Strategic Design of Bacteriochlorins as Possible Dyes for

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Strategic Design of Bacteriochlorins as Possible Dyes for Photovoltaic Applications Mannix Padayhag Balanay, and Dong Hee Kim J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07957 • Publication Date (Web): 11 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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The Journal of Physical Chemistry

Strategic Design of Bacteriochlorins as Possible Dyes for Photovoltaic Applications Mannix P. Balanaya* and Dong Hee Kimb a

Department of Chemistry, School of Science and Technology, Nazarbayev University, Astana, 010000 Kazakhstan

b

Department of Chemistry, Kunsan National University, Gunsan, Jeonbuk, 573-701, Korea

Corresponding author:

Tel. No. +7 7172 69 4657 Email: [email protected]

ACS Paragon Plus Environment


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Abstract Bacteriochlorin-based dyes, having a push-pull type of configuration similar to the YD2 dye, were theoretically designed based on the modification of the macrocycle and πconjugated bridge for use in dye-sensitized solar cells. Various parameters were assessed to determine its structure-property relationships, such as the absorption profile based on timedependent density functional theory, non-linear optical properties from (hyper)polarizability data, ground- and excited-state oxidation potentials, and the electronic properties of the free and adsorbed dyes. Based on the results, the most appropriate macrocycle would be 7,7,17,17-tetramethyl-7H,8H,17H,18H-porphyrin and for its π-conjugated bridge were either thieno[3,2-b]thiophene,

dithieno[3,2-b:2’,3’-d]thiophene,

or

4,4-diisopropyl-4H-

cyclopenta[2,1-b:3,4-b’]dithiophene. These newly designed dyes produced an absorption spectra having a range of 300 to 800 nm which could likely increase the light harvesting efficiency. It has better nonlinear properties than the reference thereby ensuring higher charge-transfer properties. And the dye regeneration efficiency is within the optimized value of 0.2 eV which could minimize the excessive loss of voltage. This shows that through theoretical approach we can deductively design analogues before synthesis to streamline the process in the design of dyes to produce an efficient dye sensitized solar cells.


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I. Introduction Dye-sensitized solar cells (DSSCs) illustrate an artificial photosynthesis method which was introduced by Grätzel in 19911. In artificial photosynthesis, instead of converting the absorbed light to chemical energy, it is transformed into electrical energy. The absorption of light in plants was done by the pigments in the leaves, such as chlorophyll – whose basic structure is porphyrin. This has an absorption range normally at around 300 to 700 nm which is very important when designing highly panchromatic dyes for DSSCs. As of to date, the dye which has the highest photon-to-current conversion efficiency (η) without co-sensitization is with a porphyrin-based dye incorporated with a benzothiadiazole unit at the π-conjugated bridge (SM315) at 13.0% 2. This is not far from the highest recorded efficiency of 14.3% when carbazole-based dye (ADEKA-1) was co-sensitized with a triphenylamine-based dye (LEG4) at one sun illumination 3. In an effort to further expand the absorption properties of the dye to increase its light harvesting efficiency (LHE), bacteriochlorins (BChl) were utilized as photosensitizers owing to their higher light absorption property at the near infrared (IR) region that was a result of the partial saturation of the two pyrrole rings which could affect the B (Soret, ~ 400 nm) and the Q (~550 nm) bands of the porphyrin system both arising from π π* transitions based on the Gouterman’s “four-orbital” system has been already some attempts to use of BChl as dyes for DSSCs

8-13

4-7

. There

with the highest

efficiency at 6.6% under one sun illumination 8. The lower efficiency of BChl dyes as compared to the highly efficient porphyrin-based dye, SM315 2 is credited to the aggregation of the dye. This was determined when 5 mM of chenodeoxycholic acid (CDCA) was used as a co-adsorbent resulted to a 6% increase in the photon-to-current conversion efficiency of the BChl dye. SM315 and SM371 utilizes 2,6-dioctyloxyphenyl at the meso-position of the porphyrin macrocycle which greatly minimizes the aggregation of the dye. The only difference between the two is that SM315 incorporates a benzothiadiazole moiety between



