Excited-state Equilibrium Research Articles

Analytic Nuclear Gradients for Complete Active Space Linearized Pair-Density Functional Theory.

Accurately modeling photochemical reactions is difficult due to the presence of conical intersections and locally avoided crossings, as well as the inherently multiconfigurational character of excited states. As such, one needs a multistate method that incorporates state interaction in order to accurately model the potential energy surface at all nuclear coordinates. The recently developed linearized pair-density functional theory (L-PDFT) is a multistate extension of multiconfiguration PDFT, and it has been shown to be a cost-effective post-MCSCF method (as compared to more traditional and expensive multireference many-body perturbation methods or multireference configuration interaction methods) that can accurately model potential energy surfaces in regions of strong nuclear-electronic coupling in addition to accurately predicting Franck-Condon vertical excitations. In this paper, we report the derivation of analytic gradients for L-PDFT and their implementation in the PySCF-forge software, and we illustrate the utility of these gradients for predicting ground- and excited-state equilibrium geometries and adiabatic excitation energies for formaldehyde, s-trans-butadiene, phenol, and cytosine.

Contrasting caging effect on the photodynamics of 1′-hydroxy-2′-acetonaphthone

Confined environments, such as cyclodextrins, offer a convenient method to modulate the excited-state (ES) photodynamics of a probe molecule, and studies on such topics have attracted tremendous attention in numerous biological and chemical fields. Current study explores the photodynamic modulations of an ESIPT probe, 1′-hydroxy-2′-acetonaphthone (1HAN), by two homologous cyclodextrin derivatives having different cavity sizes, namely hydroxypropyl-β-cyclodextrin (HPβCD) and hydroxypropyl-γ-cyclodextrin (HPγCD), using multispectroscopic techniques. 1HAN shows substantial fluorescence enhancement after forming 1:1 inclusion complexes with both of these host molecules. Time-resolved (TR) anisotropy studies indicate substantially longer rotational relaxation times for the 1HAN-HP(β/γ)CD systems compared to the free 1HAN, further supporting the formation of inclusion complexes in the current systems. Intriguingly, TR fluorescence studies reveal contrasting modulation of the ES equilibrium between keto (K1*) and twisted keto (K2*) tautomers of 1HAN by HPβCD and HPγCD hosts, respectively. In the presence of the HPβCD host, the ES equilibrium between keto tautomers of 1HAN is shifted more towards the K1* form. On the contrary, the ES equilibrium shifts more towards the K2* form of the dye in the presence of the HPγCD host. This contrasting modulation in the ES equilibrium between K1* and K2* tautomers of 1HAN indicates that the dimension of the interior cavity of the host and hydrophobic interactions play a decisive role in regulating the photodynamics of the excited 1HAN dye.

Urea-fused and π-extended single-benzene fluorophores with ultralarge Stokes shifts.

The excited-state tautomer equilibrium of the urea-fused single-benzene fluorophore was synthetically modulated to produce exceptionally large Stokes shifts (>12 400 cm-1). The key N-H⋯N hydrogen bonding motif utilizes an endogenous proton for long-wavelength emission or an exogenous proton for acid-base chemistry, the balance of which is exploited for fluorescence switching in the solid state.

Gauge-Invariant Excited-State Linear and Quadratic Response Properties within the Meta-Generalized Gradient Approximation.

