Studies of intersystem crossing dynamics in acetylene

We report a new ab initio study of the acetylene T3 potential energy surface, which clarifies the nature of its energy minimum, and present computed equilibrium geometries and diabatic frequencies. This information enables the computation of harmonic vibrational overlap integrals of T3 vibrational levels with the S1 3nu3 state. The results of this calculation support the interpretation of two local perturbations of S1 3nu3, revealed in ultraviolet laser-induced fluorescence/surface electron ejection by laser excited metastables spectroscopy and Zeeman anticrossing measurements, respectively, as arising from two rotational submanifolds of a single T3 vibrational state. We present plausible assignments for this state as a guide for future experimental work.     

Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational study

Previous studies have established that the major pathway for the first singlet excited state of 1-nitronaphthalene is intersystem crossing to the triplet manifold. In this contribution we present determinations of the decay of the S1 state of this compound in several solvents to establish the time scale of the multiplicity change as a function of the polarity and hydrogen-bonding ability of the solvent environment. The measurements were made with the femtosecond frequency up-conversion technique to follow the weak spontaneous molecular emission which precedes triplet formation. Our results show that in all environments the S1 lifetime is 100 fs or less, making 1-nitronaphthalene the organic compound with the fastest multiplicity change ever measured. We also show that the bathochromic shifts observed for the first absorption band imply changes in the relative energies of the singlet and triplet manifolds, which in turn manifest in a 2-fold increase of the fluorescence lifetime in cyclohexane compared with the polar solvents. Additionally, we performed excited-state calculations at the TD-DFT/ PBE0/6-311++G(d,p) level of theory with the PCM model for solvation. The TD-DFT theory identifies the presence of upper triplet states which can act as receiver states in this highly efficient photophysical pathway. Together, the experimental and theoretical results show that the dynamics of the S1 state in 1-nitronaphthalene represent an extreme manifestation of El-Sayed's rules due to a partial (n-pi*) character in the receiver triplets which are nearly isoenergetic with S1, determining a change in the molecular spin state within 100 fs.     

A small and stable covalently bound trianion

Stable doubly charged anions have become well known over the past decade, but the knowledge about higher-charged molecules is still sparse. Especially the minimum size of a covalently bound trianion which is still stable is an open question. Here, we present the smallest trianion of this kind known up to now, namely, B(C(2)CO(2))(3) (3-). After establishing its geometrical parameters, we investigate its stability with respect to electron autodetachment and fragmentation of the molecular framework. Our results lend strong support to the notion that this trianion indeed represents a stable compound which should be observable in the gas phase.     

Ultrafast intersystem crossing in 9,10-anthraquinones and intramolecular charge separation in an anthraquinone-based dyad

Femtosecond transient absorption spectroscopy was employed to determine quantitatively the ultrafast S1-T1 intersystem crossing in a 2-substituted 9,10-anthraquinone derivative (3), kisc = 2.5 x 10(12) s-1. Notwithstanding this rapid process, photoexcitation of dyad 1 is followed by competition between intersystem crossing and intramolecular charge separation, the latter leading to a short-lived (2 ps) singlet charge-transfer (CT) state. The local triplet state itself undergoes slower charge separation to populate a relatively long-lived (130 ns) triplet CT state. An earlier report about the formation of an extremely long-lived CT state (> 900 micros) in 1 was found to be erroneous and was related to the sacrificial photo-oxidation of the dimethylsulfoxide solvent used in that study. Finally, some important criteria have been formulated for future experimental validation of "unusually long-lived" CT states.     

Theory of collision induced intersystem crossing. Application to glyoxal

Our recent theory of collision induced intersystem crossing is applied to glyoxal where Beyer, Zittle, and Lineberger have determined single vibronic level rates. The magnitude of observed rates, their lack of deuterium isotope effect, their correlation with perturber dipole and dipole-polarizability induced collision rates, and the possible anomalous quenching efficiency of oxygen, are all explained by the theory and are correlated with the observed rotational relaxation cross sections in the ¹A_u state.     

Triplet to ground state intersystem crossing and CIS-trans isomerization in stilbene

The T₁ ? S₀ intersystem crossing process in stilbene is considered adopting a formulation in which the isomerization coordinate can be treated separately. Energy surfaces in agreement with experimental and theoretical results are employed and pertinent spin-orbit coupling is evaluated. The calculated decay rate is in good agreement with the experimental results based on the azulene quenching effect. The qualitative feature that quantum yield for cis formation tends to be higher than the counterpart trans formation is explained. Deuterium effects are discussed and explanations for them are suggested.     

On the role of methyl torsional modes in the intersystem crossing dynamics of isolated molecules

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6'=1.56 and V6'=8.28 cm(-1) in 2MP and V6'=4.11 and V6'=58.88 cm(-1) in 5MP. Many of the lines in both spectra are fragmented by couplings with lower lying triplet states. Analyses of some of these perturbations yield approximate values of the intersystem crossing matrix elements, from which it is concluded that the sigma=+/-1 torsional levels of the S1 state are significantly more strongly coupled to the T1 state than the sigma=0 torsional levels.     

