Photophysics studies of organic compounds VIII. Interaction between excited singlet state of pyrene and aromatic amines

The quenching processes of the excited singlet state of pyrene by triphenylamine, diphenylamine and N,N-dibenzylaniline in various solvents at room and lower temperature have been studied with both static state fluorescence spectroscopy and fluorescence lifetime techniques. On the basis of experimental data, a preliminary quenching mechanism has been discussed. The geometrical configuration models of the exciplexes formed between the excited pyrene and aromatic amines in nonpolar solvents were proposed.     

Excited singlet (S1)-state interactions of Nile red with aromatic amines

Both steady-state (SS) and time-resolved (TR) studies show that the fluorescence of the dye Nile red (NR) is quenched by various aromatic amines (ArA). Bimolecular quenching constants (kq) from both SS and TR measurements are observed to match well, indicating that the interaction is dynamic in nature. The quenching interaction in the present systems has been attributed to electron transfer (ET) from ArA to excited NR, based on the variations in the kq values with the oxidation potentials of the amines. The kq values calculated within the framework of Marcus' outer-sphere ET theory at different free-energy changes (deltaG0) of the ET reactions match well with the experimental ones, supporting the ET mechanism in the systems studied. The reorganization energy (lambda) estimated from the correlation of the experimental and the calculated kq values is quite similar to the solvent reorganization energy (lambda(s)), calculated on the basis of the solvent dielectric continuum model along with the assumption that the reactants are the effective spheres. Although a modest error is involved in this lambda(s) calculation, the similarity in lambda and lambda(s) values suggests that the solvent reorganization plays a dominant role in governing the ET dynamics in the present systems.     

The gold porphyrin first excited singlet state

Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetramethylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a life-time of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.     

Intracoil excited singlet state annihilation in polyvinyldiphenylanthracene

The intensity of fluorescence of poly(diphenylanthracene) (PDPA) has been found to be highly non-linear in excitation laser energy, while either diphenylanthracene or polystyrene with a low loading of covalently bound diphenylanthracene (PS-co-DPA) is linear under the same conditions. It is proposed that a Forster-type annihilation process occurs: S₁ + S₁ → S₀ + S_n → 2S₀. The R₀ for this process is estimated to be ≈ 35 Å. On the other hand the singlet exciton diffusion constant (Λ_S) is estimated to be very low, by the method of comparative quenching.     

Fluorescence from the second excited singlet of aromatic thioketones in solution

The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S₂ of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φ_f (295 K) = 5.1 × 10^?3 and φ_f(77 K) = 1.0 × 10^?2 for XS, and φ_f (295 K) = 1.5 × 10^?3 and φ_f (77 K) = 2.5 × 10^?3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S₂ is derived. It is concluded that internal conversion S₂ ? S₁ is the dominating deactivation channel of S₂ with k^{77 K}_nr(S₂ ? S₁) = 1.0 × 10¹⁰ s^?1 for XS and k^{77 K}_nr (S₂→S₁) = 2.2 × 10¹⁰ s^?1 for TXS. Between 295 and 77 K, φ_f increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φ_f is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant k_nr(S₂?S₁) with an activation energy of 190 ± 20 cm^?1 and a frequency factor k′_nr = 3 × 10¹⁰ s^?1 for the XS molecule in 3-methylpentane.     

Fluorescence from the second excited singlet state of thiophosgene vapour

Fluorescence has been observed from the second excited singlet state of thiophosgene vapour. The emission is excited mainly by transitions originating in the out-of-plane bending mode of the ground state and terminating in the lowest vibrational levels of the excited state.     

Fluorescence quenching of photoreaction products in the excited singlet state

The properties of steady-state spontaneous luminescence of a quantum system with a photoproduct with recordable fluorescence under the conditions of dynamic quenching of excited states by extraneous substances were considered. It was shown that the dependence of photoproduct fluorescence intensity and yield on quencher concentration was nontrivial and could not be conveniently used to determine the Stern-Volmer constant. At the same time, the initial form of the luminophore and its photoproduct produced in a kinetically controlled reaction are quenched in such a way that the ratio of their fluorescence intensities increases linearly as the quencher concentration grows. The corresponding equation was used to determine the constant of bimolecular quenching of reaction product excited states. The results were used in an analysis of the experimental fluorescence spectra of flavone (3-hydroxiflavone), whose fluorescence was excited under the conditions of dynamic quenching of the S ₁ state. Our analysis was shown to be applicable to a wide range of compounds with photoreactions accompanied by two-band fluorescence (charge transfer, proton transfer, phosphorescence, complex formation, etc.). It could be used to accurately determine bimolecular contact constants for excited states of photoreaction product molecules. Original Russian Text ? V.I. Tomin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 580–585.     

Adiabatic photodissociation of acenaphthylene dimers in the excited singlet state

Fluorescence spectroscopic studies of acenaphthylene dimers in saturated hydrocarbon solution have revealed that the dimer (A₂) photodissociates to     

Resonance fluorescence from the second excited singlet state of naphthalene

Resonance fluorescence exhibiting anomously long radiative decay times was observed from the second excited singlet state of the “isolated” naphthalene molecule, excited by the fourth harmonic of an Nd³⁺ laser.     

