Role of the πσ* State in Molecular Photophysics

Abstract

P hotosynthesis, 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 πσ* 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 ππ* state. Our quantum chemistry calculations, as well as time-resolved laser spectroscopies, indicate that this dark intermediate state is the πσ* state that arises from the promotion of an electron from the π orbital of the phenyl ring to the σ* orbital localized in the CtX group (where X is CH and N) or on the CsX 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 πσ*-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 ππ* character (hydrocarbons and polyenes) or nπ* and ππ* 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 πσ* configuration in these molecules.

Carbazole‐Terpyridine Donor‐Acceptor Dyads with Rigid π‐Conjugated Bridges

“…This excited‐state scenario, in which both a linear “bright” π–π* state and a trans‐bent “dark” π–σ* state are located on the S 1 potential energy surface, resembles those already investigated for diphenylacetylene and for other organic molecules with C≡X (X=C or N) groups . However, in the present case, we did not succeed in finding a minimum for the trans‐bent π–σ* state of 2 since TD‐DFT optimizations failed.…”

“…This excited-state scenario, in which both a linear "bright" π-π* state and a trans-bent "dark" π-σ* state are located on the S 1 potential energy surface, resembles those already investigated for diphenylacetylene [52,65] and for other organic molecules with C�X (X=C or N) groups. [66] However, in the present case, we did not succeed in finding a minimum for the trans-bent π-σ* state of 2 since TD-DFT optimizations failed. In fact, upon geometry relaxation, this π-σ* state rapidly approaches S 0 (i. e., the optimization failed with S 1 being only 0.54 eV above S 0 ), suggesting the presence of a peaked S 1 /S 0 conical intersection, which could be responsible for the strong luminescence quenching in this type of dyads.…”

Combined Experimental and Computational Study of Intramolecular Charge Transfer In p-N,N-Dimethylamino-p′-cyano-diphenylacetylene

“…Picosecond time-resolved fluorescence spectrum (TR-FS) was obtained by an optical Kerr-gating technique to skim an emission spectrum as a function of the delay time, as described in ref 8. Benzene as Kerr medium was filled in a static cell (1 mm thickness) and placed between two thin high-contrast polarizers (Moxtek, ProFlux PFU05C) at crossed polarization configuration.…”

Ultrafast dynamics of aniline in the 294-234 nm excitation range: The role of the πσ* state

“…20,21 Species like pyrrole, [22][23][24] indole, [25][26][27][28][29] or phenol [30][31][32] have shown a dependence of their photophysical properties to the location of the dark πσ * state. The dynamics in this electronic state has a great influence in very relevant phenomena like photodissociation, [22][23][24][25][26][27][28][29][30][31][32] charge transfer reactions, 33,34 or even internal conversion (IC) in biomolecules as DNA bases 35,36 or amino acids. 37,38 In order to gain more insight on the electronic structure of aniline and the relaxation pathways that involve the πσ * state, herein we investigate the ultrafast dynamics after direct excitation in the energy region where this state lies.…”