R EFERENCES

(1) Grätzel, M. Recent Advances in Sensitized Mesoscopic Solar Cells. Acc. Chem. Res. 2009, 42 (11), 1788–1798.

(2) Yu, Q.; Wang, Y.; Yi, Z.; Zu, N.; Zhang, J.; Zhang, M.; Wang, P. High-Efficiency Dye-Sensitized Solar Cells: The Influence of Lithium Ions on Exciton Dissociation, Charge Recombination, and Surface States. ACS Nano 2010, 4 (10), 6032–6038.

(3) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei, L.; Ngoc-le, C.-h.; Decoppet, J.-D.;

Tsai, J.-H.; Grätzel, C.; Wu, C.-G., et al. Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells. ACS Nano 2009, 3 (10), 3103–3109.

(4) Mishra, A.; Fischer, M. K.; Bauerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells:

From Structure: Property Relationships to Design Rules. Angew. Chem. Int. Ed. 2009, 48 (14), 2474–

2499.

(5) Imahori, H.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42 (11), 1809–1818.

(6) Wang, X.-F.; Tamiaki, H. Cyclic Tetrapyrrole Based Molecules for Dye-Sensitized Solar Cells.

Energy Environ. Sci. 2010, 3 (1), 94–106.

(7) Kim, J.-J.; Choi, H.; Lee, J.-W.; Kang, M.-S.; Song, K.; Kang, S. O.; Ko, J. A Polymer Gel Electrolyte to Achieve ≥6% Power Conversion Efficiency with a Novel Organic Dye Incorporating a Low-Band-Gap Chromophore. J. Mater. Chem. 2008, 18 (43), 5223–5229.

(8) Chen, L.; Li, X.; Ying, W.; Zhang, X.; Guo, F.; Li, J.; Hua, J. 5,6- Bis(octyloxy)benzo[c][1,2,5]thiadiazole-Bridged Dyes for Dye-Sensitized Solar Cells with High Open- Circuit Voltage Performance. Eur. J. Org. Chem. 2013, 2013 (9), 1770–1780.

(9) Tang, Z. M.; Lei, T.; Jiang, K. J.; Song, Y. L.; Pei, J. Benzothiadiazole Containing D-π-A Conjugated Compounds for Dye-Sensitized Solar Cells: Synthesis, Properties, and Photovoltaic Performances.

Chem. Asian J. 2010, 5 (8), 1911–1917.

(10) Velusamy, M.; Justin Thomas, K. R.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Organic Dyes Incorporating Low-Band-Gap Chromophores for Dye-Sensitized Solar Cells. Org. Lett. 2005, 7 (10), 1899–1902.

(11) Zhu, W.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang, Z.-s.; Tian, H. Organic D-A-π-A Solar Cell Sensitizers with Improved Stability and Spectral Response. Adv. Funct. Mater. 2011, 21 (4), 756–

763.

(12) Cabau, L.; Vijay Kumar, C.; Moncho, A.; Clifford, J. N.; Lopez, N.; Palomares, E. A Single Atom Change "Switches-on" the Solar-to-Energy Conversion Efficiency of Zn-Porphyrin Based Dye Sensitized Solar Cells to 10.5%. Energy Environ. Sci. 2015, 8 (4), 1368–1375.

(13) Haid, S.; Marszalek, M.; Mishra, A.; Wielopolski, M.; Teuscher, J.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M.; Bäuerle, P. Significant Improvement of Dye-Sensitized Solar Cell Performance by Small Structural Modification in π-Conjugated Donor-Acceptor Dyes. Adv. Funct.

Mater. 2012, 22 (6), 1291–1302.

(14) Yella, A.; Mai, C. L.; Zakeeruddin, S. M.; Chang, S. N.; Hsieh, C. H.; Yeh, C. Y.; Gratzel, M.

Molecular Engineering of Push-Pull Porphyrin Dyes for Highly Efficient Dye-Sensitized Solar Cells:

The Role of Benzene Spacers. Angew. Chem. Int. Ed. 2014, 53 (11), 2973–2977.

(15) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Gratzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6 (3), 242–

247.