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the ethynyl and benzoic acid moieties.2 This produced more than 36% increase in its efficiency compared to those porphyrin analogues with 3,5-di-tert-butylphenyl (YD2).14 Its design can be utilized in order to control aggregation in porphyrin-like structures. In this paper, we designed bacteriochlorin dyes having a push-pull configuration patterned from YD2 dye

14

. The theoretical screening of the analogues can be assessed

through different parameters such as the range of the absorption spectra through timedependent density functional theory (TD-DFT); the reduction-oxidation (redox) properties of the dyes which can be determined by calculating the ground- and excited-state oxidation potentials; and charge transfer properties.15-19 R

R NH

N

N

D

A N

R

HN

D

HN

NH

N

N A

D

N

R

Zn

R

R

R

BChl-1b

ZnBChl-1a

R

R

R

N

N

NH

A N

HN

D

HN

NH A

NH

A

N

N

ZnBChl-1b R

N

N A

N

N Zn

R

D

N

D

N

N

BChl-1a

D

N A

D

N Zn

A

N

HN

N

R

R

R

R

BChl-2a

BChl-2b

Porph

Ref y

t-Bu

D=

N

R=

A=

COOH

x

t-Bu

Figure 1.

Molecular structures of meso-substituted porphyrin analogues with different macrocycle units with their corresponding nomenclature.

The strategy of the design of the new dyes is based on three categories. The first category is that the dyes are designed from the changes of the structure of the porphyrin macrocycle of YD2 by replacing it with bacteriochlorin macrocycles to test their applicability


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as sensitizers for DSSCs. In addition to bacteriochlorins, the properties of free-base porphyrin (Porph) and the reference based on the YD2 structure (Ref) were included for comparison purposes (Figure 1). The second category for the design of new dyes is the variation of the structure of the π-conjugation bridge with the porphyrin and donor structure similar to YD2. It is already known that these changes could lead to different effect on the absorption spectra and its charge transfer properties of the dyes. The molecular structures of the dyes undertaken in this study are presented in Figure 2.

The last category is designing a new dye based

on the optimized structure from the previous categories in an effort to design a highly efficient bacteriocholorin-based dye for photovoltaic applications.

Figure 2. Molecular structures of meso-substituted porphyrin analogues with different πconjugation moieties. The numbers below corresponds to the labelling scheme.

II. Methodology All ground-state structures were optimized without symmetry constraints using mPWHandHPW91 exchange-correlation (xc) functional, this methodology was optimized for organic dyes having thiophene moieties

20

. mPWHandHPW91 is derived from the gradient

corrected mPWPW91 xc functional adjusted with 50 % Hartree-Fock exchange. The calculation of the excited state energies which can predict the absorption spectra of the dyes



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were done using the time-dependent DFT approach with a long-range corrected xc functional from Head-Gordon and colleagues that includes the Grimme’s dispersion model (ωB97XD)21. This functional was chosen to be the most appropriate method to calculate absorption energies in porphyrin structures with a push-pull type of configurations 18. The incorporation of solvent effects was based on the conductor-like polarized continuum model (C-PCM) 22. The ground-state oxidation potential (GSOP) is calculated based on the vertical energy difference in solution between the neutral and oxidized species, both at the geometry of the neutral species. The excited-state oxidation potential (ESOP) is calculated using the formula: ESOP = GSOP – E0–0; where E0–0 is the lowest excited-state energy calculation from the TDDFT results. The M06-2X hybrid functional of Truhlar and Zhao

23

was optimized for this

type of calculation 18. The gap between the ESOP and the conduction band (CB) of the semiconductor can be used to approximate the driving force of electron injection (–∆Ginj) by merely taking the difference of the two parameters. The same can be applied when calculating dye regeneration process (–∆Greg), by subtracting the potential of the redox couple of the electrolyte from the GSOP. We utilized the energy of the CB of TiO2 at pH 7 set at – 4.05 eV 24, while the redox couple potential was at –4.85 eV based on iodide/triiodide complex 25. The (hyper)polarizabilities were calculated at M06-2X xc functional from the groundstate optimized structures using the finite field approach with the strength of electric field set at 0.001 a.u. The mean molecular isotropic polarizability (α), anistotropy of polarizability (∆α), and the total first-order hyperpolizability (βtot) are calculated based on the following equations: = =

+ +

− + −

+ −


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= + + + + + + + + All calculations were done using Pople’s split valence double-ζ polarized basis set (631G(d)) in Gaussian 09 software 26.

