Dipole transitions, conjugated polymers

Conjugated polymers are centrosymmetric systems where excited states have definite parity of even (A,) or odd (B ) and electric dipole transitions are allowed only between states of opposite parity. The ground state of conjugated polymers is an even parity singlet state, written as the 1A... PM spectroscopy is a linear technique probing dipole allowed one-photon transitions. Non linear spectroscopies complement these measurements as they can couple to dipole-forbidden trail-... 

The optical spectra of conjugated polymers are dominated by an intense absorption due to the electric dipole allowed transition between the highest occupied and the lowest unoccupied electronic states. For solid samples the energy of this transition can lie anywhere between about 0.8 and 4 eV. Data for solid samples are listed in Table 9.1 and for solutions and solid samples in Table 9.2 on page 351. 

The Condon approximation applies term by term to any NLO coefficient. The simplest general vibronic treatment of NLO spectra thus requires frequencies and displacements of harmonic oscillators for electronic states with specified location, transition dipoles, and lifetimes. We have separate electronic and vibrational problems, and either experimental or theoretical inputs may be used in SOS expressions. To proceed, appropriate A and B excitations must be chosen. The linear absorption of conjugated polymers gives a B state or exci-ton in all models, while TPA clearly probes even-parity states whose locations have been debated. The choices [95] in Table 6.6 are two modes and three electronic excitations, a B state and two A states, that will be applied to PDA spectra. This pattern of excitations and transition moments is based on exact PPP results [96,97] for oligomers with 6 = 0.15 in Eq. (7). 

The lower panel in Fig. 6.11 shows the THG intensity and phase d (= tan Im/Re) of PDA-4BCMU films [100]. The largest SOS contribution, Eq. (30), is for A states with transition dipoles to IB. The prominent three-photon resonance at w = Eia/3 h is typical of conjugated polymers, and, as in /3-carotene [101,102], resembles the linear absorption. The indicated fit [95], with scaled intensity, is the Condon approximation for two g modes and displacements in Table 6.6. Contribu-... 

In reality there are several molecular vibrations that can couple to an electronic transition although the basic phenomenology is retained, namely the resonant character of the S St) 0-0 transition and the mirror symmetry of the vibronic satellites. In case of a jr—Mt transition the dominant vibrational modes are those of the polymer backbone, notably of the phenyl ring. An example is the absorption and fluorescence of it-conjugated molecules, such as tetracene, in the gas phase [22], In fluid solution there is interaction between the transient dipole of the molecule with the permanent and induced dipoles of the solvent. It gives rise to (i) a bathochromic shift of the spectra, (ii) a Stokes shift between the... 

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