Spontaneous Symmetry Breaking Facilitates Metal-to-Ligand Charge Transfer: A Quantitative Two-Photon Absorption Study of Ferrocene-phenyleneethynylene Oligomers

Abstract

Change of the permanent molecular electric dipole moment, Δμ, in a series of nominally centrosymmetric and noncentrosymmteric ferrocene-phenyleneethynylene oligomers was estimated by measuring the two-photon absorption cross-section spectra of the lower energy metal-to-ligand charge-transfer transitions using femtosecond nonlinear transmission method and was found to vary in the range up to 12 D, with the highest value corresponding to the most nonsymmetric system. Calculations of the Δμ performed by the TD-DFT method show quantitative agreement with the experimental values and reveal that facile rotation of the ferrocene moieties relative to the organic ligand breaks the ground-state inversion symmetry in the nominally symmetric structures.

Ground-State Structural Disorder and Excited-State Symmetry Breaking in a Quadrupolar Molecule

“…26−30 Recent investigations on platinum acetylides and ferrocene− phenyleneethynylenes pointed to the role of structural disorder in the electronic ground state as the origin of symmetry breaking in these molecules. 32,33 The change of permanent electric dipole moment upon excitation, determined from the two-photon absorption cross section, was consistent with that calculated for molecules with a 90°twist angle of either the acetylene ligand 32 or the phenyleneethynylene chain. 33 Whereas phenyleneethynylene oligomers exist with a relatively broad distribution of torsional angles in the ground state, their S 1 electronic excited state is more rigid because of conjugation, and thus, the amount of structural disorder is significantly smaller.…”

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

“…Unlike some nominally centrosymmetric organometallic systems where non-zero permanent dipole moments is caused by spontaneous symmetry-breaking conformations in the ground state [33][34][35] , the homoleptic Ru(II) complexes are structurally and energetically robust, which precludes conformation-induced lowering of symmetry. Large changes in permanent dipole can also be induced by electronic interaction with non-symmetric environments, provided that the molecular polarizability of the chromophore is different in the ground-and excited state 27 .…”

Electron-donating strength dependent symmetry breaking charge transfer dynamics of quadrupolar molecules

“…Intramolecular charge transfer (ICT) is one of the most fundamental and ubiquitous chemical reactions in organic molecules which contain electron-donor (D) and -acceptor (A) units. [1][2][3][4][5][6] The property of ICT has been proven to play a critical role in energy conversion and storage processes in photovoltaics, organic light emitting diodes (OLED) and nonlinear optics. [7][8][9][10][11][12][13][14][15] The ICT process, which takes place readily in dipolar molecules, is easily influenced by the polarity of the surrounding environments, and leads to fluorescence solvatochromism or a decrease in fluorescence quantum yield along with an increase in static dipole moment in the excited state.…”