May you define better what you mean as "excitation coefficient" ? It is not clear to me if you are interested in absorption or emission properties of your complex although in some way they are connected. Assuming that you are interested in the absorption properties of your complex it is relavant at what wavelength you are evaluating the absorption (ditto in case of emission), Both processes are complicated and depend on many factors like the electronic structure and the vibrationole properties of the ground and excited states involved in the eletronic transitions responsible of the process. If the transition you are investigating depends mainly by the ligand electronic transitions , of course modifying the ligand can give some benefit even though it could also decrease the excitation properties of the molecule. If for example the exciation properties are related to the properties of and "organic" ligand the substiturion with some inorgani ion, e.g., chlorine could make the absorption even worse. If you need more help I need to know the composition of your complex and even so it is not an elementary matter to find a simple solution of the probelm and get the best property from your kind of complexex.

Thanks professor Lelj, excitation coefficent is directly related to absorbance according to Beers Lamberts law .I do need higher values of this to get higher number of reactive oxygen species in vivo, when I excite the molecule at a particular absorption maxima.

What kind of complex are you using ? I guess that you are looking for singlet oxygen or I am wrong ?

If you can show me the complex used perhaps I can give you some hint. In past I worked with some Fe complexes of Porphyrazines but Fe was not the best we could efford. Fe is a must or you can chose other metals ?