Twisting dynamics in the excited singlet state of Michler's ketone

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.     

Interaction of the excited singlet state of 1,4- and 1,8-dimethoxynaphthalene with some organic compounds: A fluorescence-quenching study

Summary The fluoroescence quenching of 1,4-dimethoxynaphthalene (1) and 1,8-dimethoxynaphthalene (2) by tetraphenylporpyrin (3), 9,10-diphenylanthracene (4), and 3-cyano-4-phenyl-6-(p-tolyl)-pyridin-2-one (5) has been studied in chloroform solution. The quenching occursvia a resonance energy transfer mechanism. The rate constant for the energy transfer (k _ET) of donor2 is slower than that of1 by the same acceptors, indicating that the steric effect dominates the ionization potential effect in all systems. The calculated critical transfer distances (R ₀) are 17–72 Å. In contrast, charge transfer is the predominant pathway of electronic deexcitation in the fluorescence quenching of donors1 and2 by 7,7,8,8-tetracyanoquinone-dimethane (6) in chloroform. The roles of temperature and geometrical structure of the donors on the efficiency of fluorescence quenching of1 and2 by acceptor6 have also been studied.

---

Wechselwirkung des angeregten Singlett-Zustands von 1,4- und 1,8-Dimethoxynaphthalin mit einigen organischen Verbindungen: eine Untersuchung zur Fluoreszenzlöschung

Zusammenfassung Die Löschung der Fluoreszenz von 1,4-Dimethoxynaphthalin (1) und 1,8-Dimethoxynaphthalin (2) durch Tetraphenylporphyrin (3), 9,10-Diphenylanthracen (4) und 3-Cyano-4-phenyl-6-(p-tolyl)-pyridin-2-on (5) in Chloroform wurde untersucht. Die Löschung verläuft über einen Resonanzenergietransfermechanismus. Die Geschwindigkeitskonstante für den Energietransfer (k _ET) ist bei gleichem Akzeptor für den Donor2 niedriger als für1. Daraus läßt sich schließen, daß in allen untersuchten Systemen der sterische Effekt über den Effekt des Ionisierungspotentials dominiert. Die berechneten kritischen Transferdistanzen betragen 17–72 Å. Im Gegensatz zu diesen Beobachtungen verläuft der vorherrschende Relaxationsmechanismus bei der Fluoreszenzlöschung von1 und2 durch 7,7,8,8-Tetracyanochinon-dimethan (6) über einencharge-transfer-Prozeß. Die Einflüsse von Temperatur und Geometrie der Verbindungen auf die Effizienz der Fluoreszenzlöschung von1 und2 durch den Akzeptor6 wurden ebenfalls untersucht.

     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Quantitative determination of singlet oxygen generated by excited state aromatic amino acids, proteins, and immunoglobulins

Singlet oxygen quantum yields generated by excited state aromatic amino acids (tryptophan, tyrosine, phenylalanine), N-acetylated amino acids (N-acetyl-tryptophan, N-acetyl-tyrosine, N-acetyl-phenylalanine), and from selected proteins and immunoglobulins have been quantified by time-resolved phosphorescence measurements. A small, but significant, quantum yield found for proteins and immunoglobulins demonstrates that molecular oxygen can diffuse through the polypeptide matrix and can be sensitized by residues buried within the folds of protein structure.     

Interaction of secondary amines with aromatic aldehydes-efficient method for synthesis of the functionalized heterocyclic amines

A method is proposed for the benzylation of secondary heterocyclic amines with functionalized derivatives of benzaldehyde in the presence of formic acid under conditions close to amination according to the Leuckart-Wallach reaction. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1820–1823, December, 2007.     

Hydration-dependent structural deformation of guanine in the electronic singlet excited state

Theoretical study was performed to investigate how the degree of hydration affects the structures and properties of the canonical form (keto-N9H) of guanine in the ground and lowest singlet pipi* excited state. This work is the continuation of our earlier work where we have studied the hydration of guanine in the first solvation shell with one, three, five, and six water molecules. In the present investigation, we have considered 7-13 water molecules in hydrating guanine. Ground-state geometries were optimized at the Hartree-Fock level, whereas the configuration interaction-singles (CIS) method was used for the excited-state geometry optimization. The 6-311G(d,p) basis set was used in all calculations. The harmonic vibrational frequency analysis was used to determine the nature of the optimized ground- and excited-state potential energy surfaces; all geometries were found to be minima at the respective potential surfaces. It was found that the degree of hydration has a significant influence on the excited-state structural nonplanarity of guanine. It is expected that excited-state dynamics of guanine will depend on the degree of hydration. Ground- and excited-state geometries of selected hydrated species were also optimized in the bulk water solution using the polarizable continuum model (PCM). It was found that bulk water solution generally does not have significant influence on the structure of the hydrated species. Effects of hydration on different stretching vibrations in the ground and excited states are also discussed.     

Fluorescence of ammonia-d3 from its first excited singlet state

Structured emission spectra have been observed from ND₃ excited at 2139 Å and 2144 Å. The emission is short-lived (τ_{$\text{1}\text{2}$} < 10^?10 s) and has been assigned to the ND₃($\text{A}$) → ND₃($\text{X}$) fluorescence transition.