Assessing solvent effects on the singlet excited state lifetime of uracil derivatives: A femtosecond fluorescence upconversion study in alcohols and D2O

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ^* and the dark nπ^* states. This relative energy gap controls the non-radiative relaxation of the ππ^* state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D₂O where the decays are faster than in H₂O.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

High-resolution electronic spectra and luminescence properties of some benzonaphthyridines at 77 K. absorption and fluorescence spectra of 1,5-, 1,6- a

Absorption and fluorescence spectra of 1,5-, 1,6- and 4,6-benzo[h]naphthyridines (BN) were examined in Shpolskii matrices and in n-butanol at 77 K. Vibrational analysis of quasilinear spectra of 1,6- and 4,6-BN in n-hexane matrices was performed. Calcula- tions of the electronic structure of the isomers examined were done using a modified INDO CI method. The results of the experiments and calculations prove the π, π^* state to be the lowest excited singlet state of 1,6- and 4,6-BN molecules; in 1,5-BN molecule the S₁ (n, π^*) state is strongly perturbed by the nearby s₂(π π^*) state.     

Intramolecular charge delocalization in the singlet excited state of blocked pyrylium ions

The dual fluorescence emission of the pyrylium ion 3 and of the partly blocked 4 has been studied extensively under various conditions. The short-wavelength emitting species N^* of 3 is short-lived (≤200 ps at room temperature) while the long-wavelength emitting species A^* is long-lived (>3 ns, except in acetic acid). This long-wavelength fluorescence undergoes an important solvatochromic shift and the difference Δ between the absorption and fluorescence maxima versus Lippert’s solvent polarity function Δf is linear. Increasing the viscosity of the medium, or decreasing the temperature, decreases the long-wavelength emission quantum yield while that of the short-wavelength fluorescence and its lifetime (from <100 ps to >4 ns) both increase, indicating that A^* is formed from N^*. Introducing an ortho methyl group on the paraanisyl substituent (compound 4) blocks its rotation and reduces the fluorescence I_A./I_N. ratio, but it does not suppress completely the long-wavelength emission. This favors a ground state configuration where the phenyl substituent would be orthogonal to the xanthylium moiety. A strong interaction of 3 and 4 with aliphatic nitriles is characterized from the quenching of the fluorescence emission (with rate constants of ca. 2×10⁸ M⁻¹ s⁻¹). A static quenching process also occurs indicating a ground state interaction with the solvent. In pure aliphatic nitriles, this interaction is the main deactivation pathway of the singlet excited state, and practically no fluorescence nor triplet formation can be observed.     

Stereocontrol in the cationic polymerization of N-vinylcarbazole

The dependence of the steric microstructure of cationically polymerized poly(N-vinylcarbazole) (PVK) upon catalyst, polymerization temperature, and polymerization solvent has been investigated. The effect of polymerization temperature variation was found to be small, whereas the choice of catalyst and polymerization solvent was found to have a strong influence upon the PVK steric microstructure. A correlation was found between the syndiotacticities X_s and the π^* solvent polarities of the polymerization solvents for a given catalyst. A decrease in X_s with increasing π^* solvent polarity was observed using BF₃OEt₂ and AlEt₂Cl catalysts and has been interpreted in terms of propagation via contact ion-pair ring structures reversibly formed between the active end group and a preceding repeating unit. The increase in X_s with increasing π^* solvent polarity observed with several of the catalysts investigated has been interpreted in terms of chain ion pairs whose separation increases with increasing π^* solvent polarity. The influence of the various Lewis acid catalysts upon the steric microstructures of cationically polymerized PVK allowed the following order of nucleophilicity to be established:      

Intersystem crossing in 9-carbonyl derivatives of anthracene

An unusual temperature effect on the intensity of fluorescence of 9-carbonyl derivatives of anthracene is observed. This is interpreted in terms of an intersystem crossing process from the lowest excited singlet state S_ππ^* to the higher excited triplet state T_nπ^*.     

