Computational and Theoretical Chemistry 1019 (2013) 94–100

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Computational and Theoretical Chemistry

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DFT/TD-DFT studies on structural and spectroscopic properties ofmetalloporphyrin complexes: A design of ruthenium porphyrinphotosensitizer

2210-271X/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.comptc.2013.07.006

⇑ Corresponding authors.E-mail addresses: guoyuanru@gmail.com (Y.-R. Guo), panqj@163.com (Q.-J. Pan).

Ming-Jing Zhang a, Yuan-Ru Guo b,⇑, Gui-Zhen Fang b, Qing-Jiang Pan a,⇑a Key Laboratory of Functional Inorganic Material Chemistry of Education Ministry, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, Chinab Key Laboratory of Bio-based Material Science & Technology of Education Ministry, College of Material Science and Engineering, Northeast Forestry University, Harbin 150040, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 March 2013Received in revised form 3 July 2013Accepted 3 July 2013Available online 12 July 2013

Keywords:DSSCsMetalloporphyrinRuthenium sensitizerAbsorption spectraTD-DFT calculation

Given their efficacy in photosynthesis, a series of metalloporphyrins were examined using density func-tional theory (DFT) and time-dependent DFT. An experimentally known zinc porphyrin has been calcu-lated to evaluate performance of various functionals, basis sets, computational models of solvent effectand solvent sorts. With an alternative approach, absorption spectra of complexes (MLx, M = Zn, Cd andHg, x = 1–4) varying substituted porphyrin Lx and metal center were investigated. It was shown thatthe strongest peak at about 400 nm for MLx was attributed to the intra-porphyrin p ? p� transition. Sub-stitution of benzoic acid at the b-position of porphyrin core allows an extra intense peak at 450 nm in ML3

and ML4, but ML2 with the meso-substituted benzoic acid only displays some very weak low-energyabsorptions. Additionally, we theoretically designed [Ru(NCS)2Lx]2� and expected their possession ofadvantages of polypyridyl ruthenium and porphyrin-based sensitizers. The present study reveals thatthe L4 complex, [Ru(NCS)2L4]2�, exhibits extra intense absorption peaks at 500 and 582 nm, allowingfor its promising application in dye-sensitized solar cells (DSSCs).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

As a cost-effective energy-conversion device, dye-sensitized so-lar cells (DSSCs) are an alternative choice to solve shortage prob-lem of fossil fuels, decrease environmental contamination andsatisfy rapidly growing energy demand [1–7]. A typical DSSC com-prises photosensitizer, photoanode, electrolyte and counter elec-trode [1–6]. In the working process, the photosensitizer absorbssunlight and is excited to produce photoelectron. Subsequently,the photogenerated electrons inject into the semiconductor con-duction band, simultaneously causing oxidation of photosensitizer.Finally, the oxidized dye is regenerated by the redox electrolyte,which itself is regenerated at the counter electrode by recapturingelectrons passed through the external circuit.

In the last two decades, the photoelectric conversion efficiency(g) of DSSCs has been improved [8–13]. In 1991, O’Regan and Grat-zel fabricated a ruthenium dye sensitized solar cell, and its overallg reaches 7.9% [8]. Over 10% g for DSSCs was achieved in 2001[9,10]. Very recently, Yella and co-workers have developed a por-phyrin-sensitized solar cell with the cobalt(II/III) electrolyte; itsoverall efficiency has exceeded 12% [11].

To generate a large photoelectric response, photosensitizer usedin DSSCs should have absorption spectra as much as possible tomatch with solar spectrum [1,3]. The polypyridyl ruthenium-typedye such as N3 [14] and N749 [15] is one class of the most success-ful sensitizers. Its major drawback is the limited absorption in thenear-infrared region of the solar spectrum [13–16]. In contrast,porphyrin-based sensitizers exhibit intense spectral responsebands in the near-IR region and possess good chemical, photo-and thermal stability [6,17–19]. However, porphyrin-sensitized so-lar cells show lower conversion efficiency than N3 dye-sensitizedones, which has been attributed to an increased probability of ex-cited-state decay caused by porphyrin aggregation on the photoan-ode surface [20]. Therefore, a photosensitizer with combinedstructural and spectroscopic features of ruthenium-type and por-phyrin-based dyes are highly demanded.

