Photophysics of aromatic molecules with low-lying pi sigma* states: excitation-energy dependence of fluorescence in jet-cooled aromatic nitriles

Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state.     

Effect of solvents on the photophysical properties of substituted imidazonaphthyridine derivatives

The absorption and fluorescence characteristics of three substituted imidazonaphthyridine derivatives are studied in a series of organic solvents. The variation of Stokes shift with the polarity parameter of the solvent is studied and the excited state dipole moment of these derivatives is higher than the ground state dipole moment. The fluorescence lifetime profile shows single exponential decay in all the solvents. The fluorescence quantum yield, radiative and non-radiative rate constants are also calculated and these parameters show much variation in the change in substitution of these derivatives.     

MOLECULAR LUMINESCENCE OF SOME ISOALLOXAZINES IN APOLAR SOLVENTS AT VARIOUS TEMPERATURES

Abstract— –Spectral properties of isoalloxazines in organic solvents of low polarity are determined at 300 and 77 K. Vibrational structure in the spectra reveals a vibrational mode of 1250cm^-. The pure electronic transition energies are established to a greater accuracy than was done previously and comparison to theoretical data is made. Actual lifetimes up to 10 ns for fluorescence and 300 ms for phosphorescence are found. The ratio of the actual fluorescence lifetime and the radiative lifetime is found to agree well with the quantum yield. Solvent interactions hardly shift the energy of the first electronically excited singlet state but merely affect the Franck-Condon envelope of the spectrum and the non radiative decay of the chromophore. In albne solutions at 77 K isoalloxazine clusters are formed exhibiting P-type delayed fluorescence.     

D60K Mb定点突变对肌红蛋白结构及荧光量子产率的影响

为了探究肌红蛋白表面60位的Asp突变为Lys后对蛋白荧光强度的影响,在模拟生理条件下,利用荧光法和圆二色法对WT Mb及D60K Mb的结构及荧光光谱特性进行了对照研究,结果表明D60K Mb的荧光量子产率远大于WT Mb。通过研究pH值和结构对荧光量子产率的影响,发现Mb 60位天冬氨酸(Asp)突变为赖氨酸(Lys)后,蛋白表面电荷发生改变,导致蛋白的荧光量子产率明显增强;同时使蛋白内部的芳香族氨基酸较原来更暴露,极性增强,激发态分子的去激发过程由原来的无辐射能量转移为主转变为辐射能量转移为主,最终导致荧光强度增强。     

Determination of energetics and kinetics from single-particle intermittency and ensemble-averaged fluorescence intensity decay of quantum dots

Quantification of energetics and kinetics for the band-edge exciton states of quantum dots and the long-lived dark state is important for better understanding of the underlying mechanism for single-particle intermittency and ensemble fluorescence intensity decay. Based on a multistate diffusion-reaction model by extending our previous studies, we analyze experimental data from ensemble measurements and fluorescence intermittency of single quantum dots and determine important molecular-based quantities such as Stokes shift, free energy gap, activation energy, reorganization energy, and other kinetic parameters.     

Excited state properties of oligophenyl and oligothienyl swivel cruciforms

A comprehensive study has been undertaken of the electronic spectral and photophysical properties of two oligophenyl (BPH and BPHF) and one oligothienyl (BTF) swivel cruciforms involving measurements of absorption, fluorescence, and phosphorescence spectra, quantum yields of fluorescence (phiF), phosphorescence (phiPh) and triplet formation (phiT), lifetimes of fluorescence (tauF) and of the triplet state (tauT), and quantum yields of singlet oxygen production (phiDelta). From these, all radiative kF and radiationless rate constants, kIC and kISC, have been obtained in solution. The energies of the lowest lying singlet and triplet excited states were also determined at 293 K. Several of the above properties have also been obtained at low temperature and in the solid state (thin films). In general, for the phenyl oligophenyl (BPH) and for the oligothienyl (BTF) compounds, the radiationless decay channels (phiIC+phiISC) are the dominant pathway for the excited-state deactivation, whereas with the fluorene based oligophenyl BPHF the radiative route prevails. In contrast to the general rule found for related oligomers (and polymers) where radiative emission from T1 is absent, with the compounds studied, phosphorescence has been observed for all of the compounds, indicating that this type of functionalization can lead to emissive triplets. Time-resolved fluorescence decays with picosecond resolution revealed multiexponential (bi- and triexponential) decay laws compatible with the existence of more than one species or conformation in the excited state. These results are discussed on the basis of conformational flexibility in the excited state.     