Anomalous vibrational relaxation of a hot S*1 state of 3,4-benzpyrene in 2-methylpentane

Two anomalous emission bands in the fluorescence spectrum of 3,4-benzpyrene, dissolved in 2-methylpentane, have been studied as a function of temperature. These emissions originate from the second excited singlet state S₂, and from a vibrationally excited S₁ (S^*₁) respectively. From the temperature dependence of the relative yield and the decay time of the S^*₁ emission it can be concluded that the vibrational relaxation of this state is hampered. The rate constant for this relaxation process is smaller that 4 > 62;x 10⁷ sec^?1.     

Fluorescence of DPH derivatives. Evidence for emission from S2 and S1 excited states

Absorption, fluorescence excitation and emission spectra, quantum yields, and fluorescence decay times have been measured for several derivatives of all-trans 1,6-diphenyl-1,3,5-hexatriene (DPH) in a number of solvents. Emission occurs from the low-lying ¹Ag^*_g state and the ¹B^*_u which is at slightly higher energy. Accepted models for the photophysics of DPH are shown to be inadequate.     

Ruby laser induced emission from NO2

Two different types of emission from excited NO₂ were observed using pulsed ruby laser light at 6943 Å. The first type of fluorescence was seen in the near-IR and results from the single photon excitation of NO₂ from the ground (²A₁) state. By observing the emission as a function of time an unexpected behavior was observed in the near IR and could be explained by a consecutive deactivation mechanism, wherein a secondary species is preferentially detected. A second type of emission recently observed in the blue spectral region is weaker and is due to a multiphoton process. The intensity of the blue emission is a function of the cube of the laser intensity at low pressures and approaches the square at high pressures. We attribute this variation to simultaneous deactivation of the NO₂^* intermediate by collision (square) and by anti-Stokes Raman scattering off of the NO₂^* (cube).     

Studies on fluorescent species of spiro-indolinonaphthooxazines and their fluorescence spectra

Photophysical behavior of spiro[1,3,3-trimethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP1) and spiro[1,3,3,2′-tetramethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP2) were studied. The fluorescent species and their spectra of SP1 and SP2 in polar solvents (acetonitrile and methanol) and non-polar solvent (cyclohexane) were investigated. The fluorescence decay in polar solvents was studied by picosecond time-correlated single photon counting. In most cases, fluorescence decay obeyed triexponential decay kinetics. The major fluorescent species is an excited intermediate which has similar conformation as its precursor (¹ SP ^*) formed after the bond cleavage between spirocarbon and oxygen in oxazine ring. The effects of molecular structure and solvent polarity on fluorescence spectra and fluorescence decay lifetime were studied.     

Picosecond reversible reaction kinetics of excited p-dimethylaminoacetophenone and m-methyl-p-cyanodimethylaniline studied by oxygen quenching technique

Double fluorescence of p-dimethylacetophenone (DMAPh) in CH₃CN and m-methyl-p-cyanodimethylaniline (MCDMA) in CH₂Cl₂ has been observed and analyzed in terms of reversible excited state isomerisation of the primary excited form b^* to the strongly polar rotamer a^*. Using the oxygen quenching technique, the kinetics of the reactions have been solved and all rate constants separated. The “formal” lifetimes of the species b^*, τ_b ≡ (k_bf + k_bd + k_ba)^?1, are 1 ps and 2.2 ps for DMAPh and MCDMA, respectively. The first value fits well to the reorientation relaxation time of acetonitrile.     

PHOTOPHYSICS OF THE FLUORESCENT Ca2+ INDICATOR QUIN-2

The photophysics of the complex forming reaction between Quin-2 and Ca²⁺ were investigated using steady-state and time-resolved fluorescence measurements. The fluorescence decay traces were analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution with EGTA as Ca²⁺ buffer: k₀₁= 8.6 times 10⁸ s^?1, k₂₁= 1 times 10¹¹M^?1 s^?1, k₀₂= 8.8 times 10⁷ s^?1, k₁₂= 4 times 10⁴ s^?1. k₀₁ and k₀₂ denote the respective deactivation rate constants of the Ca²⁺ free and bound forms of Quin-2 in the excited state. The constant k₂₁ represents the second-order rate constant of binding of Ca²⁺ and Quin-2 in the excited state while k₁₂ is the first-order rate constant of dissociation of the excited Ca²⁺:Quin-2 complex. From the estimated values of k₁₂ and k₂₁ the dissociation constant K_d* in the excited state was calculated. It was found that pK_d* (6.4) is slightly smaller than pK_d (7.2). There was no interference of the excited-state complex forming reaction with the determination of K_d. Intracellular Ca²⁺ concentrations can thus accurately be determined from fluorometric measurements using Quin-2 as Ca²⁺ indicator.     