In this work, a series of metalloporphyrins were examinedusing density functional theory (DFT) and time-dependent DFT(TD-DFT). It is found that their absorption spectra are able to betuned via varying substituted porphyrins and metal centers (Zn/Cd/Hg and Ru). Ruthenium-porphyrin sensitizers have been theo-retically designed (in silico), which is expected to overcome thelack of near-infrared absorption of polypyridyl Ru(II) complexesand also to avoid the aggregation of excited porphyrin-basedsensitizers.

M.-J. Zhang et al. / Computational and Theoretical Chemistry 1019 (2013) 94–100 95

2. Computational details and theory

A wide variety of porphyrins have been synthesized, but nearlyall chemical modification is restricted to the meso- and b-positionof porphyrin core [3–7,17–19,21–24]. Following the rule, we haveinvestigated porphyrin and its benzoic acid substituted derivativesat meso-/b-position, see Lx (x = 1–4) in Chart 1. Due to the inclusionof porphyrin core as well as anchoring benzoic acid group, our sim-plified theoretical model can reflect intrinsic properties of experi-mentally reported complicated porphyrin dyes. Starting from Lx,complexation with zinc(II), cadmium(II), mercury(II) and ruthe-nium(II) was calculated in detail. These studied complexes were la-beled as MLx and Ru(NCS)2Lx (M = Zn, Cd and Hg, x = 1–4). It isworth noting that the ruthenium complexes carry �2 charge andRu(NCS)2Lx is used in the work for convenience.

Structures of above metalloporphyrins were fully optimizedusing Becke’s 3 parameter hybrid functional using the Lee–Yang–Parr correlation functional (B3LYP) [25,26]. Relativistic effects ofZn, Cd, Hg, Ru and S atoms were described by the Hay and Wadt[27,28] small-core effective core potentials (SC-ECPs). The LanL2DZbasis sets associated with the ECPs were employed for all atoms.

It is well established that accurate spectroscopic descriptionstrongly depends on many factors such as density functional, basissets, computational models of solvent effect, solvent sorts, and rel-ativistic effects. In the present work, an experimentally known zinctetraphenylporphyrin complex (ZnL, see L in Chart 1) [29–31] hasbeen taken as an example to evaluate performance of such factors.Optimized geometry parameters of ZnL are comparable to experi-mental values [31], suggesting the B3LYP/LanL2DZ/SC-ECPs ap-proach suitable for the present calculations. At the geometry, weperformed calculations of eighty dipole-allowed excited stateswith the variation of TD-DFT (PBE [32,33], PBE1 [34] and B3LYP[25,26]), computational model of solvent effect (PCM [35] andCPCM [36]), solvent (methanol, ethanol and dichloromethane)and basis sets (6-31G, 6-311G, 6-31G�� and SDD [37,38]). The usedPCM (the default model in Gaussian) herein performs a reactionfield calculation using the IEF-PCM model [39–41]. Additionally,the PBE1PBE functional implemented in the Gaussian code was ap-plied, and in the work we used PBE1 for short. These spectra weretheoretically simulated using Gaussian function with a full widthat half maximum (FWHM) of 0.22 eV in Fig. 1. One can see thatthe choice of functionals has a significant effect on absorptionspectra, while other factors slightly change the spectra. A betteragreement with the experimental spectra is found at the TD-PBElevel (Table 1). The strongest peak of ZnL was calculated to be

Chart 1. The investigated porphyrin-based ligands,

423 nm with the functional, close to the most intense band of419 nm in the experiment [29,30]. So the approach, TD-PBE/SDD/SC-ECPs/PCM(ethanol), will be used for spectral calculations inthe paper. In these calculations, SDD basis sets, larger thanLanL2DZ, were employed, associated with relativistic effects de-scribed by Stuttgart/Dresden SC-ECPs [38]. All the calculationswere accomplished using the GAUSSIAN03 program package [42].

