TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preparation: (claimed) The preparation of (I) from a starting material containing (II) comprises;(a) complete dissolution of the starting material in a solvent to give an analysis volume;(b) injection of this volume into a mobile phase with a predetermined analysis flow rate, consisting of an organic solvent and an aqueous acid in fractions that can be changed with a gradient DELTAAB during analysis; in an analytical separating column with a solid adsorbent as stationary phase;(c) chromatographic analysis with on-line measurement of the optical absorption spectra of the individual fractions of the dissolved starting material leaving the analytical separating column after different retention times;(d) assessment of the results with regard to the need for purification by preparative chromatography;(e) dissolution of the starting material in the solvent to maximum saturation to give a preparation volume; (f) injection of this volume into the mobile phase with a predetermined preparation flow rate, as in (b);(g) discharging the main fraction containing (I) in the relevant retention time window into a collection positive and the unwanted fractions into a disposal position;(h) repeating (e) until the required amount of main fraction is obtained;(i) possibly returning part of the main fraction to (b) for quality control; and(j) working up the main fraction to (I) and either disposal of or reusing all other fractions.Preferred Preparation: For a ruthenium dye of the formulaeRuL2(NCS)3 (IA)L = 2,2'-dipyridyl-4,4'-dicarboxylateThe solvent is methanol and water in 1:1 volume ratio; the organic solvent is acetonitrile; the acid is aqueous sulfuric acid (H2SO4); the adsorbent is alkyl-modified silica gel; the analysis volume is 20 mul, flow rate is 1.5 ml/minute and time 10 minutes; the solvent gradient DELTAAB is linear : 0 minute: 10% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 10 minutes : 50% acetonitrile in 5 . 10-4 M aqueous H2SO4; analytical separating column 120 x 4 mm; preparation volume 0.5 ml, flow rate 6.5 ml/minute and time 27 minutes; solvent gradient approximatelyDAB: linear :0 minute: 10% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 25 minutes: 50% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 27 minutes: 90% acetonitrile in 5 . 10-4 M aqueous H2SO4; and preparation separating column 250 mm x 16 mm. For the ruthenium dye RuL3((C2H5)2NCS2)2 (IB), the materials and conditions are the same as for (IA), except that the solvent is pure methanol (optionally with added dimethylformamide); the preparation volume is 2 ml and time 48 minutes; and solvent gradient approximatelyDAB in preparation: linear :0 minute: 20% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 45 minutes: 70% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 46 minutes; 90% acetonitrile in 5 . 10-4 M aqueous H2SO4 : 48 minutes: 20% acetonitrile in 5 . 10-4 M aqueous H2SO4.Apparatus: (claimed) The apparatus has a manual or automatic valve through which the main fraction is discharged into a collection position in the relevant retention time window.Preferred Apparatus: The analytical and preparative HPLC has a spectrometer as detector for on-line measurement of the optical absorption spectra of the fractions in the ultraviolet (UV) and visual range. Several preparation columns operated by a valve switch to the detector time-linked to the relevant retention window may be used. At least computer assistance, a time control, an autosampler and a threshold switch for the distributor valve are automated, so that the process can be carried out on a completely automatic cycle. SPECIFIC COMPOUNDS - Specific examples of the highly pure dye (I) are a ruthenium (Ru) dye with a dipyridyl ligand L and a thiocyanate of formula (IA), especially Ru dyes with L and ethyl thiocarbamate of formula (IB),    RuL2(NCS)3 (IA)    RuL2((C2H5)2NCS2)2 (IB)    L = 2,2'-dipyridyl-4,4'-dicarboxylate