Revisiting the basis of the excited states for solvated fluorinated benzenes and pentafluorophenylalanine

Abstract

Vapor-phase molecular simulation studies of aromatic compounds with five or more fluorine atoms on the ring reveal emission spectra characterized by S₀ → πσ* and πσ*→S₀ transitions. In this study, the absorption, excitation, and solvent-dependent emission spectra of fluorinated benzenes, including pentaflurophenyalanine (F5Phe), which is a potential marker for biochemical research, were collected and compared to the results of the simulation. Time-dependent self-consistent field (TD-SCF) density functional theory (DFT) calculations were performed to examine the nature of excited states and relevant photo-physical processes. The results show that pentafluorobenzene (PFB) and hexafluorobenzene (HFB) show behavior consistent with the vapor phase simulation studies, that tracts well with benzenes substituted with fewer fluorine atoms. For example, 1,2,3-trifluorobenzene (123TFB) and 1,2,3,4-tetrafluorobenzene (1234TFB) show emission spectra with varying intensities of tails and shoulders. Those features are attributed to πσ*→S₀ transitions where the πσ* state has been stabilized in the presence of solvents like water, acetonitrile, and isopropanol, which are different from their simulated behavior in the gas phase. The emission in water solvent especially shows a significant increase in the emission intensity at 310 nm, which is common for all studied samples. The emission spectrum of F5Phe closely reflects that of PFB, which arises from the interplay of both ππ *→S₀ and πσ*→S₀ transitions. Also, it is observed that the interaction between adjacent σ* orbitals of C-F bond for 123-TFB, 1234-TFB, 12345-PFB, and 123456-HFB contributes to further narrowing the energy gap between S₀ and S₁ states with a significant red shift on the emission spectra compared to their isomers.

Fig. 1.

Normalized absorbance (blue) and emission (green) of the 12 fluorinated benzenes in acetonitrile with the corresponding excitation spectrum (red) for their λ_{em max}. The excitation spectrum of the 305 nm emission peak of PFB is also shown (purple). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2.

Absorption and emission between ground and excited state surfaces. The dotted curves are assumed curves with a different number of fluorine atoms for the ground state, ππ * and πσ * states. Shown on the graph is the typical S₀ → S₁(ππ *) transition at 260 nm and S₁(ππ *) → S₀ transition at 280 nm. The arrow indicates the possible crossing of a conical intersection.

Fig. 3.

The energies of the λ_{em max} of fluorinated benzenes. The energy of all emissions, with the exception of 123-TFB, 1234-TFB and the 305 nm peak of PFB, closely resemble the theoretical values.

Fig. 4.

HOMO, LUMO, and πσ * orbitals of 123TFB, 1234TFB, PFB and HFB. p-orbitals in σ * of fluorine (C-F) have same phase; p-orbitals of carbons also aligned with the same phase.

Fig. 5.

Out-of-plane (e2u symmetry) of (a) PFB, (b) HFB and (c) dimer of 123TFB by TDDFT modeling for S₁ state.

Fig. 6.

The normalized emission of 13DFB, 123TFB, 1234TFB, PFB and HFB in water (blue), acetonitrile (red) and isopropanol (green). The Raman scattering of water was observed in the emission spectra of 1234TFB and PFB. The narrow peaks observed in the emission spectra of HFB at 262 nm are attributed to the second-order detection of the excitation wavelength. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7.

Charge distribution of 123TFB, 1234TFB, PFB, and HFB calculated by DFT.

Fig. 8.

The relative emission of 123TFB (red dotted line), 1234TFB (orange dotted line), PFB (green dotted line), and HFB (blue dotted line) in hexane. The quenched emission of the same benzenes with anisole (1:1 ratio): 123TFB (solid red line), 1234TFB (solid orange line), PFB (solid green line), and HFB (solid blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9.

The normalized emission spectra of Phe and F5Phe in water (blue), acetonitrile (red) and isopropanol (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)