Rationalizing the molecular origins of Ru- and Fe-based dyes for dye-sensitized solar cells

As part of an effort to design more efficient dyes for dye-sensitized solar cells (DSCs), structure–property relationships are established in the world's best-performing chemical series of dyes: 2,2′-bipyridyl-4,4′-carboxylatoruthenium(II) complexes. Statistical analysis, based on crystallographic data from the Cambridge Structural Database, is used to determine common structural features and the effects of structural change to its salient molecular constituents. Also included is the report of two new crystal structures for tris(2,2′-bipyridyl)dichlororuthenium(II)hexahydrate and tris(2,2′-bipyridyl)iron(II)dithiocyanate; these add to this statistical enquiry. Results show that the metal (M) core exhibits a distorted octahedral environment with M—N π-backbonding effects affording the propensity of the metal ion towards oxidation. The same characteristics are observed in iron-based analogues. The role of carboxylic groups in this series of dyes is assessed by comparing complexes which contain or are devoid of COOH groups. Space-group variation and large molecular conformational differences occur when COOH groups are present, while such structural features are very similar in their absence. The nature of the anion is also shown to influence the structure of COOH-containing complexes. These structural findings are corroborated by solution-based UV–vis absorption spectroscopy and DSC device performance tests. The presence of COOH groups in this series of compounds is shown to be mandatory for dye-uptake in TiO₂ in the DSC fabrication process. Throughout this study, results are compared with those of the world's most famous DSC dye, N3 (N719 in its fully protonated form): cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II). Overall, the molecular origins of charge-transfer in these complexes are ascertained. The findings have important implications to the materials discovery of more efficient dyes for DSC technology.

Computing details

For both compounds, data collection: CrystalClear (Rigaku Inc., 2008); cell refinement: CrystalClear (Rigaku Inc., 2008); data reduction: CrystalClear (Rigaku Inc., 2008). Program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) for (I); SIR92 (Altomare et al., J. Appl. Cryst. (1994). 27, 435) for (II). For both compounds, program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: WinGX publication routines (Farrugia, 1999).

Figures

(I) Tris(2,2?-bipyridyl)dichlororuthenium(II) hexahydrate

Crystal data

C30H24N6RuCl2·6(H2O)	Dx = 1.584 Mg m^−3
Mr = 748.62	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6/mcc	Cell parameters from 3247 reflections
a = 13.1383 (12) Å	θ = 5.3–55.0°
c = 20.995 (3) Å	µ = 0.72 mm^−1
V = 3138.6 (6) Å^3	T = 150 K
Z = 4	Chip, red
F(000) = 1536	0.21 × 0.16 × 0.12 mm

Data collection

Rigaku Saturn724+ (2x2 bin mode) diffractometer	1220 independent reflections
Radiation source: fine-focus sealed tube	994 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.086
Detector resolution: 10 pixels mm^-1	θmax = 27.5°, θmin = 3.7°
phi or ω oscillation scans	h = −7→17
Absorption correction: empirical (using intensity measurements) CrystalClear (Rigaku Inc., 2008)	k = −17→7
Tmin = 0.676, Tmax = 1.000	l = −24→26
11174 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.061	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.153	H-atom parameters constrained
S = 1.09	w = 1/[σ^2(Fo^2) + (0.0767P)^2 + 5.4397P] where P = (Fo^2 + 2Fc^2)/3
1220 reflections	(Δ/σ)max < 0.001
57 parameters	Δρmax = 0.98 e Å^−3
0 restraints	Δρmin = −0.56 e Å^−3

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Loop_ _platon_squeeze_void_nr _platon_squeeze_void_average_x _platon_squeeze_void_average_y _platon_squeeze_void_average_z _platon_squeeze_void_volume _platon_squeeze_void_count_electrons _platon_squeeze_void_content 1 − 0.010 − 0.009 − 0.103 930 363 ' ' 2 0.000 0.500 0.250 7 0 ' ' 3 0.000 0.500 0.750 7 0 ' ' 4 0.500 0.000 0.250 7 0 ' ' 5 0.500 0.000 0.750 7 0 ' ' 6 0.500 0.500 0.250 7 0 ' ' 7 0.500 0.500 0.750 7 0 ' '