3. Results and discussion

3.1. Structures of MLx

The structures of MLx (M = Zn, Cd and Hg; x = 1–4) in the groundstate were fully optimized. These structures are presented in Fig. 2.The selected geometry parameters are listed in Table 2. It is dem-onstrated that complexes ML1 and ML3 display an approximatelyplanar structure, due to the conjugated structures of bare porphy-rin L1 and carboxyl benzo porphyrin L3. In contrast, the benzoicacid whether substituted at meso- or b-position deviates fromthe planar porphyrin core; it has been reflected by the calculatedC3–C4–C5–C6 dihedral angles of 63� and 42� (mean value), forML2 and ML4, respectively. The N–M–N angles of all the L1 com-plexes were calculated to be 90� and the M–N bond lengths wereidentical, indicating the metal is located right in the center of por-phyrin. And other complexes with Lx (x = 2–4) still possess closeM–N distances, albeit with a slight perturbation of benzoic acidgroup.

The calculated bond lengths of MLx (M = Zn, Cd and Hg; x = 1–4)are comparable to values of experimentally known analogues [29–31]. Regarding Zn complexes, the Zn–N bond lengths were calcu-lated in the range of 2.07–2.10 Å, falling within the typical valuesof experimentally reported complexes [29–31]. The variation ofporphyrin-based ligands has a slight effect on Zn–N distancesand N–Zn–N angles. Similar cases are found in the cadmium andmercury complexes. Additionally, along Zn, Cd to Hg, the M–N dis-tances of MLx (x = 1–4) were calculated to be lengthening. Thisagrees with their respective metal ionic radii of 0.74, 0.95 and1.02 Å [43].

3.2. Electronic structures and absorption spectra of MLx

Associated with the PBE functional and SDD basis sets whileemploying PCM model and ethanol solvent, we have calculatedelectronic structures of MLx (M = Zn, Cd and Hg; x = 1–4). The de-tailed information of the molecular orbitals has been presented

where L has been experimentally synthesized.

200 300 400 500 600 7000

1

2

3

4 PBE B3LYP PBE1

a

200 300 400 500 600 7000

1

2

3

4b Ethanol

Methanol CH

2Cl

2

200 300 400 500 600 7000

1

2

3

4c

Wavelength(nm)

Absorption

PCM CPCM

200 300 400 500 600 7000

1

2

3

4

d SDD 6-31G 6-311G 6-31G**

Fig. 1. Simulated absorption spectra of experimentally obtained ZnL with the variation of (a) density functionals (GGA-PBE, Hybrid-PBE1 and Hybrid-B3LYP), (b) solvents(ethanol, methanol and CH2Cl2), (c) computational models of solvent effect (PCM and CPCM), and (d) basis sets (SDD, 6-31G, 6-311G and 6-31G��).

Table 1Calculated absorption spectra of ZnL, compared with experimental values.

PBE PBE1 B3LYP Expt. c

k (nm)a E (eV)a fb k (nm)a E (eV)a fb k (nm)a E (eV)a fb

578 2.15 0.0544 535 2.32 0.0448 546 2.27 0.0454 548 (0.079)423 2.93 1.1490 389 3.19 1.6555 397 3.12 1.5691 419 (1.98)351 3.53 0.2722 290 4.27 0.2614 302 4.11 0.2516283 4.38 0.0768 219 5.66 0.0396 238 5.22 0.0614

a Calculated absorption in nm and eV.b Oscillator strength.c Experimental absorption (nm) from Refs. [29,30], and oscillator strength listed in parentheses.

Fig. 2. Optimized structures of photosensitizers MLx and Ru(NCS)2Lx (M = Zn, Cd and Hg, x = 1–4).

