Comparative Vision of Nonlinear Thermo-Optical Features and Third-Order Susceptibility of Mechanically Flexible Metallosupramolecular Self-Repairing Networks with Isomeric Organic Acids.

Two self-healing-type supramolecular Ni(II)-metallogels are achieved. The choice of proper low-molecular-weight organic gelators such as trans-butenedioic acid (i.e., trans-BDA) and cis-butenedioic acid (i.e., cis-BDA) and triethylamine in N,N'-dimethylformamide solvent facilitates the metallogelation process. Through rheological investigations the mechanical robustness and viscoelastic properties of synthesized metallogels are explored. An in-depth exploration of thixotropic behavior also supports their self-healing features. Notably, distinct variations in morphologies of metallogels are also ascertained through field emission scanning electron microscopy studies. Furthermore, the existence of versatile noncovalent supramolecular interactions operating throughout the metallogel network is clearly revealed via Fourier transform infrared spectroscopy. Electrospray ionization-mass studies also explore the construction protocol of individual Ni(II)-metallogels. The Z-scan measurements with a 532 nm continuous wave laser were employed to unveil the nonlinear thermo-optical response of two synthesized self-healing metallogels, i.e., trans-BDA-TEA@Ni(II) and cis-BDA-TEA@Ni(II). Crucial parameters like the nonlinear refractive index, nonlinear absorption coefficient, thermo-optical coefficient, and third-order susceptibility of these metallogels are obtained. Metallogels show negative signs for the nonlinear refractive index and the nonlinear absorption coefficient. The real parts of the third-order susceptibility for these metallogels are much greater than the imaginary parts (i.e., χR(3) > χI(3)), making such metallogels very promising for all optical-switching applications.

Solvent-Driven Variations of Third-Order Nonlinear Thermo-Optical Features of Glutaric Acid-Directed Self-Healing Supramolecular Ni(II) Metallogels.

The generation of solvent-directed self-healing supramolecular Ni(II) metallogels of glutaric acid (i.e., Ni-Glu-DMF and Ni-Glu-DMSO) is described in this article. Polar aprotic solvents like N,N'-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) are separately entrapped into the Ni(II)-acetate salt and glutaric acid-mediated networks to attain the semisolid flexible scaffolds. The gel nature of the fabricated materials is experimentally proven through different rheological tests such as amplitude sweep, frequency sweep, and thixotropic (time sweep) measurements. The self-repairing strategy and load-bearing features of the synthesized metallogel are studied in this work. The different supramolecular noncovalent interactions working within the soft scaffold are clearly explored. The formation strategy and the microstructural features of these synthesized metallogels are scrutinized through a Fourier transform infrared (FT-IR) spectroscopy study and field-emission scanning electron microscopy (FESEM) morphological analyses. The FT-IR spectroscopy observation displays a considerable amount of shifting of the infrared (IR) peaks of the xerogel samples of both the metallogels Ni-Glu-DMF and Ni-Glu-DMSO. The electrospray ionization (ESI)-mass spectroscopy result demonstrates the plausible construction of the metallogel network. In order to examine the nonlinear optical characteristics of the two synthesized self-healing metallogels Ni-Glu-DMSO and Ni-Glu-DMF, Z-scan measurements are carried out with a continuous wave (CW) diode-pumped solid-state (DPSS) laser at 532 nm. The nonlinear refractive index, nonlinear absorption coefficient, thermo-optical coefficient, and third-order susceptibility of these metallogels were evaluated by analyzing the experimental data from the Sheik-Bahae formalism. The nonlinear thermo-optical study reveals that these solvent-dependent metallogels show negative signs of nonlinear refractive index and nonlinear absorption coefficient. The figure of merit calculated for these compounds shows good agreement for their use in nonlinear photonic devices.

Norepinephrine exhibits thermo-optical nonlinearity under physiological conditions.

Norepinephrine (NE), a crucial modulatory neurotransmitter, plays a significant role in human physiology. Here, we use the Z-scan technique to investigate the nonlinear properties of NE at physiological conditions. Results reveal that NE exhibits thermo-optical nonlinearity. Outcomes can be utilized to investigate noradrenergic processes in correlation with various diseases.

A spectroscopic and computational intervention of interaction of lysozyme with 6-mercaptopurine.