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.6811 (3)	0.2077 (3)	0.19802 (16)	0.0292 (8)
C1	0.7618 (3)	0.1800 (3)	0.2205 (2)	0.0287 (8)
C2	0.7861 (4)	0.1029 (4)	0.1872 (2)	0.0372 (10)
H2	0.8409	0.0843	0.2030	0.045*
C5	0.6271 (4)	0.1597 (4)	0.1426 (2)	0.0320 (9)
H5	0.5722	0.1783	0.1271	0.038*
C3	0.7288 (4)	0.0542 (4)	0.1308 (2)	0.0370 (10)
H3	0.7436	0.0017	0.1087	0.044*
C4	0.6494 (4)	0.0841 (4)	0.1076 (2)	0.0370 (10)
H4	0.6115	0.0540	0.0689	0.044*
Ru1	0.6667	0.3333	0.2500	0.0243 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0276 (17)	0.0239 (16)	0.0355 (19)	0.0124 (14)	0.0038 (14)	−0.0001 (14)
C1	0.0284 (19)	0.0230 (17)	0.035 (2)	0.0134 (15)	0.0049 (16)	0.0037 (15)
C2	0.037 (2)	0.030 (2)	0.049 (3)	0.020 (2)	0.002 (2)	−0.0011 (19)
C5	0.030 (2)	0.030 (2)	0.032 (2)	0.0130 (18)	0.0009 (17)	−0.0014 (16)
C3	0.042 (2)	0.027 (2)	0.045 (2)	0.020 (2)	0.010 (2)	0.0002 (19)
C4	0.036 (2)	0.0253 (19)	0.046 (3)	0.0122 (17)	0.0013 (19)	−0.0021 (18)
Ru1	0.0212 (3)	0.0212 (3)	0.0307 (5)	0.01058 (15)	0.000	0.000

Geometric parameters (Å, º)

N1—C5	1.345 (5)	C3—C4	1.374 (6)
N1—C1	1.366 (5)	C3—H3	0.9300
N1—Ru1	2.065 (3)	C4—H4	0.9300
C1—C2	1.393 (6)	Ru1—N1^i	2.065 (3)
C1—C1^i	1.454 (9)	Ru1—N1^ii	2.065 (3)
C2—C3	1.376 (7)	Ru1—N1^iii	2.065 (3)
C2—H2	0.9300	Ru1—N1^iv	2.065 (3)
C5—C4	1.379 (6)	Ru1—N1^v	2.065 (3)
C5—H5	0.9300
C5—N1—C1	118.2 (3)	C5—C4—H4	120.6
C5—N1—Ru1	126.5 (3)	N1—Ru1—N1^i	78.98 (19)
C1—N1—Ru1	115.1 (3)	N1—Ru1—N1^ii	94.66 (13)
N1—C1—C2	120.7 (4)	N1^i—Ru1—N1^ii	92.37 (18)
N1—C1—C1^i	115.4 (2)	N1—Ru1—N1^iii	92.37 (18)
C2—C1—C1^i	124.0 (3)	N1^i—Ru1—N1^iii	94.66 (13)
C3—C2—C1	119.9 (4)	N1^ii—Ru1—N1^iii	170.89 (18)
C3—C2—H2	120.0	N1—Ru1—N1^iv	94.66 (13)
C1—C2—H2	120.0	N1^i—Ru1—N1^iv	170.89 (18)
N1—C5—C4	123.2 (4)	N1^ii—Ru1—N1^iv	94.66 (13)
N1—C5—H5	118.4	N1^iii—Ru1—N1^iv	78.98 (19)
C4—C5—H5	118.4	N1—Ru1—N1^v	170.89 (18)
C4—C3—C2	119.3 (4)	N1^i—Ru1—N1^v	94.66 (13)
C4—C3—H3	120.3	N1^ii—Ru1—N1^v	78.98 (19)
C2—C3—H3	120.3	N1^iii—Ru1—N1^v	94.66 (13)
C3—C4—C5	118.7 (4)	N1^iv—Ru1—N1^v	92.37 (18)
C3—C4—H4	120.6

Symmetry codes: (i) −y+1, −x+1, −z+1/2; (ii) −y+1, x−y, z; (iii) −x+y+1, y, −z+1/2; (iv) −x+y+1, −x+1, z; (v) x, x−y, −z+1/2.

(II) tris(2,2'-bipyridyl) iron(II) dithiocyanate

Crystal data

C30H24FeN6·2(NCS)	Dx = 1.213 Mg m^−3
Mr = 642.28	Mo Kα radiation, λ = 0.71075 Å
Hexagonal, P6/mcc	Cell parameters from 3247 reflections
a = 13.1837 (9) Å	θ = 5.3–55.0°
c = 21.2735 (14) Å	µ = 0.20 mm^−1
V = 3202.2 (4) Å^3	T = 150 K
Z = 4	Chip, red
F(000) = 1216	0.18 × 0.12 × 0.10 mm

