Nonradiative Decay Rate Constants Research Articles

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules.

Two photon absorption (2PA) is of great interest across many disciplines and there has been a large effort to increase the two-photon cross section (σ2) via synthetic modification, especially by enhancing intramolecular charge-transfer (ICT). This work takes the previously studied (7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)diphenylamine (AF240), an asymmetric D-π-A chromophore, and intentionally appends a functional group (-OH, AF240-OH or -OCH3, AF240-OMe) to the 6-position of the fluorenyl π-bridge of the new chromophores. Electrochemical results in both dichloromethane and acetonitrile support stabilization of the highest occupied molecular orbital in the derivatives due to inductive electron donating effects of the hydroxy and methoxy groups. The lowest unoccupied molecular orbital is stabilized via intramolecular hydrogen bonding to the benzothiazole moiety in the case of AF240-OH. As previously observed for AF240, the steady-state emission spectra show significant solvatochromism as they broaden and red shift with increasing solvent polarity. The fluorescence lifetimes and quantum yields show that the non-radiative rate constant is increased for AF240-OH in all solvents, especially in nonpolar media. The results suggest there is forced intramolecular hydrogen bonding to the benzothiazole in nonpolar solvents because the solvent poorly solubilizes the hydroxy group. This increases the non-radiative decay rate constant (knr) via additional vibrational decay pathways. While not as dramatic, the increase in knr in polar solvents supports some deactivation via hydrogen bonding to the solvent. Steric effects are also observed in the methoxy derivative, which inhibits planarization of the benzothiazole with the fluorene, increasing the energy of the excited state. Ultrafast transient absorption spectroscopy in tetrahydrofuran solution supports stabilization of the excited state in a few ps as solvent and structural reorganizations occur. In the case of AF240-OH, no evidence of proton transfer is observed. The decrease in emission energies in the case of AF240-OH support increased ICT driven by higher degree of coplanarity and the quinoidal structure in the excited state. However, a moderate increase in the intrinsic 2PA cross-section is resulted. It is likely because of the two possible and competing solvent-stabilized ICT processes (PICT and TICT) in AF240-OH. Nevertheless, the strategic presence of a hydroxide group capable of intramolecular hydrogen bonding in AF240-OH provides a much broader 2PA sensitivity window than AF240.

Synthesis and Optical Properties of Excited-State Intramolecular Proton Transfer Active π-Conjugated Benzimidazole Compounds: Influence of Structural Rigidification by Ring Fusion.

Two excited-state intramolecular proton transfer (ESIPT) active benzimidazole derivatives (1 and 2) were synthesized by acid-catalyzed intramolecular cyclization. The steady-state fluorescence spectrum in THF revealed that ring-fused derivative 1 exhibits a dual emission, namely, the major emission was from the K* (keto) form (ESIPT emission) at 515 nm with a large Stokes shift of 11 100 cm-1 and the minor emission was from the E* (enol) form at below 400 nm. In contrast, the normal emission from the E* form was dominant and the fluorescence quantum yield was very low (Φ ∼ 0.002) for nonfused derivative 2. The time-resolved fluorescence spectroscopy of 1 suggested that ESIPT effectively occurs due to the restricted conformational transition to the S1-TICT state, and the averaged radiative and nonradiative decay rate constants were estimated as ⟨kf⟩ = 0.15 ns-1 and ⟨knr⟩ = 0.60 ns-1, respectively. The fluorescence emission of 1 was influenced by the measurement conditions, such as solvent polarity and basicity, as well as the presence of Lewis base. The ESIPT process and solvatochromic behavior were nicely reproduced by the DFT/TDDFT calculation using the PCM model. In the single-crystal fluorescent spectra, the ESIPT emissions were exclusively observed for both fused and nonfused compounds as a result of hydrogen-bonding interactions.

Fluorescence Behavior of Schiff Base-N, N'-bis(salicylidene) Trans 1, 2-Diaminocyclohexane in Proteinous and Micellar Environments.