Photophysics and Inverted Solvatochromism of 7,7,8,8-Tetracyanoquinodimethane (TCNQ)

We report absorption, fluorescence, and Raman spectroscopy of 7,7,8,8-tetracyanoquinodimethane (TCNQ) in a variety of solvents. The fluorescence quantum yields (QYs) of linear alkane solutions are similar to one another, but QY is shown to acutely decrease in other solvents with increasing polarities. The slope of the solvatochromic plot of absorption maxima is inverted from negative to positive with an increase in solvent polarity. A significant change in the frequency of carbon-carbon double bond stretching modes is not observed in Raman spectra of TCNQ in different solvents. The molar absorption coefficient is determined to calculate the oscillator strength of the absorption band. The radiative decay rate constant calculated from the oscillator strength is approximately ten times larger than that elucidated from the fluorescence lifetime and QY. These spectroscopic parameters reveal that the relaxation occurs from a Franck-Condon excited state to a distinct fluorescence emissive state with a smaller transition dipole moment.     

Swelling-controlled polymer phase and fluorescence properties of polyfluorene nanoparticles

Highly fluorescent nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene) (PFO) with distinct phases were prepared, and their photophysical properties were studied by steady state and time-resolved fluorescence spectroscopy. An aqueous suspension of PFO nanoparticles prepared by a reprecipitation method was observed to exhibit spectroscopic characteristics consistent with the glassy phase of the polymer. We demonstrate that controlled addition of organic solvent leads to partial transformation of the disordered polymer chains into the planarized conformation (beta-phase), with the fractions of each component phase dependent on the amount of solvent added. Fluorescence spectroscopy of the PFO nanoparticles containing beta-phase indicates efficient energy transfer from the glassy-phase regions of the nanoparticles to the beta-phase regions. Salient features of the nanoparticles containing beta-phase include narrow, red-shifted fluorescence and increased fluorescence quantum yield as compared to the glassy-phase nanoparticles. Fluorescence lifetime measurements indicate that the increased quantum yield of the beta-phase PFO originates from a decrease in the nonradiative decay rate, with little change in the radiative rate. This decrease is likely due to exciton trapping by the beta-phase, which leads to a reduction in the energy transfer efficiency to quencher species present within the nanoparticle.     

Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non‐radiative Decay Pathway

To broaden the application of aggregation‐induced emission (AIE) luminogens (AIEgens), the design of novel small‐molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non‐radiative decay pathways, a series of bridged stilbenes was designed, and their non‐radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.     

Spectroscopic properties and hydration of the Cm(III) aqua ion from 10 to 85 °C

The optical absorption, fluorescence excitation, and emission spectra of the Cm(III) aqua ion in 0.001 M perchloric acid were studied in pure H(2)O, pure D(2)O, and in mixtures of H(2)O-D(2)O at temperatures from 10 to 85 °C. The quantum yield of the fluorescence of the Cm(III) aqua ion in pure H(2)O and D(2)O was also measured in this temperature range and the radiative decay rate constant was obtained from these data. The results indicate that, from 10 to 85 °C, the effect of temperature on the absorption, excitation, and emission spectra is very small. By correcting the observed decay rate constant for the radiative rate constant, a set of correlations between the observed fluorescence decay rate constant and the hydration number of Cm(3+) in H(2)O at temperatures from 10 to 85 °C was developed. A weak temperature dependence was observed for the nonradiative decay rate constant for the (6)D'(7/2)-(8)S'(7/2) transition and described by the Arrhenius equation. The activation energy of the nonradiative decay was measured to be 0.9 kJ mol(-1), approximately matching the energy gap between the first and the second (A(1) and A(2)) levels of the metastable (6)D'(7/2) multiplet of the Cm(III) aqua ion. On the basis of these observations, it is postulated that the slight increase in the observed fluorescence decay rate constant as the temperature increases from 10 to 85 °C is due to the effect of thermal population of the A(2) level.     