Data collection

Rigaku Saturn724+ (2x2 bin mode) diffractometer	1259 independent reflections
Radiation source: fine-focus sealed tube	1165 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.043
Detector resolution: 10 pixels mm^-1	θmax = 27.5°, θmin = 3.6°
phi or ω oscillation scans	h = −17→17
Absorption correction: empirical (using intensity measurements) CrystalClear (Rigaku Inc., 2008)	k = −16→17
Tmin = 0.857, Tmax = 1.000	l = −27→25
26491 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.049	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.115	H-atom parameters constrained
S = 1.13	w = 1/[σ^2(Fo^2) + (0.0452P)^2 + 3.3496P] where P = (Fo^2 + 2Fc^2)/3
1259 reflections	(Δ/σ)max < 0.001
57 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.27 e Å^−3

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.67794 (13)	0.21313 (14)	0.19921 (8)	0.0230 (4)
C4	0.78247 (19)	0.10828 (18)	0.18768 (11)	0.0320 (5)
H4A	0.8363	0.0889	0.2035	0.038*
C5	0.75801 (15)	0.18462 (15)	0.22066 (10)	0.0235 (4)
C1	0.62321 (18)	0.16518 (17)	0.14468 (10)	0.0282 (4)
H1A	0.5676	0.1831	0.1300	0.034*
C3	0.7263 (2)	0.06123 (18)	0.13113 (11)	0.0351 (5)
H3A	0.7426	0.0109	0.1083	0.042*
C2	0.64568 (19)	0.09035 (18)	0.10931 (11)	0.0330 (5)
H2A	0.6070	0.0602	0.0714	0.040*
Fe	0.6667	0.3333	0.2500	0.0189 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0212 (8)	0.0185 (7)	0.0269 (8)	0.0080 (6)	0.0015 (6)	0.0005 (6)
C4	0.0328 (11)	0.0257 (10)	0.0409 (12)	0.0172 (9)	0.0057 (9)	0.0022 (9)
C5	0.0208 (8)	0.0183 (8)	0.0317 (10)	0.0101 (7)	0.0039 (7)	0.0028 (7)
C1	0.0277 (10)	0.0244 (9)	0.0301 (11)	0.0112 (8)	−0.0026 (8)	−0.0038 (8)
C3	0.0434 (12)	0.0234 (10)	0.0391 (12)	0.0170 (10)	0.0108 (10)	−0.0029 (8)
C2	0.0338 (11)	0.0256 (10)	0.0332 (11)	0.0101 (9)	0.0010 (9)	−0.0061 (8)
Fe	0.0170 (2)	0.0170 (2)	0.0229 (3)	0.00848 (12)	0.000	0.000

Geometric parameters (Å, º)

N1—C1	1.345 (3)	C3—C2	1.378 (3)
N1—C5	1.364 (2)	C3—H3A	0.9300
N1—Fe	1.9841 (16)	C2—H2A	0.9300
C4—C3	1.386 (3)	Fe—N1^i	1.9841 (16)
C4—C5	1.391 (3)	Fe—N1^ii	1.9841 (16)
C4—H4A	0.9300	Fe—N1^iii	1.9841 (16)
C5—C5^i	1.460 (4)	Fe—N1^iv	1.9841 (16)
C1—C2	1.386 (3)	Fe—N1^v	1.9841 (16)
C1—H1A	0.9300
C1—N1—C5	117.79 (17)	C1—C2—H2A	120.5
C1—N1—Fe	127.46 (13)	N1^i—Fe—N1	81.67 (9)
C5—N1—Fe	114.53 (13)	N1^i—Fe—N1^ii	93.16 (6)
C3—C4—C5	119.7 (2)	N1—Fe—N1^ii	92.47 (9)
C3—C4—H4A	120.1	N1^i—Fe—N1^iii	92.47 (9)
C5—C4—H4A	120.1	N1—Fe—N1^iii	93.16 (6)
N1—C5—C4	121.47 (19)	N1^ii—Fe—N1^iii	172.56 (9)
N1—C5—C5^i	114.54 (11)	N1^i—Fe—N1^iv	93.16 (6)
C4—C5—C5^i	123.98 (13)	N1—Fe—N1^iv	172.56 (9)
N1—C1—C2	123.20 (19)	N1^ii—Fe—N1^iv	93.16 (6)
N1—C1—H1A	118.4	N1^iii—Fe—N1^iv	81.67 (9)
C2—C1—H1A	118.4	N1^i—Fe—N1^v	172.56 (9)
C2—C3—C4	118.81 (19)	N1—Fe—N1^v	93.16 (6)
C2—C3—H3A	120.6	N1^ii—Fe—N1^v	81.67 (9)
C4—C3—H3A	120.6	N1^iii—Fe—N1^v	93.16 (6)
C3—C2—C1	119.0 (2)	N1^iv—Fe—N1^v	92.47 (9)
C3—C2—H2A	120.5

Symmetry codes: (i) −y+1, −x+1, −z+1/2; (ii) −x+y+1, y, −z+1/2; (iii) −y+1, x−y, z; (iv) x, x−y, −z+1/2; (v) −x+y+1, −x+1, z.