Gauge invariance is a fundamental symmetry connected to charge conservation and is widely accepted as indispensable for any electronic structure method. Hence, the gauge variance of the time-dependent kinetic energy density τ used in many meta-generalized gradient approximations (MGGAs) to the exchange-correlation (XC) functional presents a major obstacle for applying MGGAs within time-dependent density functional theory (TDDFT). Replacing τ by the gauge-invariant generalized kinetic energy density τ̂ significantly improves the accuracy of various functionals for vertical excitation energies [R. Grotjahn, F. Furche, and M. Kaupp. J. Chem. Phys. 2022, 157, 111102]. However, the dependence of the resulting current-MGGAs (cMGGAs) on the paramagnetic current density gives rise to new exchange-correlation kernels and hyper-kernels ignored in previous implementations of quadratic and higher-order response properties. Here we report the first implementation of cMGGAs and hybrid cMGGAs for excited-state gradients and dipole moments, as well as an extension to quadratic response properties including dynamic hyperpolarizabilities and two-photon absorption cross sections. In the first comprehensive benchmark study of MGGAs and cMGGAs for two-photon absorption cross sections, the M06-2X functional is found to be superior to the GGA hybrid PBE0. Additionally, two case studies from the literature for the practical prediction of nonlinear optical properties are revisited and potential advantages of hybrid (c)MGGAs compared to hybrid GGAs are discussed. The effect of restoring gauge invariance varies depending on the employed MGGA functional, the type of excitation, and the property under investigation: While some individual excited-state equilibrium structures are significantly affected, on average, these changes result in marginal improvements when compared against high-level reference data. Although the gauge-variant MGGA quadratic response properties are generally close to their gauge-invariant counterparts, the resulting errors are not bounded and significantly exceed typical method errors in some of the cases studied. Despite the limited effects seen in benchmark studies, gauge-invariant implementations of cMGGAs for excited-state properties are desirable from a fundamental perspective, entail little additional computational cost, and are necessary for response properties consistent with cMGGA linear response calculations such as excitation energies.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation.

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some TbIII and EuIII complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of TbIII /EuIII emission, EuIII /SmIII emission, EuIII /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

TDDFT versus GW/BSE Methods for Prediction of Light Absorption and Emission in a TADF Emitter.

Design concepts for organic light emitting diode (OLED) emitters, which exhibit thermally activated delayed fluorescence (TADF) and thereby achieve quantum yields exceeding 25%, depend on singlet-triplet splitting energies of order kT to allow reverse intersystem crossing at ambient temperatures. Simulation methods for these systems must be able to treat relatively large organic molecules, as well as predict their excited state energies, transition energies, singlet-triplet splittings, and absorption and emission cross sections with reasonable accuracy, in order to prove useful in the design process. Here we compare predictions of TDDFT with M06-2X and ωB97X-D exchange-correlation functionals and a GoWo@HF/BSE method for these quantities in the well-studied DPTZ-DBTO2 TADF emitter molecule. Geometry optimization is performed for ground state (GS) and lowest donor-acceptor charge transfer (CT) state for each functional. Optical absorption and emission cross sections and energies are calculated at these geometries. Relaxation energies are on the order of 0.5 eV, and the importance of obtaining excited state equilibrium geometries in predicting delayed fluorescence is demonstrated. There are clear trends in predictions of GoWo@HF/BSE, and TDDFT/ωB97X-D and M06-2X methods in which the former method favors local exciton (LE) states while the latter favors DA CT states and ωB97X-D makes intermediate predictions. GoWo@HF/BSE suffers from triplet instability for LE states but not CT states relevant for TADF. Shifts in HOMO and LUMO levels on adding a conductor-like polarizable continuum model dielectric background are used to estimate changes in excitation energies on going from the gas phase to a solvated molecule.

Intersystem Crossing Rates in Photoexcited Rose Bengal: Solvation versus Isolation.

We compare the intersystem crossing rate, kISC, of Rose Bengal (RB) in an aqueous pH 12 solution with the corresponding relaxation rates of four different RB-derived anion and dianion species isolated in the gas phase: the doubly deprotonated dianion ([RB-2H]2-), the singly deprotonated monoanion ([RB-H]-), and the corresponding singly negatively charged sodium and cesium adducts ([RB-2H + Na]- and [RB-2H + Cs]-, respectively). Each of them was probed following photoexcitation of their first singlet excited states (S1) at or near room temperature. The solution was studied by transient absorption spectroscopy, whereas the mass-selected anions were characterized by time-resolved photoelectron spectroscopy─all with ca. 50 femtosecond temporal resolution. [RB-H]- shows an S1 lifetime of ca. 80 ps; the solution ensemble, thought to consist primarily of solvated dianion chromophores, shows a similar lifetime of ca. 70 ps. By contrast, the isolated dianion, [RB-2H]2-, has a much longer lifetime. Superimposed on S1 decay attributable mainly to intersystem crossing, all four isolated anions also show some rapid oscillatory features of the transient photoelectron signal on a 4-5 ps timescale after excitation. Interestingly, an analogous phenomenon is also seen in the transient absorption measurements. We attribute it to a librational oscillation as the S1 state, initially populated in the S0 geometry, relaxes into its excited state equilibrium structure. Some implications of these observations for RB photophysics and interpretation of solution measurements are discussed─also in terms of density functional theory and time-dependent density functional theory calculations of ground and excited states.

