Excited State Decay Dynamics Research Articles

Rise and Decay of Photoluminescence in Upconverting Lanthanide-Doped Nanocrystals.

Nanocrystals (NCs) doped with lanthanides are capable of efficient photon upconversion, i.e., absorbing long-wavelength light and emitting shorter-wavelength light. The internal processes that enable upconversion are a complex network of electronic transitions within and energy transfer between dopant centers. In this work, we study the rise and decay dynamics of upconversion emission from β-NaYF4 NCs codoped with Er3+ and Yb3+. The rise dynamics of the red and green upconverted emissions are nonlinear, reflecting the nonlinear nature of upconversion and revealing the mechanisms that populate the emitting states. The excited-state decay dynamics are nonexponential. We unravel the underlying decay pathways using photonic experiments. These reveal the contributions of different upconversion pathways visually, as each pathway exhibits a distinct response to systematic variation of the local density of optical states. Moreover, the effect of the local density of optical states on core-only NCs is qualitatively different from core-shell NCs. This is due to the different balance between feeding and decay of the electronic levels that produce upconverted emission. The understanding of the upconversion dynamics provided here could lead to better imaging and sensing methods relying on upconversion lifetimes or guide the rational optimization of the dopant concentrations for brighter upconversion.

Nuclear Quantum Effects on Nonadiabatic Dynamics of a Green Fluorescent Protein Chromophore Analogue: Ring-Polymer Surface-Hopping Simulation.

Herein, we have used the "on-the-fly" ring-polymer surface-hopping simulation method with the centroid approximation (RPSH-CA), in combination with the multireference OM2/MRCI electronic structure calculations to study the photoinduced dynamics of a green fluorescent protein (GFP) chromophore analogue in the gas phase, i.e., o-HBI, at 50, 100, and 300 K with 1, 5, 10, and 15 beads (3600 1 ps trajectories). The electronic structure calculations identified five new minimum-energy conical intersection (MECI) structures, which, together with the previous one, play crucial roles in the excited-state decay dynamics of o-HBI. It is also found that the excited-state intramolecular proton transfer (ESIPT) occurs in an ultrafast manner and is completed within 20 fs in all the simulation conditions because there is no barrier associated with this ESIPT process in the S1 state. However, the other excited-state dynamical results are strongly related to the number of beads. At 50 and 100 K, the nuclear quantum effects (NQEs) are very important; therefore, the excited-state dynamical results change significantly with the bead number. For example, the S1 decay time deduced from time-dependent state populations becomes longer as the bead number increases. Nevertheless, an essentially convergent trend is observed when the bead number is close to 10. In contrast, at 300 K, the NQEs become weaker and the above dynamical results converge very quickly even with 1 bead. Most importantly, the NQEs seriously affect the excited-state decay mechanism of o-HBI. At 50 and 100 K, most trajectories decay to the S0 state via perpendicular keto MECIs, whereas, at 300 K, only twisted keto MECIs are responsible for the excited-state decay. The present work not only comprehensively explores the temperature-dependent photoinduced dynamics of o-HBI, but also demonstrates the importance and necessity of NQEs in nonadiabatic dynamics simulations, especially at relatively low temperatures.

Formulation of transition dipole gradients for non-adiabatic dynamics with polaritonic states.

A general formulation of the strong coupling between photons confined in a cavity and molecular electronic states is developed for the state-interaction state-average spin-restricted ensemble-referenced Kohn-Sham method. The light-matter interaction is included in the Jaynes-Cummings model, which requires the derivation and implementation of the analytical derivatives of the transition dipole moments between the molecular electronic states. The developed formalism is tested in the simulations of the nonadiabatic dynamics in the polaritonic states resulting from the strong coupling between the cavity photon mode and the ground and excited states of the penta-2,4-dieniminium cation, also known as PSB3. Comparison with the field-free simulations of the excited-state decay dynamics in PSB3 reveals that the light-matter coupling can considerably alter the decay dynamics by increasing the excited state lifetime and hindering photochemically induced torsion about the C=C double bonds of PSB3. The necessity of obtaining analytical transition dipole gradients for the accurate propagation of the dynamics is underlined.