In the present work, biophysical insight into the binding interactions of the protein, hen egg white (HEW) lysozyme (Lyz) with an anticancer drug, 6-mercaptopurine (6-MP)' was investigated by using a combination of spectroscopic and computational tools. 6-MP, a synthetic analog of natural purines, is a well-known anticancer drug and antiviral agent that inhibits the synthesis of RNA, DNA, and proteins. Lysozyme is a single-chain protein that can combine with endogenous and exogenous substances to exert its antiviral, antibacterial, and antitumor effects. The intrinsic fluorescence of lysozyme was quenched with the increased addition of 6-MP. The quenching mechanism was found to be static in nature as shown by the fluorescence lifetime and excitation spectrum measurements. The conformational changes of Lyz in the presence of 6-MP were monitored both at the ensemble and single-molecule level by using synchronous fluorescence spectroscopy, circular dichroism (CD), and fluorescence correlation spectroscopy (FCS). Molecular docking results predicted the probable binding sites for 6-MP on Lyz. The experimental findings are in good agreement with the results obtained by the molecular dynamics (MD) simulation study. Graphical abstract.

Nonlinear study of indolamines: A hidden property that might have possible implications in neurodegeneration.

Indolamines (e.g., serotonin and melatonin) are tryptophan-derived class of neurotransmitters and neuromodulators that play crucial roles in mood regulation, sleep-wake cycles, and gastrointestinal functions. These biogenic amines exert their effects by binding to specific receptors in the central nervous system, influencing neuronal activity and signalling cascades. Indolamines are vital in maintaining homeostasis, and imbalances in their levels have been implicated in various neurological and psychiatric disorders. Hence, in the present study, we have investigated the nonlinear properties of indolamines under a continuous wave (CW) and pulsed laser excitation using the closed-aperture (CA) Z-scan technique. The CA Z-scan is a cost-effective and sensitive analytical tool for investigating nonlinear properties. It is observed that indolamines show negative refractive and positive absorptive nonlinearity under in vitro physiological conditions. The origin of nonlinearity is ascribed to the thermo-optical effect governed by the saturated atomic absorption and molecular orientation mechanisms under CW and pulsed laser excitation, respectively. The strength of nonlinearity is found to vary linearly with the concentration of indolamines. Overall, serotonin possesses stronger nonlinearity than melatonin. The maximum nonlinearity (refractive index (n2) & absorption coefficient (ß)) for melatonin under CW and pulsed laser excitations are (-1.266 × 10-12 m2W-1 and -1.883 × 10-17 m2W-1) & (8.046 × 10-8 mW-1 and 1.516 × 10-13 mW-1), respectively. Meanwhile, the maximum n2 and ß under pulsed laser excitation for serotonin are obtained as -3.195 × 10-17 m2W-1 and 6.149 × 10-12 mW-1, respectively. The outcome of the results may be utilized in understanding processes mediated by indolamines and designing therapeutic interventions.

Hematin anhydride (ß-hematin): An analogue to malaria pigment hemozoin possesses nonlinearity.

Hematin anhydride (ß-hematin), the synthetic analogue of the malaria pigment, "hemozoin", is a heme dimer produced by reciprocal covalent bonds among carboxylic acid groups on the protoporphyrin-IX ring and the iron atom present in the two adjacent heme molecules. Hemozoin is a disposal product formed from the digestion of hemoglobin present in the red blood cells infected with hematophagous malaria parasites. Besides, as the parasites invade red blood cells, hemozoin crystals are eventually released into the bloodstream, where they accumulate over time in tissues. Severe malaria infection leads to significant dysfunction in vital organs such as the liver, spleen, and brain in part due to the autoimmune response to the excessive accumulation of hemozoin in these tissues. Also, the amount of these crystals in the vasculature correlates with disease progression. Thus, hemozoin is a unique indicator of infection used as a malaria biomarker and hence, used as a target for the development of antimalarial drugs. Hence, exploring various properties of hemozoin is extremely useful in the direction of diagnosis and cure. The present study focuses on finding one of the unknown properties of ß-hematin in physiological conditions by using the Z-scan technique, which is simple, sensitive, and economical. It is observed that hemozoin possesses one of the unique material properties, i.e., nonlinearity with a detection limit of âˆ¼ 15 µM. The self-defocusing action causes ß-hematin to exhibit negative refractive nonlinearity. The observed data is analyzed with a thermal lensing model. We strongly believe that our simple and reliable approach to probing the nonlinearity of ß-hematin will provide fresh opportunities for malaria diagnostics & cure in the near future.

