Internal Conversion Rate Constants Research Articles

Application of vibrational correlation formalism to internal conversion rate: case study of Cu(n) (n = 3, 6, and 9) and H2/Cu3.

This work reports non-radiative internal conversion (IC) rate constants obtained for Cun with n = 3, 6, and 9 and H2 on Cu3. The Time-Dependent Density Functional Theory (TDDFT) method was employed with three different functionals in order to investigate the electronic structures and the absorption spectra. The performance of the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) and the hybrid B3LYP and PBE0 exchange correlation functionals in combination with the SVP and the def2-TZVP basis sets was examined. TDDFT results were used as input data to compute internal conversion rate constants. For this purpose, we have developed a program package. A description of the theoretical background used in our numerical implementation and the program input file is presented. In view of future applications of this program package in photoinduced catalysis, we present the analysis of the IC rate processes for the photodissociation of H2 on Cu3. These results showed the applicability of the method and the computational program to identify the vibrational modes in transition metal clusters giving rise to the largest IC rate constant due to their interactions with the excited electronic states occurring in the hot-electron induced dissociation phenomena.

Strong impact of the solvent on the photokinetics of a 2(1H)-pyrimidinone

Pyrimidinones are part of the (6-4) photolesions which may be formed from two pyrimidine bases adjacent on a DNA strand. In relation to the secondary photochemistry of the (6-4) lesion, i.e. its transformation into a Dewar valence isomer, photophysical and photochemical properties of 1-methyl-2(1H)-pyrimidinone (1MP) in water, acetonitrile, methanol, and 1,4-dioxane are reported here. As deduced from steady state fluorescence and femtosecond transient absorption spectroscopy the S1 lifetime of 1MP is strongly affected by the solvent. The lifetimes range from 400 ps for water to 40 ps for 1,4-dioxane. Internal conversion (IC) and intersystem crossing (ISC) contribute to the S1 decay. The solvent effect on the IC rate constant is more pronounced than on the ISC constant. The quantum yields for the consumption of 1MP (values for nitrogen purged solvents) are large for methanol (0.35) and 1,4-dioxane (0.24) and small for acetonitrile (0.02) and water (0.003). Hydrogen abstraction from the solvent by the triplet state of 1MP may rationalize this.

Protein Influence on Charge-Asymmetry of the Primary Donor in Photosynthetic Bacterial Reaction Centers Containing a Heterodimer: Effects on Photophysical Properties and Electron Transfer

The substantial electronic distinctions between bacteriochlorophyll (BChl) and its Mg-free analogue bacteriopheophytin (BPh) are exploited in two sets of Rhodobacter capsulatus reaction center (RC) mutants that contain a heterodimeric BChl-BPh primary electron donor (D). The BPh component of the M-heterodimer (Mhd) or L-heterodimer (Lhd) obtains from substituting a Leu for His M200 or for His L173, respectively. Lhd-β and Mhd-β RCs serve as the initial templates in the two mutant sets, where β denotes that the L-side BPh acceptor (HL) has been replaced by a BChl (due to substituting His for Leu M212). Three variants each of Lhd-β and Mhd-β mutants were constructed: (1) a swap (denoted YF) of the native Phe (L181) and Tyr (M208) residues, which flank D and the nearby M- and L-side monomeric BChl cofactors, respectively, giving Tyr (L181) and Phe (M208); (2) addition of a hydrogen bond (denoted L131LH) to the ring V keto group of the L-macrocycle of D, via replacing the native Leu at L131 with His; (3) the combination of 1 and 2. A low yield of electron transfer (ET) to the M-side BPh (HM) is observed in all four Lhd-containing RCs. Comparison with the yield of ET to β on the L-side shows that electron density on the L-macrocycle of D* favors ET to the M-side cofactors and vice versa. Increasing or decreasing the electronic asymmetry of D* via the YF, L131LH mutations or the combination results in consistent trends in the characteristics of the long-wavelength ground state absorption band of D, the rate constant of internal conversion of D* to the ground state, and the rate constants for ET to both the L- and M-side cofactors. A surprising correlation is that an increase in the charge asymmetry in D* not only increases the D* internal-conversion rate constant, but also the rate constants for ET to both the L- and M-side cofactors, spanning time scales of tens of picoseconds to several nanoseconds. The YF swap has a previously unrecognized effect on the electronic asymmetry of D*, resulting in increased charge asymmetry for the Mhd and decreased charge asymmetry for the Lhd. This result indicates that the native Tyr (M208) and Phe (L181) in the wild-type RC promote an electron distribution in P* that is the reverse of that favorable for ET to the photoactive L-branch. This conclusion reinforces the view that the native configuration of these residues promotes ET to the L branch primarily by poising the free energies of the charge-separated states. Overall, this work addresses the extent to which electronic couplings complement energetics in underpinning the directionality of ET in the bacterial RC.