Effective Photosensitization in Excited-State Equilibrium: Brilliant Luminescence of TbIII Coordination Polymers Through Ancillary Ligand Modifications.

Molecular photosensitizers provide efficient light-absorbing abilities for photo-functional materials. Herein, effective photosensitization in excited-state equilibrium is demonstrated using five TbIII coordination polymers. The coordination polymers are composed of TbIII ions (emission center), hexafluoroacetylacetonato (photosensitizer ligands), and phosphine oxide-based bridges (ancillary ligands). The two types of ligand combinations induces a rigid coordination structure via intermolecular interactions, resulting in high thermal stability (with decomposition temperatures above 300 °C). Excited-triplet-state lifetimes of photosensitizer ligands (τ=120-1320 μs) are strongly dependent on the structure of the ancillary ligands. The photosensitizer with a long excited-triplet-state lifetime (τ≥1120 μs) controls the excited state equilibrium between the photosensitizer and TbIII , allowing the construction of TbIII coordination polymer with high TbIII emission quantum yield (≥70 %).

AIE mechanism of 2-(2-hydroxyphenyl) benzothiazole derivatives: CASPT2 and spin-flip study

Wang et al. synthesized phenylacetylene modified HBT derivative (HBT-s-Ph) and diphenylacetylene modified HBT derivative (HBT-d-Ph) with aggregation-induced emission (AIE) phenomenon. In this work, we have theoretically compared the photochemical and photophysical processes of HBT-s-Ph and HBT-d-Ph in the gas phase, toluene and solid phase using several theoretical methods. The ground-state, excited-state equilibrium structures, the potential energy curves and the excited-state properties were comparatively calculated for a better understanding of the AIE mechanism. The excited-state intramolecular proton transfer (ESIPT) process within HBT derivatives is very fast in both gas phase, toluene, and solid phase due to the small energy barrier, inducing the formation of Keto tautomer which is responsible for emission peaks observed in experiment in both gas phase, toluene and solid phase. But, the reaction paths are very different from there. In toluene, the non-radiative path via HBT intramolecular rotation along the C–C bond is favorable to occur, leading to low Φf of HBT-s-Ph and HBT-d-Ph in toluene. In contrast, the non-radiative path of HBT intramolecular rotation is strongly suppressed in solid phase due to the high energy barriers, which is responsible for high Φf of HBT-s-Ph and HBT-d-Ph in solid phase. The substitution effect on AIE and the large difference of Φf in HBT-s-Ph and HBT-d-Ph in solid phase have been revealed. The presents results contribute to a better understanding of the different photophysical properties of HBT derivatives in toluene and solid phase, and unveil the AIE mechanism in HBT derivatives, which provides deep insight into molecular design of organic photoelectric materials with high Φf.

Quantum-Classical Approach for Calculations of Absorption and Fluorescence: Principles and Applications.

Absorption and fluorescence spectroscopy techniques provide a wealth of information on molecular systems. The simulations of such experiments remain challenging, however, despite the efforts put into developing the underlying theory. An attractive method of simulating the behavior of molecular systems is provided by the quantum–classical theory—it enables one to keep track of the state of the bath explicitly, which is needed for accurate calculations of fluorescence spectra. Unfortunately, until now there have been relatively few works that apply quantum–classical methods for modeling spectroscopic data. In this work, we seek to provide a framework for the calculations of absorption and fluorescence lineshapes of molecular systems using the methods based on the quantum–classical Liouville equation, namely, the forward–backward trajectory solution (FBTS) and the non-Hamiltonian variant of the Poisson bracket mapping equation (PBME-nH). We perform calculations on a molecular dimer and the photosynthetic Fenna–Matthews–Olson complex. We find that in the case of absorption, the FBTS outperforms PBME-nH, consistently yielding highly accurate results. We next demonstrate that for fluorescence calculations, the method of choice is a hybrid approach, which we call PBME-nH-Jeff, that utilizes the effective coupling theory [GelzinisA.;J. Chem. Phys.2020, 152, 05110332035455] to estimate the excited state equilibrium density operator. Thus, we find that FBTS and PBME-nH-Jeff are excellent candidates for simulating, respectively, absorption and fluorescence spectra of real molecular systems.