An apparent angle dependence for the nonradiative deactivation of excited triplet states of sterically constrained, binuclear ruthenium(II) bis(2,2':6',2' '-terpyridine) complexes

The photophysical properties are reported for a series of binuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes built around a geometrically constrained, biphenyl-based bridge. The luminescence quantum yield and lifetime increase progressively with decreasing temperature, but the derived rate constant for nonradiative decay of the lowest-energy triplet state depends on the length of a tethering strap attached at the 2,2'-positions of the biphenyl unit. Since the length of the strap determines the dihedral angle for the central C-C bond, the rate of nonradiative decay shows a pronounced dependence on angle. The minimum rate of nonradiative decay occurs when the dihedral angle is 90 degrees, but there is a maximum in the rate when the dihedral angle is about 45 degrees. This effect does not appear to be related to the extent of electron delocalization at the triplet level but can be explained in terms of variable coupling with a low-frequency vibrational mode associated with the strapped biphenyl unit.     

Listening to Lanthanide Complexes: Determination of the Intrinsic Luminescence Quantum Yield by Nonradiative Relaxation

The photophysical properties of lanthanide complexes have been studied extensively; however, fundamental parameters such as the intrinsic quantum yield as well as radiative and nonradiative decay rates are difficult or even impossible to measure experimentally. Herein, a photoacoustic (PA) method is proposed to determine the intrinsic quantum yield of lanthanide complexes with lifetimes in the order of milliseconds. This method is used to determine the intrinsic quantum yields for europium(III)‐containing metallomesogens as well as terbium(III) complexes. The results show that the PA signal is sensitive to both the lifetime and the ratio of the fast‐to‐slow heat component of the samples. It is found that there is an efficient ligand sensitization and a moderate intrinsic quantum yield for the complexes. The intrinsic quantum yield of Eu³⁺ in the metallomesogens exhibits an obvious increase upon the isotropic liquid to smectic A transition. The proposed PA method is quite simple, and can contribute to a clearer understanding of the photophysical processes in luminescent lanthanide complexes.     

Synthetic control of excited-state properties in cyclometalated Ir(III) complexes using ancillary ligands