Prototropically Controlled Dynamics of Cytosine Photodecay

The effect of the existence of several prototropic tautomers of cytosine on its UV/vis spectra and the excited state decay dynamics is studied by spectral and nonadiabatic molecular dynamics (NAMD) simulations in connection with the mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method. Simulated UV/vis spectra provide a strong indication that the H3N keto-amino cytosine tautomer (the least anticipated species) may be present under experimental conditions. The NAMD simulations yield a wide range of excited state decay constants for various tautomers of cytosine, ranging from ∼1.3 ps for the biologically relevant H1N keto-amino tautomer to ∼0.1 ps for the keto-imino tautomer. The slowness of the H1N decay dynamics follows from the presence of a barrier on the excited state energy surface separating the Franck-Condon structure from the major decay funnel, the conical intersection seam. It is suggested that the experimentally observed photodecay dynamics may result from a combination of the decay processes of various tautomers (H3N in particular) present simultaneously under the experimental conditions.

Modulation of the Photophysics of Nucleotide-Functionalized Copper Nanoclusters Using Aqueous Binary Mixtures

Nanomaterials have been steadily gaining importance over the past few decades. Metal nanoclusters comprised of gold or silver have been thoroughly studied and well documented. Only recently, the focus has begun to shift to copper nanoclusters (CuNCs). However, there are very few studies that focus on the modulation of the photophysics of metal nanoclusters, especially CuNCs. Dimethyl sulfoxide (DMSO) is an industrially and biologically important solvent. Aqueous DMSO binary mixtures have been frequently used for modulation of excited-state dynamics of many conventional fluorophores. Here, by performing the time-resolved fluorescence spectroscopy of nucleotide-functionalized CuNCs in aqueous DMSO binary mixtures, we have demonstrated that DMSO negatively affects the excited-state decay dynamics of CuNCs by destabilizing the excited state, leading to faster decay dynamics. We have proposed that DMSO disrupts the water-mediated internanocluster (interNC) cuprophilic interactions, which are mostly responsible for stabilization of the CuNC excited state. These results will provide significant physical insight on techniques to modulate the radiative decay dynamics of CuNCs and other nanomaterials.

Computational studies on the excited state decay rates in aggregates of two‐coordinate Cu (I) complexes: Thermally Activated Delayed Fluorescence and Aggregation Induced Emis

AbstractTwo‐coordinate Cu (I) complexes have attracted great interest recently because of the rich photophysical property in solid state, including the aggregation‐induced thermal activated delayed fluorescence. Here, we summarize our theoretical investigations on the excited state structure and decay dynamics for the two‐coordinate Cu (I) complexes in solution phase and solid state by the thermal vibration correlation function rate formalism we developed earlier coupled with time‐dependent density‐functional theory within polarizable continuum model and hybrid quantum and molecular mechanics. First, for the CAACCu (I)Cl complex, we found that the nature of the excited state undergoes a change from metal‐to‐ligand charge transfer (MLCT) in solution to hybrid halogen‐to‐ligand charge transfer and MLCT in solid state. The bending vibrations of the CCuCl and CuCN bonds are restricted in aggregates, reducing the non‐radiative decay rate to cause strong solid‐state fluorescence. Second, for CAACCu (I)Cz, we found that both intersystem crossing (ISC) and reverse intersystem crossing (rISC) are enhanced by 2–4 orders of magnitudes upon aggregation, leading to highly efficient thermally activated delayed fluorescence (TADF). The enhanced ISC and rISC rates can be attributed to the increase of the metal proportion in the frontier molecular orbitals, leading to an enhanced spin−orbit coupling between S1 and T1. The reaction barriers for ISC and rISC are much lower in solution than that in aggregate phase resulting in a decrease in energy gap and an increase in the relative reorganization energy through bending the angle ∠C − Cu − N for T1. Our theoretical studies provide a clear rationalization for the highly efficient solid‐state luminescence character of two‐coordinate Cu (I) complexes and may clarify the ongoing dispute on the understanding of the high TADF quantum efficiency.

Through-Space Interaction of Tetraphenylethylene: What, Where, and How.

Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission character, fluoresces blue emission in the aggregate state, and such photoluminescence is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C═C bond. However, in this work, systematic spectroscopic studies and DFT theoretical simulation reveal that the intramolecular through-space interaction (TSI) between two vicinal phenyl rings generates the bright blue emission in TPE but not the TBC effect. Furthermore, the evaluation of excited-state decay dynamics suggests the significance of photoinduced isomerization in the nonradiative decay of TPE in the solution state. More importantly, different from the traditional qualitative description for TSI, the quantitative elucidation of the TSI is realized through the atoms-in-molecules analysis; meanwhile, a theoretical solid-state model for TPE and other multirotor systems for studying the electronic configuration is preliminarily established. The mechanistic model of TSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC and expands the quantum understanding of molecular behavior to the aggregate level.

Excited-state properties of newly sensitized imidazole-arylamine-based organic DSSC sensitizers in solvent and adsorbed on TiO2/FTO support

Six recently synthesized arylamine-imidazole combo dyes, utilized in dye-sensitized solar cells staining TiO2 semiconductor, were spectroscopically investigated. Studies were performed with application of steady-state and time-resolved absorption spectroscopies and were done on dyes dissolved in dichloromethane and anchored on TiO2/FTO film support. The investigations allowed to characterize the excited state decay dynamics of individual compounds, determine time constants of dye-to-TiO2 electron injection process for dyes adsorbed on TiO2/FTO support and subsequent recombination and dye regeneration dynamics. These studies were performed on bare dye/TiO2/FTO photoanodes in air environment and, if possible, on the same samples with dye layer immersed in electrolyte solution. These evaluations allowed to determine approximate time constants for dye regeneration and directed toward dye that reveals most advantageous properties and hypothetically should outperform its counterparts as TiO2 sensitizer.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution.

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the 1ππ*, 1nπ*, 3ππ*, 3nπ*, and S0 states, we have identified several feasible excited-state relaxation pathways for the initially populated S2(1ππ*) state to decay to the initial enol isomer' S0 state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless 1ππ* ESIPT process to generate the 1ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S1 → S0 internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S2(1ππ*) minimum, an S2(1ππ*)/S1(1nπ*)/T2(3nπ*) three-state intersection region helps the S2 system branch into the T1 state through a S2 → S1 → T1 or S2 → T2 → T1 process. Once it has reached the T1 state, the system may relax to the S0 state via direct ISC or via subsequent nearly barrierless 3ππ* ESIPT to yield the T1 keto tautomer and ISC. The resultant S0 keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T1/S0 crossing points, the ISC to S0 state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S1(1ππ*) state and the lower-lying T1 and T2 states. In the major relaxation pathway, the photoexcited S1 keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S1 enol minimum, near which a favorable S1/S0 conical intersection decays the system to the S0 state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S0 keto species. In the minor one, an S1/T2/T1 three-state intersection in the keto region makes the T1 state populated via direct and T2-mediated intersystem crossing (ISC) processes. In the T1 state, an ESIPT occurs, which is followed by ISC near a T1/S0 crossing point in the enol region to the S0 state and finally back to the S0 keto species. In addition, a T1/S0 crossing point near the T1 keto minimum can also help the system decay to the S0 keto species. However, small spin-orbit couplings between T1 and S0 at these T1/S0 crossing points make ISC to the S0 state very slow and make the system trapped in the T1 state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Ultrafast decay dynamics of N-ethylpyrrole excited to the S1 electronic state: A femtosecond time-resolved photoelectron imaging study

N -ethylpyrrole is one of ethyl-substituted derivatives of pyrrole and its excited-state decay dynamics has never been explored. In this work, we investigate ultrafast decay dynamics of N - ethylpyrrole excited to the S1 electronic state using a femtosecond time-resolved photoelectron imaging method. Two pump wavelengths of 241.9 and 237.7 nm are employed. At 241.9 nm, three time constants, 5.0±0.7 ps, 66.4±15.6 ps and 1.3±0.1 ns, are derived. For 237.7 nm, two time constants of 2.1±0.1 ps and 13.1±1.2 ps are derived. We assign all these time constants to be associated with different vibrational states in the S1 state. The possible decay mechanisms of different S1 vibrational states are briefly discussed.

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole.

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S1(1πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 11ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S1(1πσ*) state might be one of the decay mechanisms of the 11ππ* state.

