Nonradiative S1 Research Articles

Modulation of lanthanide luminescence by carbamoylmethylphosphine oxide ligand: A theoretical study

The antenna capacity of Phenacyldiphenylphosphine oxide (Phenac), combining C=O and P=O donor groups was established by theoretical examination of its chromophore properties, binding ability to encapsulate and stabilize lanthanide complexes and potential to sensitize the EuIII and TbIII luminescence in Eu(Phenac)2(NO3)3(H2O) (1), Eu(Phenac)2(NO3)3 (2), Eu(Phenac)3(NO3)3 (3) and Tb(Phenac)2(NO3)3(H2O) (4) complexes in vacuum, solution and solid phase. The ligand-centered singlet and triplet excited state energies, the rates and lifetimes for the rival nonradiative (S1→T1,2 intersystem crossing (ISC)) and radiative and nonradiative S1→S0 and T1→S0 relaxation were predicted by means of DFT/TDDFT/ωB97XD calculations. Similar relaxation paths were found for all complexes. The absorbed energy by Sn > 1(π-π*) states relaxed to T1min by major ISC mechanism S1→T2→T1. The long radiative lifetime of S1→S0 due to n(p(C)(O))π-π*,p(P) character facilitated the forward ISC process. The environment, Phenac's number and LnIII size affect the T1 energy ∼0.1 eV. The calculated (Phenac→EuIII) energy transfer rates suggested the most likely T1 → 5D1/5D0 channels for a population of emitting metal-centered states. The small Φ of EuIII and larger luminescence of TbIII were discussed and explained. The importance of the P=O and C=O groups for the coordination ability and absorption properties of Phenac has been elucidated.

Dynamics and mechanisms of nonradiative photodissociation of glycine: A theoretical study

Photochemistry of small biomolecules is fundamental to understand the photochemical reactions of larger molecules, such as proteins. In this work, the dynamics and mechanisms of photodissociations of glycine conformers were studied using the DFT, TD-DFT and CASPT2 methods with the aug-cc-pVDZ basis set and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This study emphasized unimolecular dissociations and isomerization, nonradiative S1 → S0 relaxations that generate molecular products and the interplay between photoenergies and thermal energies. Four nonplanar conformers were suggested by the TD-DFT/B3LYP/aug-cc-pVDZ and CASPT2(8,14)/aug-cc-pVDZ methods to be stable in the S1 state. The potential energy curves obtained from the TD-DFT/B3LYP/aug-cc-pVDZ and CASPT2(8,14)/aug-cc-pVDZ methods and SA-CPMCSCF/6-31G(d) conical intersection optimizations suggested S0/S1 intersections for distorted glycine structures, Cα–N and NH dissociations and NH → OC isomerization. The NVE-MDSH results showed that although no dissociated molecular product was observed in the S1 state, isomerization-mediated dissociation and exothermic S1 → S0 relaxation temperature could lead to eight molecular product channels, in which the vertically excited Wigner-sampled structures generated eleven reactive and stable molecular products in different temperature ranges. These thermally selective reactions favored the lowest energy conformer (structure Ip) as the precursor, from which reactive and stable molecular products were formed above the threshold exothermic relaxation temperature. These theoretical results showed in detail for the first time how unimolecular photodissociations and nonradiative S1 → S0 relaxations create molecules in cold interstellar media without heat transfer from the surroundings. The results in this work could be used in future theoretical and experimental studies on the same or similar systems.

Solvent Mediated Excited State Proton Transfer in Indigo Carmine.

Excited state proton transfer (ESPT) is thought to be responsible for the photostability of biological molecules, including DNA and proteins, and natural dyes such as indigo. However, the mechanistic role of the solvent interaction in driving ESPT is not well understood. Here, the electronic excited state deactivation dynamics of indigo carmine (InC) is mapped by visible pump-infrared probe and two-dimensional electronic-vibrational (2DEV) spectroscopy and complemented by electronic structure calculations. The observed dynamics reveal notable differences between InC in a protic solvent, D2O, and an aprotic solvent, deuterated dimethyl sulfoxide (dDMSO). Notably, an acceleration in the excited state decay is observed in D2O (<10 ps) compared to dDMSO (130 ps). Our data reveals clear evidence for ESPT in D2O accompanied by a significant change in dipole moment, which is found not to occur in dDMSO. We conclude that the ability of protic solvents to form intermolecular H-bonds with InC enables ESPT, which facilitates a rapid nonradiative S1 → S0 transition via the monoenol intermediate.

Integration of Cyanine, Merocyanine and Styryl Dye Motifs with Synthetic Bacteriochlorins.