The synthesis and photophysical characterization of a series of (N,C(2')-(2-para-tolylpyridyl))2 Ir(LL') [(tpy)2 Ir(LL')] (LL' = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)2 Ir(LL')] (dfppy = N,C(2')-2-(4',6'-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)2 Ir(CN)2- and (ppy)2 Ir(NCS)2- (ppy = N,C(2')-2-phenylpyridyl). The (tpy)2 Ir(PPh2CH2)2 BPh2 and [(tpy)2 Ir(CN-t-Bu)2](CF3SO3) complexes have been structurally characterized by X-ray crystallography. The Ir-C(aryl) bond lengths in (tpy)2 Ir(CN-t-Bu)2+ (2.047(5) and 2.072(5) A) and (tpy)2 Ir(PPh2CH2)2 BPh2 (2.047(9) and 2.057(9) A) are longer than their counterparts in (tpy)2 Ir(acac) (1.982(6) and 1.985(7) A). Density functional theory calculations carried out on (ppy)2 Ir(CN-Me)2+ show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-pi and Ir-d orbitals, while the lowest unoccupied molecular orbital is localized primarily on the pyridyl-pi orbitals. Electrochemical analysis of the (tpy)2 Ir(LL') complexes shows that the reduction potentials are largely unaffected by variation in the ancillary ligand, whereas the oxidation potentials vary over a much wider range (as much as 400 mV between two different LL' ligands). Spectroscopic analysis of the cyclometalated Ir complexes reveals that the lowest energy excited state (T1) is a triplet ligand-centered state (3LC) on the cyclometalating ligand admixed with 1MLCT (MLCT = metal-to-ligand charge-transfer) character. The different ancillary ligands alter the 1MLCT state energy mainly by changing the HOMO energy. Destabilization of the 1MLCT state results in less 1MLCT character mixed into the T1 state, which in turn leads to an increase in the emission energy. The increase in emission energy leads to a linear decrease in ln(k(nr)) (k(nr) = nonradiative decay rate). Decreased 1MLCT character in the T1 state also increases the Huang-Rhys factors in the emission spectra, decreases the extinction coefficient of the T1 transition, and consequently decreases the radiative decay rates (k(r)). Overall, the luminescence quantum yields decline with increasing emission energies. A linear dependence of the radiative decay rate (k(r)) or extinction coefficient (epsilon) on (1/deltaE)2 has been demonstrated, where deltaE is the energy difference between the 1MLCT and 3LC transitions. A value of 200 cm(-1) for the spin-orbital coupling matrix element 3LC absolute value(H(SO)) 1MLCT of the (tpy)2 Ir(LL') complexes can be deduced from this linear relationship. The (fppy)2 Ir(LL') complexes with corresponding ancillary ligands display similar trends in excited-state properties.     

Single particle versus ensemble average: from power-law intermittency of a single quantum dot to quasistretched exponential fluorescence decay of an ensemble

Light-induced diffusion-controlled electron transfer is proposed as an underlying mechanism for the intermittency (power law and breakdown) of a single quantum dot and ensemble-averaged fluorescence decay. The intensity decay can be approximated to a stretched exponential expression. The physical links to the free energy gap, reorganization energy, electronic coupling, and diffusion correlation times are discussed. A procedure is described for extracting these molecular-based parameters from experiments and is demonstrated with examples using existing data.     

Experimental and theoretical studies of potassium cation interactions with the acidic amino acids and their amide derivatives

The binding of K(+) to aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), and glutamine (Gln) is examined in detail by studying the collision-induced dissociation (CID) of the four potassium cation-bound amino acid complexes with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Formed by electrospray ionization, these complexes have energy-dependent CID cross sections that are analyzed to provide 0 K bond energies after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Quantum chemical calculations for a number of geometric conformations of each K(+)(L) complex are determined at the B3LYP/6-311+G(d,p) level with single-point energies calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311+G(2d,2p) basis set. Theoretical bond dissociation energies are in good agreement with the experimental values. This coordinated examination of both experimental work and quantum chemical calculations allows for a comprehensive understanding of the molecular interactions of K(+) with the Asx and Glx amino acids. K(+) binding affinities for the amide complexes are systematically stronger than those for the acid complexes by 9+/-1 kJ/mol, which is attributed to an inductive effect of the OH group in the carboxylic acid side chain. Additionally, the K(+) binding affinity for the longer-chain amino acids (Glx) is enhanced by 5+/-1 kJ/mol compared to the shorter-chain Asx because steric effects are reduced. Further, a detailed comparison between experimental and theoretical results reveals interesting differences in the binding of K(+) and Na(+) to these amino acids.     

Structure and Decay of Gaseous Organic Radical Cations Studied by Their Radiative Decay,Exemplified by the 1,3-Pentadiyne Cation

The radiative decay of over a hundred open-shell organic radical cations has now been established. As a result, the spectral structure of such cations in their ground and excited electronic states can be probed with resolutions of the order of ? 1 cm^?1. This is achieved by means of emission and laser-induced fluorescence techniques. The analysis of the emission and excitation spectra provides the vibrational frequencies of many of the totally symmetric fundamentals of the cations in the two electronic states. In order to study the relaxation behavior of these cations under “isolated conditions”, the lifetimes and fluorescence quantum fields can be obtained by means of photoelectron-photon coincidence measurements. These data yield the radiative and non-radiative rate constants as a function of the internal energy of the cations. The structural and decay information obtained from each of these techniques is illustrated using the 1,3-pentadiyne radical cation as example.     