Revealing the interaction mechanism between bovine serum albumin (BSA) and a fluorescent coumarin derivative: A multispectroscopic and in silico approach.

The interaction of biomacromolecules with each other or with the ligands is essential for biological activity. In this context, the molecular recognition of bovine serum albumin (BSA) with 4-(Benzo[1,3]dioxol-5-yloxymethyl)-7-hydroxy-chromen-2-one (4BHC) is explored using multispectroscopic and computational techniques. UV-Vis spectroscopy helped in predicting the conformational variations in BSA. Using fluorescence spectroscopy, the quenching behaviour of the fluorophore upon interaction with the ligand is examined, which is found to be a static type of quenching; fluorescence lifetime studies further verify this. The binding constant is discovered to be in the range of 104 M-1, which indicates the moderate type of association that results in reversible binding, where the transport and release of ligands in the target tissue takes place. Fourier-transform infrared spectroscopy (FT-IR) measurements validate the secondary structure conformational changes of BSA after complexing with 4BHC. The thermodynamic factors obtained through temperature-dependent fluorescence studies suggest that the dominant kind of interaction force is hydrophobic in nature, and the interaction process is spontaneous. The alterations in the surrounding microenvironment of the binding site and conformational shifts in the structure of the protein are studied through 3D fluorescence and synchronous fluorescence studies. Molecular docking and molecular dynamics (MD) simulations agree with experimental results and explain the structural stability throughout the discussion. The outcomes might have possible applications in the field of pharmacodynamics and pharmacokinetics.

Optimization of malaria detection based on third harmonic generation imaging of hemozoin.

The pigment hemozoin is a natural by-product of the metabolism of hemoglobin by the parasites which cause malaria. Previously, hemozoin was demonstrated to have a very high nonlinear optical response enabling third harmonic generation (THG) imaging. In this study, we present a complete characterization of the nonlinear THG response of natural hemozoin in malaria-infected red blood cells, as well as in pure isostructural synthesized hematin anhydride, in order to determine optimal imaging parameters for detection. Our study demonstrates the wavelength range for optimal pulsed femtosecond laser excitation of THG from hemozoin crystals. In addition, we show the hemozoin crystal detection as a function of crystal size, incident laser power, and the emission response of the hemozoin crystals to different incident laser polarization states. Our systematic measurements of the nonlinear optical response from hemozoin establish detection limits, which are essential for the optimal design of malaria detection technologies that exploit the THG response of hemozoin.

Phytoconstituents of Ashwagandha as potential inhibitors of human islet amyloid polypeptide (hIAPP): an in silico investigation.

Amylin or human islet amyloid polypeptide (hIAPP) is a small peptide co-secreted with insulin. Its peripheral aggregation on the lipid bilayer leads to fibril formation. The formation of hIAPP fibrils is hypothesized to rupture the membrane of ß -cells, which culminates in ß-cell death. Following additional studies, amylin fibril formation is a hallmark of T2DM and is also implicitly responsible for Alzheimer's disease. This study reports the virtual screening of 1000 phytoconstituents of traditional Indian medicinal plants to get potential inhibitors of amylin, which will likely restrict and block amyloid aggregation, preventing the progression of T2DM and Alzheimer's illness. The compounds having drug-likeness properties (acquired from ADMET calculations) and highest binding affinities (from molecular docking) are subjected to molecular dynamics (MD) simulation to investigate the temporal stability of the conformations of the complexes. This study discovers that Withaferin A and Withacoagulin have the highest binding affinity for amylin, and their stability with amylin was verified further by parameters such as RMSD, RMSF, number of H-bonds and MMPBSA. Individual principle component analysis (PCA) confirms the stable complex formation of amylin with Withaferin A and Withacoagulin. We strongly believe that wet-lab experiments and clinical trials will help to validate our computational findings.Communicated by Ramaswamy H. Sarma.