Multiple-Valence and Visible to Near-Infrared Photoluminescence of Manganese in ZnGa2O4: A First-Principles Study

The transition metal manganese is attractive for magnetism and luminescence engineering due to the nontrivial interplay of charge, spin, lattice, and orbital freedom degrees. Here, we report the manganese site occupancy and valence states by taking experimental synthesis conditions into consideration and the optical features of target intra-atomic Mn2+, Mn3+, and Mn4+ in the framework of density functional theory in a prototype spinel ZnGa2O4 host. The formation energy results show that Mn2+ dominates in tetrahedral ligands at the Zn2+ site and Mn3+ and Mn4+ dominate in octahedral ligands at the Ga3+ site under the corresponding different chemical potential conditions, and the distortion of the local coordination environment of Mn3+ in octahedral ligands due to the Jahn–Teller effect is studied in detail. The excited-state equilibrium geometries and energies are obtained via spin or orbital occupancy constraints, which are further complemented by the Tanabe–Sugano diagram to predict the energy spectra and to interpret the experimental results on Mn2+, Mn4+, and Mn3+. In particular, the competition of 5T2 and 1T2 excited states and the relaxation processes leading to near-infrared luminescence of rarely reported Mn3+ are analyzed. First-principles calculations complemented by the Tanabe–Sugano diagram analysis may serve as an effective and predictive tool for exploring valence states, energy structures, and luminescence of complexes containing 3dn transition-metal ions.

Excited states of six oxazine 1 conformers in aqueous solution: TD-DFT/DFT study

The paper analyzes the electronic-vibrational states of six possible conformers of the dye oxazine 1 (also known as oxazine 725) in an aqueous solution using the SMD solvent model and the 6-31++G(d,p) basis set. Vibronic absorption spectra are calculated using a wide range of hybrid functionals. The X3LYP functional gives the best agreement with the experiment for the positions of the main maximum and shoulder. According to the calculations, the shoulder in the experimental spectrum is of vibronic origin. Various characteristics of the ground and excited states (Duschinsky matrices, IR spectra, atomic charges, dipole moments, maps of the distribution of the electrostatic potential) were obtained. All six investigated conformers have similar vibronic absorption spectra and frontier molecular orbitals. Significant differences between them are observed only for the set of vibronic transitions and relaxation from nonequilibrium to an equilibrium excited state. The effect of two strongly bound water molecules on the excitation of the dye was analyzed.

Tuning the dual emission of keto/enol forms of excited-state intramolecular proton transfer (ESIPT) emitters via intramolecular charge transfer (ICT)

Herein disclosed the adjustment of the dual emission of keto and enol forms of excited-state intramolecular proton transfer (ESIPT) emitters via intramolecular charge transfer (ICT) effects. Introducing electron-donating triphenylamine (TPA) and electron-withdrawing triphenylboron (TPB) substituents into the para-position of the phenolic hydroxyl group or the side of the oxazole of 2-(2′-hydroxyphenyl)oxazole skeleton endows the resulting compounds (6a-6d) with different photophysical properties. Owing to the ICT effect from electronic donor to acceptor, introducing TPA into the side of oxazole and TPB into the para-position of phenolic hydroxyl is conducive to an enol-form emission (6a). Exchanging the two substituents, namely introducing TPB into the side of oxazole and TPA into the para-position of phenolic hydroxyl, would be beneficial to a keto-form emission (6b). Synchronously introducing two identical substituents, whether TPA or TPB, into two sides of 2-(2′-hydroxyphenyl)oxazole skeleton (6c and 6d) would lead to the dual emission of keto and enol forms due to the excited-state equilibrium of ESIPT reactions, which are further verified by DFT calculation. The organic light-emitting diode (OLED) devices with 6a and 6c as emitters were fabricated, both of which exhibit hybridized local and charge transfer (HLCT) excited-state characters with high external quantum efficiencies (EQEs) of 4.9% and 5.6%, respectively.