Excited-state dynamics of isolated and (micro)solvated methyl sinapate: the bright and shady sides of a natural sunscreen

ABSTRACT The increasing awareness of the adverse effects of exposure to UV radiation in combination with the conclusion that presently employed sunscreen agents are far from optimal has led to the need to develop novel UV filters with improved absorption and stability characteristics. Studies of natural sunscreens can provide fundamental insight into strategies to come to a rational design of such filters. Here, we use high-resolution laser spectroscopic methods to study the spectroscopy and excited-state dynamics of methyl sinapate, a prominent plant sunscreen derivative. We find that our experimental approach based on two-color Resonance Enhanced Two-Photon Ionisation spectroscopy enables us to observe a hitherto not observed decay pathway leading to a long-lived state, albeit with a significantly lower yield than in cinnamates and coumarates. In combination with extended excitation spectra and employing the results of quantum chemical calculations a comprehensive picture is obtained of the internal conversion pathways that are accessible to this compound. Similar high-resolution studies on clusters of methyl sinapate with water show how solvent-solute interactions affect the electronic structure and excited-state decay dynamics of the chromophore.

Mechanistic Insight into Selective Sensing of Hazardous Hg2+ and Explosive Picric Acid by Using a Pyrene‐Azine‐Hydroxyquinoline Framework in Differential Media

AbstractA pyrene‐hydroxyquinoline tethered ingeniously designed and developed azine based Schiff base luminophoric system, 2‐((Z)‐((E)‐(pyren‐1‐ylmethylene)hydrazono)methyl)quinolin‐8‐ol (PHQ) has been explored as a selective ‘TURN ON’ chromofluorogenic sensor for hazardous Hg2+ ions in ethanol/H2O (9 : 1,v/v) medium with detection limit of 0.22 μM and binding constant of 1.09 x 103 M−1. The detail scrutiny of mechanism of interaction between PHQ and Hg2+ is rationalized through various techniques like 1H NMR titration, FT‐IR, mass spectral analysis, theoretical computations, Job's plot, dynamic light scattering (DLS), transmission electron microscopy (TEM), time resolved excited state decay dynamics and fluorescence reversibility studies, which concretely declare a synergistic effect of supramolecular aggregation induced enhanced emission and deactivation of photo‐induced electron transfer (PET) processes. On the other hand, a ‘TURN OFF’ luminescence feature of highly fluorescent PHQ hydrosol is utilized for the selective screening of powerful explosive picric acid (PA) in aqueous medium and mechanism is deciphered by means of steady state, time‐resolved emission as well as computational studies which altogether reflect a combined effect of PET and ground state complexation between probe and PA. The commendable detection limit of 55 nM and Stern‐Volmer quenching constant of 1.48 x 104 M−1 with high quenching efficiency of 84% increases the pertinency of the nano‐probe.

Improving oxygen sensing performance via inner-molecular π-π stacking in a series of phosphorescent Cu(I) complexes.

In this paper, six phosphorescent Cu(I) complexes with three diamine ligands and two phosphorous ligands were prepared. Detailed discussion was performed on these complexes, including single crystals, quantum mechanics theoretical calculation, absorption spectra, emission spectra, emission quantum yields and excited state decay dynamics. Large conjugation planes and π-π stacking were found in these complexes. Their emission was originated from MLCT excited state. Long-lived emissive center was observed due to this MLCT-based decay and the help from π-π stacking. Such long-lived emissive state and the large conjugation planes in these complexes offered enough collision probability with O2 molecules, making themselves potential oxygen sensing probes. These six complexes were then doped into silica supporting matrix MCM-41 spheres. The resulting composite samples and their emission sensing response towards O2 were discussed in detail. The optimal sample showed sensitivity as high as 7.80 with response time of 14s. A careful discussion between Cu(I) complex molecular structure and sensing performance was performed. It was concluded that both a long lifetime and a large conjugation plane lead to improved sensing sensitivity since they increased the collision probability with O2 molecules. On the other hand, it was found that both sensing response and recovery times were mainly controlled by O2 diffusion in supporting matrix.