Dual Fluorescence of isolated aromatic molecules

Expressions are derived for the S^*₂ and S^*₁ fluorescence quantum yields and response functions of isolated excited aromatic molecules in the statistical limit. The S^*₂ and S^*₁ fluorescences have a common decay time, a property previously considered characteristic of intermediate strong coupling. Data on pyrene, naphthalene, and 3:4-benzopyrene vapours are discussed in relation to the two models.     

Excited State Intramolecular Proton Transfer of New Diphenylethylene Derivatives Bearing Imino Group: A Combination of Experimental and Theoretical Investigation

In this paper, we described the synthesis and characterization of new diphenylethylene bearing imino group. We concentrated particularly on the investigation of the possibility of the excited state intramolecular charge transfer (ESIPT) of the new dyes experimentally and theoretically. The absorption and fluorescence spectroscopy of the dyes were determined in various solvents. The results showed that the maximal absorption wavelength of 2‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C1 ) and 4‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C2 ) exhibited almost independence on the solvent polarity. While as contrast, the maximal fluorescence wavelength of the dyes showed somewhat dependence on the solvent polarity. In particular, C1 displayed well‐separated dual fluorescence spectroscopy. The second fluorescence peak was characterized with an "abnormal" fluorescence emission wavelength in aprotic solvents with large Stokes shift (ca. 140 nm in THF), which was much more than normal Stokes shift (ca. 30 nm in THF). This emission spectroscopy could be assigned to ESIPT emission. On the other hand, the ESIPT fluorescence of C1 was much reduced or lost in the protic solvents. While, only normal fluorescence emission was detected in various solvents. Although the absorption maxima of C1 exhibited about 10 nm red‐shift with respect to those of C2 , the normal fluorescence maxima of C1 and C2 were almost identical in various solvents. These results suggested that C1 could undergo ESIPT, but C2 was not able to proceed ESIPT. The molecular geometry optimization of phototautomers in the ground electronic state (S₀) was carried out with HF method (Hartree‐Fock) and at DFT level (Density Functional Theory) using B3LYP both, while the CIS was employed to optimize the geometries of the first singlet excited state (S₁) of the phototautomers of C1 and C2 respectively. The properties of the ground state and the excited state of the phototautomers of C1 and C2 , including the geometrical parameter, the energy, the frontier orbits, the Mulliken charge and the dipole moment change were performed and compared completely. The data were analyzed further based on our experimental results. Furthermore, the absorption and fluorescence spectra were calculated in theory and compared with the measured ones. The rate constant of internal proton transfer (9.831×10¹¹ s^?1) of C1 was much lower than that of salicylidene methylamine ( C3 , 2.045×10¹⁵ s^?1), which was a typical Schiff base compound and was well demonstrated to undergo ESIPT easily under photoexcitation.     

NH(A3Π → X3Σ−) Chemiluminescence from the CH(X2 Π) + NO reaction

NH(A³Π → X³Σ^?) and OH(A²Σ⁺ → X²Π) chemiluminescences from the reaction of CH(X²Π) with NO and O₂, respectively, have been observed at room temperature. From the decay of such emissions we have measured the rate constants for these two reactions: k_NO = (2.5 ± 0.5) × 10^?10 and k_O2 = (8 ± 3) × 10^?11 cm³ molecule ^?1 s^?1, which are in agreement with previously reported rates determined by direct CH(X) detection using, laser-induced fluorescence. This indicates that a four-centered mechanism generating these excited species is operative in both reactions. The CH generation from 266 nm photolysis of CHBr₃ has also been investigated via analysis of CH* emissions.     