Characterizing the dynamical phase diagram of the Dicke model via classical and quantum probes

We theoretically study the dynamical phase diagram of the Dicke model in both classical and quantum limits using large, experimentally relevant system sizes. Our analysis elucidates that the model features dynamical critical points that are distinct from previously investigated excited-state equilibrium transitions. Moreover, our numerical calculations demonstrate that mean-field features of the dynamics remain valid in the exact quantum dynamics, but we also find that in regimes where quantum effects dominate signatures of the dynamical phases and chaos can persist in purely quantum metrics such as entanglement and correlations. Our predictions can be verified in current quantum simulators of the Dicke model including arrays of trapped ions.

Controlling Thermally Activated Delayed Photoluminescence in CdSe Quantum Dots through Triplet Acceptor Surface Coverage.

Quantum-dot/molecule composites (QD/mol) have demonstrated useful photochemical properties for many photonic and optoelectronic applications; however, a comprehensive understanding of these materials remains elusive. This work introduces a series of cadmium(II) selenide/1-pyrenecarboxylic acid (CdSe/PCA) nanomaterials featuring bespoke PCA surface coverage on CdSe585 (coded by the peak of the first exciton absorption band) to glean insight into the QD/mol photophysical behavior. Tailoring the energy gap between the CdSe585 first exciton band (2.1 eV) and the lowest PCA triplet level (T1 = 2.0 eV) to be nearly isoenergetic, strong thermally activated delayed photoluminescence (TADPL) is observed resulting from reverse triplet-triplet energy transfer. The resultant average decay time constant (τobs) of the photoluminescence emanating from CdSe585 is deterministically controlled with surface-bound PCAn chromophores (n = average number of adsorbed PCA molecules) by shifting the triplet excited state equilibrium from the CdSe585 to the PCA molecular triplet reservoir as a function of n.

Temperature and solvent-dependent photoluminescence quenching in [Ru(bpy)2(bpy-cc-AQ)]2.

I have herein investigated the solvent-dependent photoluminescence quenching mechanism of [Ru(bpy)2(bpy-cc-AQ)]2+ using variable temperature emission spectroscopies. The photophysics of this complex are dominated by an excited-state thermal equilibrium between a photoluminescent 3MLCT state and a charge-separated state that lies higher in energy relative to the 3MLCT state in low polarity solvents and approximately isoenergetic in high polarity solvents. Furthermore, an unusual photoluminescence temperature-dependence in high polarity solvents is shown to arise from competition between enthalpic factors favouring the charge-separated state and entropic factors favouring the photoluminescent 3MLCT state, analogous to the molecular light-switch effect of [Ru(bpy)2(dppz)]2+. The solvent-dependent photoluminescence quenching of [Ru(bpy)2(bpy-cc-AQ)]2+ is attributed to two key solvent-dependent factors: (1) the excited-state equilibrium position and (2) the rate of charge-recombination from the charge-separated state.

Nonequilibrium steady-state picture of incoherent light-induced excitation harvesting.

We formulate a comprehensive theoretical description of excitation harvesting in molecular aggregates photoexcited by weak incoherent radiation. An efficient numerical scheme that respects the continuity equation for excitation fluxes is developed to compute the nonequilibrium steady state (NESS) arising from the interplay between excitation generation, excitation relaxation, dephasing, trapping at the load, and recombination. The NESS is most conveniently described in the so-called preferred basis in which the steady-state excitonic density matrix is diagonal. The NESS properties are examined by relating the preferred-basis description to the descriptions in the site or excitonic bases. Focusing on a model photosynthetic dimer, we find that the NESS in the limit of long trapping time is quite similar to the excited-state equilibrium in which the stationary coherences originate from the excitation-environment entanglement. For shorter trapping times, we demonstrate how the properties of the NESS can be extracted from the time-dependent description of an incoherently driven but unloaded dimer. This relation between stationary and time-dependent pictures is valid, provided that the trapping time is longer than the decay time of dynamic coherences accessible in femtosecond spectroscopy experiments.