Fluorescence properties of N, N'-bis(salicylidene) trans 1, 2-diaminocyclohexane (H 2 L) is used to probe the anionic (SDS), cationic (CTAB) and nonionic (TX-100) micelles as well as in serum albumins (BSA and HSA) and chicken egg white lysozyme (LYZ) by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found to increase with concomitant blue-shift with gradual addition of different surfactants. All the experimental results suggest that the probe molecule resides in the micelle-water interface rather than going into the micellar core. However, the penetration is more towards the micellar hydrocarbon core in nonionic surfactant (TX-100) while comparing with ionic surfactants (SDS and CTAB). Several mean microscopic properties such as critical micelle concentration, polarity parameters and binding constant were calculated in presence of different surfactants. The decrease in nonradiative decay rate constants in micellar environments indicates restricted motion of the probe inside the micellar nanocages with increasing fluorescence emission intensity and quantum yields. Further in this work, we also investigated the interaction behavior of the probe with different proteins at low concentrations under physiological conditions (pH = 7.4). Stern-Volmer analysis of the tryptophan (Trp) fluorescence quenching data in presence of probe reveals Stern-Volmer constant (Ksv) as well as bimolecular quenching rate constant (Kq). The binding constant as well as the number of binding sites of the probe with proteins were also monitored and found to be 1:1 stoichiometry ratio.

Excited state intersystem crossing and the relaxation dynamics of phosphorescent Ir(III) complexes bearing bipyridine-based C^N ligand

Abstract The photo-deactivation mechanism of heteroleptic Ir(III)(C^N)2 (LX) complexes [(dfpypy)2Ir(pic)](1) and [(dfppy)2Ir(pic)](2) [where dfpypy = 2′,6′-difluoro-2,3′-bipyridine, dfppy = 2-(2,4-difluorophenyl)pyridine, pic = 2-picolinate] were elucidated by density functional theory/time-dependent density functional theory (DFT/TD-DFT) to rationalize the nature of the significant differences in luminescence efficiency between 1 and 2. Specifically, the radiative deactivation was formulated through the calculation of zero-field splitting (ZFS) and the radiative decay rate constant (kr) based on spin-orbit coupling (SOC). Meanwhile, the non-radiative deactivation was estimated by considering the structural distortion, the SOC between the emitting state and the ground state, as well as the thermal deactivation process via metal-centered excited 3MC state. Based on the results, 1 has much smaller SOC matrix elements between T1 and S0 than 2, which determines its small non-radiative decay rate constant, thus one may understand why 1 has higher phosphorescence quantum efficiency than 2. To further explore the structure-property relationship of Ir(III) complexes, four other new complexes 3–6 were designed by incorporating trimethylphenyl(R1), phenyl(R2), ter-butyl (R3), diphenylamine(R4) to the pyridine rings of dfpypy ligand of 1, respectively. Through analyzing, complex 4 with larger radiative decay rate and smaller non-radiative decay rate may be considered as a potential candidate as robust blue-emitting material applied in organic light emitting diodes (OLEDs).

TDDFT study on recognition mechanism for the oxygen sensing of the cyclometalated platinum (II) complex

The influence of oxygen molecule on the luminescent properties of a cyclometalated Pt(II) complex Lxp1, was investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Analysis of frontier molecular orbitals and electronic configuration indicated that the highest-occupied molecular orbital of the Lxp1 has a significant mixture of metal Pt (d) as well as 2-phenylpyridine and acetyl acetone(π). The lowest-unoccupied orbital of the Lxp1 primarily locates on π* of 2-phenylpyridineligands. The emission mechanism of the cyclometalated Pt(II) complex Lxp1 is assigned to the mixing of ligand-to-metal charge transfer and ligand-to-ligand charge transfer. The emission mechanism of the Lxp1–O2 complex can be attributed to the charge transfer from the oxygen molecule to the luminescent material Lxp1. Our study showed that intermolecular hydrogen bonding between the Lxp1 and oxygen molecule was strengthened by the calculation of electronic excitation, leading to a luminescence-decreasing phenomenon. The calculation of the radiative and non-radiative decay rate constants of the Lxp1 and the Lxp1–O2 complex demonstrates that the phosphorescence from T1-S0 of the Lxp1 would alter to the internal conversion from T1-T0 of the Lxp1–O2 complex. This alteration further explains the luminescence quenching phenomenon of the cyclometalated Pt(II) complex Lxp1 after interacting with oxygen molecule.

Excitonic coupling effect on the nonradiative decay rate in molecular aggregates: Formalism and application

We present here an analytical thermal vibration correlation function formalism to calculate the nonradiative decay rate constant (knr) considering excitonic coupling effect (ECE) for molecular aggregates based on split-operator approximation. Combining with first-principles calculations, we found that knr is enhanced by ECE for both H- and J-aggregates. In addition, ECE is found to be minor for the AIEgens (aggregation-induced emission luminogens).