Picosecond proton ejection: an ultrafast pH jump

The rates of proton ejection from 2-naphthol-3,6-disulfonate (pK^* = 0.5 ± 0.1) and 8-hydroxy 1,3,6-pyrene trisulfonate (pK^* = 0.4 ± 0.1) have been found to be 3.1 × 10¹⁰ s^?1 and 3.2 × 10¹⁰ s^?1, respectively. This is in keeping with the scaling of the ejection rate inversely with the excited state pK^*.     

Photoinduced Excited State Intramolecular Proton Transfer of New Schiff Base Derivatives with Extended Conjugated Chromophores: A Comprehensive Theoretical Survey

This paper presented comprehensive theoretical investigation of excited state intramolecular proton transfer (ESIPT) of four new large Schiff base derivatives with extended conjugated chromophores. The properties of the ground state and the excited state of phototautomers of C1 to C4 [ C1 : 2‐(4′‐nitro‐stilbene‐4‐ylimino)methylphenol; C2 : 2‐(4′‐cyano‐stilbene‐4‐ylimino)methylphenol; C3 : 2‐(4′‐methoxyl‐stilbene‐4‐ylimino)methylphenol; C4 : 2‐(4′‐N,N‐diethylamino‐stilbene‐4‐ylimino)methylphenol], which included geometrical parameter, energy, rate constant, frontier orbit, Mulliken charge, dipole moment change, were studied by DFT (density functional theory), CIS (configuration interaction singles‐excitation), TDDFT (time‐dependent DFT) methods to analyze the effects of chromophore part on the occurrence of ESIPT and the role of substituent groups. The structural parameter calculation showed that the shorter R_{H? N} and larger R_{O? H} from enol to enol* form, and less twisted configuration in the excited state implied that these molecules could undergo ESIPT as excitation. Stable transition states and a low energy barrier were observed for C1 to C4 . This suggested that chromophore part increased some difficulty to undergo ESIPT for these molecules, while the possibility of occurrence of ESIPT was quite high. The negative ΔE^* (?9.808 and ?9.163 kJ/mol) of C1 and C2 and positive ΔE^* (0.599 and 1.029 kJ/mol) of C3 and C4 indicated that withdrawing substituent groups were favorable for the occurrence of ESIPT. The reaction rate constants of proton transfer of these compounds were calculated in the S₀ and S₁ states respectively, and the high rate constants of these compounds were observed at S₁ state. C1 even reached at 1.45×10¹⁵ s^?1 in the excited state, which is much closed to 2.05×10¹⁵ s^?1 of the parent moiety (salicylidene methylamine). Electron‐donating and electron‐withdrawing substituent groups had different effects on the electron density distribution of frontier orbits and Mulliken charges of the atoms, resulting in different dipole moment changes in enol*→keto* process. These differences in turn suggested that C1 and C2 had more ability to undergo ESIPT than C3 and C4 . The ultraviolet/visible absorption spectra, normal fluorescence emission spectra and ESIPT fluorescence emission spectra of these compounds were predicted in theory.     

Photodissociation Dynamics of 2-Iodotoluene Investigated by Femtosecond Time-Resolved Mass Spectrometry

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 38±50 fs was measured from the rising time of the co-fragments of toluene radical (C₇H₇) and iodine atom (I), which is attributed to the averaged time needed for the C-I bond breaking for the simultaneously excited nσ^* and ππ^* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ^* and ππ^* states. And a rise time of 40±50 fs is extracted from the fitting of time-dependent I⁺ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.     

Solvent-Dependent Photophysics of Indoloquinoxaline

Rapid internal conversion (6×10⁸ s^-1) from the S₁ (π π^*) state of indolo [2,3-b] quinoxaline is observed in ethanol, apparently due to a specific hydrogen bonding interaction. In other solvents (heptane, dioxane and acetonitrile) radiationless transition occurs via intersystem crossing.     

Absorption spectrum,lifetime and reactivity of the excited triplet state of dimesitylcarbene

The first excited triplet state of dimesitylcarbene has been generated in solution at room temperature. It has a lifetime of 60 ns and shows fluorescence with λ_max = 501 nm and absorption with λ_max = 360 nm. This species is quenched by oxygen and carbon tetrachloride with rate constants of (4.4 ± 0.8) × 10¹⁰ and (7.3 ± 0.6) × 10⁹ M⁻¹ s⁻¹, respectively.