3. Results and Discussion 3.1. Effect of change of macrocycle As shown in Figure 1, the differences between the “a” and “b” structures of the BChl or ZnBChl is the relative position of the methyl moieties with respect to the donor and acceptor groups. The difference between the BChl-1 against BChl-2 is the addition of a conjugation at the pyrrole ring of the latter which affects the planarity of the macrocycle. Based on the stability of these two positions, the conformers, BChl-1a and ZnBChl-1a are more stable that their “b” counterparts (BChl-1b and ZnBChl-1b) by only 0.10 and 0.28 kcal mol-1. However, the introduction of the added conjugation resulted to the added strain at the pyrrole ring where the methyl groups are attached which makes the difference between the energies of the two conformers greater than those observed with BChl and ZnBChl analogues. Nevertheless, the same trend is observed wherein BChl-2a is more stable as compared to BChl-2b conformer by 4.36 kcal mol-1.

3.1.1. Absorption characteristics The Q-bands

27-28

of all the analogues, except for BChl-2, were all red-shifted and

produced higher oscillator strengths compared to the reference. Unfortunately, the B-band undergoes hypsochromic shift accompanied with the decrease in oscillator strength. The representative absorption spectra of the analogues with different porphyrin macrocycle simulated at 2000 cm-1 peak half-width at half-height using a GaussView 5 software


29

are


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presented in Figure 3. The simulated absorption spectra of the reference was slightly blue shifted as compared to the experimental bands wherein, at the Q-bands, it produced a blueshifting of less than 0.20 eV, while larger blue-shifting at the B-band of 0.40 eV was observed. Nevertheless it falls within the theoretical limits when compared to experimental results.

Figure 3. Simulated absorption spectra of meso-porphyrin analogues with different macrocycles compared to reference structure. (a) Porph; (b) BChl-1a and BChl1b; (c) BChl-2a and BChl-2b; and (d) ZnBChl-1a and ZnBChl-1b. The relative shifts of the absorption spectra between two conformations, “a” and “b”, the “b” conformations (BChl-1b, BChl-2b, and ZnBChl-1b) were all red-shifted compared to the “a” analogues (BChl-1a, BChl-2a, and ZnBChl-1a) by 7.0, 21.5, and 3.0 nm for BChl-1, BChl-2, and ZnBChl-1, respectively. All analogues undergo transitions consistent with the Gouterman’s four-orbital model28. The BChl-1 and ZnBChl-1 analogues show the same absorption characteristics, but the inclusion of the metal produced a more red-shifted spectrum of an average of 36 nm for


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the lowest transition state with an increase in oscillator strength which resulted in a much higher light harvesting efficiency. This is also observed for regular porphyrin macrocycles where the metallation resulted in a red-shifting of about 34 nm but decreased in oscillator strength. To support further the optical properties, a calculation of light harvesting efficiency of the analogues under consideration at the lowest transition state compared to the result obtained for the reference analogue. The LHE is related to the short-circuit current density (JSC), which is defined as:

J = e LHEλΦ!"# λη%&'' η()* I, λdλ .

where Φinj is the quantum yield of charge injection; ηcoll is the charge collection efficiency at the back contact; ηreg is the regeneration efficiency of the oxidized dye; and Is is the incident spectral photon flux density. The LHE can be calculated theoretically based on 1–10–f, where f is the oscillator strength of a given transition. In order to account for majority of the spectra, all the LHE’s for electronic transitions greater than 300 nm were summed together ∞ ∑λλ3 3455 012 . To compare the difference with the reference dye, the relative LHE (RLHE)

∞ λ3∞ was considered, wherein it represents the ratio of ∑λλ3 3455 012678 / ∑λ3455 012;8< . As

shown in Table 1, all the RLHE where slightly below the reference, with BChl-2a decreased by 18 %. The ZnBChl analogues were the least to be affected due to its slightly similar in structure, having a decreased RLHE value of not more than 4 %.