Photodissociation of gaseous CH3COSH at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Observation of three dissociation channels

Upon one-photon excitation at 248 nm, gaseous CH(3)C(O)SH is dissociated following three pathways with the products of (1) OCS + CH(4), (2) CH(3)SH + CO, and (3) CH(2)CO + H(2)S that are detected using time-resolved Fourier-transform infrared emission spectroscopy. The excited state (1)(n(O), π*(CO)) has a radiative lifetime of 249 ± 11 ns long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of collision-induced internal conversion is estimated to be 1.1 × 10(-10) cm(3) molecule(-1) s(-1). Among the primary dissociation products, a fraction of the CH(2)CO moiety may undergo further decomposition to CH(2) + CO, of which CH(2) is confirmed by reaction with O(2) producing CO(2), CO, OH, and H(2)CO. Such a secondary decomposition was not observed previously in the Ar matrix-isolated experiments. The high-resolution spectra of CO are analyzed to determine the ro-vibrational energy deposition of 8.7 ± 0.7 kcal/mol, while the remaining primary products with smaller rotational constants are recognized but cannot be spectrally resolved. The CO fragment detected is mainly ascribed to the primary production. A prior distribution method is applied to predict the vibrational distribution of CO that is consistent with the experimental findings.

Theoretical investigation of fluorescence properties of EDTA and DTPA substituted tetraphenylporphyrin molecules

The matrix elements of spin–orbit coupling operator (HSO) and non-adiabatic coupling operator (Ω) for the H2ATPP–EDTA and H2ATPP–DTPA molecules were calculated using INDO method. Rate constants of internal conversion and of intersystem crossing were calculated using the values of matrix elements of HSO and Ω operators and excitation energies both calculated with the ADC(2) and TDDFT methods and obtained from experimental data. Results of calculations show that the probability of radiationless deactivation in internal conversion channel is greater than the probability of radiationless transition in intersystem crossing channel for considered molecules. Obtained theoretical values of fluorescence quantum yield have good agreement with their experimental values.

Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v = 1, 2) and CO(v = 1-3) are detected in one-photon dissociation of acetyl cyanide (CH(3)COCN) at 308 nm. The S(1)(A(")), (1)(n(O), π(∗) (CO)) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 μs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1-7) × 10(-12) cm(3) molecule(-1) s(-1). The measurements of O(2) dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJ∕mol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN + CH(2)CO, in which the CH(2)CO moiety may further undergo secondary dissociation to release CO. The production of CO(2) in the reaction with O(2) confirms existence of CH(2) and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH(3) fragments that dominate the dissociation products at 193 nm are not detected.

Computational studies of photophysical properties of porphin, tetraphenylporphyrin and tetrabenzoporphyrin

The molecular photonics of porphyrins are studied using a combination of first-principle and semi-empirical calculations. The applicability of the approach is demonstrated by calculations on free-base porphyrin, tetraphenylporphyrin, and tetrabenzoporphyrin. The method uses excitation energies and oscillator strengths calculated at the linear-response time-dependent density functional theory (TDDFT) or the corresponding values calculated at the linear-response approximate second-order coupled-cluster (CC2) levels. The lowest singlet excitation energies obtained in the TDDFT and CC2 calculations are 0.0-0.28 eV and 0.18-0.47 eV larger than the experimental values, respectively. The excitation energies for the first triplet state calculated at the TDDFT level are in excellent agreement with experiment, whereas the corresponding CC2 values have larger deviations from experiment of 0.420.66 eV. The matrix elements of the spin-orbit and non-adiabatic coupling operators have been calculated at the semi-empirical intermediate neglect of differential overlap (INDO) level using a spectroscopic parameterization. The calculations yield rate constants for internal conversion and intersystem crossing processes as well as quantum yields for fluorescence and phosphorescence. The main mechanism for the quenching of fluorescence in tetraphenylporphyrin and tetrabenzoporphyrin is the internal conversion, whereas for free-base porphyrin both the internal conversion and the intersystem crossing processes reduce the fluorescence intensity. The phosphorescence is quenched by a fast internal conversion from the triplet to the ground state.