96 M.-J. Zhang et al. / Computational and Theoretical Chemistry 1019 (2013) 94–100

in Tables S1–S12. It is shown that the variation of Lx and metalcenters has a significant effect on character of orbitals. ZnL1 hasporphyrin-based HOMO and HOMO-1 (H-1), and its H-2 is contrib-uted by porphyrin (75%) mixed with zinc (25%) composition. Unoc-cupied orbitals of LUMO�L+2 of ZnL1 are made up of

p�(porphyrin), while predominant zinc with minor porphyrinforms L+3. The similar character is found in the occupied orbitalsof ZnLx (x = 2, 3 and 4); however, the addition of benzoic acid leadsto its minor and predominant participation in LUMO and L+2,respectively. Compared with respective ZnLx (x = 1–4), CdLx and

Table 2Optimized geometry parameters of photosensitizers MLx and Ru(NCS)2Lx (M = Zn, Cd and Hg, x = 1, 2, 3 and 4) using the B3LYP functional in the ground state (Distances inangstrom and angles in degree).

Zn Cd Hg Ru

L1 L2 L3 L4 L1 L2 L3 L4 L1 L2 L3 L4 L1 L2 L3 L4

M–N1 2.069 2.070 2.064 2.072 2.156 2.156 2.153 2.160 2.196 2.240 2.194 2.243 2.066 2.066 2.068 2.069M–N2 2.069 2.068 2.096 2.063 2.156 2.157 2.176 2.154 2.199 2.237 2.219 2.235 2.068 2.064 2.061 2.059M–N3 2.069 2.068 2.063 2.077 2.156 2.157 2.152 2.163 2.197 2.244 2.193 2.247 2.066 2.064 2.070 2.070M–N4 2.069 2.070 2.073 2.066 2.156 2.156 2.162 2.155 2.199 2.237 2.206 2.235 2.065 2.067 2.068 2.062M–N5 – – – – – – – – – – – 2.084 2.081 2.079 2.078M–N6 – – – – – – – – – – – 2.084 2.082 2.079 2.078N5–C1 – – – – – – – – – – – 1.189 1.189 1.190 1.190C1–S1 – – – – – – – – – – – 1.706 1.704 1.702 1.702N6–C2 – – – – – – – – – – – 1.189 1.189 1.190 1.190C2–S2 – – – – – – – – – – – 1.707 1.704 1.702 1.702N1–M–N2 90.0 90.7 90.0 90.0 90.0 90.9 90.1 89.8 90.0 88.1 90.1 86.7 90.0 90.4 90.5 89.9N2–M–N3 90.0 89.1 90.0 89.9 90.0 88.8 90.1 89.9 90.0 85.5 90.2 86.7 89.9 89.6 90.2 89.8N3–M–N4 90.0 90.7 90.0 90.3 90.0 90.9 89.9 90.3 90.1 87.5 89.8 87.1 90.1 90.4 89.7 90.6N4–M–N1 90.0 89.1 90.0 89.9 90.0 88.8 89.9 89.8 90.0 86.4 89.9 86.9 90.0 89.6 89.6 89.7C3–C4–C5–

C665.2 43.3 62.0 42.0 63.3 43.0 80.2 31.9

M.-J. Zhang et al. / Computational and Theoretical Chemistry 1019 (2013) 94–100 97

HgLx show similar character of unoccupied orbitals but differ inoccupied ones. Filled orbitals with the p(porphyrin) and metalcharacter are raised to form HOMOs in CdLx and HgLx, being theH-2 in ZnLx. This is caused by stronger relativistic effects of cad-mium and mercury than of zinc.

To improve the overall conversion efficiency of light into elec-tricity, exploration on absorption spectra of the sensitizer is of ut-most importance. Electronic absorptions of MLx (M = Zn, Cd andHg; x = 1–4) calculated at the TD-PBE/SDD/SC-ECPs/PCM level arelisted in Tables S13–S24. We have simulated their spectra inFig. 3. It is illustrated that the variation of ligands from L1 to L4

greatly changes absorption while varying metal center (Zn, Cdand Hg) has a small effect. With respect to L1 ligated complexes(Fig. 3a), a very strong band was calculated to be around 400 nm.According to Tables S1, S5 and S9, it is assigned as a p ? p� transi-tion localized on porphyrin-based ligand.