3.1.2. (Hyper)polarizabilities The (hyper)polarizabilities determines the intramolecular charge delocalization in pushpull configuration which is a very vital characteristic to design an efficient sensitizer for DSSCs. Table 1 lists the CT properties such as polarizabilities (α), polarizability anisotropy (∆α), the highest β tensor component which is found to be at βxxx, and the total first-order



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hyperpolarizability (βtot). Between the “a” and “b” conformers of each bacteriochlorins, it produced almost the same isotropic polarizabilities. Comparing the different bacteriochlorins, BChl-1 and ZnBChl-1 are more polarizable than BChl-2 analogues. All analogues also undergo a unidirectional electron transport when subjected to an electrical field. It was said that the large values of first hyperpolarizabilities shows enhanced CT properties which is related to the electronic intramolecular CT excitation

18, 30-34

. Among the analogues which

exhibit the largest βtot value is BChl-1b followed closely by BChl-1a.

Table 1.

The relative light harvesting efficiencies (RLHE), mean molecular isotropic polarizabilities (α), polarizability anisotropies (∆α), highest first-order hyperpolarizability tensor (βxxx), and total first-order hyperpolarizabilities (βtot) values with different types of macrocycle Ref

Porph

BChl-1

BChl-2

a b RLHE 1.000 1.073 0.909 0.901 1.15 0.90 0.94 0.94 α (103) 2 6.54 6.83 6.23 6.26 ∆α (10 ) 4 1.76 1.87 1.50 1.61 βxxx (10 ) 1.61 1.70 1.75 1.87 βtot (104) Note: all values are expressed in a.u. except for RHLE

a 0.821 0.89 5.57 1.62 1.39

b 0.901 0.89 5.65 1.26 1.15

ZnBChl-1 a b 0.962 0.987 0.93 0.94 5.85 6.03 1.02 1.16 1.20 1.31

3.1.3. Oxidation potentials The driving force of the electron injection is an important property that could help determine if there is an increase in the short-circuit current density value. In general, the –

∆Ginj is directly proportional to the electron injection efficiency and in order for the dyes to approach unity, it should be more than 0.20 eV 35 . For an efficient dye regeneration process, the –∆Greg should be within 0.20 to 0.25 eV

36

. In the case of almost similar ruthenium-

polypyridyl compounds; such as N719 (η = 11.2%)37-38, CYC-B11 (η = 11.5%)39, and C106 (η = 11.7%)

40-41

, which produced almost identical efficiencies but the redox properties of


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N719 and CYC-B11 is different from C106. The electron injection efficiencies of N719 and CYC-B11 are 0.15 and 0.12 V vs. normal hydrogen electrode (NHE), respectively which are almost at the ideal value of the driving force for electron injection. However, their dye regeneration efficiencies are 0.62 and 0.61 V vs. NHE, respectively which could cause voltage losses in the form of heat. This excessive voltage loss was also observed in porphyrin analogues when comparing different electrolytes

42

. For C106 dye, the values of electron

injection and dye regeneration are 0.62 and 0.20 V vs. NHE, respectively. The very high electron injection efficiency of C106 compared to N719 and CYC-B11 could result to an increase in charge recombination process. Thus, the most ideal value for both –∆Ginj and –

∆Greg should be close to 0.20 V vs NHE. -2

ESOP

-5

1.685

1.692

2.199

2.285

1.767

1.974

-4

CB TiO2

1.785

-3

2.089

Oxidation potentials (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I-/I3-

GSOP

-6

Figure 4

Oxidation potentials (eV) of porphyrin analogues with different macrocycle.

