Ruthenium absorption coefficient

As described in Section II.A.3, dx-bis(4,4 -dicarboxy-2,2 -bipyridine)dithiocya-nato ruthenium(II) (N3 dye) and trithiocyanato 4,4 4"-tricarboxy-2,2 6, 2"-terpyr-idine ruthenium(II) (black dye) exhibit quite good performance therefore, the systems using these two Ru dyes have been intensively investigated. However, other Ru dye photosensitizers have also been synthesized and characterized, and their performances as photosensitizers in DSSCs have been reported by many researchers [98-116]. Several kinds of electron withdrawing ligand for Ru dye photosensitizers are shown in Fig. 13. Figure 14 shows the structures of newly synthesized Ru dye photosensitizers and their absorption properties. The axis shows the molar absorption coefficient, e (i.e., absorption coefficient per M unit M 1 cm-1). 

We have seen from the above work that the nonradiative rate constants dominate the luminescence behavior of ruthenium(II) complexes. If one can increase the value of the radiative rate constant, kr, without substantial increases in knr, then emission efficiency can be improved. The radiative rate constant is, in theory at least, related to the molar absorption coefficient, epsilon187. Demas and Crosby188, made a number of assumptions and calculated radiative lifetimes based on observed epsilon values, which were in good agreement with the experimental kr values. Watts and Crosby1895 went on to comment on the possible implications of the epsilon value. 

Ruthenium and osmium polyazine complexes have been widely used as light absorbers. The complex [Ru(bpy)3] (Fig. 2) is a well studied LA with high extinction coefficients in the ultraviolet and visible regions of the spectrum. This complex shows intense absorptions in the UV region that are intraligand n- n (IL) and in the visible region that are MLCT transitions. Upon optical excitation at 450 nm, population of the Ru(d7i)- bpy(ji ) MLCT occurs. 

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