(16) Chai, Z.; Wu, M.; Fang, M.; Wan, S.; Xu, T.; Tang, R.; Xie, Y.; Mei, A.; Han, H.; Li, Q., et al.

Similar or Totally Different: the Adjustment of the Twist Conformation Through Minor Structural Modification, and Dramatically Improved Performance for Dye-Sensitized Solar Cell. Adv. Energy Mater. 2015, 5 (18), 1500846.

(17) Han, H.-G.; Weerasinghe, H. C.; Kim, K. M.; Kim, S. J.; Cheng, Y.-B.; Jones, D. J.; Holmes, A.

B.; Kwon, T.-H. Ultrafast Fabrication of Flexible Dye-Sensitized Solar Cells by Ultrasonic Spray- Coating Technology. Sci. Rep. 2015, 5, 14645.

(18) Jin, M. Y.; Kim, B.-M.; Jung, H. S.; Park, J.-H.; Roh, D.-H.; Nam, D. G.; Kwon, T.-H.; Ryu, D. H.

Indoline-Based Molecular Engineering for Optimizing the Performance of Photoactive Thin Films. Adv.

Funct. Mater. 2016, 6876–6887.

(19) Bessho, T.; Zakeeruddin, S. M.; Yeh, C.-Y.; Diau, E. W.-G.; Grätzel, M. Highly Efficient Mesoscopic Dye-Sensitized Solar Cells Based on Donor–Acceptor-Substituted Porphyrins. Angew.

Chem. Int. Ed. 2010, 49 (37), 6646–6649.

(20) Lin, R. Y.-Y.; Lee, C.-P.; Chen, Y.-C.; Peng, J.-D.; Chu, T.-C.; Chou, H.-H.; Yang, H.-M.; Lin, J.

T.; Ho, K.-C. Benzothiadiazole-Containing Donor-Acceptor-Acceptor Type Organic Sensitizers for Solar Cells with ZnO Photoanodes. Chem. Commun. 2012, 48 (99), 12071–12073.

(21) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136 (3B), B864–B871.

(22) Salvatori, P.; Agrawal, S.; Barreddi, C.; Malapaka, C.; de Borniol, M.; De Angelis, F. Stability of Ruthenium/Organic Dye Co-Sensitized Solar Cells: A Joint Experimental and Computational Investigation. RSC Adv. 2014, 4 (101), 57620–57628.

(23) Kanetada, N.; Matsumura, C.; Yamazaki, S.; Adachi, K. Mechanism of Peripheral Substituent Effects on Adsorption–Aggregation Behaviors of Cationic Porphyrin Dyes on Tungsten(VI) Oxide Nanocolloid Particles. ACS Appl. Mater. Interfaces 2013, 5 (24), 12991–12999.

(24) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Graetzel, M. Conversion of Light to Electricity by Cis-X2bis(2,2'-bipyridyl-4,4'- dicarboxylate)ruthenium(II) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on Nanocrystalline Titanium Dioxide Electrodes. J. Am. Chem. Soc. 1993, 115 (14), 6382–6390.

(25) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.;

Moser, J.-E.; Yi, C., et al. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat. Commun. 2012, 3, 631.

(26) Zhang, J.; Li, H.-B.; Sun, S.-L.; Geng, Y.; Wu, Y.; Su, Z.-M. Density Functional Theory Characterization and Design of High-Performance Diarylamine-Fluorene Dyes with Different π Spacers for Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22 (2), 568–576.

(27) Schlichthörl, G.; Park, N. G.; Frank, A. J. Evaluation of the Charge-Collection Efficiency of Dye- Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 1999, 103 (5), 782–791.

(28) Wang, Q.; Moser, J.-E.; Grätzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye- Sensitized Solar Cells. J. Phys. Chem. 2005, 109 (31), 14945–14953.

(29) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Sero, I.; Bisquert, J. Characterization of Nanostructured Hybrid and Organic Solar Cells by Impedance Spectroscopy. Phys. Chem. Chem. Phys.

2011, 13 (20), 9083–9118.

(30) Katoh, R.; Furube, A.; Mori, S.; Miyashita, M.; Sunahara, K.; Koumura, N.; Hara, K. Highly Stable Sensitizer Dyes for Dye-Sensitized Solar Cells: Role of the Oligothiophene Moiety. Energy Environ.