Table 2

Amount of electron injection (–∆Ginj, in eV), electron regeneration (–∆Greg in eV), and the total dipole moment (µtotal, in debye) of analogues with different macrocycle BChl-1 BChl-2 ZnBChl-1 Ref Porph a b a b a b

–∆Ginj

0.85

0.68

0.81

0.78

1.42

1.34

0.96

0.95

–∆Greg

0.44

0.49

0.18

0.18

0.07

0.06

-0.07

-0.06

4.4969

4.5311

3.4710

3.9342

5.5614

4.3834

3.5991

4.2331

µtotala



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

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All vectors of the dipole moment are pointing away from the TiO6H5.

As shown in Figure 4 and Table 2, it was only the BChl-1 and Porph analogues have efficient dye regeneration processes. The GSOP levels of the metallated analogues (ZnBChl1a and b) were higher than the oxidation potential of the redox couple which is not good thermodynamically for dye regeneration process. The GSOP of the BChl-2 analogues were about 0.06 eV lower from the redox couple which would not be enough to push electrons to regenerate the dye. For the electron injection efficiency, all the analogues were above the CB band with a difference of more than 0.78 eV, except for Porph analogue, which was only 0.68 eV. The ESOP of the BChl-1 and ZnBChl-1 analogues produced almost the same oxidation potential as the reference. Of all the studied macrocycles, the BChl-1 analogues are the most promising sensitizers for DSSCs based on their good optical properties, charge transfer characteristics, and proper level of the oxidation potentials. Another macrocycle that could be considered are the ZnBChl-1 analogues provided that the GSOP level can be lowered to an appropriate level by attaching some substituents, such as changing the π-conjugation.

3.1.4. Influence of dipole moment to open circuit voltage The shift of the conduction band of the semiconductor, such as TiO2, is related to the

µnormal which is the dipole moment of the individual molecule perpendicular to the surface of the semiconductor and due to the bulkiness of the molecules into consideration, especially if longer alkyl groups are attached, it is assumed to be almost perpendicular to the TiO2 surface, the µnormal can be approximately equal to the total dipole moment µtotal

17-18

. Various

researches have established theoretically that the larger µnormal of the adsorbed molecules pointing outward from the semiconductor surface, the larger the open circuit voltage (VOC) 43-


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44

. In order to do a faster screening process, we utilized a smaller dye-TiO2 model which was

proven enough to calculate the µnormal of the system

18, 43

, wherein the dyes are bound to a

bidentate system to the TiO6H5 model as shown in Figure 5.

Figure 5.

Bidentate chelating modes of the dyes: (a) schematic diagram (b) representative optimized structure of the dye focusing at the dye-TiO6H5 portion.

As shown in Table 2, µtotal increases in the order of BChl-1a < ZnBChl-1a < BChl-1b < ZnBChl-1b < BChl-2b < Porph < Ref < BChl-2a. From these data, BChl-2a could produce a higher VOC than the rest of the analogues, however, the absorption spectra of the BChl-2 analogues (Figure 3) were unfavorable and also there was an improper matching of the GSOP and the redox couple (Table 2). The other analogue that could be taken into consideration for high VOC is the ZnBChl-1b analogue, even though it has a very good absorption spectra, the GSOP was higher than the redox couple which makes it unfavorable for dye regeneration (Figure 4).

3.2. Effect of the change of the bridge-acceptor group This section discusses the effect of the replacement of the benzoic acid with different πconjugated bridges as shown in Figure 2. The basic structure of the reference, the reference macrocycle is mostly retained except for the bridge and acceptor group. The structural changes with respect to the planarity of the porphyrin macrocycle and π-conjugated bridge are shown in Table 3. It is interesting that analogue 3 produced that largest dihedral angle (C2-C3-C4-C5) with 10.76o which is more than 4o larger than the other analogues. This is probably due to the break of the flow of the π-conjugation wherein the S atom is in line with



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the conjugation chain. Analogue 3 also produced the largest bond length between C3-C4 that reduces the π-conjugated character. Thus, it is very important to have an efficient flow of electrons by proper positioning the heteroatom as can be seen in analogues 1 to 4. Analogues 10 to 12 produced the most planar configuration with respect to the porphyrin macrocycle.

Table 3.