Theoretical and experimental investigations of photophysics of the nile red molecule and its protonated structures

Results of quantum-chemical studies of the nile red (NR) molecule and its protonated structures by the INDO/S method are presented. It is demonstrated that the best agreement between the calculated and experimental data is obtained for the flat molecule in the ground electron state. Energies of the strongest singlet and triplet electronic states, molecular nature of these states, transition oscillator force, dipole moments in the ground and excited states, electron density distribution around atoms and molecular fragments in the S0 and S1 states, and rate constants of radiative, internal, and intercombination conversion are presented for the NR molecule and its protonated structures. The most probable NR protonation centers are analyzed using the molecular electrostatic potential (MESP) method. It is established that the reaction of proton addition to the NR molecule proceeds at the cyclic nitrogen atom. As demonstrated the results of quantum-chemical calculations, low fluorescent properties of the protonated NR structures (with a quantum yield of 4%) are due to a high rate of the S1 – T4 intercombination conversion.

Excited-State Dynamics of trans,trans-1,3,5,7-Octatetraene: Estimation of Decay Rate Constants of 11Bu ⇝ 21Ag and 21Ag ⇝ 11Ag Internal Conversions

The general expressions we previously derived for calculating internal conversion rate constants between two adiabatic displaced-distorted-rotated potential energy surfaces, by including all vibratinal modes, are applied to estimate the decay rate constants of 1(1)B(u) ⇝ 2(1)A(g) and 2(1)A(g) ⇝ 1(1)A(g) internal conversions in trans,trans-1,3,5,7-octatetraene molecule. The minimal models with respect to the number and types of vibrational modes are determined for these processes. Our calculations show that in the low temperature limit the 1(1)B(u) ⇝ 2(1)A(g) internal conversion takes place on a 232-290 fs time scale in the condensed phase and 2 ps in the gas phase, whereas 2(1)A(g) ⇝ 1(1)A(g) internal conversion takes place on a 2 μs time scale under the isolated conditions.

Kinetic Analysis for the Effect of Intramolecular Hydrogen Bonding on Photophysical Properties of N-Hydroxyalkyl-1,8-naphthalimides

Abstract N-Methyl-, N-(2-hydroxyethyl)-, N-(2-methoxyethyl)-, N-(3-hydroxypropyl)-, and N-(3-methoxypropyl)-1,8-naphthalimide (1, 2a, 2b, 3a, and 3b, respectively) were prepared and their photophysical properties examined. The UV and IR spectra of 2a and 3a in dichloromethane showed the presence of intramolecular hydrogen bonding between the carbonyl group and the hydroxy group. In addition, the fluorescence intensities of 2a and 3a in dichloromethane were found to be about two times larger than those of 1, 2b, and 3b. Furthermore, the fluorescence lifetimes of 2a and 3a, determined by picosecond single photon counting, were about two times longer than those of 1, 2b, and 3b. The rate constants of the intersystem crossing (kisc) for 2a and 3a, calculated based on the quantum yields of the intersystem crossing (Φisc) determined by the time-resolved thermal lensing technique, were about one half of those obtained for 1, 2b, and 3b, while the rate constants of fluorescence emission (kf) and internal conversion (kic) were minimally affected by the presence of intramolecular hydrogen bonding. Enhancement of the fluorescence quantum yield and the lifetime of 2a and 3a was thus explained by a decrease in the efficiency of the intersystem crossing from 1(ππ*) to 3(nπ*), that results from an increase in the energy of the 3(nπ*) level due to the presence of intramolecular hydrogen bonding.

Tetra(β-phenothiazinyl) zinc phthalocyanine: An easily prepared D 4–A system for efficient photoinduced electron transfer

Four identical electron donor (D) moieties, phenothiazines (PTZs), were covalently attached onto the same acceptor (A), zinc phthalocyanine (ZnPc), to form ZnPc(β-PTZ) 4, i.e. D 4–A. The symmetrical D 4–A was synthesized by the condensation method to examine intra-molecular photoinduced electron transfer (PET). The common electron donor-spacer–acceptor (D–A) photosynthetic models, which involve the asymmetrical synthesis of a mono-substituted porphyrin or its analogs, suffer from a low yield and arduous isolation, since D–A is only one of several products (D n –A or A n –D, n = 0, 2–4). The D 4–A preparation, however, can be carried out without the problem. The steady state and time-resolved fluorescence of the D 4–A were measured and compared with that of ZnPc(β-R) 4 (R = H, OPh). The result showed that the excited singlet state of phthalocyanine moiety in the D 4–A molecule was efficiently quenched by phenothiazine units owing to intra-molecular PET. The rate constant of PET ( k et) was calculated and the value is much higher than the rate constant of fluorescence emission, intersystem crossing and internal conversion of ZnPc moiety. The laser flash photolysis study revealed the presence of a long-lived charge-separated state due to PET. The results suggest that a D 4–A system can be a more efficient artificial photosynthetic model than D–A towards the practical commercial use.