Sci. 2009, 2 (5), 542–546.

4

Electronic Coupling Engineering for Realizing Vectorial Electron Transfer in Photoanodes

This chapter includes the published contents:

Deok-Ho Roh^†, Jun-Hyeok Park^†, Hyun-Gyu Han^†, Ye-Jin Kim^†, Daiki Motoyoshi, Eunhye Hwang, Wang-Hyo Kim, Joseph I. Mapley, Keith C. Gordon, Shogo Mori*, Oh-Hoon Kwon*, and Tae-Hyuk Kwon* Chem. 2022, 8, 1121-1136. DOI: 10.1016/j.chempr.2022.01.017. Reproduced with permission of Copyright © 2022 Elsevier.

4.1.INTRODUCTION

Natural photosystems (NPSs) sustain life by converting sunlight into chemical energy. In the NPSs, a multi-step electron transfer process occurs called the Z-scheme.¹ Notably, the quantum efficiency for the charge-separation process in the NPSs can achieve close to 100% by virtue of vectorial electron transfer (VET). The NPSs induce the VET by controlling the electronic coupling from relatively strong to very weak between the redox centers.^1-5 Artificial photosystems (APSs), such as photovoltaics,⁶ photoelectrochemical cells,⁷ photocatalysts,⁸ and photo-rechargeable batteries,⁹ have been developed with inspiration from the photochemistry of the NPSs. Conventionally, the π-conjugated organic materials in APSs have been designed with strong donor and acceptor in a planar structure for inducing strong electronic coupling (Figure 4.1a).^6,10 The main advantage of strong electronic coupling induces efficient ICT properties of the π-conjugated organic materials, facilitating charge separation (k1

in Figure 4.1a) and improving light-harvesting ability. Unfortunately, the strong electronic coupling also causes rapid backward charge transfer (k−1 in Figure 4.1a), called charge recombination, which is

a factor in making inferior APSs.^11,12 This charge recombination is far from the VET behavior needed in an ideal APS. For realizing VET, the weakly coupling should be considered in designing π-conjugated organic materials for suppressing charge recombination, as the NPSs control the degree of electronic coupling (k−1 in Figure 4.1b).^13,14 While such approaches should be considered in developing ideal APSs, studies on weak electronic coupling in photosensitizers are sparse because of inferior forward charge transfer and light-harvesting ability.^13-17

DSC is one of the fascinating APSs due to its relatively less toxic compounds, cheap materials, simple fabrication, and high photovoltaic performances in ambient light and moderate PCE in outdoor condition.^6,9,18,19 Conventionally, the photosensitizers for DSCs have been designed with strong electronic coupling to extend the light-harvesting range. For this purpose, in designing the photosensitizers, strong acceptor units were used, such as diketopyrrolopyrrole (DPP),¹² BT,^11,18 and isoindigo²⁰. Unfortunately, the weak electronic coupling was not considered in the design of photosensitizers, even though these strong acceptors cause serious BET problems, as shown in Figure 4.1c, which significantly reduces the PCE.11,12,18,20 To retard BET, π-spacers that decrease the electronic interaction between oxidized sensitizers and TiO2 have been introduced in the photosensitizers.^11,15-17 However, while previous research has focused mainly on retarding non-geminate BET, molecular- design strategies for suppressing ultrafast BET, which will maximize the extent of the usable charge carrier, have yet to be established. Therefore, new approaches similar to those in NPSs are necessary to realize VET in DSCs and achieve ideal APSs with performances beyond the current PCE limits of DSCs (Figure 4.1d and 4.1e).