Understanding the effects of substituents on spectral properties is essential for the rational design of tailored bacteriochlorins for light-harvesting and other applications. Toward this goal, three new bacteriochlorins containing previously unexplored conjugating substituents have been prepared and characterized. The conjugating substituents include two positively charged species, 2-(N-ethyl 2-quinolinium)vinyl- (B-1) and 2-(N-ethyl 4-pyridinium)vinyl- (B-2), and a neutral group, acroleinyl- (B-3); the charged species resemble cyanine (or styryl) dye motifs whereas the neutral unit resembles a merocyanine dye motif. The three bacteriochlorins are examined by static and time-resolved absorption and emission spectroscopy and density functional theoretical calculations. B-1 and B-2 have Qy absorption bathochromically shifted well into the NIR region (822 and 852 nm), farther than B-3 (793 nm) and other 3,13-disubstituted bacteriochlorins studied previously. B-1 and B-2 have broad Qy absorption and fluorescence features with large peak separation (Stokes shift), low fluorescence yields, and shortened S1 (Qy ) excited-state lifetimes (~700 ps and ~100 ps). More typical spectra and S1 lifetime (~2.3 ns) are found for B-3. The combined photophysical and molecular-orbital characteristics suggest the altered spectra and enhanced nonradiative S1 decay of B-1 and B-2 derive from excited-state configurations in which electron density is shifted between the macrocycle and the substituents.

Theory of Excited State Decays and Optical Spectra: Application to Polyatomic Molecules

General formalism of absorption and emission spectra, and of radiative and nonradiative decay rates are derived using a thermal vibration correlation function formalism for the transition between two adiabatic electronic states in polyatomic molecules. Displacements, distortions, and Duschinsky rotation of potential energy surfaces are included within the framework of a multidimensional harmonic oscillator model. The Herzberg-Teller (HT) effect is also taken into account. This formalism gives a reliable description of the Q(x) spectral band of free-base porphyrin with weakly electric dipole-allowed transitions. For the strongly dipole-allowed transitions, e.g., S(1) --> S(0) and S(0) --> S(1) of linear polyacenes, anthracene, tetracene, and pentacene, the HT effect is found to enhance the radiative decay rates by approximately 10% compared to those without the HT effect. For nonradiative transition processes, a general formalism is presented to extend the application scope of the internal conversion theory by going beyond the promoting-mode approximation. Numerical calculations for the nonradiative S(1) --> S(0) decay rate of azulene well explain the origin of the violation of Kasha's rule. When coupled with first-principles density functional theory (DFT) calculations, the present approach appears to be an effective tool to obtain a quantitative description and detailed understanding of spectra and photophysical processes in polyatomic molecules.

Nonradiative S-T intersystem crossing in α and β chlorine derivatives of naphthalene

The rate constants KSTs of nonradiative S1 ↝ T1stransitions to triplet sublevels s in molecules of chlorine derivatives of naphthalene (1,4-dichloro- and monosubstituted α-chloro- and β-chloronaphthalene) have been calculated within the model of vibronically induced spin-orbit (VISO) couplings. The contribution of the spin-orbit couplings in Cl atoms and carbon framework of the molecule to VISO couplings is determined. A dependence of the heavy-atom effect on the constant KSTsin relation to the type of sublevel Ts and α and β positions of chlorine in the molecule is established and explained.

Probability estimate for intersystem crossings in naphthalene and acenaphthene in the presence of benzophenone

To a first approximation in perturbation theory, we have obtained expressions for the rate constants for intersystem crossings in acceptor molecules. We show that exchange interactions between the components of the donor-acceptor pair can change the probability of intersystem crossings for both radiative T → S0 and nonradiative S1 → T transitions. The theoretical conclusions are supported by the results of experimental studies.

Properties of the lowest singlet states of dimethylaminobenzonitrile in liquid dielectrics: Quantum-mechanical calculations

The energies of the ground (S 0) and first two excited (S 1 and S 2) singlet states, their electric dipole moments, and the oscillator strengths for different conformers of dimethylaminobenzonitrile having different orientations of the dimethyl group are calculated for different solvents assuming a universal character of the intermolecular interactions. The results obtained show that, as the polarity of the solvent increases, both the energy gap between the S 1 and S 2 levels, which determines their vibronic interaction, and the barrier for the nonradiative S 1 → S 2 transition, which can be interpreted as the barrier for the reaction of charge transfer, strongly decrease. In strongly polar solvents, the inversion angle along the twist coordinate, which determines the mutual position of the first two singlet states, decreases from 55° in free molecules to ∼16° in a solvent with a permittivity of 47, which radically facilitates charge transfer. The dipole moments of all the conformations in the singlet states are shown to depend considerably on the dielectric properties of the solvent, demonstrating a strong polarizability of the dimethylaminobenzonitrile molecule. The strongest increase in the dipole moments with increasing solvent polarity is observed for the planar geometry in the S 1 state and for the perpendicular orientation of the dimethyl group in the S 2 state. An important result is that the spectral properties of different conformations, first of all, the energies of the 0-0 transitions, are different, which leads to the necessity of taking into account this factor for the correct description of the light absorption and photoreactions in systems with charge transfer. In the calculations, the semiempirical software packages HyperChem 5.0 and MOPAC are used.