Analogues Ref 1 2 3 4 5 6 7 8 9 10 11 12

Selected geometric parameters of the meso-substituted porphyrin analogues including their dipole moment in their ground state configuration

φ (C2-C3-C4-C5) (°) 5.86 6.02 4.21 10.76 5.34 2.26 3.42 2.56 6.70 1.32 0.90 0.61 2.05

r (C3-C4) (Å) 4.039 4.040 4.038 4.042 4.037 4.041 4.040 4.019 4.020 4.020 4.040 4.039 4.039

r (H1-H5) (Å) 3.153 3.150 3.111 3.147 3.113 3.112 3.110 3.463 3.479 3.479 3.129 3.125 3.119

µtotal (Debye) 3.59 5.11 7.69 5.93 5.62 2.65 4.77 7.41 6.56 7.65 3.27 6.74 6.91

As observed in Figure 6, all the analogues were red-shifted at both Q- and B-bands as compared to the reference molecule, which resulted in higher light harvesting efficiencies as shown in Table 4. This trend is different from the one observed when changing the type of porphyrin macrocycle wherein the shifts are not the same for the Q- and B-bands. The analogues which produced a good coverage of the absorption spectrum are those presented in Figure 6c. These analogues have multiple thiophene moieties which are fused together. However, the downside of this configuration is that it can also encourage dye aggregation due to the planarity of the system. This is addressed by introducing an alkyl chain between the


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two thiophene moieties as seen in analogue 9, which produced a broader B-band region and more red-shifted and higher oscillator strength for the Q-bands.

Figure 6.

Simulated absorption spectra of meso-porphyrin analogues with different πconjugation group compared to YD2 structure. The analogues with the same base structure of the π-conjugation are grouped together. (a) 1, 2, 3, and 4 analogues; (b) 5 and 6 analogues; (c) 7, 8, and 9 analogues; and (d) 10, 11, and 12 analogues.

All the analogues produced better CT properties, except for analogues 1, 3, 5, and 10 which have lower βtot values as compared to the reference (Table 4). They also have very large unidirectional properties with 5 times higher than the next β tensor. For analogues 1 and 2 wherein the proper location of the S atom was investigated showed that the conjugation which is in line with the porphyrin macrocycle is beneficial for better CT characteristics as indicated with an increase of more than 2 times with regards to the βtot. The change of acceptor group from carboxylic to cyanoacrylic acid also resulted to an increase in CT properties due to the additional electron withdrawing group. The cyanoacrylic acid as its acceptor moiety produced better results than the cyano and carboxylic



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acid were attached at different positions at the ring. The order of increasing βtot values are 1 < 3 < 10 < 5 < 8 < 11 < 2 < 6 < 12 < 9 < 4 < 7. Thus, the group which has higher CT properties are those analogues with fused thiophene groups due to increase in coupling of the donor and acceptor groups. This is consistent with the red-shifting of the dyes and better optical properties as shown in Figure 6c.

The relative light harvesting efficiencies (RLHE), mean molecular isotropic polarizabilities (α), polarizability anisotropies (∆α), highest first-order hyperpolarizability tensor (βxxx), and total first-order hyperpolarizabilities (βtot) values of porphyrin analogues with different π-conjugation units. Analogues Ref 1 2 3 4 5 6 RLHE 1.000 1.025 1.030 1.246 1.122 1.046 1.101 1.15 1.21 1.23 1.27 1.31 1.36 1.44 α (103) ∆α (102) 6.54 7.28 7.87 8.52 9.43 8.21 9.74 1.76 -1.34 -3.02 -1.40 -5.25 -1.64 -4.21 βxxx (104) βtot (104) 1.61 1.23 2.88 1.33 5.12 1.50 4.06

Table 4.

7 8 9 RLHE 1.139 1.059 1.131 α (103) 1.34 1.39 1.50 10.40 10.82 10.98 ∆α (102) βxxx (104) 5.89 -5.59 -4.93 5.70 5.50 5.02 βtot (104) Note: all values are expressed in a.u. except for RHLE.