Symmetry forbidden vibronic spectra and internal conversion in benzene

The spectra of symmetry-forbidden transitions and internal conversion were investigated in the present work. Temperature dependence was taken into account for the spectra simulation. The vibronic coupling, essential in the two processes, was calculated based on the Herzberg-Teller theory within the Born-Oppenheimer approximation. The approach was employed for the symmetry-forbidden absorption/fluorescence, and internal conversion between 1(1)A(1g) and 1(1)B(2u) states in benzene. Vibrational frequencies, normal coordinates, electronic transition dipole moments, and non-adiabatic coupling matrix elements were obtained by ab initio quantum chemical methods. The main peaks, along with the weak peaks, were in good agreement with the observed ones. The rate constant of the 1(1)A(1g)← 1(1)B(2u) internal conversion was estimated within the order of 10(3) s(-1). This could be regarded as the lower limit (about 4.8 × 10(3) s(-1)) of the internal conversion. It is stressed that the distortion effect was taken into account both in the symmetry-forbidden absorption/fluorescence, and the rate constants of internal conversion in the present work. The distortion effects complicate the spectra and increase the rate constants of internal conversion.

Ultrafast Excited-State Dynamics of Kynurenine, a UV Filter of the Human Eye

The excited-state dynamics of kynurenine (KN) has been examined in various solvents by femtosecond-resolved optical spectroscopy. The lifetime of the S(1) state of KN amounts to 30 ps in aqueous solutions, increases by more than 1 order of magnitude in alcohols, and exceeds 1 ns in aprotic solvents such as DMSO and DMF, internal conversion (IC) being shown to be the main deactivation channel. The IC rate constant is pH independent but increases with temperature with an activation energy of about 7 kJ/mol in all solvents studied. The dependence on the solvent proticity together with the observation of a substantial isotope effect indicates that hydrogen bonds are involved in the rapid nonradiative deactivation of KN in water. These results give new insight into the efficiency of KN as a UV filter and its role in cataractogenesis.

Fluorescent characteristics of coumarin photosensitizers

We present a theoretical and experimental study of the fluorescent ability of new coumarin sensitizers. Using the INDO method, we have drawn the level diagram for the photophysical processes in the coumarin molecules. We have calculated the rate constants for internal conversion and intersystem crossing and the radiative rate constants. We have determined the experimental and theoretical fluorescence yields of the test compounds.

Efficient Singlet‐State Deactivation of Cyano‐Substituted Indolines in Protic Solvents via CNHO Hydrogen Bonds

The photophysical properties of indoline (I) and three of its derivatives, namely, N-methylindoline (MI), 5-cyanoindoline (CI), and 5-cyano-N-methylindoline (CMI), are studied in H-donating solvents of varying polarity. Based on measurements of fluorescence yield and lifetime, and of triplet yield and hydrated-electron formation, two distinct mechanisms of solvent-induced fluorescence quenching are evidenced. The first mechanism involves the cyano substituent and leads to an increase in the rate constant of internal conversion of one order of magnitude in ethanolic solution and of more than two orders of magnitude in water, as compared to solutions in n-hexane or acetonitrile. A similar trend had previously been observed in the case of 4-N,N-dimethylaminobenzonitrile (DMABN). The second mechanism reduces the fluorescence lifetimes of the non-cyanated derivatives in aqueous solution by one order of magnitude and is related to the formation of hydrated electrons. Neither of these mechanisms is influenced by methylation at the ring nitrogen. Quantum chemical calculations are performed on the ground and excited states of the hydrogen-bonded complexes between protic solvents and MI as well as CMI. Stable hydrogen-bonded configurations involving the CN substituent and a solvent OH group are found; these configurations are stable both in the ground and the first excited singlet states, whereas the corresponding complex at the ring amino nitrogen is stable in the ground state only. The CN--HO configuration is therefore a prime candidate for a mechanistic explanation of the observed quenching by the first mechanism. These findings may have useful applications for the design of fluorescence probes for water in biological systems.

Dynamics of Radiationless Transitions: Effects of Displacement−Distortion−Rotation of Potential Energy Surfaces on Internal Conversion Decay Rate Constants,

General expressions for calculating the internal conversion decay rate constants between two adiabatic electronic states and between two diabatic electronic states are derived. The expressions include the displacements, distortions, and rotations of potential energy surfaces as well as the temperature. For illustration, internal conversion rate constants between various singlet electronic states of ethylene and between the first excited S1 and the ground S0 singlet electronic states of azulene are calculated.

