Edward C. Lim

Ab initio study of a biradical radiationless decay channel of the lowest excited electronic state of cytosine and its derivatives

A theoretical model for the ultrafast S1-->S0 internal conversion of cytosine is presented, in which a state switch from the initially prepared 1pipi* state to the out-of-plane deformed excited state of biradical character controls the rate of the S1(1pipi*) decay. This mechanism successfully accounts for the dramatically longer S1 lifetimes of 5-fluorocytosine and N-acetylcytosine relative to cytosine. The replacement of the C5 hydrogen atom by a methyl group is predicted to lead to a substantial, but not dramatic, increase in the S1 lifetime, also consistent with experiment. It is this ability to correctly predict the substituent effects that distinguishes the present model from the previously proposed mechanisms.

Naphthalene dimer: Electronic states, excimers, and triplet decay

Computations have been performed for the singlet and triplet electronic states of varying orientations of naphthalene dimer. The dependence of exciton splitting upon orientation and intermonomer distance was explored. Splittings of triplet states are seen to be nontrivial at typical bonding distances, commensurate with the splittings of weakly allowed singlet states. Charge-transfer interaction with the excimer states is seen to be most significant in face-to-face orientations which can allow closer approach of the two monomers. Predictions of the prominent features of the singlet–singlet and triplet–triplet absorption spectra agree well with experimental findings. A spin-orbit channel-counting scheme is introduced to account for observed radiative and nonradiative decay of the T1 triplet state of the monomer, and then applied to the dimer. The mechanism has been found for the observed more rapid phosphorescence of the T1 state of the dimer when placed in orientations lacking inversion symmetry.

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

We present here the results of time-resolved absorption and emission experiments for 4-(dimethylamino)benzonitrile in solution, which suggest that the fluorescent intramolecular charge transfer (ICT) state may differ from the twisted ICT (TICT) state observed in transient absorption.

The T1(nπ*)←S0 laser induced phosphorescence excitation spectrum of acetaldehyde in a supersonic free jet: Torsion and wagging potentials in the lowest triplet state

The laser induced T1(nπ*)←S0 phosphorescence excitation spectrum of jet‐cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion‐wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 609.68 and 869.02 cm−1, respectively. Minima in the potential hypersurface at θ=61.7° and α=42.2° defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding S1‐state parameters showed that the torsion barrier (in cm−1) does not greatly change, S1/T1=710.8/609.7, whereas the barrier height for the wagging‐inversion barrier increases dramatically, 574.4/869.0.

Toluene internal-rotation: Measurement and simulation of the high-resolution S1–S0 fluorescence excitation spectrum at 0.5 K

Rotational structure in the origin band of the S1–S0 fluorescence excitation spectrum of toluene has been measured at 0.012 cm−1 resolution with a rotational temperature of 0.5 K using a pulsed beam apparatus. Such spectra have been obtained for the parent isotopomer and for the isotopomers with mono- and di-deuterated methyl groups. These, and previously known forbidden bands in which the internal-rotation quantum number Ki changes, are simulated here using ab initio internal-rotation-angle-dependent geometries, potential functions, and electronic transition moment function. An adjustment of some of the ab initio parameters allows a close fitting of the spectra to be made, and this can only be achieved if bond lengths and angles are allowed to vary with internal rotation. The resulting geometries for ground and excited-state toluene are the most accurate to date.