Fine tuning phosphorescent properties of platinum complexes via different N-heterocyclic-based CˆNˆN ligands

In this article, series of platinum(II) complexes with different [CˆNˆN] cyclometalated ligand scaffolds was elaborated via density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods to mainly explore how the N atomic number, the atomic number of cyclization and the position of N atom influence phosphorescent processes (radiative and nonradiative decay processes). Thereinto, the factors which determine the radiative processes, including μ(Sn) for S0–Sn transitions, ΔE(Sn–T1) and SOC matrix elements〈T1| HSOC| Sn〉were calculated. In addition, according to the energy gap law, the absorption-emission Stokes shifts as well as energy gaps between T1 and S0 states were also computed to describe the nonradiative decay processes qualitatively. As revealed, compared with six-membered N-heterocyclic compounds, five-membered N-heterocyclic compounds exhibit obvious blue-shifted behaviors, smaller nonradiative decay rate constants and similar radiative decay rate constant, especially complex 2b with the other N atom at the 4-position of five-membered N-heterocyclic ligand, which leads to a maximum hypsochromic shift of phosphorescence band and relatively larger radiative decay rate constant and smaller nonradiative decay rate constant among these complexes. These findings successfully illustrated the structure–property relationship and provide valuable information to design highly efficient phosphorescent material.

Modulating FRET in Organic–Inorganic Nanohybrids for Light Harvesting Applications

The energy transfer efficiencies of organic–inorganic nanohybrids comprised of two structurally similar squaraine dyes and CdSe nanoparticles were studied in detail and compared. Carbazole based unsymmetrical squaraine dyes (CTSQ-1 and CTSQ-2) having modified absorption characteristics were considered for modulating the effect of the overlap integral on energy transfer rate with the designed QDs. CTSQ-2 with ∼1.75 times higher molar extinction coefficient and 35 nm red-shift in absorption resulted in an ∼2.4 times faster energy transfer rate with QD. The calculated energy transfer rates (kT = 1.35 × 108 s–1 and 3.26 × 108 s–1 respectively for QD:CTSQ-1 and QD:CTSQ-2 nanohybrids) are at least one order of magnitude higher than both radiative (kr = 5.97 × 106 s–1) and nonradiative decay rate constants (knr = 1.89 × 107 s–1) of QDs yielding very high FRET efficiency. The Stern–Volmer analysis of the quenching data indicated mainly static interaction of dyes with the QDs thus suggesting formation of organic–i...

Theoretical Investigation and Design of Highly Efficient Blue Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Groups

Aromatic sulfonyl groups have attracted increasing interest due to their unique electronic features. In this article, a series of IrIII complexes bearing fluorinated phenylsulfonyl groups were evaluated by density functional theory and time-dependent density functional theory methods. To explore their phosphorescence efficiencies, factors that determine the radiative decay rate constant, kr , and the nonradiative decay rate constant, knr , were computed. As demonstrated by the results, complex 4, which has fluorinated phenylsulfonyl groups at the 5-positions of the phenyl rings for all three C^N ligands, was found to have the highest phosphorescence efficiencies with the largest kr and smallest knr values among these complexes. Moreover, it was found to exhibit significantly blueshifted behavior relative to complex 1 and emits in the blue region, and thus, it can serve as a highly efficient blue emitter for application in organic light-emitting diodes. These findings successfully illustrated the structure-properties relationship and provided valuable information for the development of future highly efficient blue-emitting phosphors.

Computational Study of the Influence of the Binding Geometries of Organic Ligands on the Photoluminescence Quantum Yield of CdSe Clusters

This article describes density-functional-theory- (DFT-) based calculations of the rate constants for radiative (kR) and nonradiative (kNR) decay from the lowest singlet excited state (S1) to the ground state (S0) of a Cd16Se13 cluster ligated with various molecules in various binding geometries. The value of kR is suppressed by ligands whose localized orbitals, as a result of their binding geometry, become the cluster’s frontier orbitals and, thereby, decrease the overlap of the electron densities of the HOMO and LUMO necessary for efficient dipole coupling. Thiolate ligands in a monodentate geometry and dithioate ligands in a bridging geometry decrease kR in this manner. The value of kNR is also sensitive to the binding geometries of the ligands: binding geometries that are less rigid yield a greater change in nuclear coordinates between the S1 and S0 electronic states, which, in turn, increases the rate of nonradiative decay by maximizing the vibronic coupling between the band-edge exciton and the vibr...

