Best one, which is not a) excited through same wavelenght/energy or/and b) emit in same wavelenght/energy, or close to, respectivly c) interact less with your complex (in the excited form).
So it is strongly depending on your particular compound. May a look at Ru(bpy)3]2+ help...3MLCT with excitation throug 1MLCT at 450 nm with emission at 610nm in water (values and solvent with best of my knowlege, should be easy to test).
Is spectroscopy in solid form possible (as thin film)?
For polar compounds, ethanol is an ideal solvent with an addition of small quantities of base or acid in order to produce a clear solid. whereas for non-polar compounds, the ideal solvent is a mixture of diethyl ether, isopentane and ethanol in the ratio
of 5 : 5 : 2 respectively, called EPA. Some of the other solvents used are, hexane, isopentane and heptane.