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First-Principle Characterization of the Adsorption Configurations of Cyanoacrylic Dyes on TiO2 Film for Dye-Sensitized Solar Cells Hui-Hsu Gavin Tsai,* Jia-Cheng Hu, Chun-Jui Tan, Yung-Ching Sheng, and Chih-Chiang Chiu Department of Chemistry National Central University No. 300, Zhongda Road, Zhongli District, Taoyuan City 32001, Taiwan S Supporting Information *

ABSTRACT: The loading of sensitizers on a semiconductor is crucial for determining the light-harvesting efficiency of dyesensitized solar cells (DSSCs). The interfacial properties of dyes adsorbed on a TiO 2 film, such as adsorption configurations and adsorption energy, can influence the total amount of dye sensitizers that loads and the stability of a DSSC device. Therefore, it is important to characterize the adsorption properties of sensitizers on TiO2 films atomically and electronically to ensure rational structure-based dye design for high-performance DSSCs. Due to the complex properties of interfacial dyes, previous works on the identification of adsorption configurations of dyes on TiO2 have sometimes been controversial, in particular, the essential IR band assignments. In this study, we employed density functional theory to investigate the adsorption energies, geometries, and vibrational frequencies of various adsorption configurations of 2cyano-3-(thiophen-2-yl)acrylic acid adsorbed on TiO2. We performed a comparative assignment of the calculated vibrational peaks of tridentate and bidentate configurations to the experimental FT-IR spectra simultaneously. Our work backs up the coexistence of tridentate and bidentate bridging configurations, first proposed by Meng and co-workers. Moreover, our comparative IR mode assignments provide clues for further studies of the interfacial properties of dyes adsorbed on TiO2. Study of the transformation mechanisms between tridentate and bidentate modes suggests that the bidentate bridging configuration is a kinetically trapped adsorption mode and the tridentate configuration is thermodynamically the most stable one. Finally, we investigated the photophysical properties of a D−π−A dye in tridentate and bidentate adsorption configurations.

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INTRODUCTION Limited natural resources and overconsumption of fossil fuels has led to a search for renewable energy sources. Over the past decade, the dye-sensitized solar cell (DSSC) has been widely recognized as a promising candidate because of its low manufacturing cost and high efficiency. This device was invented by Grätzel and O’Regan in 1991 and is also known as the Grätzel cell.1 Over the past decade, through research and evolution, several types of DSSCs have been developed. One of the most successful and popular frameworks for metal-free organic dyes is a dipolar donor, π-bridge, and acceptor (D−π− A) structure based on the electronic push−pull architecture.2−4 Generally, D−π−A dyes have efficient intramolecular charge transfer (ICT) properties. On the basis of this strategy, many high-performance dye sensitizers have been synthesized and tested for DSSC applications. In general, an ideal dye sensitizer should have a broad absorption spectrum and high molar extinction coefficient for efficient light harvesting, and the energy alignment of the excited state of the dye sensitizer between the conduction bands of the semiconductor also needs to be finely tuned so that it can provide sufficient driving force for electron injection. Furthermore, the dye sensitizers should have suitable anchoring groups, for grafting the dye sensitizers © XXXX American Chemical Society

on the semiconductor surface, to ensure the electronic coupling between the excited state of dye sensitizers and the conduction band of the semiconductor. Intuitively, dye sensitizers adsorbed on a TiO2 surface with different adsorption configurations will lead to different surface coverages and thus will have different total amounts of dye loading. A dye molecule with large surface coverage/molecule may lead to a smaller overall amount of dye loading and vice versa. For example, a linear-shaped and/or planar dye molecule with a carboxylate anchoring group in a bidentate binding mode using carboxylate group bonding to the surface will be aligned in a more upright fashion. In contrast, the dye in a monodentate binding mode will have a larger tilting angle. Therefore, the dye with a bidentate mode is expected to have smaller surface coverage per molecule than the one with monodentate mode. Moreover, a dye sensitizer in a given adsorption configuration can also have different surface coverage/molecule if it adopts different geometrical conformations. For example, a dye with a bent conformation is expected Received: August 30, 2016 Revised: October 9, 2016 Published: October 20, 2016 A

DOI: 10.1021/acs.jpca.6b08752 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

accepted bidentate bridging configuration. In terms of their theoretical calculations, they reassigned the COO stretching bands of previous work10−12 and have a Δν of about 300 cm−1. Meng and colleagues reassigned the νasym(COO) stretching mode of 2-cyano-3-(thiophen-2-yl)acrylic acid (denoted as ThCAA in this study) in their calculated tridentate configuration and identified the νsym(COO) stretching mode of their calculated bidentate bridging configuration for their experimental FT-IR spectra. On the basis of these reassignments, they concluded the coexistence of tridentate and bidentate bridging configurations. They reported that the Th-CAA molecule in the tridentate configuration adopts a Zconfiguration; on the other hand, in the bidentate bridging configurations, the Th-CAA molecule adopts E-configurations; the adsorption energies of different binding modes of Th-CAA with different molecular configurations were compared directly. Previous works had significantly improved our understanding of the interfacial structures of cyanoacrylic dyes on TiO2; however, some important questions remained unclear. First, previous experimental and theoretical works13,14 on the assignments of COO stretching modes of cyanoacrylic dyes are inconsistent, affecting their further applications. Second, to conclude the coexistence of tridentate and bidentate bridging configurations, we need a complete and comparative assignment of the νasym(COO) and νsym(COO) stretching modes of the tridentate configuration and these two vibrational modes of bidentate bridging configurations at the same time. Third, if the cyanoacrylate dyes can be adsorbed on TiO2 with both tridentate and bidentate bridging configurations simultaneously, it is unclear why the energetically less stable bidentate bridging mode can exist. Fourth, how do the adsorption configurations influence the photophyiscal properties such as absorption spectra and electron injection properties? In this study, we aimed to unravel the above questions. We employed density functional theory (DFT) to investigate adsorption energies, geometries, and vibrational frequencies of the Th-CAA molecule adsorbed on TiO2 with various adsorption configurations. A comparative band assignment of two sets of νasym(COO) and νsym(COO) stretching modes of tridentate and bidentate bridging configurations simultaneously was obtained. The energy profiles of transformation between significant adsorption modes were calculated. Finally, we investigated the photophysical properties of a D−π−A dye with tridentate and bidentate configurations.