Apart from the strong p ? p� absorption at 400 nm, benzoicacid meso-substituted L2 complexes display several weak lower-energy bands at 460, 495 and 570 nm. The first two are related

200 400 600 8000.0

0.2

0.4

0.6

0.8

1.0 ZnL1

CdL1

HgL1

Ru(NCS)2L1

a

Absorption

Waveleng200 300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0 c ZnL3

CdL3

HgL3

Ru(NCS)2L3

Fig. 3. Simulated absorption spectra of MLx and Ru(NCS)2Lx (M = Zn, Cd and Hg, x = 1–spectra were normalized.

to charge transfer from porphyrin to benzoic acid and the lastone is featured with the intra-porphyrin transition. Regarding b-substituted porphyrin, carboxyl benzo L3 ligand is designed in sil-ico, whereas L4 and its analogous derivatives have been experi-mentally synthesized [3–7,17–19]. It is found in Fig. 3 that bothL3 and L4 complexes display two strong peaks at 400 and450 nm, and one weak peak around 560 nm. Apparently, the b-sub-stitution in ML3 and ML4 gives rise to strong 450 nm peaks, differ-ent from the meso-substituted ML2 having weak peaks.

3.3. Structures and absorption spectra of Ru(NCS)2Lx

On the basis of prevalent metalloporphyrins ZnLx (x = 1–4), Ru-porphyrin photosensitizers, Ru(NCS)2Lx, were designed and calcu-lated using the same approach. We expect that the Ru-porphyrindyes are able to combine advantages of existing polypyridyl Ru(II)and porphyrin-based dyes on the one hand, and also overcome thelack of near-infrared absorption of the former and avoid the aggre-gation of the latter on the other hand.

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 b

th(nm)

ZnL2

CdL2

HgL2

Ru(NCS)2L2

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 d ZnL4

CdL4

HgL4

Ru(NCS)2L4

4) in ethanol, which are grouped by the ligand. For convenient comparison, all the

Fig. 4. The single electron transition (CI coefficient >0.2) for the 582 nm absorptionof Ru(NCS)2L4 in ethanol under the TD-PBE/SDD/SC-ECPs/PCM calculation.

Fig. 5. The single electron transition (CI coefficient >0.2) for the 514 nm absorptionof Ru(NCS)2L4 in ethanol under the TD-PBE/SDD/SC-ECPs/PCM calculation.

98 M.-J. Zhang et al. / Computational and Theoretical Chemistry 1019 (2013) 94–100

As shown in Fig. 2, Ru(NCS)2Lx were optimized to exhibit octa-hedral coordination geometry of ruthenium center, coordinated bytwo nitrogen atoms of NCS donor ligands and four nitrogen atomsof porphyrin. The calculated Ru-N distances fall within 2.06 and2.09 Å (in Table 2), agreeing well with typical values of rutheniumcomplexes reported by experimental [1–3,10,12] and theoreticalstudies [13,16,44–46]. Ru(NCS)2L2 has a greater deviation of thebenzoic acid plane from the porphyrin core (80� dihedral angle)than those of ML2 (M = Zn, Cd and Hg), whereas Ru(NCS)2L4 showsa smaller angle of 32� than those of ML4, Table 2.

The calculated absorptions and electronic structures ofRu(NCS)2Lx (x = 1–4) in ethanol are listed in Tables S25–S32.Ru(NCS)2Lx is better considered as the combination of conven-tional ruthenium dye and porphyrin sensitizer, having higher-en-ergy occupied orbitals with the anti-bonding d(Ru), p(porphyrin)and p(NCS) character and lower-energy unoccupied orbitals of p�(-porphyrin). Except for Ru(NCS)2L1 with the bare porphyrin, otherruthenium-porphyrin complexes show character of major benzoicacid and minor p�(porphyrin) in LUMO. For example, LUMO ofRu(NCS)2L4 is composed of 63% benzoic acid, 30% p�(porphyrin)and 7% Ruthenium.