In silico investigation of spice molecules as potent inhibitor of SARS-CoV-2.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel infectious disease that is in rapid growth. Several trials are going on worldwide to find a solution for this pandemic. The viral replication can be blocked by inhibiting the receptor-binding domain (RBD) of SARS-CoV-2 spike protein (SARS-CoV-2 RBD Spro) and the SARS-CoV-2 main protease (SARS-CoV-2 Mpro). The binding of potential small molecules to these proteins can inhibit the replication and transcription of the virus. The spice molecules that are used in our food have antiviral, antifungal and antimicrobial properties. As spice molecules are consumed in the diet, hence its antiviral properties against SARS-CoV-2 will benefit in a significant manner. Therefore, in this work, the molecular docking of 30 selected spice molecules (screened through ADME property) was performed to identify the potential inhibitors for the RBD Spro and Mpro of SARS-CoV-2. We have found that though all the molecules bind actively with the SARS-CoV-2 RBD Spro and Mpro, but Piperine has the highest binding affinity among the 30 screened molecules. Besides, the comparative study between Piperine and currently used drugs show that Piperine is more effective. The interaction of Piperine with RBD Spro and Mpro is further validated by the molecular dynamics (MD) simulation studies. The free energy landscape and binding free energy results also, support for the stable complex formation of Piperine with RBD Spro and Mpro. We anticipate immediate wet-lab experiments and clinical trials in support of this computational study that might help to inhibit the SARS-CoV-2 virus. Communicated by Ramaswamy H. Sarma.

Interaction of vitamin B12 with ß-lactoglobulin: a computational study.

The ß-Lactoglobulin (ßLG) is a major whey protein that has the potential to bind various ligands; hence it is used as a model protein in protein-ligand interaction studies. Vitamin B12 is an essential nutrient for the human body, which helps in the synthesis of DNA, proteins, and the production of red blood cells. Binding interaction of vitamin B12 with ßLG will help to understand the potency of ßLG as a transporter for vitamin B12. Our experimental findings already showed that ßLG binds with vitamin B12 successfully (Swain et al., 2020). Nevertheless, to further support our experimental results firmly, here, we have employed computational tools such as molecular docking and molecular dynamics (MD) simulation. The molecular docking technique was used to elucidate the probable binding sites and binding affinity of vitamin B12 on ßLG. The docked complex of vitamin B12 with ßLG was subjected to MD simulation to investigate its stability and other interaction properties over a time frame. The study revealed that the compound is stable, and vitamin B12 imposes no change to the secondary structure of the ßLG. The computational results agree reasonably well with our experimental study.

Z-scan analysis and theoretical studies of dopamine under physiological conditions.

Dopamine (DA) is a widely researched catecholamine best known for its role in motor, motivation, addiction, and reward. Disruption in dopamine homeostasis and signaling within the central nervous system (CNS) can lead to disorders such as attention deficit hyperactivity disorder (ADHD), schizophrenia, Parkinson's disease, and obsessive-compulsive disorder. In the periphery, circulating DA is stored in blood platelets, and its disruption correlates with pathological conditions such as head and neck paragangliomas, Huntington's chorea, and schizophrenia. Various methods to sensitively and selectively detect dopamine have been reported, but sparse attempts have been made to exploit its intrinsic properties. Previously, we have harnessed dopamine's natural mid-ultraviolet auto-fluorescence to carry out its label-free imaging in live brain tissues. Recently, we used the closed-aperture (CA) Z-scan method to provide the first line of evidence on the existence of dopamine nonlinearity. Here, we utilized this simple, sensitive, and straightforward CA Z-scan technique and coupled this with theoretical simulations to further investigate the nonlinear photophysical properties of DA under physiological conditions. Our combined approach revealed that the nonlinear property of dopamine is governed by the thermo-optical effects, and the CA Z-scan profiles can be modulated by parameters such as phase-shift, orders of absorption, and time dependency. Simple and physiologically relevant systems, such as the platelets, are amenable to Z-scan analysis, thereby empowering us to scrutinize in the future if nonlinearity and its alterations, if any, have a direct bearing on DA homeostasis and associated diseases.

Spectroscopic and computational insight into the conformational dynamics of hemoglobin in the presence of vitamin B12.