to have larger surface coverage/molecule than a linear-shaped one. Moreover, a nonplanar dye will have larger surface coverage/molecule than a planar dye. No doubt, dye loading is crucial in DSSC performances as it determines the efficiency of light harvesting. Furthermore, the strength with which the dye molecules bind to the TiO2 surface can affect the stability and duration time of DSSC devices. Therefore, characterization of dye binding configurations as well as their adsorption energies is crucial for a rational structure-based dye design for highperformance DSSC applications. In addition, how the dye binding configurations influence the electron transfer to the semiconductor remained poorly understood. Experimental and theoretical studies have been performed to investigate the binding configurations of dyes adsorbed on the TiO2 surface.5−7 For a carboxyl or cyanoacrylic dye, a bidentate bridging mode with the COO group bonded to two different five-coordinated Ti atoms of a TiO2 film has been widely accepted in many previous studies.8,9 The FT-IR technique has been commonly used to probe the vibrational spectra of dye molecules adsorbed on TiO2 surfaces and further to investigate the binding modes. Sun and colleagues10 employed the resonance FT-IR technique to probe the interfacial binding between 2-cyano-3-(4-(4-(diphenylamino)styryl)phenyl)acrylic acid (TPC1) dye with a cyanoacrylic anchoring group and semiconductor TiO2. They utilized the asymmetric COO (νasym(COO)) and the symmetric COO (ν sym (COO)) stretching modes of TPC1 to determine the possible binding modes; they first assigned the νasym(COO) and the νsym(COO) stretching modes of TPC1, taking the FT-IR spectra of the TPC1 sodium state as a reference. Their assignments of IR bands show similar Δν (νasym(COO) − νsym(COO)) values (230 cm−1) in polar and nonpolar solvents, which is also similar to the Δν observed in the FT-IR of TPC1 sodium salt (243 cm−1). On the basis of this comparison and other studies,11 they suggested that the bidentate bridging mode between the semiconductor surface and the anchored dye is the majority. Arakawa and colleagues also suggested that NKX-2311 (with a cyanoacrylic group) is adsorbed on the TiO2 surface with bidentate carboxylate coordination12 based on FT-IR absorption spectroscopy and a calculation analysis; however, Δν is 140 cm−1, which is significantly smaller than that reported by Sun and colleagues.10 The IR band assignments of cyanoacrylic dye adsorbed on TiO2 are generally based on the COO stretching bands of its salt state. However, when the cyanoacrylic dye is adsorbed on the TiO2 surface, the COO group may coordinate to the Ti atoms, which may have a different chemical bonding environment in relation to that of its salt state. Moreover, the dye molecules may not only be coordinated to the TiO2 surface; the dye molecules in a free or salt state can also be adsorbed on the coordinated dyes. In other words, the measured IR spectra may also contain the signature peaks of free dye. Therefore, the interfacial structure of the dye adsorbed on the surface is complex, making the IR band assignment a challenging task. Recently, Meng and colleagues13,14 found that the CN groups of cyanoacrylic dyes are involved in interfacial Ti−N anchoring on TiO2 inferred from adsorption energetics of theoretical calculations and vibrational identifications. They proposed a new tridentate adsorption configuration with CO and CN coordinated to TiO2 and one CO bonded to the surface through a hydrogen bond. Therefore, the bonding strengths of two CO groups in the tridentate adsorption configuration should be different from those of the commonly

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COMPUTATIONAL METHODS The (TiO2)48 unit cell with the anatase (101) surface containing a six-layer TiO2 slab was used for the calculations of Th-CAA adsorption energies in different adsorption configurations and the potential energy surfaces (PESs) of the transformation between different adsorption configurations. The calculated dimensions of the (TiO2)48 unit cell with the anatase (101) surface are a = 21.11, b = 11.53, and c = 5.88 Å. The slab is separated by a vacuum layer of ∼20 Å from its neighboring images under the periodic boundary condition. The first-principle calculations based on DFT were performed using the DMol3 software package.15,16 Perdew−Burke− Ernzerhof (PBE) parametrization of the generalized gradient approximation (GGA),17 in conjunction with the double numerical basis sets with polarization (DNP) and all-electron core treatment, was adopted in the geometry optimization.18 The fine criteria were used in geometry optimization. The O and/or N atoms of Th-CAA are bonded to the five-coordinated B


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The Journal of Physical Chemistry A Ti atoms of TiO2. If the proton is transferred to the TiO2 surface, it bonds to the oxygen atom of TiO2.9,19 The ground-state molecular geometries of the Th-CAA (Figure 1) were optimized using Becke’s three-parameter

