Minor chromophore modifications

To date, we have attempted only minor chromophore modifications which can be effected in a cost effective manner necessary to produce the quantities of materials (grams to kilograms) necessary for prototype device studies (Figure 4). 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

This hypothesis is reinforced by the fact that even minor modifications in the chromophor composition and structure can exert remarkable effects on energy transfer, as recently verified by a combinatory chemistry study [9]. 

These systems can be taken as good examples of the general behavior of free-base porphyrins (Fb), zinc-porphyrins (Zn), and ruthenium-porphyrins (Ru). Minor quantitative changes in the spectroscopic and photophysical behaviors of these chromophores are only expected to occm as a consequence of chemical modifications such as, for example, replacement of peripheral phenyl with pyridyl groups or axial coordination of an additional Ugand (e.g. pyridine) to the metal in the case of Zn. 

Minor modification of the chromophore structure also is likely to result in the least perturbation of chromophore electronic structure so that we do not have to worry about... 

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