Photophysical Properties of Oligo(2,3-Thienyleneethynylene)s

Photophysical properties of oligo(2,3-thienyleneethynylene)s (nTE, n denotes the number of thiophene rings, n = 2, 3) in benzene were investigated using steady-state, time-resolved fluorescence, and transient absorption spectroscopies. For 2TE, generation of the radiative S2 and nonradiative S1 states was confirmed. Upon excitation, the S2 state was initially generated and deactivated to the S1 state within 10 ps. The S1 state exhibited the transient absorption band at 470 nm, of which the lifetime was estimated to be 5.3 ns. In the case of 3TE, on the other hand, it was revealed that the radiative S1 state with a transient absorption peak at 650 nm was generated upon excitation. The T1 states of nTE were generated from the S1 states. The quantum yields were estimated to be 0.52 and 0.54 for 2TE and 3TE, respectively. Extremely fast reactions in the higher triplet excited state were indicated for both 2TE and 3TE.

Photophysics of pyrene-labelled compounds of biophysical interest

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py-NH-CO-C2H5, Py-NH-CO-Leu-Boc, Py-CH2-NH-CO-C2H5, Py-CH2-NH-CO-Leu-Boc, Py-CO-NH-C3H7, Py-CO-NH-Leu-OMe, Py-CH2-CO-NH-C3H7, Py-CH2-CO-NH-Leu-OMe and Py-C3H6-CO-NH-Leu-OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, kr, and non-radiative decay constants of their S1 states and the values of these constants correlated with the structures of the tethers. Non-radiative S1 decay rates (mainly intersystem crossing to T1) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between [epsilon]max of the S1-S0 transition and either kr or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the 'normal' radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S1 states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S1-T1 electronic energy spacings of all the derivatives exceed the O2(1Deltag-3Sigmag-) electronic excitation energy of ca. 1 eV.

Electronic States of a Novel Smaragdyrin Isomer: Polarized Spectroscopy and Theoretical Studies

Spectral and theoretical techniques were applied to investigate the electronic structure and spectra of two recently synthesized pentapyrrolic macrocycles, isomers of smaragdyrin: 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-[22]pentaphyrin-(1.1.1.0.0) (1) and 16,20-dibutyl-2,3,6,7,10,11,15,21-octamethyl-5-oxa-[22]pentaphyrin-(1.1.1.0.0) (2). Combined use of linear dichroism, magnetic circular dichroism, fluorescence anisotropy and INDO/S calculations resulted in the location and assignments of a number of electronic transitions (eleven for 1 and seven for 2). Even though the spectral pattern differs somewhat from that characteristic of most porphyrins, the results show that the four lowest excited electronic singlet states of both compounds are very well described by a four-orbital model, widely used for the interpretation of spectra in this type of compounds. Fluorescence and transient absorption/bleaching measurements enabled the determination of the rate constants of the radiative and nonradiative S1 depopulation processes. These photophysical properties are consistent with a rigid, nearly planar excited state geometry that is essentially unchanged with respect to the ground-state structure.

ChemInform Abstract: Semi-empirical PM3 Calculations Predict the Fluorescence Quantum Yields (Φ) of 4-Monosubstituted Benzofurazan Compounds

We have elucidated the relationship between the fluorescence quantum yield (Φ) of 4-monosubstituted benzofurazan (2,1,3-benzoxadiazole) compounds and the substituent group at the 4-position using a group of eleven such compounds. The absorption and fluorescence spectra of these compounds were measured in fifteen different solvents. Semi-empirical PM3 calculations were carried out to obtain the optimized geometry, energies, charges and dipole moments of this group in the ground and excited states. The S1–T2 energy gaps obtained from the PM3-CAS/CI calculations, which reflected the probability of S1→T2 intersystem crossing, related well to the fluorescence quantum yield (Φ) of the 4-monosubstituted benzofurazan compounds in nonpolar solvents. It also appeared that the decrease in the S1–T1 energy gaps was related to the change in the fluorescence quantum yield (Φ) due to solvent effects. In highly polar solvents, non-radiative S1→T1 intersystem crossing occurs, in addition to non-radiative S1→T2 intersystem crossing, in 4-monosubstituted benzofurazan compounds, as they have a larger dipole moment in the S1 state than in the T2 state. These studies have enabled us to predict the fluorescence quantum yield (Φ) for a series of 4-monosubstituted benzofurazan compounds.