Protein-ligand interactions play a significant role in all living organisms, thereby affecting the design and application of drugs and other biomaterials. The current study reports the binding of vitamin B12 to hemoglobin, employing optical spectroscopy and computational methods. It is observed that vitamin B12 quenched the intrinsic fluorescence of hemoglobin. The nature of quenching appears to be static according to the steady-state and time-resolved fluorescence measurements. The conformational changes of hemoglobin caused by vitamin B12 interactions were studied by synchronous fluorescence spectroscopy and protein secondary structure analyses. The synchronous fluorescence spectra indicate the tryptophan residue microenvironment change while no secondary structural change is observed from circular dichroism spectra and molecular dynamics (MD) simulation study. The computational molecular docking elucidated the probable binding of vitamin B12 at the active site of hemoglobin, whereas the stability of the hemoglobin-vitamin B12 complex was studied by MD simulation. The study might be helpful for the treatment of pernicious anemia, hereditary transcobalamin deficiency, and performance enhancement of elite athletes.

Conformational dynamics of myoglobin in the presence of vitamin B12: A spectroscopic and in silico investigation.

Myoglobin is an essential transport protein of heart and muscle tissues that acts as a local oxygen reservoir and a marker in different diseased conditions. On the other hand, Vitamin B12 is a vital nutrient that helps synthesize red blood cells, DNA, and proteins. To understand the ability of vitamin B12 to bind to the excess of myoglobin produced in the body under certain conditions (muscle injuries, severe trauma, etc.), it is essential to dig into the interaction between them. Therefore, the present study reports the binding interaction of vitamin B12 and myoglobin employing different spectroscopic and computational methods. The myoglobin's intrinsic fluorescence is quenched by vitamin B12 via static nature as observed from steady-state as well as time-resolved fluorescence measurements. The microenvironment of myoglobin's tryptophan residue gets affected, but there is no change observed in its α-helical content by vitamin B12 as seen from synchronous fluorescence and circular dichroism measurements. The probable binding of vitamin B12 on myoglobin was elucidated through molecular docking, and the interaction stability was studied by molecular dynamics simulation. The determination of vitamin B12's affinity to myoglobin and its effect on the conformational transitions of myoglobin might afford valuable insight for clinical pharmacology.

Spectroscopic insight into the interaction of dopamine with spherical gold nanoparticles.

Dopamine (DA) is a monoamine neurotransmitter of phenethylamine and catecholamine families, which is present in the central nervous system (CNS) and its periphery. Since DA is associated with several functions in the brain and body (motivational salience, reward, motor control, paracrine messenger, etc.), any imbalance in the DA level can trigger several neurodegenerative and other diseases. On the other hand, the spherical gold nanoparticles (AuNPs) can be used for drug delivery in several parts of the body. In addition, AuNPs also have the potentiality to penetrate through the blood-brain barrier and interact with the central nervous system without causing any toxicity. In view of many applications, it is important to look into the interaction between DA and AuNPs for a potential drug delivery model in DA related diseases. Here, we have used the steady-state and time-resolved fluorescence spectroscopic tools to investigate the binding interaction of DA with AuNPs. The nature of the quenching mechanism was confirmed through both steady-state and time-resolved fluorescence measurements. The binding constants along with the number of binding sites were estimated from the steady-state fluorescence measurements. The distance between DA and AuNPs was calculated using Förster's theory to verify the possibility of fluorescence resonance energy transfer (FRET) from DA to AuNPs.

A simulation study on the influence of energy migration and relative interaction strengths of homo- and hetero-FRET on the net FRET efficiency.

Förster resonance energy transfer (FRET) is a powerful method for probing biomolecular conformations and dynamics in bulk as well as at a single-molecule level. FRET utilizes non-radiative mechanisms to transfer energy between fluorophores, donor and acceptor when placed in close proximity. The FRET efficiency has a strong distance dependence and serves as a direct read-out for molecular interaction. In case of a significant overlap of donor emission and absorption spectra, the excited state energy can be exchanged between the identical donors in close proximity, which eventually migrates back and forth until it gets dissipated. This form of energy transfer is called energy migration or homo-FRET. Here, we have simulated FRET efficiency by considering the donor-donor interaction strength (ξDD) and donor-acceptor interaction strength (ξDA) under conditions of non-uniform distribution of molecules. Our earlier studies indicate that energy migration modulate the FRET efficiency for various values of ξDD and ξDA. We, therefore, determined the limiting values of acceptor concentration (CLA) that will allow the determination of FRET efficiency in the absence and presence of energy migration. Taken together, our study optimizes the conditions for meaningful FRET efficiency for a given FRET pair for better reporting of molecular interactions.