Therefore, a stable configuration is needed for correct IR band assignment of calculated vibrational modes of Th-CAA adsorbed on TiO2 to the experimental spectra. We first determined the relative stability of E- and Z-configurations of Th-CAA by the B3LYP/6-31G(d,p) method. Our calculations show that the E-configuration of Th-CAA with a Th ring and a CN group located on the same side is 6.61 kcal/mol more stable than its Z-configuration. Th-CAA adopts a planar structure with all of its dihedral angles close to 0 or 180° in its E-/Z-configurations. Previous work by Kim and co-workers also shows that (E)-Th-CAA is the most stable one.31 On the basis of these results, we use the stable E-configuration of ThCAA for the following calculations. Binding Modes of Th-CAA on TiO2. The bidentate configuration of cyanoacrylic and acrylic dyes with both oxygen atoms in the carboxyl group binding to the TiO2 surface is the most studied and widely accepted configuration in the literature.10−12,19,32,33 Recently, Meng and co-workers further investigated the roles of the cyano group of cyanoacrylic dyes in adsorption configurations.13,14 They investigated possible adsorption configurations of Th-CAA on TiO2 with an anatase (101) surface using molecular dynamics (MD) simulations. The adsorption configurations of Meng and colleagues can be classified into two groups: (i) those with the cyano group bonded to the TiO2 and (ii) those with a free cyano group. They found that the tridentate configuration with an additional hydrogen bond (H-bond) connected to the surface is 4.45 kcal/ mol more stable than the widely accepted bidentate bridging configuration with both oxygen atoms in the carboxyl group bonded to the substrate. These results can be understood because the tridentate configuration forms one more bond with the substrate than the bidentate configurations. Nevertheless, the binding configurations of Meng and colleagues have different E-/Z-configurations in the Th-CAA molecule. For example, the Th-CAA molecule in the tridentate configuration adopts a Z-configuration; in contrast, the bidentate bridging configuration with two oxygen atoms bonded to the substrate adopts a E-configuration, and the bidentate configuration with one oxygen atom and −CN bonded to the substrate has a Zconfiguration. As discussed above, the Th-CAA in its Econfiguration is more stable than its Z-configuration. These results indicate that the Th-CAA molecule in the tridentate configuration adopts a less stable configuration, making direct ranking of the adsorption energies of different adsorption configurations much more difficult. Moreover, the adsorption configurations with a different Th-CAA configuration may further affect the vibrational spectra calculations, which makes band assignment of experimental IR spectra difficult. It is, therefore, necessary to calculate the relative energies of different adsorption configurations having the Th-CAA in the thermodynamically most stable E-configuration. Figures 2−4 show our calculated adsorption configurations of Th-CAA adsorbed on an anatase (101) TiO2 substrate. These binding configurations can be grouped according to the following three features: (i) binding number with substrate; (ii) binding atoms of Th-CAA; and (iii) protonated or deprotonated state of Th-CAA. For convenience, we abbreviate these binding configurations as binding number (protonated or deprotnated state of Th-CAA)-binding atoms to TiO2/binding atoms···proton on TiO2. For a monodentate, the binding number is abbreviated as “M”; the binding number of a bidentate mode is abbreviated as “B”; and the binding number of a tridentate mode is abbreviated as “T”. The protonated Th-

Figure 1. Chemical structures and atom labeling of the dye sensitizers. (a) Th-CAA and (b) TPA-Th-CAA.

exchange functional,20 the Lee−Yang−Parr gradient-corrected correlation functional,21 and the 6-31G(d,p) basis set,22 as implemented in the program Gaussian 09.23 To model the experimental vibrational spectra of Th-CAA adsorbed on TiO2 thin films, Th-CAA adsorbed on a (TiO2)22 cluster was studied. The dye was adsorbed on the anatase (101) surface of the (TiO2)22 cluster24 with different adsorption configurations. This model system was chosen as a compromise between computational cost and the stated purpose of predicting the properties of dyes adsorbed on TiO2 thin films. The UV−vis spectra of (2-cyano-3-(5-(4-(diphenylamino)phenyl)thiophen2-yl)acrylic acid) (denoted TPA-Th-CAA) adsorbed on TiO2 was modeled using a (TiO2)38 cluster. The ground-state molecular geometries of the dye−(TiO2)38 complexes were optimized using the DMol3 software package.15,16 PBE parametrization of the GGA17 in conjunction with the DNP was adopted in the geometry optimization.18 The quality of Gaussian 09 default optimization and all-electron core treatment was used in geometry optimization of the dye−(TiO2)38 systems. Finally, optimized structures of all of the studied dye− (TiO2)38 complexes were used to calculate their UV−vis spectra, using the CAM-B3LYP/6-31G(d,p)25 method with Gaussian 0923 in dichloromethane (modeled by C-PCM). The electron density difference map (EDDM)26−28 corresponding to the electronic transitions that visually revealed the changes in electron density before and after transitions was generated using GaussSum (v. 2.2.6).29

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RESULTS AND DISCUSSION Configurations of 2-Cyano-3-(thiophen-2-yl)acrylic Acid. Th-CAA (Figure 1) contains the same cyanoacrylic acceptor/anchoring group as most important organic dyes. ThCAA with its simplicity is suitable for a model molecule to investigate the binding configurations of cyanoacrylic dye on TiO2. The CAA moiety of Th-CAA contains a CC double bond (C6C7); therefore, Th-CAA has E-/Z-isomers, which have different photophysical properties. Th-CAA in the Econfiguration (νasym(COO) = 1751 cm−1; νsym(COO)) = 1351 cm−1) has higher COO vibrational frequencies than those of Zconfiguration (νasym(COO) = 1741 cm−1; νsym(COO)) = 1304 cm−1). The vibrational frequencies shown here are calculated at the B3LYP/6-31G(d,p) level and are scaled by 0.9613.30 C