Excited state properties, fluorescence energies, and lifetime of a poly(fluorene-pyridine) copolymer, based on TD-DFT investigation

The structural and electronic properties of the fluorene-pyridine copolymer (FPy)(n), (n = 1-4) were investigated theoretically by means of quantum mechanical calculations based on density functional theory (DFT) and time-dependent DFT (TD-DFT) using the B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest excited state. It was found that (FPy)(n) is nonplanar in its ground state, whereas a more pronounced trend toward planarity is observed in the S(1) state. Absorption and fluorescence energies have been extrapolated to infinite chain length making use of their good linearity with respect to 1/n. An extrapolated value of 2.64 eV is obtained for vertical excitation energy. The S(1)<--S(0) electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is dominating in terms of oscillator strength. Fluorescence energies and radiative lifetime were calculated as well. The obtained results indicate that the fluorescence energy and radiative lifetime of (FPy)(n) are 2.16 eV and 0.38 ns, respectively. The decrease of fluorescence energy and radiative lifetime with the increase in the chain length is discussed.     

Optical and photophysical properties of indolocarbazole derivatives

We present a study of the optical and photophysical properties of five ladder indolo[3,2-b]carbazoles, namely, M1, M2, M3, M4, and M5. The ground-state optimized structures were obtained by B3LYP/6-31G* density functional theory (DFT) calculations, whereas the optimization (relaxation) of the first singlet excited electronic state (S1) was performed using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. The excitation to the S1 state does not cause important changes in the geometrical parameters of the compounds, as corroborated by the small Stokes shifts. The excitation and emission energies have been obtained by employing the time-dependent density functional theory (TDDFT). For all the compounds, excitation to the S1 state is weakly allowed, whereas the S2 <-- S0 electronic transition of each oligomer possesses a much larger oscillator strength. The absorption and fluorescence spectra of the compounds have been recorded in chloroform. A reasonable agreement is obtained between TDDFT vertical transition energies and the (0,0) absorption and fluorescence bands. On one hand, the pattern of the aliphatic side chains does not affect the absorption and fluorescence maxima of the compounds. On the other hand, the replacement of aliphatic chains by phenyl or thiophene rings induces hypsochromic shifts in the absorption and fluorescence spectra. Finally, the fluorescence quantum yield and lifetime of the compounds in chloroform have been obtained. From these data, the radiative and nonradiative rate constants of the deactivation of the S1 state have been determined.     

The special five-membered ring of proline: An experimental and theoretical investigation of alkali metal cation interactions with proline and its four- and six-membered ring analogues

The interaction of the alkali metal cations, Li+, Na+, and K+, with the amino acid proline (Pro) and its four- and six-membered ring analogues, azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip), are examined in detail. Experimentally, threshold collision-induced dissociation of the M+(L) complexes, where M = Li, Na, and K and L = Pro, Aze, and Pip, with Xe are studied using a guided ion beam tandem mass spectrometer. From analysis of the kinetic energy dependent cross sections, M(+)-L bond dissociation energies are measured. These analyses account for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations for a number of geometric conformations of the M+(L) complexes were determined at the B3LYP/6-311G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. Theoretical bond energies show good agreement with the experimental bond energies, which establishes that the zwitterionic form of the alkali metal cation/amino acid, the lowest energy conformation, is formed in all cases. Despite the increased conformational mobility in the Pip systems, the Li+, Na+, and K+ complexes of Pro show higher binding energies. A meticulous examination of the zwitterionic structures of these complexes provides an explanation for the stability of the five-membered ring complexes.     

Charge shift and triplet state formation in the 9-mesityl-10-methylacridinium cation

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state. The triplet decays via first-order kinetics with a lifetime of ca. 30 micros at room temperature in the absence of oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually long-lived CT state.