The research interests of this group are in the areas of electronic spectroscopy and photophysics of organic molecules. Various forms of laser spectroscopy are combined with high-level quantum chemistry calculations to deduce electronic structures and excited-state dynamics of molecules of fundamental importance to chemistry and biology.
Understanding energy and charge transfers in electronically excited molecules is of fundamental importance to photochemistry and photobiology. Traditionally, excited-state dynamics of organic molecules are described in terms of the low-lying ππ* and nπ*states. The biradical state, with geometry that arises from the stretching of a single bond, twisting of a double bond, or bending of a triple bond, is not usually considered in the description of the photophysics of closed-shell molecules. Our recent work on a number of molecular systems, including halogenated benzenes, aminobenzonitriles, nucleobases, as well as phenylethynylbenzenes, indicates that a biradical state lies near or below the conventional excited states. Because of the greatly different equilibrium geometry of the biradical state relative to ππ*, nπ* or electronic ground state, the biradical state can cross the initially prepared excited state as well as the ground state, thus leading to an ultrafast non-adiabatic photoprocesses. During the past few years, a significant portion of our research efforts has been directed towards the identification of the low-lying dark electronic states (i.e., biradical states), and the role they play in the ultrafast excited-state dynamics of molecular systems of fundamental importance to chemistry.1,2
Ultrafast radiationless transition of DNA/RNA bases
Because of their biological importance, nucleic acid bases have been the subjects of many spectroscopic and photophysical studies over the past three decades. The most striking photophysical property of the nucleobases is the extremely short excited-state (S1) lifetime, caused by ultrafast radiationless decay (internal conversion) to the ground state (S0). It is generally recognized that the highly efficient internal conversion of nucleic acid components provides DNA with a high degree of intrinsic photostability.
To probe the origin of the subpicosecond internal conversion, we have carried out CIS, CC2 and CCSD(T) calculations of the potential energy profiles for pyrimidine bases (cytosine, uracil, and thymine) and their derivatives at optimized CIS geometries.3,4 The results indicate that the S1(ππ*) ® S0 internal conversion occurs through a low-barrier (or barrierless) state switch from the initially excited 1ππ* to a biradical state, which in turn intersects the ground state at lower energy. In the biradical state, the C5 and C6 hydrogen atoms are almost perpendicular to the average ring plane and are displaced in opposite directions. As a result, the pz orbitals of the C5 and C6 carbon atoms are decoupled from the π-electron system and are singly occupied, giving the state a biradical character.3,4
We have also performed CC2, EOM-CCSD and CR-EOM-CCSD(T) calculations of the potential energy profiles of the purine bases, adenine and guanine, at optimized CIS geometries using the cc-pVDZ basis set.5 The results of this study indicate that the internal conversion of purine bases is also mediated by a biradical state, which arises from the ππ* state upon out-of-plane deformation of the pyrimidine ring. The biradical state is characterized by a strongly puckered six-membered ring and the C2-H bond (for adenine), or the C2-N bond (for guanine), which is nearly perpendicular to the average ring plane, as would result from the twist of the N3-C2 bond.
A compelling experimental evidence in support of the biradical-mediated internal conversion comes from the observation that covalent modification of nucleic acid bases leading to hindrance of the C5-C6 twist in the pyrimidines and N3-C2 twist in the purines leads to a dramatic retardation of the internal conversion rate. Thus, 5,6-trimethylenecytosine (TMC),6 5,6-trimethyleneuracil (TMU), 6 and propanodeoxyguanosine (PdG),7 which have stable planar equilibrium geometries, are strongly fluorescent compounds with nanosecond excited-state lifetimes.
Intramolecular charge transfer in electron donor-acceptor molecules
Intramolecular charge transfer (ICT) in electron donor-acceptor (EDA) molecules has long been a topic of great interest in photochemistry. 4-Dimethylaminobenzonitrile (4-DMABN) is a prototype of EDA molecules that exhibit dual fluorescence, related to ICT, in polar solvents. The shorter-wavelength emission with a small Stokes shift was assigned to the normal fluorescence from the lowest-energy ππ* state (Lb), whereas the 'anomalous' longer-wavelength emission was attributed to the fluorescence from a highly polar CT state. The charge transfer from the dimethyl group to the benzonitrile moiety is believed to occur in concert with the 90° twisting of the amino group with respect to the benzonitrile moiety (TICT).
The long-standing belief in the field is that the ICT state is formed from the lowest-energy Lb (ππ*) singlet state, often referred to as the locally excited (LE) state. The LE → ICT reaction mechanism would be entirely reasonable, if not for the complication that the ππ* (Lb and La) states are not the only low-lying excited electronic states of the molecule.