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the B-CO/CO mode with a stable (E)-Th-CAA (IIb in the original literature), which is 4.45 kcal/mol less stable than the T-CO/CN/CO···H. Taking the energy difference of E- and Zconfigurations into consideration, the tridentate is more than 4.45 kcal/mol more stable than the B-CO/CO mode when the Th-CAA adopts a stable E-configuration. The third most stable configuration is the B-CO/CN mode with a less stable (E)-ThCAA (Ib in the original literature). The adsorption configurations with different E-/Z-configuration of Th-CAA make a direct comparison between adsorption energies of different adsorption configurations difficult. Nevertheless, our calculations and the work of Meng and colleagues suggest that the tridentate T-CO/CN/CO···H configuration is the most stable one. Adsorption Configurations of Th-CAA on TiO2. Different adsorption configurations of dye on TiO2 have different occupation areas/molecule of the TiO2 film and can thus affect the total amount of dye loading. A dye with a CAA anchor that binds to the TiO2 surface in a tilting and/or bending fashion is expected to shadow more area on the TiO2 surface than one that binds in a vertical fashion. We, therefore, estimated the bending and tilting extent of all adsorption configurations. We first defined the coordination system by setting the origin at the five-coordinated Ti atom of TiO2 with which the O11 atom of Th-CAA commonly binds (Figure 5). The vector of the z-axis is set to (0, 0, 1). The y-axis is set to the vector from the origin to its neighboring five-coordinated Ti atom, the widely accepted binding sites of CAA dyes on TiO2. The molecular vector of Th-CAA is defined as from the origin to the C2 atom, where extended dye molecules may connect to the donor and/ or spacer. We define the α angle as the angle between the projection vector of the molecular vector on the yz plane and z vector (see Figure 6). For a planar dye, a large α angle indicates that the dye shadows more Ti atoms. The α angle arise not only from the adsorption configuration of a dye but also from the configuration and topology of a dye. The β angle is the angle between the projection vector of the molecular vector on the xz plane and the z vector (see Figure 6). For a planar dye, the β angle mainly arises from the adsorption configuration. Table 2 lists the α and β angles and C8−C7−C10−O11 dihedral angles (τ) of Th-CAA adsorbed on TiO2 with different configurations. The τ dihedral angles listed in Table 2 show that the Th-CAA remains in a nearly planar structure with all adsorption configurations except for the T-CO/CN/CO···H configuration. The T-CO/CN/CO···H configuration has the largest β angle of 27.5°, indicating that the Th-CAA molecule is

Figure 2. Adsorption of the Th-CAA molecule on an anatase TiO2(101) surface with a tridentate T-CO/CN/CO···H configuration (a) viewed through the yz plane and (b) viewed through the xz plane.

CAA is abbreviated as “p”, and the deprotonated dye, where its proton is transferred to the TiO2, is not noted. If the Th-CAA atom forms a hydrogen bond with the transferred proton on TiO2, it is abbreviated as atom···H. For example, the tridentate configuration (Figure 3) where CO and CN bind to Ti atoms of TiO2 and another CO forms a hydrogen bond with the proton transferred to TiO2 is abbreviated as T-CO/CN/CO··· H. Table 1 lists the relative energies and adsorption energies for different configurations of the Th-CAA adsorbed on TiO2. Generally, the adsorption energies are in the order of tridentate > bidentate > monodentate modes. The tridentate T-CO/CN/ CO···H configuration is the most stable one, ∼5.4 kcal/mol more stable than those of the second stable bidentate configurations, B-CO/CO and B-CO/CN. This result can be understood because the T-CO/CN/CO···H has an additional hydrogen bond with the surface. The adsorption energies of the bidentate configurations can be classified into two groups; the B-CO/CO and B-CO/CN configurations have similar adsorption energies, and the B(p)-CO/CN and B-CO/CO··· H configurations have close adsorption energies, which are ∼3 kcal/mol less stable than those of the B-CO/CO and B-CO/ CN. Interestingly, the adsorption configuration with deprotonated Th-CAA has a larger adsorption energy than its protonated counterpart (e.g., B-CO/CN vs B(p)-CO/CN). This result indicates that the proton prefers to transfer onto the TiO2 surface. In the work of Meng and colleagues,13,14 the tridentate TCO/CN/CO···H configuration (Ic in the original literature) is identified as the most stable; however, the Th-CAA adopts a less stable Z-configuration. The second stable configuration is

Figure 3. Adsorption of a Th-CAA molecule on an anatase TiO2(101) surface in the bidentate mode for (a) B-CO/CN, (b) B-CO/CO, (c) B(dp)CO/CO···H, and (d) B(p)-CO/CN. D


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Figure 4. Adsorption of a Th-CAA molecule on the anatase TiO2(101) surface in the monodentate mode for (a) M-CO, (b) M(p)-CO, (c) M-CN, and (d) M(p)-CN.

Table 1. Relative Energies and Adsorption Energies for Different Configurations of Th-CAA Adsorbed on TiO2 adsorption configuration

relative energy (kcal/mol)

adsorption energy (kcal/mol)

T-CO/CN/CO···H B-CO/CN B-CO/CO B(p)-CO/CN B-CO/CO···H M-CO M(p)-CO M-CN M(p)-CN

0.00 5.42 5.37 8.17 8.58 9.40 10.86 11.77 12.13

30.38 24.96 25.01 22.21 21.80 20.98 19.51 18.61 18.25

Figure 6. Tilt angles (a) α and (b) β, which are composed of the projection vector of the molecular vector on the yz plane and the xz plane with the z vector, respectively.