Semi-empirical PM3 calculations predict the fluorescence quantum yields (Φ) of 4-monosubstituted benzofurazan compounds

We have elucidated the relationship between the fluorescence quantum yield (Φ) of 4-monosubstituted benzofurazan (2,1,3-benzoxadiazole) compounds and the substituent group at the 4-position using a group of eleven such compounds. The absorption and fluorescence spectra of these compounds were measured in fifteen different solvents. Semi-empirical PM3 calculations were carried out to obtain the optimized geometry, energies, charges and dipole moments of this group in the ground and excited states. The S1–T2 energy gaps obtained from the PM3-CAS/CI calculations, which reflected the probability of S1→T2 intersystem crossing, related well to the fluorescence quantum yield (Φ) of the 4-monosubstituted benzofurazan compounds in nonpolar solvents. It also appeared that the decrease in the S1–T1 energy gaps was related to the change in the fluorescence quantum yield (Φ) due to solvent effects. In highly polar solvents, non-radiative S1→T1 intersystem crossing occurs, in addition to non-radiative S1→T2 intersystem crossing, in 4-monosubstituted benzofurazan compounds, as they have a larger dipole moment in the S1 state than in the T2 state. These studies have enabled us to predict the fluorescence quantum yield (Φ) for a series of 4-monosubstituted benzofurazan compounds.

Deuterium Isotope Effect on 4-Aminophthalimide in Neat Water and Reverse Micelles

Picosecond time-resolved emission of 4-aminophthalimide (4-AP) in neat water and D2O and in the water pool of aerosol OT reverse micelles in n-heptane is reported. It is proposed that in protic solvents 4-AP undergoes ultrafast solvent-mediated proton transfer and the emission originates from the proton-transferred species. The abrupt decrease in the emission quantum yield and lifetime of 4-AP in protic solvents is attributed to the reduction in the S1−S0 energy gap in the proton-transferred species. Nearly 4-fold increase in the emission quantum yield (φf) and lifetime (τf) of 4-AP is observed in neat D2O compared to water. This is ascribed to the retardation of the nonradiative S1 → S0 internal conversion processes on deuteration of the imide proton. In neat water or D2O the solvation dynamics is too fast to be detected using our picosecond setup. However, in the water pool of reverse micelles the relaxation of the water (or D2O) slows down appreciably. The initial component of the solvation dynamics in...

Subpicosecond population decay time of the first excited singlet state of thioxanthione in fluid solution

The population decay time of the first excited singlet state of an aromatic thione, thioxanthione, has been measured for the first time in fluid solution at room temperature, using a laser pump-probe method. The average value of τ(S1) is 0.53±0.12 ps in three solvents, acetonitrile, acetone and 1-octanol giving a nonradiative S1 decay constant of (1.9 ± 0.4) × 1012 s−1. Using these data and a calculated value of the radiative lifetime of S1, a quantum yield of S1 → S0 fluorescence, φf < 5 × 10−8, is estimated, a value which is consistent with the failure to observe prompt fluorescence in these compounds. The short S1 lifetime of thioxanthione is attributed to fast S1(1A2)−Tn(3A1) intersystem crossing, which can occur by a first order spin-orbit coupling mechanism because Tn(3A1) (n = 1 or 2) has a lower energy than S1(1A2) in the polar solvents employed.

Ein theoretischer Beitrag zur radiativen und nicht-radiativen S1-Desaktivierung der isomeren Terphenyle unter Ber�cksichtigung ihrer stabilen Konformationen

A Theoretical Contribution to the Radiative and Non-Radiative S1 Deactivation of the Isomeric Terphenyls by Including their Stable Conformations On the basis of completely optimized S0 and S1 molecular geometries of isomeric terphenyls the radiative and non-radiative S1 deactivations were investigated theoretically within the Condon and Herzberg-Teller approach, respectively. The theoretical vibronic S1 S0 ‘line’ spectra are compared with the experimental fluorescence data. The influence of stable conformations on the vibronic spectral behaviour is discussed. For the internal S1 S0 conversion the rate constants were calculated by including the promoting and accepting modes, and the electronic factor as well. In all cases the photophysically relevant vibrational modes are analyzed.