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.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 114–125.Original Russian Text Copyright © 2005 by Kuzmin, Soboleva, Dolotova, Dogadkin.This revised version was published online in April 2005 with a corrected cover date.     

Picosecond time-resolved spectroscopy and the intersystem crossing rates of anthrone and fluorenone

The rise time of the T_n ← T₁ absorption of anthrone in benzene is determined to be 70 ps. The mechanism of efficient intersystem crossing of anthrone has been discussed and is compared with that of benzophenene. The rise time of fluorenone has been found to be sensitive to solvent, changing from 140 ps in cyclohexane to 12 ns in acetone. This tendency is explained in terms of the change in the character of the lowest excited singlet state of fluorenone from ππ^* in acetone to nπ^* in cyclohexane.     

A TDDFT investigation of bay substituted perylenediimides: Absorption and intersystem crossing

The conformational structure and electronic spectra properties of a series of bay substituted perylenediimides (PDI) derivatives have been investigated by means of density functional theory (DFT) and time‐dependent DFT. The B3LYP and PBE0 hybrid exchange‐correlation functionals were applied in conjunction with the double‐ζ quality SVP basis set. These compounds are interesting for organic materials science and as photosensitizers in cancer phototherapy (PDT), because of their intense absorption in the visible region. Results show that the substitution at the bay position of the PDI parent molecule with N‐alkyl groups shifts the absorption maxima towards the red part of the visible spectrum (around 650–700 nm) as required for the applications in PDT. The main PDT action mechanisms have been investigated by computing of electron affinities, ionization potentials, triplet energies and spin‐orbit matrix elements between singlet and triplet excited states. © 2012 Wiley Periodicals, Inc.     

The role of intersystem crossing steps in singlet oxygen chemistry and photo-oxidations

Singlet oxygen chemistry and photo-oxidation reactions, in general, often require one or more critical reaction steps that involve an intersystem crossing from a singlet state to a triplet state or vice versa. This paper considers two important intersystem crossing mechanisms, electron spin-electron orbit (spin-orbit) coupling and electron spin-nuclear spin (spin-spin) coupling, and how they may be involved : (1) in the deactivation of ¹O₂ to ³O₂ ; (2) in the thermal catalytic conversion of ³O₂ to ¹O₂; and (3) in the fragmentation of aromatic endoperoxides to yield O₂ and an aromatic substrate.     

Proximity effect in spin sublevel dynamics: Possible influence of a low-lying3 A2 (nπ*) state: On singlet → triplet intersystem crossing of Pyrimidine

Comparison of spin sublevel population rates in pyrimidine and 5-methylpyrimidine suggests that the vibronic perturbation of ³B₂ and/or ³A₁ ππ* states by a higher-lying ³A₂nπ* state plays an important role in S₁ → T₁ intersystem crossing of pyrimidine. The result also implies that the n₊π*—n_?π* splitting in pyrimidine is significantly smaller than the corresponding gap in pyrazine. The appearance of the 0.0 band in the τ_x spectrum indicates that the triplets state symmetry is lower tha C_2v.     

Femtosecond excited-state dynamics of 4-nitrophenyl pyrrolidinemethanol: evidence of twisted intramolecular charge transfer and intersystem crossing involving the nitro group

Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.     

Application of the collision-complex model to collision-induced intersystem crossing and collisional magnetic quenching of polyatomic molecules

The purpose of this paper is to apply the collision-complex model to collision-induced intersystem crossing in poly-atomic molecules (glyoxal, propynal, pyrimidine, etc.), and to the collisional magnetic quenching of polyatomic molecules, with particular emphasis on glyoxal.     

Characterization of photoinduced isomerization and intersystem crossing of the cyanine dye Cy3

Several important photophysical properties of the cyanine dye Cy3 have been determined by laser flash photolysis. The triplet-state absorption and photoisomerization of Cy3 are distinguished by using the heavy-atom effects and oxygen-induced triplet --> triplet energy transfer. Furthermore, the triplet-state extinction coefficient and quantum yield of Cy3 are also measured via triplet-triplet energy-transfer method and comparative actinometry, respectively. It is found that the triplet --> triplet (T1-->Tn) absorptions of trans-Cy3 largely overlap the ground-state absorption of cis-Cy3. Unlike what occurred in Cy5, we have not observed the triplet-state T1-->Tn absorption of cis-Cy3 and the phosphorescence from triplet state of cis-Cy3 following a singlet excitation (S0-S1) of trans-Cy3, indicating the absence of a lowest cis-triplet state as an isomerization intermediate upon excitation in Cy3. The detailed spectra of Cy3 reported in this paper could help us interpret the complicated photophysics of cyanine dyes.