Theoretical study on the neutral and ionic Cu(I) phosphorescent complexes with 2-(2′-quinolyl)benzimidazole and phosphine mixed ligand

The electronic structures and photophysical properties of a series of the neutral and ionic Cu(I) complexes with 2-(2′-quinolyl)benzimidazole and phosphine mixed ligand were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. To investigate the differences between the neutral and ionic complexes, the highest occupied molecular orbital (HOMO) energy, the lowest virtual molecular orbital (LUMO) energy, ΔEH-L (the energy gap between HOMO and LUMO), the ionization potentials (IPs), electron affinities (EAs) and reorganization energies (λ) were computed. To explain the reason of the metal-to-ligand charge transfer (MLCT) hardly detection in the neutral complexes, the absorption and emission spectra were studied in detail. To evaluate the photoluminescence quantum yield (PLQY), the non-radiative and radiative decay rate constants (knr and kr) were also presented. As a result, these calculations reveal that the structural distortion between the ground and excited states plays an important role in governing the PLQY. The charge transfer and the transport equilibrium property were markedly improved due to the lack of mobile counterions in the neutral form. Introducing the ether linkage together with the enhanced NˆN π-conjugation in 2b had obviously faster kr but slower knr, which led to its higher PLQY relative to the introducing the ether linkage only or lacking of a proton and the counterions in complexes.

Fluorescence characteristics of Schiff base-N,N/-bis(salicylidene) trans 1,2-diaminocyclohexane in the presence of bile acid host

Understanding the formation of bile acid micellar aggregates is of great importance due to its wide biological and pharmacological applications. In this paper, we study the fluorescence properties of Schiff base-N,N/-bis(salicylidene) trans 1,2-diaminocyclohexane (H2L) to understand micelle formation by aggregation of different important bile acids — cholic acid, deoxycholic acid, chenodeoxycholic acid and glycocholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found to increase with concomitant blue shift with gradual addition of different bile acids. Several mean microscopic properties such as critical micelle concentration, polarity parameters and binding constant were estimated in the presence of bile acids. The increase in fluorescence quantum yields, fluorescence decay times and substantial decrease in nonradiative decay rate constants in bile acid micellar environment points to the restricted motion of the fluorophore inside the micellar subdomains.

Fluorescence properties of Schiff base – N,N′-bis(salicylidene) – 1,2-Phenylenediamine in presence of bile acid host

Fluorescence properties of Schiff base – N,N′-bis(salicylidene) – 1,2-phenylenediamine (LH2) is used to study the micelles formed by aggregation of different important bile acids like cholic acid, deoxycholic acid, chenodeoxycholic acid and glycocholic acid by steady state and picosecond time-resolved fluorescence spectroscopy. The fluorescence band intensity was found out to increase with concomitant red shift with gradual addition of different bile acids. Binding constant of the probe with different bile acids as well as critical micelle concentration was obtained from the variation of fluorescence intensity on increasing concentration of bile acids in the medium. The increase in fluorescence quantum yields, fluorescence decay times and substantial decrease in nonradiative decay rate constants in bile acids micellar environment points to the restricted motion of the fluorophore inside the micellar subdomains.

Theoretical studies on the photophysical properties of luminescent pincer gold(iii) arylacetylide complexes: the role of π-conjugation at the C-deprotonated [C^N^C] ligand.