Table 2. Geometrical Parameters for Configurations of ThCAA Adsorbed on TiO2 adsorption mode

α (deg)a

β (deg)a

τ (deg)a

T-CO/CN/CO···H B-CO/CN B-CO/CO B-CO/CO···H B(p)-CO/CN M-CO M(p)-CO M-CN M(p)-CN

41.40 37.63 22.55 4.61 37.08 28.12 28.57 48.09 48.09

27.47 10.11 5.10 7.33 11.34 5.46 5.25 8.13 8.13

−25.32 5.61 2.90 −1.19 3.58 0.00 5.84 0.00 0.00

Figure 5. Vector of Th-CAA ( ) in the three-dimensional Cartesian coordinate system, with the titanium (Ti5c) atom as the origin O and axis lines X, Y, and Z.

a

significantly tilting along the x-axis and has a large α angle of 41.4°. Vibrational Spectra of Th-CAA Adsorbed on TiO2. IR spectroscopy is one means by which the adsorption configurations of dyes on TiO2 can be probed. Because the Th-CAA binds to the surface locally with the cyanoacrylic acid group, we expect that the vibrational spectra of the cyanoacrylic acid group, in particular, the CO and CN stretching modes, will be altered with different adsorption configurations. Meng and colleagues measured the IR spectrum of Th-CAA adsorbed on a TiO2 film at pH 5.9.14 In terms of the analysis of the CN stretching frequency change before and after the Th-CAA is adsorbed on TiO2, Meng and colleagues concluded that the CN group is involved in the binding and T-CO/CN/CO···H is the adsorption configuration.13 By further analysis of the CO

stretching modes, they concluded that the possibility of T-CO/ CN/CO···H and B-CO/CO coexistence is high.14 They assigned a strong peak at 1606 cm−1 to the asymmetric COO (νasym(COO)) mode and the medium-intense peak at 1312 cm−1 to the symmetric COO (νsym(COO)) mode observed in their measured IR spectra. Applying a scaling factor of 1.015, a calculated νasym(COO) peak at 1608 cm−1 in the T-CO/CN/ CO···H configuration with the Z-form of Th-CAA is identified, which is very close to the experimental value, implying the existence of the T-CO/CN/CO···H configuration. On the other hand, the B-CO/CO configuration with the E-form of Th-CAA shows a calculated IR peak at 1315 cm−1, suggesting the existence of the B-CO/CO configuration. The B-CO/CN configuration has a calculated blue-shifted νasym(COO) at 1691 E

For definitions, see the main text.


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Table 3. Spectral Data and Vibrational Assignmentsa of Th-CAA Dye in B-CO/CO and T-CO/CN/CO···H Adsorption Configurations expt14b 2222 w 1606 s 1563 bd sh 1500 1397 1312 1250 1222

vw s m vw m

T-CO/CN/CO···Hb 2170 1607 1556 − 1477 − 1306 1215 1278

m m m vw s vw m

vib. modec

B-CO/COb

ν(CN) νasym (COO) ν(CC) − ν(ring CC), ν(CC) − νsym (COO) ν(ring CC) C−H rocking

2216 − 1562 1505 1475 1360 − 1218 1284

vw m m vw s vw m

vib. modec ν(CN) − ν(CC) νasym (COO) ν(ring CC), C−H rocking νsym (COO) − ν(ring CC) C−H rocking

a Units: cm−1. bAbbreviations of intensity: s: strong; m: medium; w: weak; vw: very weak; bd; broader, sh; shoulder. cνsym: symmetric stretching band; νasym: asymmetric stretching band.

cm−1, while no obvious peak is observed in this frequency region, indicating a very low population of the B-CO/CN configuration. Because the adsorption modes are derived from the band assignment and vibrational analysis, a consistent and comparative band assignment and analysis in conjunction with accurate theoretical calculations are needed. Moreover, for a coexistence system of different adsorption modes, it is expected that both νasym(COO) and νsym(COO) peaks of T-CO/CN/ CO···H and B-CO/CO configurations should be observed in the measured IR spectra at the same time. In addition, the eigenvectors of calculated vibrational modes have to be taken into considerations for conclusive band assignment. On the basis of the above reasons, we carefully and comparatively assign our calculated vibrational spectra in the range of 1000− 2300 cm−1 for both T-CO/CN/CO···H and B-CO/CO configurations to the measured IR spectra of Meng and colleagues.13 First, we obtained a scaling factor of 0.9495 by calibrating our calculated CN stretching ν(CN) peak at 2336 cm−1 of 4-chlorocinnamonitrile dyes13 to the measured ν(C N) stretching mode at 2218 cm−1. Table 3 lists the experimentally observed IR peaks and calculated vibrational frequencies (scaled by 0.9495) of T-CO/CN/CO···H and BCO/CO configurations. The vibrational modes were obtained by visualizing the eigenvectors of vibrations. Figure 7 displays the simulated IR spectra of T-CO/CN/CO···H and B-CO/CO configurations. Here, we describe the details of how we assign the vibrational modes. We first examined the existence of the BCO/CN configuration. For the B-CO/CN configuration, our calculated νasym(COO) was located at 1706 cm−1, and no obvious peak was observed in the measured spectra. Therefore, no significant population of B-CO/CN configuration existed, which is consistent with the work of Meng and co-workers. For the T-CO/CN/CO···H configuration, the calculated νasym(COO) frequency was 1607 cm−1, which was obviously assigned to the experimental mode of 1606 cm−1. Similarly, the calculated νsym(COO) frequency was 1306 cm−1, which was assigned to the experimental mode of 1312 cm−1. In the T-CO/ CN/CO···H configuration, the calculated and experimental Δν values of the νasym(COO) and νsym(COO) modes were 301 and 294 cm−1, respectively, and are good agreement with each other. The experimental mode with strong intensity at 1397 cm−1 was assigned to the calculated intense νsym(COO) mode of 1360 cm−1 in the B-CO/CO configuration. In the B-CO/ CO configuration, the calculated νasym(COO) frequency of 1505 cm−1, which was buried in the experimental mode of 1563 cm−1, was a broader shoulder of the 1606 cm−1 band. The

