Chromophore state optical excitation

The chromophores of optical brighteners are Jt electron systems in which Jt-Jt transitions occur. The chromophores must be rigid [5], and their conformations should differ only slightly in the electronic ground state and in the first excited state. 

The challenges outlined above still await a solution. In this section, we show how some of the theoretical limitations employed in traditional formulations of the band shape analysis can be lifted. We discuss two extensions of the present-day band shape analysis. First, the two-state model of CT transitions is applied to build the Franck-Condon optical envelopes. Second, the restriction of only two electronic states is lifted within the band shape analysis of polarizable chromophores that takes higher lying excited states into account through the solute dipolar polarizability. Finally, we show how a hybrid model incorporating the electronic delocalization and chromophore s polarizability effects can be successfully applied to the calculation of steady-state optical band shapes of the optical dye coumarin 153 (C153). We first start with a general theory and outline the connection between optical intensities and the ET matrix element and transition dipole. 

Sum-over-state (SOS) expressions [35] relate static susceptibilities to optical excitations. Of course (hyper)polarizabilities obtained as field derivatives of P coincide with SOS results, provided that the calculations refer to the same Hamiltonian. In this respect it is particularly interesting to discuss the merit of the excitonic approximation in the calculation of susceptibilities. Our best excitonic approximation to the Hamiltonian for interacting molecules in Eq. (10) defines the vacuum states for the excitonic model as the mf gs, i.e. adopts as gs the best uncorrelated solution of the total Hamiltonian. Therefore exactly the same OGM and mf-OGM susceptibilities discussed above can be obtained also in the liM approach from the derivatives or the relevant gs polarization. As a matter of fact mf-OGM results can also be obtained from a SOS calculation, summing over the excited states of the cluster described by the mf Hamiltonian in Eq. (15), or, equivalently, by summing up the SOS susceptibilities calculated for the chromophores in their mf gs. Instead, susceptibilities obtained by summing over the excited states calculated in the excitonic approximation (i.e. by diagonalizing H j- + are different, and... 

Fig. 3. Jablonski energy-level diagram typical for chromophores involved in ONP processes (specifically for pentacene in naphthalene, after Ref. [39]), showing the respective roles of optical excitation, ISC, OEP, electron-nuclear polarization transfer (here driven by mw radiation), and subsequent decay to the diamagnetic ground state. The relative populations of the triplet magnetic sublevels are given at the far right (assuming the crystal is oriented such that the long molecular axis of pentacene is parallel to the external magnetic field ).

The molecular mechanism of photoacidity of phenol has been investigated theoretically by Sobolewski and coworkers [100-102]. The ab initio calculations on phenol-ammonia cluster predicted that the increased acidity of excited phenol is not due to a property of the optically excited jot state, but rather arises from the nonadiabatic interaction of the jiji state with an optically dark state of jta character. The jot potential energy function is crossed by the jta function, and the jro energy is strongly stabilized when the proton moves from the chromophore to the solvent (ammonia) as illustrated in Figure 2.6 [100]. Hence, they consider that jia state plays a key role in the ESPT reaction of phenol-ammonia cluster. 

The optical properties of organic dyes (Fig. ld-f, Table 1) are controlled by the nature of the electronic transition(s) involved [4], The emission occurs either from an electronic state delocalized over the whole chromophore (the corresponding fluorophores are termed here as resonant or mesomeric dyes) or from a charge transfer (CT) state formed via intramolecular charge transfer (ICT) from the initially excited electronic state (the corresponding fluorophores are referred to as CT dyes) [4], Bioanalytically relevant fluorophores like fluoresceins, rhodamines, most 4,4 -difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPY dyes), and cyanines (symmetric... 

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