We have performed theoretical analyses of the photophysical properties of a series of cyclometalated gold(iii) arylacetylide complexes, [(C^N^C)AuIIIC[triple bond, length as m-dash]CPh-4-OMe], with different extents of π-conjugation at the doubly C-deprotonated [C^N^C] ligand via replacement of one of the phenyl moieties in the non-conjugated CH^N^C ligand (1) by a naphthalenyl (2) or a fluorenyl moiety (3-exo and 3-endo; HCH^N^CH = 2,6-diphenylpyridine). Conforming to the conventional wisdom that extended π-conjugation imposes rigidity on the structure of the 3IL(ππ*(C^N^C)) excited state (IL = intraligand), the calculated Huang-Rhys factors for the 3IL → S0 transition follow the order: 1 > 2 > 3-exo ∼ 3-endo, which corroborates qualitatively the experimental non-radiative decay rate constants, k nr: 1 ≫ 2 > 3-exo, but not 3-endo. Density Functional Theory (DFT) calculations revealed that there is an additional triplet excited state minimum of 3LLCT character (LLCT = ligand-to-ligand charge transfer; 3[π(C[triple bond, length as m-dash]CPh-4-OMe) → π*(C^N^C)]) for complexes 1 and 3-endo. This 3LLCT excited state, possessing a large out-of-plane torsional motion between the planes of the C^N^C and arylacetylide ligands, has a double minimum anharmonic potential energy surface along this torsional coordinate which leads to enhanced Franck-Condon overlap between the 3LLCT excited state and the ground state. Together with the larger spin-orbit coupling (SOC) and solvent reorganization energy for the 3LLCT → S0 transition compared with those for the 3IL → S0 transition, the calculated k nr values for the 3LLCT → S0 transition are more than 690- and 1500-fold greater than the corresponding 3IL → S0 transition for complexes 1 and 3-endo respectively. Importantly, when this 3LLCT → S0 decay channel is taken into consideration, the non-radiative decay rate constant k nr could be reproduced quantitatively and in the order of: 1 ≫ 3-endo, 2 > 3-exo. This challenges the common view that the facile non-radiative decay rate of transition metal complexes is due to the presence of a low-lying metal-centred 3dd or 3LMCT excited state (LMCT = ligand-to-metal charge transfer). By analysis of the relative order of MOs of the chromophoric [C^N^C] cyclometalated and arylacetylide ligands, one may discern why complexes 1 and 3-endo have a low-lying 3LLCT excited state while 3-exo does not.

Solvent effects on the optical spectra and excited-state decay of triphenylamine-thiadiazole with hybridized local excitation and intramolecular charge transfer.

The triphenylamine-thiadiazole molecule (TPA-NZP) is a newly popular, highly efficient OLED fluorescent emitter with exciton utilization efficiency exceeding the upper limit of spin statistics (25%). In this work, the optical spectra and the radiative and nonradiative decay rate constants have been investigated theoretically for TPA-NZP in hexane, ethyl ether, tetrahydrofuran, and dimethylformamide solvents, in comparison with the gas phase. We observed the evolutions of the excited states from the hybridized local and charge-transfer (HLCT) character to complete intramolecular charge transfer (CT) character with the increase of the solvent polarities. It is found that upon increasing the solvent polarity, the amount of red shift in the absorption peak is much less than that of emission, resulting in breakdown of the mirror symmetry. This is because that 0-0 transition energy is red-shifted but the vibrational relaxation increases with the solvent polarity, leading to subtraction in absorption while addition in emission. The radiative decay rate constant is calculated to be almost independent of polarity. The nonradiative decay rate increases by almost one order of magnitude from that in nonpolar hexane to the strongly polarized dimethylformamide, which is attributed to the dual effects of the red shift in the gap and enhancement of the vibrational relaxation by solvent polarity.

Solvent effect on the relative quantum yield and fluorescence quenching of a newly synthesized coumarin derivative.

We estimated the relative florescence quantum yield (Φ) of 8-methoxy-3-[1-(4,5-dicarbomethoxy-1,2,3-triazoloacetyl)]coumarin [8MDTC] using a single-point method with quinine sulfate in 0.1 M of sulfuric acid used as a standard reference. The fluorescence lifetimes, radiative and non-radiative decay rate constants are calculated. Relative quantum yields were found to be less in the non-polar solvents, indicating that the solute exhibits less fluorescence in a non-polar environment. The fluorescence quenching of [8MDTC] by aniline was studied at room temperature by examining the steady state in five different solvents in order to explore various possible quenching mechanisms. The experimental results show a positive deviation in Stern-Volmer plots in all solvents. Ground state complex and sphere of action static quenching models were used to interpret the results. Many quenching rate parameters were calculated using these models. The values of these parameters suggest that the sphere of action static quenching model agrees well with the experimental results. Further, a finite sink approximation model was used to check whether these bimolecular reactions were diffusion limited or not. The values of the distance parameter R' and the diffusion coefficient D were determined and are compared with the values of the encounter distance R and diffusion coefficient D calculated using the Stokes-Einstein equation.

Platinum(II) complexes of N^C^N-coordinating 1,3-bis(2-pyridyl)benzene ligands: thiolate coligands lead to strong red luminescence from charge-transfer states.