Role of the πσ* State in Molecular Photophysics

Photosynthesis, which depends on light-driven energy and electron transfer in assemblies of porphyrins, chlorophylls, and carotenoids, is just one example of the many complex natural systems of photobiology. A fuller understanding of the spectroscopy and photophysics of simple aromatic molecules is central to elucidating photochemical processes in the more sophisticated assemblies of photobiology. Moreover, developing a better grasp of the photophysics of simple aromatic molecules will also enhance our ability to create and improve practical applications in photochemical energy conversion, molecular nanophotonics, and molecular electronics. In this Account, we present a concerted experimental and theoretical study of aromatic ethynes, aromatic nitriles, and fluorinated benzenes, illustrating the important roles that the low-lying pisigma* state plays in the electronic relaxation of these aromatic compounds. Diphenylacetylene, 4-dialkylaminobenzonitriles, 4-dialkylaminobenzethynes, and fluorinated benzenes exhibit fluorescence that strongly quenches as the excitation energy is increased for gas-phase systems and at elevated temperatures in solution. Much of this interesting photophysical behavior can be attributed to the presence of a dark intermediate state that crosses the fluorescent pipi* state. Our quantum chemistry calculations, as well as time-resolved laser spectroscopies, indicate that this dark intermediate state is the pisigma* state that arises from the promotion of an electron from the pi orbital of the phenyl ring to the sigma* orbital localized in the C[triple bond]X group (where X is CH and N) or on the C-X group (where X is a halogen). These crossings not only lead to the strong excitation energy and temperature dependence of fluorescence but also induce highly interesting pisigma*-mediated intramolecular charge transfer in 4-dialkylaminobenzonitriles. Most previous studies on the excited-state dynamics of organic molecules have examined aromatic hydrocarbons, nitrogen heterocycles, aromatic carbonyl compounds, and polyenes, which have low-lying excited states of pipi* character (hydrocarbons and polyenes) or npi* and pipi* character (carbonyls and N-heterocycles). These studies have revealed important involvement of selection rules (promoting vibrational modes and spin-orbit coupling) and Franck-Condon factors for radiationless transitions, which have important effects on photophysical properties. The recent experimental and time-dependent density functional theory (TDDFT) calculations of aromatic ethynes, nitriles, and perfluorinated benzenes described in this Account demonstrate the importance of the bound excited state of a pisigma* configuration in these molecules.

Photophysics of aromatic molecules with low-lying πσ* states: Fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S1) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S1 state of ππ* character for fluorinated benzenes with four or less F atoms, and S1 state of πσ* character for PFB and HFB. The πσ* character of the S1 state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the π1π* (Lb)←S0 transition, which can be identified with the predicted low-energy π1σ*←S0 absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the π1σ* state and efficient S1 (πσ*)→S0 internal conversi...

Electronic spectra and photophysics of the two stable conformers of anthracene dimer: evaluation of an ab initio structure prediction

Abstract It is shown that the spectral features and excited-state dynamics of the two Van der Waals dimers of anthracene, generated by supersonic free expansion, can be accounted for if the dimers have the crossed (D 2d ) and the parallel-displaced (C 2h ) structures predicted by an ab initio calculation with MP2 correlation.

The cavity ringdown spectrum of the visible electronic system of thiophosgene: An estimation of the lifetime of the T1(ã 3A2) triplet state

To obtain insights into the photophysical properties of collision-free T1(a 3A2) thiophosgene, Cl2CS, the cavity ringdown (CRD) spectrum of the T1←S0 absorption system was recorded under supersonic jet conditions and compared with the corresponding excitation spectrum of the total emission. It was found that none of the T1←S0 bands in the CRD spectrum appears in the excitation spectrum, indicating that the T1 thiophosgene decays almost exclusively by the nonradiative T1→S0 intersystem crossing (ISC). An estimation of the T1 nonradiative lifetime was made using the T1–S0 spin-orbit coupling and the Franck–Condon factors for the T1→S0 ISC based on the ab initio equilibrium structures and vibrational frequencies for the T1 and S0 states, computed at the MP2/6-31G(d,p) and MP4/6-31G(d,p) level of theory. The nonradiative life, calculated as the 1/e of the survival probability, is approximately 20 ps for barrier heights of 770–845 cm−1 and out-of-plane angles of 32.07°–32.69°. (The thiophosgene adopts a pyrami...

INVERSION POTENTIALS IN THE GROUND AND EXCITED ELECTRONIC STATES OF 9,10-DIHYDROANTHRACENE AS PROBED BY THE ABSORPTION AND FLUORESCENCE SPECTRA OF JET -COOLED MOLECULES

The inversion potentials in the ground and the lowest excited singlet states of 9,10‐dihydroanthracene were deduced from the fit of the absorption and fluorescence spectra of jet‐cooled molecules to model potentials. The barrier height in the ground state (615 cm−1 ) is substantially larger than in the excited state (80 cm−1 ). Correspondingly, the folding angle in the ground state is smaller (144.6° vs 164.1°). Comparison of the calculated line intensities with the experimental absorption spectrum indicates that the transition dipole moment is dependent on the nuclear coordinate of the inversion mode. The measured quantum yield of the single vibronic level fluorescence is essentially independent of the quantum number and the symmetry of the inversion level excited.