 exhibits a much higher antitumor efficacy and selectivity, which was confirmed by in vitro cell experiments.

Use of Cyclodextrins in Anticancer Photodynamic Therapy Treatment.

Photodynamic therapy (PDT) is mainly used to destroy cancerous cells; it combines the action of three components: a photoactivatable molecule or photosensitizer (PS), the light of an appropriate wavelength, and naturally occurring molecular oxygen. After light excitation of the PS, the excited PS then reacts with molecular oxygen to produce reactive oxygen species (ROS), leading to cellular damage. One of the drawbacks of PSs is their lack of solubility in water and body tissue fluids, thereby causing low bioavailability, drug-delivery efficiency, therapeutic efficacy, and ROS production. To improve the water-solubility and/or drug delivery of PSs, using cyclodextrins (CDs) is an interesting strategy. This review describes the in vitro or/and in vivo use of natural and derived CDs to improve antitumoral PDT efficiency in aqueous media. To achieve these goals, three types of binding modes of PSs with CDs are developed: non-covalent CD–PS inclusion complexes, covalent CD–PS conjugates, and CD–PS nanoassemblies. This review is divided into three parts: (1) non-covalent CD-PS inclusion complexes, covalent CD–PS conjugates, and CD–PS nanoassemblies, (2) incorporating CD–PS systems into hybrid nanoparticles (NPs) using up-converting or other types of NPs, and (3) CDs with fullerenes as PSs.

Femtosecond stimulated Raman line shapes: Dependence on resonance conditions of pump and probe pulses

Resonance enhancement has been increasingly employed in the emergent femtosecond stimulated Raman spectroscopy (FSRS) to selectively monitor molecular structure and dynamics with improved spectral and temporal resolutions and signal-to-noise ratios. Such joint efforts by the technique- and application-oriented scientists and engineers have laid the foundation for exploiting the tunable FSRS methodology to investigate a great variety of photosensitive systems and elucidate the underlying functional mechanisms on molecular time scales. During spectral analysis, peak line shapes remain a major concern with an intricate dependence on resonance conditions. Here, we present a comprehensive study of line shapes by tuning the Raman pump wavelength from red to blue side of the ground-state absorption band of the fluorescent dye rhodamine 6G in solution. Distinct line shape patterns in Stokes and anti-Stokes FSRS as well as from the low to high-frequency modes highlight the competition between multiple third-order and higher-order nonlinear pathways, governed by different resonance conditions achieved by Raman pump and probe pulses. In particular, the resonance condition of probe wavelength is revealed to play an important role in generating circular line shape changes through oppositely phased dispersion via hot luminescence (HL) pathways. Meanwhile, on-resonance conditions of the Raman pump could promote excited-state vibrational modes which are broadened and red-shifted from the coincident ground-state vibrational modes, posing challenges for spectral analysis. Certain strategies in tuning the Raman pump and probe to characteristic regions across an electronic transition band are discussed to improve the FSRS usability and versatility as a powerful structural dynamics toolset to advance chemical, physical, materials, and biological sciences.