A new family of platinum(II) complexes of the form PtL(n)SR have been prepared, where L(n) represents a cyclometalating, N^C^N-bound tridentate ligand and SR is a monodentate thiolate ligand. The complexes fall into two groups, those of PtL(1)SR where HL(1) = 1,3-bis(2-pyridyl)benzene, and those of PtL(2)SR, where HL(2) = methyl 3,5-bis(2-pyridyl)benzoate. Each group consists of five complexes, where R = CH3, C6H5, p-C6H4-CH3, p-C6H4-OMe, p-C6H4-NO2. These compounds, which are bright red, orange, or yellow solids, are formed readily upon treatment of PtL(n)Cl with the corresponding potassium thiolate KSR in solution at room temperature. The replacement of the chloride by the thiolate ligand is accompanied by profound changes in the photophysical properties. A broad, structureless, low-energy band appears in the absorption spectra, not present in the spectra of PtL(n)Cl. In the photoluminescence spectra, the characteristic, highly structured phosphorescence bands of PtL(n)Cl in the green region are replaced by a broad, structureless emission band in the red region. These new bands are assigned to a πS/dPt → π*N^C^N charge-transfer transition from the thiolate/platinum to the N^C^N ligand. This assignment is supported by electrochemical data and TD-DFT calculations and by the observation that the decreasing energies of the bands correlate with the electron-donating ability of the substituent, as do the increasing nonradiative decay rate constants, in line with the energy-gap law. However, the pair of nitro-substituted complexes do not fit the trends. Their properties, including much longer luminescence lifetimes, indicate that the lowest-energy excited state is localized predominantly on the arenethiolate ligand for these two complexes. Red-emitting thiolate adducts may be relevant to the use of PtL(n)Cl complexes in bioimaging, as revealed by the different distributions of emission intensity within live fibroplast cells doped with the parent complex, according to the region of the spectrum examined.

Photophysics of a Coumarin in Different Solvents: Use of Different Solvatochromic Models

This study reported the photophysics of 7-(diethylamino)coumarin-3-carboxylic acid N-succinimidyl ester (7-DCCAE) in different neat solvents of varying polarity using steady-state absorption, fluorescence emission and picosecond time-resolved spectroscopy. In nonpolar solvents, the dye molecule predominantly exists in nonpolar structure and exhibits very low value of nonradiative decay rate constant (k(nr)), demonstrating the emission takes place from S(1) -LE to S(0) ground state. The fluorescence quantum yields, lifetime values of 7-DCCAE in different solvents are rationalized on the basis of intramolecular charge transfer (ICT) followed by twisted intramolecular charge transfer state formation (TICT) as well as specific solute-solvent interactions. Several solvatochromic models (such as Lippert, Dimroth, Kamlet-Taft, Catalán 3P and Catalán 4P models) were used to analyze the solvatochromic shift of 7-DCCAE in different solvents. The different empirical models show that the observed results are better correlate for nonchlorinated solvents and provide statistically significant best-fit result. A comparison was done between comparatively new solvatochromic model (Catalán 3P and Catalán 4P model) with Kamlet-Taft model. The ground state structure of the said molecule was optimized by using Density Functional Theory (DFT).

Tunable fluorescence upon aggregation: Photophysical properties of cationic conjugated polyelectrolytes containing AIE and ACQ units and their use in the dual-channel quantification of heparin

A series of cationic conjugated polyelectrolytes named as PTPExFyN, which consist of tetraphenylethylene (TPE) as aggregation-induced emission (AIE) units and fluorene (F) as aggregation-caused quenching (ACQ) units, have been synthesized by the quaternization of the corresponding neutral polymers. These cationic conjugated polyelectrolytes exhibited aggregation-induced emission enhancement (AIEE) and dual-channel fluorescence response (DCFR) when aggregated in solution, either by adding non-solvent or by adding heparin. The fluorescence intensity of PTPEN or PTPE0.5F0.5N in the fluorescence titration curves showed a linear increasing relationship with the concentration of heparin, while the fluorescence intensity of PTPE0.3F0.7N, PTPE0.2F0.8N and PTPE0.1F0.9N showed DCFR (i.e., two independent linear relationships) with the concentration of heparin, one attributed to the ACQ signal and the other attributed to the AIEE signal. The mechanism of AIEE was investigated by calculating the radiative and nonradiative decay rate constants using the experimental data of photoluminescence (PL) quantum yields and lifetimes of PTPExFyN in solution and aggregation state. At last, the successful quantification of heparin by the PTPE0.2F0.8N solution was demonstrated, which showed good linearity and sensitivity.

The Role of Ligands in Determining the Exciton Relaxation Dynamics in Semiconductor Quantum Dots

This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.