NOVELTY - Photosensitizer dye comprising a ruthenium (Ru) complex compound (I), is new.    USE - The photosensitizer dye is useful as a material for a dye layer in a dye-sensitized solar cell.    ADVANTAGE - The photosensitizer dye has a high absorption coefficient for enhancing the photoelectrical conversion efficiency of a dye-sensitized solar cell, which has enthusiastically pursued in the dye-sensitized solar cell industry.    DETAILED DESCRIPTION - Photosensitizer dye comprising a ruthenium (Ru) complex compound of formula (I), is new.    X1 = thiophene moiety of formula (II), benzo(c)thiophene group of formula (III), 2,3-dihydro-thieno(3,4-b)(1,4)dioxine group of formula (IV), 3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine group of formula (V), thieno(3,4-b)(1,4)dioxine group of formula (VI), 2,3-dihydro-thieno(3,4-b)(1,4)dithiine group of formula (VII), thieno(3,4-b)(1,4)dithiine group of formula (VIII), 1-thiophen-2-yl-benzo(c)thiophene group of formula (IX), 5-thiophen-2-yl-2,3-dihydro-thieno(3,4-b)(1,4)dioxine group of formula (X), 6-thiophen-2-yl-3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine group of formula (XI), 5-thiophen-2-yl-thieno(3,4-b)(1,4)dioxine group of formula (XII), 5-thiophen-2-yl-2,3-dihydro-thieno(3,4-b)(1,4)dithiine group of formula (XIII), 5-thiophen-2-yl-thieno(3,4-b)(1,4)dithiine group of formula (XIV), ((E)-2-propenyl)-thiophene group of formula (XV), ((E)-5-propenyl)-2,3-dihydro-thieno(3,4-b)(1,4)dioxine group of formula (XVI), diphenyl-(((E)-5-propenyl)-thiophen -2-yl)-amine group of formula (XVII), or diphenyl-(((E)-7-propenyl)-2,3-dihydro-thieno(3,4-b)(1,4)dioxin-5-yl)-amine group of formula (XVIII); either    X2 = H; or    X1X2 = (II)-(XVIII);    R1-R36, R39-R49 = H, CmH2m+1 or OCpH2p+1;    m, p = 1-15;    n = 1-4;    Y1, Y2 = H, CiH2i+1, OCjH2j+1, SCkH2k+1, pyrrolidine moiety, piperidine moiety, 1H-pyrrole group of formula (Ia), 2,3-dihydro-1H-indole group of formula (IIa), 6-methyl-5,6-dihydro-4H-thieno(2,3-b)pyrrole group of formula (IIIa), 5,6-dihydro-4H-thieno(3,2-b)pyrrole group of formula (IVa), 9H-carbazole group of formula (Va), 1H-indole group of formula (VIa), 4H-dithieno(3,2-b,2',3'-d)pyrrole group of formula (VIIa), 10H-phenothiazine group of formula (VIIIa), 10H-phenoxazine group of formula (IXa) or naphthalen-2-yl-phenyl-amine group of formula (Xa);    i, j, k = 1-15;    R37, R38 = H, CmH2m+1 (where m is 1-6) or OCpH2p+1 (where p is 1-6);    R40 = CqH2q;    q = 1-3;    Z1 = -COOA1, -PO3HA1, -H2C=CH2-COOA1, thiophene group of formula (XIa), 2,3-dihydro-thieno(3,4-b)(1,4)dioxine group of formula (XIIa), ((E)-2-propenyl)-thiophene group of formula (XIIIa) or 2,3-dihydro-thieno(3,4-b)(1,4)dioxine group of formula (XIVa);    Z2 = H or Z1;    A1 = H, Li, Na, K or tetra-alkyl ammonium groups of formula ((R52)(R55)N+(R53)(R54)); and    R52-R55 = H or CmH2m+1.    Provided that when Z1 and Z2 both represent -COOA1 and X1 represents one of (I)-(IV) and X2 represents H, Y2 does not represent H, CiH2i+1, OCjH2j+1 or SCkH2k+1, and when Z1 and Z2 both represent -COOA1 and X1 and X2 both represent one of (II)-(IV), Y2 does not represent H, CiH2i+1, OCjH2j+1 or SCkH2k+1.