Figure 7. Simulated vibrational spectra of calculated (a) B-CO/CO and (b) T-CO/CN/CO···H adsorption configurations.

calculated Δν value in the B-CO/CO configuration was 145 cm−1, which was approximately half of that in the T-CO/CN/ CO···H configuration. This is because the two CO bonds in the B-CO/CO configuration have more similar bond lengths (1.291 and 1.272 Å) than those (1.292 and 1.246 Å) in the T-CO/CN/CO···H configuration. Experimental IR spectra14 showed three significant modes with strong to medium intensities related to the CO stretching, 1606 (s), 1397 (s), and 1312 (m) cm−1. Our calculated vibrational spectra of BCO/CO and T-CO/CN/CO···H configurations reproduce these three modes well, confirming their coexistence. Our band assignments are different from the work of Meng and co-workers13,14 and further clarify some questions. The strongest peak at 1397 cm−1 in the measured IR spectra is reasonably assigned and thus provides evidence of the existence of the B-CO/CO configuration. We assigned the measured peak with medium intensity at 1312 cm−1 to the calculated F


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Table 4. TPC1 Experimental IR Spectra and Assignments of Major Vibrational Modesa of TPC Dye with the T-CO/CN/CO··· H and B-CO/CO Adsorption Configurations expt10b 2170 1618 1450 1375 1310

vw s s m m

T-CO/CN/O····Hb

modec

B-CO/COb

modec

2170 m 1607 m − − 1306 s

ν(CN) νasym (COO) − − νsym (COO)

2216 vw − 1505 m 1360 s −

ν(CN) − νasym (COO−) νsym (COO−) −

Units: cm−1. bAbbreviations of intensity: s: strong; m: medium; w: weak; vw: very weak. cνsym: symmetric stretching band; νasym: asymmetric stretching band.

a

Figure 8. PES of the transformation between (a) B-CO/CN and T-CO/CN/CO···H configurations, (b) B-CO/CO and T-CO/CN/CO···H configurations, and (c) B-CO/CO and B-CO/CN configurations.

mode at 1306 cm−1 in the T-CO/CN/CO···H configuration, further confirming the existence of the T-CO/CN/CO···H configuration. Our band assignments are also compatible with the IR spectra of TPC1 adsorbed on TiO2.10 Table 4 lists the experimentally observed IR peaks of TPC1 adsorbed on TiO2 and calculated vibrational modes of T-CO/CN/CO···H and BCO/CO configurations. Because the binding between dyes and TiO2 is confined to the acceptor moiety of the dye, the characteristic vibrational frequencies of the acceptor are expected to be similar for cyanoacrylic dyes with different spacer and donor. Table 4 shows that we can identify four ν(COO−) bands with strong-to-medium intensities. These four ν(COO−) bands can be assigned to the two sets of νasym(COO−) and νsym(COO−) peaks for the T-CO/CN/ CO···H and B-CO/CO configurations, respectively. These results further confirm the coexistence of T-CO/CN/CO···H and B-CO/CO configurations of TPC1 dye. There are two sources of systemic error in theoretically calculated normal-mode frequencies, which make accurate band assignments challenging. Our vibrational spectra are calculated using the uncoupled harmonic approximation, whereas actual vibrations have a certain degree of anharmonicity. This approximation causes calculated frequencies to be higher than experimental values. Another source of error arises from the inaccurate description of the electron−electron interactions. In order to correct the calculated frequencies to match the experimental vibrational frequencies, we employed an experimentally calibrated scaling factor34 to reduce the errors of calculated vibrational spectra. Moreover, frequency differences (e.g., Δν), which are critical to characterize different adsorption configurations, benefit from the cancellation of errors. Therefore, band assignments of different adsorption configurations investigated here are expected to be reliable. The accuracy of vibrational spectra can be further improved by taking the anharmonicity and coupling of modes into consideration. Using