Excited State Decay Dynamics in 3‐Formyl‐4‐hydroxy Benzoic Acid: Understanding the Global Picture of an ESIPT‐Driven Multiple‐Emissive Species

AbstractRelaxation dynamics of excited state intramolecular proton transfer (ESIPT) probe 3‐formyl‐4‐hydroxy benzoic acid (FHBA) was studied by steady state, nanosecond time‐resolved fluorescence (TRF) and femtosecond transient absorption (TA) experiments in combination with adequate level of quantum chemical calculations. Global analysis of the transient data with target model reveals that enol conformers of both the neutral FHBA (I) and proton dissociated mono‐anionic species (III) undergo ESIPT in picosecond time scale (τESIPT ≈1.2 ps) towards the formation of corresponding vibrationally hot keto structure. Rapid intramolecular vibrational cooling (τ≈20 ps) follows singlet deactivation of the corresponding conformers with fluorescence lifetime (τfluo) of ∼5.1 and 2.0 ns, respectively as confirmed from stimulated emission (SE) and/or time resolved fluorescence experiments. Selective excitation of the di‐anionic structure (II) at 400 nm follows relatively simple excited state non‐ESIPT decay dynamics having fluorescence emission at 500 nm with τfluo ≈ 1.2 ns. Complete optimization of individual structures and construction of ESIPT profile was done by HF(CIS) as well as (TD)DFT calculation using 6–311++G(d,p) basis set and found to be consistent with the experimental observations.

Deactivation of Excited States in Transition-Metal Complexes: Insight from Computational Chemistry.

Investigation of the excited-state decay dynamics of transition-metal systems is a crucial step for the development of photoswitchable molecular based materials with applications in growing fields as energy conversion, data storage, or molecular devices. The photophysics of these systems is an entangled problem arising from the interplay of electronic and geometrical rearrangements that take place on a short time scale. Several factors play a role in the process: various electronic states of different spin and chemical character are involved, the system undergoes important structural variations and several nonradiative processes can occur. Computational chemistry is a useful tool to get insight into the microscopic description of the photophysics of these materials, since it provides unique information about the character of the electronic spin states involved, the energetics and time evolution of the system. In this review article, we present an overview of the state of the art methodologies available to address the several aspects that have to be incorporated to properly describe the deactivation of excited states in transition-metal complexes. The most recent developments in theoretical methods are discussed and illustrated with examples.

Spin-Vibronic Mechanism for Intersystem Crossing.

Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin-orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin-orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin-orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the spin-vibronic mechanism for ISC in both fundamental as well as applied research fields has been revealed with significant impact across chemistry, physics, and biology. In this review article, we present the theory and fundamental principles of the spin-vibronic mechanism for ISC. This is followed by empirical rules to estimate the rate of ISC within this regime. The most recent developments in experimental techniques, theoretical methods, and models for the spin-vibronic mechanism are discussed. These concepts are subsequently illustrated with examples, including the ISC mechanisms in transition metal complexes, small organic molecules, and organic chromophores.

Ultrafast excited state dynamics of six-branched styryl derivatives based on 1,3,5-triazine: Dissolved in chloroform solution or formed spin-coating film

Nondegenerate pump-probe experiment was performed to study the effect of the environment on the excited state decay dynamics of six-branched styryl derivatives based on 1,3,5-triazine. The two compounds were dissolved in CHCl3 solutions for research in solvent environment, and the spin-coating films were also prepared for reasearch in agglomerate state. Obvious different excited state dynamics were observed under two conditions. A fast process about several ps and a long process ∼100ps were observed for compounds in CHCl3 solutions. However, another ultrafast process ∼0.8ps was observed for compounds in spin-coating film condition, indicating some other decay channels and some processes of other mechanism.

Different ultrafast dynamics of several dithienylethene photochromism derivatives: Under parallel and perpendicular polarization configurations

In this work, two-color pump-probe measurements, were performed to study the effect of the structural configuration on the excited state decay dynamics of dithienylethene photochromism systems under ring-closure state. Obvious different transient absorption dynamics were observed under parallel and perpendicular polarization configurations. The rising part of the perpendicular component probed at wavelength of 800nm was clearly delayed compared with the parallel component, indicating some overlap between pure excited state decay dynamic and some processes of other mechanism.