Our TDDFT/cc-pVDZ and CIS/cc-pVDZ calculations on 4-DMABN and 4-aminobenzonitrile (4-ABN) indicate that the lowest-energy ps* state of bent geometry (with C-C-N angle of about 120°) is in fact lower in energy than the lowest-energy ππ* state of Lb type, at their respective optimized geometries.8 Because the vertical transition from the ground state of linear geometry to the πσ* state of bent geometry is Franck-Condon forbidden, direct excitation of the ground-state molecule to the πσ* state is not allowed, and the radiative decay of the πσ* state is essentially dipole forbidden. The πσ* state is therefore a dark state that is formed, by radiationless transitions from the initially excited ππ* state.8 The predicted state switch from the initially excited ππ* state to the dark πσ* state is supported by the observation of an abrupt break-off (loss) of the gas phase LE fluorescence following higher-energy excitation9 and the detection and identification of the ππ* state by picosecond transient absorption and time-resolved resonant Raman spectroscopy.10
We have proposed that the highly polar πσ* state, which has a large dipole moment, is the intermediate state of the sequential ICT reaction that takes the initially excited πσ* state to the fully charge-separated ICT state in 4-DMABN.8 Consistent with this proposal, we have found that the ps rise time of the benzonitrile-anion-like ICT-state absorption at 410 nm is identical to the decay time of the πσ*-state picosecond transient at 700 nm.11 Extension of the experimental and theoretical studies to other dialkylaminobenzonitriles and 4-(dimethylamino)phenylacetylene (4-DMAPA), shows that 2-DMABN and 3-DMABN, which possess high-lying πσ* state (relative to the initially excited ππ* state), do not exhibit the 700 nm transient or the ICT reaction. For molecules with the 1ps* state below the low-lying 1ππ* states, the lifetime of the 700 nm transient was found to be very short (a few picoseconds or less) for molecules that exhibit ICT, and very long (a few nanoseconds) for those that do not (e.g., 4-ABN and 4-DMAPA.11 These results corroborates the important role the πσ* state plays in the ICT reaction of the EDA molecules.
Time-resolved spectroscopic probe of the excited-state dynamics of halogenated Benzenes
The third class of biradicaloid geometry is that arises from the stretching of a single bond. The πσ* state of halogenated benzenes, which arise from the promotion of an electron from the ring-centered π orbital to the σ* orbital, localized on the CX bond is a good example.
To probe the nature of the low-lying excited state of fluorinated benzenes, we have carried out TDDFT calculations of vertical excitation energies and oscillator strengths for the lowest-energy ππ* and the lowest-energy πσ* singlet states at optimized ground-state geometry.12 The results of the computation show that while the vertical excitation energy of the ππ* state decreases slowly with increasing number of fluorine atoms, the vertical excitation energy of the πσ* state decreases rather rapidly with increasing degree of fluorination. As a consequence, the 1πσ* state is expected to lie very close to the 1ππ* state in pentafluorobenzene (PFB), and below the 1ππ* state in hexafluorobenzene (HFB).12 The calculation also indicates that the dark πσ* state is a highly polar electronic state, with strongly non-planar equilibrium geometry and a very long C-F bond length.12 Thus, following the optical excitation to the Franck-Condon 1ππ* state, electronic charge distribution shifts from the benzene ring to the C-F bond through π* → σ* electron transfer. The πσ* state of the fluorinated benzenes are therefore charge-transfer state as well as a biradicaloid state.12
Consistent with the prediction of the calculation, supersonic-jet laser spectroscopy demonstrates that the lowest excited singlet (S1) state of pentafluorobenzene (PFB) and hexafluorobenzene (HFB) is indeed the πσ* state, from which fluorescence originates.12
Remarkably, excitation of HFB to S2 (ππ*) and S3 (ππ*) states by a femtosecond laser pulse leads to the appearance of large-amplitude oscillations (of frequency ~100 cm-1) both in the transient absorption in solution, and in the multiphoton ionization in a supersonic free jet. The most surprising aspect of the quantum coherence in jet-cooled HFB is that oscillatory behavior (quantum beats) is preserved for the entire range of the pump laser wavelength (265-217 nm) used to excite the S2 (nπ*) and S3 (ππ*) states. Although the question of whether the oscillations originate from the electronic coherence (between the optically excited ππ* state and the πσ* state) or from the vibrational coherence (involving a low-frequency out-of-plane mode in the πs* state) is unclear, there is little doubt that coupling between the close-lying ππ* and πσ* states is the reason for these highly interesting observations. Experimental and theoretical studies are under way to deduce the origin of the large-amplitude quantum beats.
Our long-standing research programs on the structure and excited-state dynamics of aromatic clusters, and photophysical study of small molecules (thiophosgene, in particular), are continuing, albeit at slower paces. We have successfully identified and characterized the geometries of dimmers and higher clusters of 1-cyanonaphthalene,13 and mixed clusters of 1-cyanonaphthalene with water,14 generated by supersonic expansion, for which there have been significant controversy. Thiophosgene (Cl2CS) continue to reveal many important facets of intramolecular electronic relaxation.15-17 Our most recent study demonstrates that photophysics of the gas-phase molecule in its lowest triplet state belongs to the "intermediate case" of the classification scheme for electronic relaxation, in which the decay time varies with the coherence width of the laser used for excitation. This approach is believed to be the first observation of its kind.
B.S., 1954, St. Procopius College
M.S., 1957, Oklahoma State University
Ph.D., 1959, Oklahoma State University