the parametrized Hermite functions as a basis, Manzhos, Carringtonm, and co-workers have obtained sub-cm−1 accuracy for the frequencies of acetic acid adsorbed on the anatase (101) TiO2 surface.35 In the cases with complicated vibrational spectra, more accurate calculated frequencies considering anharmonicity and coupling will be needed to obtain a clear band assignment. Transformation between Different Adsorption Modes. On the basis of FT-IR analysis, our work and previous study14 suggest the coexistence of B-CO/CO and T-CO/CN/ CO···H configurations. However, our calculations also show that the T-CO/CN/CO···H configuration is approximately 5.4 kcal/mol more stable than the B-CO/CO and B-CO/CN configurations. Therefore, some significant questions need to be clarified: (i) Because the B-CO/CN configuration has similar thermodynamic stability as the B-CO/CO configuration, why is the B-CO/CN configuration not observed? (ii) Why does the B-CO/CO configuration have a certain population even though it is less stable than the T-CO/CN/ CO···H configuration? To clarify these two questions, we calculated the PES of the transformation of these three configurations. Figure 8a shows the PES of the transformation between the B-CO/CN and T-CO/CN/CO···H configurations as a function of the distance r(O11···H) of O11 to the H at the surface. It is seen that the energy barrier for the transformation from the B-CO/CN to T-CO/CN/CO···H configuration is 1.23 kcal/mol only. This transformation does not involve significant bond breaking; instead, it has H-bond formation. As discussed above, the significant configuration changes between the B-CO/CN and T-CO/CN/CO···H configurations are the τ dihedral angle. Our calculation of free deprotonated Th-CCA molecules shows that the energy barrier from the planar ThCAA form (τ = 5.6°) in the B-CO/CN configuration and the twisted form (τ = −25.3°) is 0.75 kcal/mol. This result indicates that the less stable B-CO/CN configuration can be converted to the most stable T-CO/CN/CO···H configuration G


Article


Table 5. Characteristics of the Electron Density of TPA-Th-CAA Dye Adsorbed on TiO2 in Different Adsorption Modes Before and After Transition percent contribution (%) λmax

change

TPA

Th

CAA

(TiO2)38

B-CO/CN

459 nm f = 1.19

B-CO/CO

461 nm f = 1.71

T-CO/CN/CO···H

486 nm f = 0.96

before after net before after net before after net

75 4 −71 74 5 −69 75 4 −71

16 10 −6 16 12 −4 16 11 −5

8 14 +6 9 16 +7 9 14 +5

0 74 +74 0 67 +67 1 71 +70

adsorption mode

at room temperature, supporting that no significant amount of B-CO/CN configuration is observed experimentally. In contrast, the energy barrier for transformation from the BCO/CO to T-CO/CN/CO···H configuration is 8.72 kcal/mol (Figure 8b), which first requires O···Ti bond breaking in the BCO/CO configuration. The large energy barrier for the transformation from the B-CO/CO to T-CO/CN/CO···H configuration indicates that the B-CO/CO configuration is a kinetic-controlled adsorption mode. On the other hand, the energy barrier for the transformation from the B-CO/CO configuration to the B-CO/CN configuration is 11.9 kcal/mol (Figure 8c), retarding their free transformation at room temperature. Transformation among the T-CO/CN/CO···H, B-CO/CO, and B-CO/CN states via other high-energy states listed in Table 1 is also possible; however, it is expected that the probability is low for the stable state to convert to a less stable state at room temperature. Taken together, the coexistence of T-CO/CN/CO···H and B-CO/CO configurations can be understood in terms of the transformation PESs between adsorption configurations. The T-CO/CN/CO···H configuration is the thermodynamically most stable adsorption mode. The less stable B-CO/CO configuration is observed because it is trapped kinetically. In contrast, the less stable B-CO/CN configuration is not observed because it is easily transformed to the most stable T-CO/CN/CO···H configuration. Effects of Adsorption Configurations on Photophysical Properties. Different adsorption configurations have different degrees of coupling with the TiO2 and thus have different photophysical properties. To investigate the effects of the adsorption configurations on the photophysical properties, we calculated TPA-Th-CAA adsorbed on a (TiO2)38 cluster with the anatase (101) surface in the T-CO/CN/CO··· H, B-CO/CO, and B-CO/CN configurations. The diphenylaminophenyl group is a commonly used electrondonating one, and thus, the TPA-Th-CAA molecule has an electronic push−pull D−π−A structure. Table 5 shows the UV−vis spectra and characters of electron density of TPA-ThCAA dye adsorbed on a (TiO2)38 cluster in different adsorption modes before and after transition. The B-CO/CO and B-CO/ CN configurations have a similar λmax value, while the λmax of the T-CO/CN/CO···H configuration is red-shifted by ∼26 nm in relation to those of B-CO/CO and B-CO/CN configurations. The EDDMs listed in Table 5 show that before the transition the electron densities involved in the transition of TPA, Th, and CAA moieties for the T-CO/CN/CO···H, BCO/CO, and B-CO/CN configurations are similar, while the electron densities transferred to the TiO2 after photoexcitation are different. The electron probability transferred to the TiO2

cluster is in the order of B-CO/CN (74%) > T-CO/CN/CO··· H (71%) > B-CO/CO (67%). These results may arise from the fact that the electron-withdrawing −CN group in the B-CO/ CN configuration (N···Ti distance = 2.36 Å) binds to the Ti atom stronger than that of T-CO/CN/CO···H (N···Ti distance = 2.41 Å). Table 6 lists the aligned excited-state energy levels Table 6. Energy Alignments of the TPA-Th-CAA Molecule Adsorbed on a (TiO2)38 Cluster in the B-CO/CN, B-CO/ CO, and T-CO/CN/CO···H Modes along with the GSOP, Excitation Energy (Eabs), Aligned First Singlet Excited State (S1), Driving Force for Electron Injection, and ΔEreg (in eV)a adsorption modes

GSOP

Eabs

aligned S1b

driving forcec

ΔEreg

B-CO/CN B-CO/CO T-CO/CN/CO···H

−5.38 −5.34 −5.34

2.69 2.69 2.55

−2.69 −2.65 −2.79

1.25 1.29 1.15

0.53 0.49 0.49

a

These results are calculated at the CAM-B3LYP/6-31G(d,p) theoretical level in dichloromethane. bE(aligned S1) = E(GSOP) + Eabs. cDriving force = E(aligned S1) − E(TiO2 C.B. EXP.); E(TiO2 C.B. EXP.) = −3.94 eV.