In this chapter, I present the electronic coupling engineering strategy of the following DPP-based sensitizers composing three molecular moieties with a donor, acceptor (DPP), and π-spacer (Figure 4.1f), in contrast to the typical two-state system comprising D–A–anchoring groups. By modulating the magnitude of electronic coupling (strong–weak), this electronic coupling engineering can independently suppress the BET and maximize the LHE of the photosensitizers. The magnitude of electronic coupling is modulated by different dihedral angles between the π-spacer and DPP unit, which is also visualized by spin density plots. Femto- and nanoseconds transient-absorption (TA) spectroscopies elucidate that DD-DPP-DMP comprised of strong and relatively weak electronic coupling retards both ultrafast and non-geminate BET and delivers a rapid charge injection process, realizing VET. Relatively weakly coupling preferentially reduces ultrafast BET in comparison with non- geminate BET. Therefore, DD-DPP-DMP exhibits a dramatic ~60% improvement in PCE compared with strong coupled DD-DPP-Ph. This result demonstrates that balancing strong coupling and weak coupling is a successful strategy for improving the performance of APSs. By use of the same strategy, the donor-optimized sensitizer, bTPA-DPP-DMP, achieves a high photovoltaic efficiency (10.3%) under a tandem electrolyte and has a unique and aesthetic blue color, which is essential for developing

sensitization with a D35 sensitizer.²¹ This electronic coupling engineering strategy of combining strongly and weakly coupling in a π-conjugated organic material to realize VET will pave the way for the development of ideal APSs.

Figure 4.1. Electronic coupling engineering strategy inducing VET. Schematic free-energy surfaces for (a) the stongly and (b) weakly coupled donor-acceptor materials. (c) Schematic rapid BET in a strongly coupled sensitizers (d) Schematic VET in a electronic coupling engineered sensitizer. (e) Electronic coupling engineering strategy of combinding strong and weak coupling. (f) Chemical structures of the DPP-based sensitizers. (g) The DPP-based sensitizers in solution and on TiO2 films.

4.2.RESULTS AND DISCUSSION

4.2.1. Electronic Coupling Engineering Strategy of the DPP-sensitizers

DPP moiety has been widely used in various applications due to its unique photoelectrochemical property and stability.²² Unfortunately, DSCs with DPP-based sensitizers showed very low PCE values of 1−5%, owing to a serious BET problem.¹² To overcome this BET problem, electronic coupling engineering strategy (Figure 4.1f) includes the following factors: i) introducing three molecular moieties, as shown in Figure 4.1e, for controlling the LEH property and BET kinetics independently, and ii) retarding BET via modulating the magnitude of electronic coupling by different dihedral angles (Figure 4.2), but the weakened electronic coupling of DPP–π moiety decreases the LHE property. This inferior light-harvesting ability can be mitigated by iii) enhancing the electronic coupling of donor–

DPP moiety by introducing a strong donor and a planar structure. This is because the strongly coupled the D–DPP moiety can enhance the ICT property.^23-25

Figure 4.3 shows the synthetic steps for the DPP-based sensitizers. The first step is the synthesis of DPP units (compound 2). Compound 4 is an important intermediate compound, which was synthesized by amine alkylation and bromination reactions. Compounds 5a–c possessing aldehyde group were obtained by the Suzuki coupling reaction using phenylboronic acid and compound 4 in yields over 80%. Compounds 6a–c were prepared by monobromination reaction of compounds 5a–c in 70–85% yields. Compounds 7a–d were obtained by the Suzuki coupling reaction with compounds 6a–

c and boron compounds (D1 or D2). Lastly, the DPP-based sensitizers were synthesized using the Knoevenagel condensation reaction with compounds 7a–d and 2-cyanoacetic acid in 50–90% yields.

Figure 4.2. Ground-state optimized geometries and dihedral angles of the DPP-based sensitizers.

Geometry optimizations of ground-state DPP-sensitizers were performed to confirm their different stable structures based on the π-spacer. All DPP-sensitizers had a similar low dihedral angle (15–21°) between the donor and DPP, whereas the dihedral angles differed based on the π-spacer.

Figure 4.3. Synthesis scheme of the DPP-based sensitizers.