Novel gallium(III) phthalocyanine derivatives – Synthesis, photophysics and photochemistry

The syntheses of gallium(III) chloride phthalocyanine {(Cl)GaPc}, octaphenoxyphthalocyaninato gallium(III) chloride {(Cl)GaOPPc} and octakis(4-tert-butylphenoxy)phthalocyaninato gallium(III) chloride {(Cl)GaOTBPPc}; as well as their photophysical and photochemical parameters are hereby presented. Fluorescence quantum yields do not vary much among the three metallophthalocyanines (MPcs); therefore it was concluded that the effect of the substituents is not significant amongst (Cl)GaPc, (Cl)GaOPPc and (Cl)GaOTBPPc. Solvents effects, however, had an effect on the results. Triplet quantum yields were found to be lower in DMSO than in DMF and toluene. The rate constants for fluorescence, intersystem crossing and internal conversion as well as fluorescence and triplet lifetimes are reported. We have also reported photodegradation and singlet oxygen quantum yields. There was no clear correlation between the later parameters. It was, however, established that the three MPcs were stable.

Effects of interaction with CTAB micelles on photophysical characteristics of meso-tetrakis(sulfonatophenyl) porphyrin

The whole set of photophysical characteristics of the metal-free water-soluble meso-tetrakis( p-sulfonatophenyl) porphyrin (TPPS 4) in its protonated and nonprotonated states in homogeneous aqueous solutions and in the presence of cationic cetyltrimethylammonium bromide micelles (CTAB) was investigated with nonlinear optical Z-scan technique, UV–vis absorption and fluorescence spectroscopies, time-resolved fluorescence and flash-photolysis. The characteristics were: the cross-sections of the ground ( σ 0) and the excited singlet ( σ S) and triplet ( σ T) states; the rate constants of intersystem-crossing ( k isc), internal conversion ( k ic) and radiative one ( k r); the quantum yields ( Φ fl and Φ T) and lifetimes ( τ S and τ T) of singlet and triplet states and quantum yield of internal conversion ( Φ ic). The study was realized at pH 7.0 and 4.0, where TPPS 4 in homogeneous solutions is nonprotonated and biprotonated, respectively. It was observed that, in spite of the fact that protonation changes all these characteristics, their values for TPPS 4 bound with CTAB micelles were at both pH close to those of nonprotonated porphyrin in the solution. It is due to the fact that binding with CTAB shifts the p K of TPPS 4 from 5.0 in homogeneous solution to 2.5 in the micellar one and the bound porphyrin appears in nonprotonated state at both pH used. We can conclude also that the change of the quality of the environment from homogeneous aqueous solution to the micellar one affects weakly the TPPS 4 photophysical characteristics when it is not protonated.

Photophysical and photochemical studies of sulphonated non-transition metal phthalocyanines in aqueous and non-aqueous media

The photophysical and photochemical parameters for mixed sulphonated metallophthalocyanine complexes (AlPcS mix, SiPcS mix, GePcS mix, SnPcS mix, and ZnPcS mix) are reported in phosphate buffer saline (PBS, pH 7.4), PBS containing the surfactant Triton X-100, and in dimethylsulphoxide (DMSO). The ground state spectra of SiPcS mix, GePcS mix and SnPcS mix show splitting of the Q-band in DMSO, but the fluorescence spectra have only one band, suggesting that only some components of the mixed complexes fluoresce. In general the quantum yields of fluorescence ( Φ F) were smaller in DMSO compared to the aqueous solvents, while quantum yields of triplet state ( Φ T) were larger in DMSO. Triplet lifetimes were much lower in aqueous solutions (compared to DMSO) due to the fact that water absorbs strongly around 1108 nm, which corresponds to the triplet energy of a metallophthalocyanine complex. The MPcS mix complexes quenched hydroquinone, and the Stern–Volmer constants follow the order: AlPcS mix > SiPcS mix > GePcS mix > ZnPcS mix > SnPcS mix which is the order of the extinction coefficients (of the low energy band for complexes with split Q-band) of these molecules. The rate constants for fluorescence, intersystem crossing, internal conversion, and photodegradation were determined from the hydroquinone quenching data.

The nature of internal conversion and intersystem crossing in exciplexes

Experimental data obtained by the authors themselves and other published data on the dependence of the rate constants of internal conversion and intersystem crossing in exciplexes (back electron transfer) on the value of energy gap were critically revisited. The conclusion that these processes occur according to the mechanism of radiationless diabatic quantum transitions, rather than the preceding thermally activated solvent and reactant reorganizations was substantiated.