It is seen from Fig. 3, Ru(NCS)2L1 and Ru(NCS)2L2 were calcu-lated to exhibit absorption bands similar to ML1 and ML2

(M = Zn, Cd and Hg), respectively. In contrast, Ru(NCS)2L4 withthe b-substituted benzoic acid displays four peaks at 400, 426(shoulder), 500 and 582 nm, which match well with solar spectra.The first two are very strong and merge into one band ranging from350 to 460 nm, having intra-porphyrin p ? p� transition modifiedby the charge transfer from donors (porphyrin, NCS and Ru) toacceptor (benzoic acid). The 500 nm band is composed of severaltransitions between 504 and 514 nm as seen in Table 3, which isattributed to charge transfer from NCS and porphyrin donors tobenzoic acid. A strong lower-energy band at 582 nm was obtained

Table 3Calculated absorptions of Ru(NCS)2L4 in the ethanol solution.

Band (nm)a k (nm)b E (eV)b fc Configuration |CI Coef.| > 0.2

582 582 2.13 0.2285 H-4 ? L 0.64677500 514 2.41 0.0131 H-7 ? L 0.53489

H-6 ? L 0.44101510 2.43 0.0127 H-8 ? L 0.58019

H-5 ? L �0.34163508 2.44 0.0285 H-5 ? L 0.52719

H-8 ? L 0.30442504 2.46 0.0239 H ? L + 4 0.53095

H-3 ? L + 1 �0.26419H-1 ? L + 3 0.21620

426 (sh)d 426 2.91 0.1472 H-8 ? L + 1 �0.38093H-9 ? L 0.34564H-5 ? L + 1 0.25893

425 2.92 0.1263 H-5 ? L + 1 �0.42676H-9 ? L 0.35144H-8 ? L + 1 0.25652

400 401 3.09 0.5092 H-10 ? L 0.49042H-4 ? L + 2 0.24965

391 3.17 0.4073 H-4 ? L + 1 0.31909H-10 ? L �0.30483H-3 ? L + 2 0.23271H-9 ? L + 2 0.22075

379 3.27 0.1197 H-11 ? L 0.37024H-9 ? L 0.27593H-9 ? L + 1 �0.24558H-10 ? L 0.23662H-4 ? L + 2 �0.20061

a Absorption band in the simulated spectra.b Calculated absorption in nm and eV.c Oscillator strength.d Shoulder band.

Fig. 6. The single electron transition (CI coefficient >0.2) for the 508 nm absorptionof Ru(NCS)2L4 in ethanol under the TD-PBE/SDD/SC-ECPs/PCM calculation.

for Ru(NCS)2L4. This visible-region absorption is originating fromtransition of p(porphyrin) to benzoic acid. Electron-density dia-grams shown in Figs. 4–6 provide evidence for above assignment.Apparently, all these bands are featured with transition from elec-tron-donating (porphyrin)/NCS/Ru to electron-withdraw benzoicacid. The lower-energy 582 nm absorption, for instance, is contrib-uted by H-4 ? L excitation configuration with CI coefficient of0.647. Electron density of H-4 is mainly located around porphy-rin-core ligand with the p character, while most electrons is

M.-J. Zhang et al. / Computational and Theoretical Chemistry 1019 (2013) 94–100 99

distributed over benzoic acid in LUMO. Therefore, upon harvestingsunlight, the Ru(NCS)2L4 sensitizer is excited. The photogeneratedelectrons are promoted to orbitals of benzoic acid anchoring onelectrode of semiconductor. Subsequently, the electrons are easyto inject into the semiconductor conduction band. Building on itsstrong absorptions ranging from 350 to 610 nm (Fig. 3d) and fea-tured transition properties from donor to acceptor, Ru(NCS)2L4 isexpected to be a promising photosensitizer for DSSCs.