10 1.031 1.37 8.33 -1.63 1.49

11 1.023 1.38 8.50 -2.89 2.75

12 1.151 1.47 10.41 -4.95 4.81

The GSOP levels of the analogues were all higher by 0.02 to 0.06 eV than the reference, except for analogue 2 which is only slightly below the reference. The effect of changes of the acceptor moiety has also a slight effect on the oxidation potentials. The changing of carboxylic acid to cyanoacrylic acid produced lower GSOP and ESOP, as shown in Figure 7. But generally, the changes of the character of the π-conjugated group only has slight effect on the oxidation potentials, which also shows minimal changes in the electron-transfer processes shown in Table 5. The π−conjugated moieties that could be a candidate for use as sensitizers for DSSCs are analogues 7, 8, and 9 due to its better absorption profile and CT properties.


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-3

ESOP

-5

2.061

2.069

2.070

2.014

2.035

2.034

2.068

2.074

2.061

2.088

2.077

CB TiO2

2.087

-4

2.089

Oxidation potentials (eV)

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I-/I3-

GSOP

-6 ref

Figure 7.

Table 5.

–∆Ginj –∆Greg µtotala

–∆Ginj –∆Greg µtotala a

1

2

3

4

5

6

7

8

9

10

11

12

Oxidation potentials (eV) of porphyrin analogues with different π-conjugated units.

Amount of electron injection (–∆Ginj, in eV) and electron regeneration (–∆Greg, in eV), and total dipole moment (µtotal, in Debye) of analogues with different πconjugation units Analogues Ref 1 2 3 4 5 6 0.85 0.86 0.83 0.87 0.84 0.88 0.87 0.44 0.42 0.44 0.42 0.42 0.39 0.40 4.4969 4.8118 6.5206 7.7038 6.5688 4.1326 6.4075 7 0.79 0.45 9.0774

8 0.82 0.42 8.0834

9 0.83 0.39 9.6310

10 0.88 0.39 4.5660

11 0.86 0.41 6.6850

12 0.86 0.40 8.1156

All vectors of the dipole moment are pointing away from the TiO6H5.

With the same theoretical methodology applied for porphyrin with different macrocycles as presented in section 3.1.4, the calculated µtotal values for the dyes with different π-conjugated group are presented in Table 5. The increase in its ability to transfer electrons through the π-conjugated group produced higher µtotal values, especially with the use of a cyanoacrylic acid as an acceptor group. Among the dyes considered, those with µtotal



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> 8 Debye were the analogues 7, 8, 9, and 12. These dyes are very ideal as π-spacers for dyes in DSSCs.

Figure 8.

Ground-state optimized structures of analogues calculated at mPWHandHPW91/6-31G(d) in vacuo. Also shown are the selected dihedral angles between the macrocycle and π-conjugated group.

3.3. Design of new BChl dyes from screening of analogues From the results obtained in sections 3.1 and 3.2, new bacteriochlorin-based sensitizers were designed in an effort to produce a highly efficient dye. The structure was based on BChl-1b for its porphyrin macrocycle and analogues 7, 8, and 9 as its π-conjugated groups, herein labeled as BChl-A, BChl-B, and BChl-C, respectively, collectively referred in this manuscript as tBu analogues. We also include a design based on the highest porphyrin analogue having alkyl groups at the ortho positions of the meso-phenyl rings

42

, herein

labeled as BChl-oC2-A, BChl-oC2-B, and BCh-oC2-C. These molecules are labeled generally as oC2 analogues to easily distinguish the different groups of analogues. The optimized geometries calculated at mPWHandHPW91/6-31G(d) in vacuo are presented in Figure 8.


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Figure 9.

Simulated absorption spectra of the new designed meso-porphyrin analogues. (a) tBU analogues and (b) oC2 analogues