Modulation of excited-state photodynamics of ESIPT probe 1′-hydroxy-2′-acetonaphthone (HAN) on interaction with bovine serum albumin

The 1′-hydroxy-2′-acetonaphthone (HAN) is a well-known excited-state intramolecular proton-transfer (ESIPT) dye that has been used as a useful fluorescence probe to understand various homogenous and microheterogenous media. However, interaction of HAN with proteins is still largely less explored. The present study is aimed to investigate the interaction of HAN with the most abundant animal circulatory protein, bovine serum albumin (BSA), using multi-spectroscopic techniques and molecular docking studies. The photophysical properties of HAN undergo significant modulation upon its interaction with BSA protein. The confinement and reduced polarity experienced by bound HAN into BSA binding pocket causes a large fluorescence enhancement. The binding constant for HAN-BSA complex is evaluated to be ∼4.3 × 104 M−1. Time-resolved fluorescence measurements reveal the exclusive presence of only keto (K*) form and twisted keto rotamer (KR*) form of the dye in the excited state inside the BSA binding pocket, though in the ground state the dye exists entirely in its enol (E) form. Results suggest a quantitative ESIPT reaction for HAN even inside the BSA pocket. Time resolved fluorescence anisotropy studies indicate very tight binding of HAN into the BSA pocket such that independent rotational diffusion of the dye is completely ceased. Molecular docking studies provide best docking conformation and involved interaction forces, suggesting the preferred binding of HAN to the site IIA (Sudlow's site I) of the BSA protein. Observed photophysical results provide valuable insights of HAN–BSA binding interaction, revealing for the first time that the intriguing excited-state equilibrium between K* and KR* forms of HAN is largely shifted towards KR* form, as the dye binds to the BSA pocket. To the best of our knowledge such an intriguing modulation in the excited state prototautomeric conversion process has not been reported before for the studied dye in the presence of any other host. Results of the present study can be useful in understanding the pharmacology of structurally similar dyes/drugs, having relevance to drug delivery, pharmaceutical sciences, sensors, and so on.

Position isomers of Ru(II) polypyridine complexes with tunable photophysical properties, aggregation-induced phosphorescence enhancement and application in triplet-triplet annihilated upconversion

Three Ru(II) polypyridine complex isomers (Ru-2-DP, Ru-3-DP and Ru-5-DP) have been synthesized using phenanthroline-based isomers containing 4-(2,2-diphenyl-vinyl)-phenyl unit. Their photophysical properties are systematically investigated. Notably, three coordination isomers exhibit much longer triplet lifetimes compared to the model complex [Ru(bpy)2(Phen)]2+. The density function theory calculations reveal that the extension of triplet lifetimes is attributed to the establishment of an excited-state equilibrium between the intraligand (3IL) state and the metal-to-ligand charge-transfer (3MLCT) state. In aggregates, Ru-2-DP shows an interesting aggregation-induced phosphorescence enhancement (AIPE) property. The phosphorescence intensity of Ru-2-DP increases by 16.2-fold from acetonitrile solution to aggregated state. However, Ru-3-DP and Ru-5-DP are AIPE-inactive. Single-crystal analyses show substituent position has a dramatic effect on intramolecular steric hindrance, leading to different molecular conformation and packing pattern. Using three coordination isomers as triplet sensitizers for triplet-triplet annihilated (TTA) upconversion, Ru-3-DP and Ru-5-DP display obvious upconversion properties, but it's quite the opposite for Ru-2-DP. Experimental data demonstrate 3- and 5-positions of phenanthroline, especially 3-position, are beneficial to enhance intersystem crossing and triplet-triplet energy transfer and for the resulting upconverted efficiency enhancement. This work definitely suggests that minor structural change may have major effects upon the solid-state spectroscopic properties and TTA upconversion performances, which provides a rational basis for designing excellent solid phosphorescent materials and triplet sensitizers.

