JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 8, No. 5, October 2006, p. 1951 - 1955 Electric dipole moments in the excited states determined by means of spectral methods I. A. RUSU, Ş. FILOTI, D. O. DOROHOI * , M. TOMA Department of Physics, “Al.I.Cuza” University, Bd. Carol I nr.11, 700506-Iasi, Romania The electric dipole moments involved in the electronic transitions responsible for the visible band appearance in the absorption and/or emission spectrum of some cycloimmonium ylids are estimated by two methods: -the dipole moments in the ground electronic state were computed by using PM3 procedure; - the dipole moments in the excited state were estimated from the spectral shifts measured in different solvents, related to the gaseous phase of the spectrally active molecule. The Bakhshiev theory was used to express these shifts versus the solution parameters. (Received May 9, 2006; accepted September 13, 2006) Keywords: Cycloimmonium ylids, Dipole moments in electronic excited states, Visible absorption and fluorescence, NMR spectra 1. Introduction The spectral shifts ( ν ∆ ) of the absorption and/or the fluorescence bands recorded in a given solvent ( . solv ν ) related to the gaseous phase ( . vap ν ) of the spectrally active molecule; . . vap solv ν ν ν − = ∆ (1) are correlated in theories about the liquid solutions with physical parameters of the solvent and spectrally active molecules [1-3]. The spectral shift measures the difference between the solvation energies of the spectrally active molecule in the electronic states involved in the appearance of the corresponding electronic bands. The theories [1,2] regarding solvent influence on the electronic absorption or emission bands express the spectral shifts by relations of the type [3]: f a pl f a f a f a n n C n n C , . sup 2 2 , 2 2 2 , 1 , 2 1 2 1 2 1 ν ε ε ν ∆ + + − ⋅ + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − − + − ⋅ = ∆ (2) where n and ε are the refractive index and the electric permittivity of the solvent and and are coefficients depending on the microscopic parameters (dipole moment, polarizability, ionization potential) of the molecules. The third term in relation (2) expresses the supply of the specific interactions at the total spectral shift. This term is usually neglected in the theories of the liquid solutions. Relation (2) can be used when the solvents have high values of the relaxation time, as compared with the life-time of the excited state of the spectrally active molecule. f a C , 1 f a C , 2 The contribution of the orientation forces at the solvatochromic shift is expressed by the first term in (2), while the contribution of the inductive-dispersive and/or polarization forces is represented by the second term. The third term checks the contribution of the specific interactions to the total spectral shift. It is difficult to separate the contribution induction-polarization and dispersive forces in (2), because they depend on the same function of the refractive index. The first coefficient, noted with a and f for absorption and fluorescence spectra in (2), has the following expressions: 2 1 2 ) cos ( 2 2 2 3 1 + + ⋅ − ⋅ ⋅ = n n r C e g g a ϕ µ µ µ ; (3) 2 1 2 ) cos ( 2 2 2 3 1 + + − ⋅ ⋅ ⋅ = n n r C e g e f µ ϕ µ µ ; (4) These coefficients can be used to determine the molecular dipole moments in the electronic states involved in the electronic transition. Measurements both in absorption and emission spectra must be made in order to estimate the values of the dipole moments in the excited electronic state, the dipole moment of the ICT transition and the angles between them when the dipole moment in the ground state of the spectrally active molecule is estimated by other methods. In order to estimate the dipole moments in the excited states, some carbanion monosubstitutes p-nitro- phenacylids having as heterocycle benzo-[f]-quinolinium, p-phenyl-pyridazinium and p-cumyl-pyridazinium were used. They belong to the cycloimmonium ylids [4], substances with amphionic nature, in which a sp 2 hybridized nitrogen atom from a heterocycle is covalently bound to a negative charged carbon. The ylid carbanion has two attached substituents. The bigger the electronegativity of the carbanion substituents, the higher the cycloimmonium ylids stability is [5]. When the hydrogen atom is one of the two substituents, cycloimmonium ylids are called carbanion monosubstituted.