Theoretical parameters for assessment of PV performances Analogues Propertiesa BChlBChlBChlRef BChl-A BChl-B BChl-C oC2-A oC2-B oC2-C RLHE 1.000 1.502 1.599 1.627 1.500 1.719 1.501 3 1.15 1.13 1.18 1.30 1.18 1.23 1.35 α (10 au) 2 6.54 10.11 10.47 10.69 9.73 10.02 10.30 ∆α (10 au) 4 1.76 5.78 -5.27 4.81 6.83 -6.47 6.20 βXXX (10 au) 1.61 5.96 5.53 5.11 7.02 6.75 6.50 βtot (104 au) inj 0.85 0.72 0.73 0.74 0.82 0.82 0.83 –∆G (eV) reg 0.44 0.21 0.19 0.18 0.13 0.11 0.10 –∆G (eV) 4.497 8.108 7.722 9.281 8.531 8.332 9.831 µtotal (Debye) a RLHE = relative light harvesting efficiency; α = mean molecular isotropic polarizabilities; ∆α = polarizability anisotropies; βxxx = highest first-order hyperpolarizability tensor; βtot = total first-order hyperpolarizabilities; –∆Ginj = electron injection, in eV; –∆Greg = electron regeneration, in eV; µtotal = total dipole moment, in Debye. All vectors of the dipole moment are pointing away from the semiconductor surface. Table 6

As shown in Figure 9, the newly designed dyes produced more red-shifted spectra near the IR region with high oscillator strengths. Unfortunately, the red-shifting also produced the lowering of the oscillator strength at the B-band, but still the strengths of the B-band are still in the order of 1 x 105 M-1 cm-1, which is higher than the molar extinction coefficients of the Ru-based dyes. The incorporation of two ethyl groups at the ortho position at each mesophenyl ring undergoes slight blue-shifting between 3 to 5 nm at the first transition state with a decrease of 2% in its oscillator strengths which is also observed in the comparison of optical



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properties of YD2 and YD2-o-C8 42. All the theoretical parameters presented in Table 6 were higher compared to the reference. The RHLE value for all the analogues were more than 50% higher than the reference analogue. This is consistent with the observed simulated spectra shown in Figure 9. The hyperpolarizabilities were also 3 times higher which could result to higher CT properties. The CT properties of the dyes are also well depicted in the HOMOLUMO spatial orientation presented in Figure 10.

Figure 10. HOMO-LUMO spatial orientation of the dyes calculated at TD-ωB97X-D/631G(d)//mPWHandHPW91/6-31G(d) in ethanol using C-PCM framework.

The µtotal of all the newly proposed dyes were about 2 times higher than the reference which is very beneficial for increasing VOC. The µtotal of the oC2 analogues were all higher compared to the tBu analogues which means that the VOC of the oC2 analogues were higher than tBu analogues as also observed from the references, where YD2-o-C8 has a higher VOC than YD2 42. The use of BChl as the porphyrin macrocycle in combination with the fused thiophene


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moieties as its π-conjugated bridge increases the ground-state potential making the dye regeneration process falls within the 0.2 eV limit which could minimize the excessive loss of voltage. However, the oC2 analogues are too close to the oxidation potential of the redox couple which could inhibit the drive for dye regeneration. The major purpose of the use of the alkoxy group at the ortho-position of the meso-phenyl moieties is to block the iodide species to come in contact with the porphyrin macrocycle to avoid the strong formation of the metaliodide complex and also could help control the aggregation between the dyes.

4. Conclusion After the identification of the proper xc functionals to be used for calculations, a screening of possible candidate porphyrin analogues were undertaken. This is based on two categories: first it involves the use of different types of the porphyrin macrocycle. This category employs the bacteriochlorin as a candidate replacement of the regular porphyrin structure owing to its high extinction coefficient at the Q-band region. The other category involves an assessment of different π-conjugated molecules with YD2 as its base structure. Based on the results, the BChl-1 analogues are the most promising for the set of dyes and for π-conjugation, it is analogues 7, 8, and 9 which involve at least two fused thiophene moieties, owing to their enhanced photophysical properties. For the results obtained from the screening procedure, a new set of porphyrin dyes based on the optimized structures was proposed and the results show that all the properties that were assessed were all much larger compared to the reference analogue. This research has proved that the effective use of theoretical calculations can design and propose a new set of sensitizers that has a possibility of attaining highly efficient DSSCs.

Acknowledgement



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This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funding by the ministry of Education, Science and Technology (2010-0021818) and the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KCS-202-C3-44).

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Table of Contents Graphic


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Strategic Design of Bacteriochlorins as Possible Dyes for

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