Biophysical study on complex formation between ß-Lactoglobulin and vitamin B12.

Biophysical insight into the binding interaction between the major whey protein, ß-Lactoglobulin (ßLG) and vitamin B12, was studied using different spectroscopic tools such as steady-state & time-resolved fluorescence spectroscopy, Circular Dichroism (CD) and Fluorescence Correlation Spectroscopy (FCS). The intrinsic fluorescence of ßLG was quenched by vitamin B12. From the time-resolved fluorescence experiment, the nature of quenching was found to be static suggesting ground-state complex formation between ßLG and vitamin B12, which was also supported by the excitation spectra. Synchronous fluorescence spectra revealed that the tryptophan residue microenvironment of ßLG was affected by the vitamin B12. The CD spectra suggested that the secondary structure of the ßLG remains unaffected by vitamin B12. From the FCS experiment, the tertiary structure of ßLG was observed to be stable in the presence of vitamin B12 at the single-molecule level. The outcome of this study might have potential applications in the food and pharmaceutical industry.

A Novel Tool to Investigate the Early and Late Stages of α-Synuclein Aggregation.

The accumulation of an inherently disordered protein α-synuclein (α-syn) aggregates in brain tissue play a pivotal role in the pathology and etiology of Parkinson's disease. Aggregation of α-syn has been found to be complex and heterogeneous, occurring through multitudes of early- and late-stage intermediates. Because of the inherent complexity and large dynamic range (between a few microseconds to several days under in vitro measurement conditions), it is difficult for the conventional biophysical and biochemical techniques to sample the entire time window of α-syn aggregation. Here, for the first time, we introduced the Z-scan technique as a novel tool to investigate different conformations formed in the early and late stage of temperature and mechanical stress-induced α-syn aggregation, in which different species showed its characteristic nonlinear characteristics. A power-dependent study was also performed to observe the changes in the protein nonlinearity. The perceived nonlinearity was accredited to the thermal-lensing effect. A switch in the sign of the refractive nonlinearity was observed for the first time as a signature of the late oligomeric conformation, a prime suspect that triggers cell death associated with neurodegeneration. We validate Z-scan results using a combination of different techniques, like thioflavin-T fluorescence assay, fluorescence correlation spectroscopy, Fourier-transform infrared spectroscopy, and atomic force microscopy. We believe that this simple, inexpensive, and sensitive method can have potential future applications in detecting/monitoring conformations in other essential peptides/proteins related to different neurodegenerative and other human diseases.

Ground- and excited-state dynamics of aluminum and gallium corroles.

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).

Mechanisms of low-power noncoherent photon upconversion in metalloporphyrin-organic blue emitter systems in solution.

The mechanisms of noncoherent photon upconversion that involve triplet-triplet annihilation (TTA) in solution have been investigated for two model systems. ZnTPP (meso-tetraphenylporphine zinc) is used as the model visible light-absorbing metalloporphyrin because its S(1) fluorescence intensity can be used to monitor the initial rate of porphyrin triplet state production and because its S(2) fluorescence intensity can be used as a direct measure of the rate of porphyrin TTA. When perylene, which has a triplet energy lower than that of ZnTPP, is added as a signaling blue emitter (BE), the mechanism of photon upconversion involves triplet energy transfer from the porphyrin to the BE followed by TTA in the BE to form the fluorescent perylene S(1) state. The kinetics of this process have been characterized and are unremarkable. When coumarin 343 (C343), which has photophysical properties similar to those of perylene except that it has a much higher triplet energy than ZnTPP, is added as the signaling BE, emission from the ZnTPP S(2) state is quenched and fluorescence from the C343 grows in. Contrary to previous suggestions, the mechanism of photon upconversion in this system does not involve singlet energy transfer from the porphyrin S(2) state to the BE. Instead, ground-state C343 complexes with the ZnTPP triplet to form a triplet exciplex, which then undergoes TTA with a second ZnTPP triplet to give the fluorescent state of the BE in a three-center process.