Additionally, Ru(NCS)2L3 was calculated to exhibit bands at 466and 554 nm in the visible region. These bands are weaker thanthose of Ru(NCS)2L4, and occur in relatively higher-energy region.As far as these calculated absorptions are concerned, Ru(NCS)2L3

may be a sensitizer candidate, albeit not as good as Ru(NCS)2L4.One can note that the PBE functional was used in the present

work. This GGA-PBE is known not to describe the charge-transfer(CT) excitations well. As a matter of fact, In our previous study[47], we have calculated ZnLA (LA = N,N-di(picolyl)aminoethyl-imi-nocoumarin, see Fig. S1 of Supplementary information) in theaqueous solution using various TD-DFT including GGA (PBE,PW91 and BLYP), LDA (SVWN), hybrid (PBE1, B3LYP, BH&H andBH&HLYP) and long range corrected (LC-wPBE, LC-BLYP andCAM-B3LYP) functionals. The simulated spectra are presented inFig. S2. One can see that all functionals present general spectralcharacteristics for the lowest-energy intense absorption. Moreover,this absorption is contributed from HOMO ? LUMO excitationconfiguration. The compositions of HOMO and LUMO calculatedwith different functionals are also quite similar. As far as the calcu-lated wavelength of the intense absorption is concerned, a bestagreement with the experimental spectra of ZnLA [48] is found atthe GGA level. For example, the GGA-PBE calculations presentthe lowest-energy absorption peak at 517 nm, agreeing well withthe experimentally measured 524 nm one. Similarly, we have cho-sen PBE in the work.

4. Conclusions

The structures and spectroscopic properties of MLx andRu(NCS)2Lx (M = Zn, Cd and Hg; x = 1–4) were investigated byDFT and TD-DFT. By evaluating its performance on the experimen-tally known ZnL, an alternative approach, TD-PBE/SDD/SC-ECPs/PCM(ethanol), has been used in the work.

It is shown that absorption spectra of MLx (M = Zn, Cd and Hg;x = 1–4) can be tuned by varying substituted porphyrins Lx, but areslightly affected via changing metal center. All the complexes exhi-bit the strongest peak at about 400 nm, attributed to the intra-por-phyrin p ? p� transition. Substitution of benzoic acid at the b-position of porphyrin core allows an extra intense peak at450 nm in ML3 and ML4, but ML2 with the meso-substituted ben-zoic acid only displays some very weak low-energy absorptions.

Complexes Ru(NCS)2Lx (x = 1–4) were theoretically designedand calculated. As expected, they possess advantages of both poly-pyridyl ruthenium and porphyrin-based sensitizers, i.e. containingabsorptions with not only intra-porphyrin transition but alsocharge transfer from donor ligand (NCS) and metal center (Ru).The calculated results reveal that Ru(NCS)2L4 exhibits absorptionspectra matching well with solar spectra. Apart from having strong350–460 nm absorption band, the complex exhibits intenseabsorption peaks in the low-energy visible region between 460and 610 nm. What’s more, all the obtainable absorptions are fea-tured with the charge transfer from donor (porphyrin, NCS andRu) to acceptor (benzoic acid), where the carboxyl acid is the mostefficient and the most commonly-used anchoring group in DSSCs.In brief, the designed Ru(NCS)2L4 may have potential applicationas a photosensitizer in DSSCs.

Acknowledgements

This work is supported by Program for New Century ExcellentTallents of Common Universities of Heilongjiang Province (1154-NCET-010), National Natural Science Foundation of China(21273063, 30901136), Program for New Century Excellent Talentsin University (NECT-11-0958), Key Project of Chinese Ministry ofEducation (211048), and Program for Innovative Research Teamin University (IRT-1237). Foundations of State Education Ministryfor the Returned Overseas Chinese Scholars and of HeilongjiangProvince (LC2011C22) are greatly acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.comptc.2013.07.006.

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