The effect of heteroatoms and end groups of polymethines on the all-optical switching processing application: a CC2 calculation

The structure–properties relationship of heterocyclic polymethines on the all-optical switching processing (AOSP) is studied by CC2 method. In particular, the excitation energies of the first and second excited states (E ge and E ge′ ) and the corresponding ratio (R ee′ = E ge′ /E ge, energy window width) are evaluated. The comparable energy gap between HOMO and HOMO − 1 (π type) and between LUMO and LUMO + 1 (π* type) is the key in keeping R ee′ large. Moreover, only a significant increase of such energy gap can result in the increase in R ee′ because the energy gap is a poor description of excitation energy. The results show that the heavy heteroatoms decrease the E ge and E ge′ as expected compared to light atoms such as carbon. For five-membered heterocyclic polymethines and their derivatives (two heteroatoms in total), the R ee′ is stable around 1.7 reflecting the nature of this system: The additional heteroatoms have little effects on the width of energy window. In the case of six-membered heterocyclic polymethines and their derivatives, the decrease in excitation energy is more effective compared to the former due to the relative large effective length of the end group, which can thus help avoiding the symmetry breaking. The effect of heteroatoms on R ee′ is clear that it helps keeping the energy window open compared to carbon atom. The heavier the heteroatom is, the higher R ee′ value will be when it is on the ortho position. Consequently, at least one heteroatom is necessary on the design of polymethines for AOSP application.