Identification and Quantification of Bacterial Pathogens with a Ratiometric Fluorescent Sensor Array

Timely and efficient identification of pathogenic microorganisms is of high importance for public health and safety. Current bacterial pathogen analysis relies on phenotypic identification of the bacterial species by Gram staining and culturing in combination with other biochemical methods and PCR. These methods, however, are sometimes inconclusive, time-consuming and equipment-demanding, which delays treatment and limits their application to laboratory settings. The development and use of molecular sensing techniques has a potential for overcoming the limitations of the traditional phenotypic approaches. Most of currently developed sensing methods are based on optical and/or electrochemical techniques for analysis of biomarkers or whole bacteria [1], where selectivity is usually achieved by using antibodies, their fragments, natural or engineered peptides, or aptamers. Important limitations for using these elements as a part of the sensor are cost, stability and adaptability: they often require special handling and are specific to only a narrow range of analytes. Multivariative analysis using sensor arrays has been actively developed for various applications, including detection and analysis of small molecules and ions, proteins and even whole microorganisms [2]. Ratiometric dyes have been long explored as sources of a self-referenced analytical signal that is independent from the concentration of the probe. However, relatively few sensor arrays have been explored leveraging this principle.Derivatives of 3-hydroxyflavone have been widely explored as powerful and sensitive tools to study different parameters of their microenvironment [3]. Owing to the excited-state intramolecular proton transfer (ESIPT) taking place in their molecules, they exhibit two distinct emission bands in their fluorescence spectra. In addition to that, these molecules can participate in other protolytic equilibria. For example, in basic media or under the influence of strong external hydrogen bond acceptors (solvent or other molecules), 3-hydroxyflavones form anions whose fluorescence is located between the emission bands of the normal and tautomer forms of the neutral molecule undergoing ESIPT. Leveraging both ground- and excited-state equilibria, two independent ratiometric signals can be obtained from a single probe dye.Sensor array components were designed by introducing various substituents into the 3-hydroxyflavone core, thus leading to differences in the specific interactions of the dyes with their microenvironment, and potentially in their localization in bacteria [4]. The response patterns were investigated upon interaction with eight different bacterial species equally representing Gram positive and Gram negative strains. Linear discriminant analysis is usually used for data processing, however, other methods such as support vector machines can also be applied. Analysis of the canonical score plots using the first two discriminants results in excellent separation of all groups of sensor responses originating from different bacterial strains. An interesting detail can be further observed: signals from the Gram-positive bacteria tend to group on the left side of the canonical score plot, whereas those coming from the Gram-negative bacterial species cluster on the right side. This evidences for some distinctions in the spectral features of the dyes fluorescence upon interaction with bacteria of different Gram status. The discriminant analysis of these data revealed the ability of the sensor array to correctly identify bacterial species and perfectly recognize the Gram status of the analyzed bacteria. This illustrates the potential versatility of applications of the proposed sensor.From the general perspective, being “trained” to recognize the responses of the eight bacterial species, the system is intrinsically incapable of recognizing other strains beyond this list. However, this limitation is not necessarily valid for the broader application of the obtained dataset, which can be exemplified by Gram status recognition and “one against the rest” analysis. In these cases, samples representing bacterial species excluded from the training dataset will still belong to one of the “known” classes. To test this hypothesis with Gram status determination, a truncated training dataset was used. The results show that the system was able to correctly recognize the Gram status in 100% of the cases, even when the samples represented bacterial species excluded from the training dataset. A similar test for “one against the rest” discrimination showed correct attribution in 89% of the cases.We further expand the functionality of a ratiometric sensor array by demonstrating its successful application for the analysis of mixed bacterial samples. Using pattern recognition methods and data from pure bacterial species, we demonstrate that this approach can be used to quantify the composition of mixtures, as well as to predict their components with the accuracy of approximately 80% without the need to acquire additional reference data.Thus, the proposed sensor array not only can be used for classification and multiparametric analysis of the samples, but is also capable of characterizing the unknown samples beyond the species used in training datasets. The sensor’s functionality can be expended to allow analysis and quantification of mixed samples, and even prediction of the mixture components. The described approach also provides important insights into data processing for the analysis of other complex samples.