(S1 states) of the TPA-Th-CAA molecule adsorbed on a (TiO2)38 cluster in the B-CO/CN, B-CO/CO, and T-CO/CN/ CO···H modes. We followed the methods proposed by De Angelis and colleagues36,37 for the energy alignment of the S1 state. The driving force for electron injection to TiO2 is calculated by the aligned energy of the S1 state in relation to the experimental energy level of the TiO2 conduction band (TiO2 C.B., −3.94 eV). It is observed that the T-CO/CN/CO···H mode has the lowest driving force mainly arising from its redshifted excitation energy. Nevertheless, all three adsorption modes have sufficient driving forces for electron injection. Because the T-CO/CN/CO···H mode is more red-shifted in absorption than that of the B-CO/CO mode, the T-CO/CN/ CO···H mode has better spectral match with solar radiation. Moreover, the T-CO/CN/CO···H mode has more electron density transferred to TiO2 than that of B-CO/CO upon photoexcitation. These results suggest that the T-CO/CN/ CO···H mode might have better electron injection efficiency than that of the B-CO/CO mode. The dye regeneration energy ΔEreg is calculated as electrolyte dye ΔEreg = Eredox − EGSOP

where the standard redox potential of the I−/I3− electrolyte Eelectrolyte is −4.85 eV (0.35 eV versus NHE)38 and the Edye redox GSOP is H


Article


bridging (B-CO/CO) configurations, which are well matched with the measured FT-IR spectra simultaneously. Our work confirms the coexistence of tridentate and bidentate bridging configurations, which was first proposed by Meng and colleagues. Moreover, our work re-examines previous assignments of COO band stretching, providing detailed spectroscopic analysis of the interfacial structure of a cyanoacrylic dye adsorbed on TiO2. Our calculations show that the tridentate configuration TCO/CN/CO···H is thermodynamically the most stable adsorption mode, which is ∼5.4 kcal/mol more stable than those of B-CO/CO and B-CO/CN configurations. On the other hand, the B-CO/CO configuration is a kinetically trapped adsorption mode; the energy barrier for the transformation from the B-CO/CO to the tridentate mode is 10.5 kcal/mol, hindering the free transformation at room temperature. In contrast, the energy barrier for the transformation of B-CO/ CN to the tridentate mode is only 1.23 kcal/mol, supporting the low population of the B-CO/CN mode. The tridentate mode has red-shifted absorption in relation to that of the BCO/CO mode. Our work can be extended to characterize the interfacial structure of the complex dye system and further provide clues for rational structured-based dye design for highefficiency DSSCs applications.

the ground-state oxidation potential (GSOP). The energy difference (ΔEreg) between the dye and electrolyte can affect the rate of dye regeneration. The B-CO/CN mode has the lowest GSOP and thus has the largest ΔEreg. Graphic presentations of the alignments of excited-state energy levels of TPA-Th-CAA-(TiO2)38 in the B-CO/CN, B-CO/CO, and T-CO/CN/CO···H modes and their electron density distributions before and after photoexcitation are displayed in Figure 9.

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ASSOCIATED CONTENT

* Supporting Information

Figure 9. Energy levels of TPA-Th-CAA adsorbed on a (TiO2)38 anatase cluster with different adsorption modes; the bottom bars are the GSOP, and the upper ones are aligned S1 states.

S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b08752. Reference 23 with the complete author list (PDF)

■

Our results above are based on equilibrium static absorption configurations without a temperature effect. At finite temperatures (300 and 350 K), Manzhos and co-workers39,40 observed that 2E,4E-2-cyano-5-(4-dimethylaminophenyl)penta-2,4-dienoic acid and 2E,4E-2-cyano-5-(4-diphenylaminophenyl) penta-2,4-dienoic acid tend to tilt from their vertical equilibrium adsorption configurations in ab initio MD simulations. Tilted and vertical equilibrium adsorption configurations have different distances (z) between the electron-donating group and the top layer of oxygen atoms of the TiO2 surface and are therefore expected to affect the recombination rates. On the other hand, the lowest value of z is observed to be larger in the case of the water-covered surface than that of the corresponding water-free one, indicating that the coadsorbates (and/or water) can reduce the recombination rate. Thermal energy populates all vibrational modes, in particular, the low-frequency modes, and thus promotes the dynamics of low-frequency orientational degrees of freedom. In the actual case of the TiO2 surface with a significant amount of dye molecules loaded (like coadsorbates), the degree of dye tilting from the equilibrium vertical configuration is expected to be reduced from that of an isolated dye molecule. Whether the tilted adsorption configuration is a minimum or a transient state at an infinite temperature would need to be quantified by a long-time-scale dynamical study and/or an investigation of the PES of dye tilting.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan (Grant No. MOST 104-2113-M-008-005) for financial support and the National Center for High-Performance Computing and the V’ger computer cluster at the National Central University of Taiwan for allowing access to computer time and facilities.

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REFERENCES

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CONCLUSION We performed a computational study of various adsorption configurations of Th-CAA adsorbed on TiO2 to analyze the measured vibrational spectra. We rationalized and identified two sets of νasym(COO) and νsym(COO) stretching modes of calculated tridentate (T-CO/CN/CO···H) and bidentate I


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