4.2.2. Materials Characterization

All DPP-based sensitizers show light-harvesting ability from 300 to 750 nm with a gap of 450–

500 nm (Figure 4.4a), which provides a blue color to the DPP-based sensitizers (Figure 4.1g). The DPP-based sensitizers show absorption peaks at 300–450 nm and 600–650 nm, corresponding to local π–π* transitions and an ICT, respectively. A hypochromic shift for the ICT absorption peaks (λmax) occurs due to the electronic coupling change. The order is as follows: 621 nm for DD-DPP-Ph > 616 nm for DD-DPP-MP > 614 nm for DD-DPP-DMP > 603 nm for bTPA-DPP-DMP. This order can be explained by the decrease in electronic coupling arising from twisting π-conjugation structure. It is noteworthy that all DPP-based sensitizers show close to 100% LHE at 400–700 nm, barring the 465–

515 nm region (Figure 4.4b). This is because the strongly coupled donor–DPP moiety mitigates the inferior LHE problems of previously reported relatively weakly coupled materials.^16,17

Figure 4.4. Optical properties of the DPP-based sensitizers. (a) Electronic absorption profiles in solutions (20 mM in tetrahydrofuran). (b) LHE of the DPP-based sensitizers on TiO2 film (1.8 μm). The LHE at each wavelength (λ) was estimated from absorbance (A) using the following equation:²⁶ LHE (λ) = 1 − 10^−A(λ).

CV measurements (Figure 4.5) were performed to evaluate the electrochemical properties of the DPP-based sensitizers. Figure 4.6 and Table 4.1 show the summarized energy levels of the DPP-based sensitizers. The DPP-based sensitizers have different excited-state oxidation potentials (ES+/S*) from

−0.97 to −0.75 V vs. NEH, dependent on the π-spacers, but their ground-state oxidation potentials (ES+/S) show similar values of 0.89–0.96 V vs. NHE. These electrochemical and photophysical results of the DPP-based sensitizers are also consistent in DFT calculation results (Figure 4.7 and 4.12).

Figure 4.5. CV curves of the DPP-based sensitizers. The CV curves were performed in a three- electrode configuration under a scan rate (10 mV/s). The dashed line box indicates the CV curve of Fc/Fc⁺ against Ag/AgCl. The +0.63 V vs. NHE for the Fc/Fc⁺ was used to correct the measured values to NHE.²⁷

Figure 4.6. Energy level diagram for the DPP-based sensitizers.

Table 4.1. Summarized spectroscopic and electrochemical properties.

Photosensitizer λmax (nm) εmax (M⁻¹ cm⁻¹) E0–0 (eV) ES/S+ (V) ES*/S+ (V)

DD-DPP-Ph 621, 385 82600, 63200 1.71 0.96 −0.75

DD-DPP-MP 616, 380 72600, 50600 1.76 0.95 −0.81

DD-DPP-DMP 614, 372 70000, 35000 1.85 0.89 −0.96

bTPA-DPP-DMP 603, 333 62000, 62000 1.89 0.92 −0.97

Based on DFT calculations, we investigated the photophysical properties of the DPP-sensitizers.

Geometry optimizations of ground-state DPP-sensitizers were performed to confirm their different stable structures based on the π-spacer. As shown in Figure 4.2, all DPP-sensitizers had a similar low dihedral angle (15–21°) between the donor and DPP, whereas the dihedral angles of the DPP–π-spacer moiety differed based on the π-spacer. The dihedral angle changed the energy levels and molecular orbitals (MOs) of the DPP-sensitizers (Figure 4.7). The HOMO of the DPP-sensitizers was delocalized mainly on the donor–DPP moiety, resulting in similar HOMO energy levels. However, the LUMO energy levels were shifted upward with an increasing dihedral angle between DPP and π-spacer because the twisted π-spacers distorted the π-conjugated orbital overlap. As shown in Figure 4.8, all DPP- sensitizers exhibited a delocalized LUMO along the DPP–π–anchoring group moiety, including twisted sensitizers of DD-DPP-DMP and bTPA-DPP-DMP. We further investigated the contributions of MOs (Figure 4.9) and projected density-of-states (PDOS, Figure 4.10) based on the fragments (donor, DPP, and π-spacer) in the DPP-sensitizers. The LUMO of DD-DPP-DMP mainly consisted of DPP (51%) and π-spacer (44%) (Figure 4.9), which are similar to those of other planar sensitizers. The percentage contribution of the π-spacer in the LUMO+1 followed the order: DMP (64% for bTPA-DPP-DMP and 54% for DD-DPP-DMP) > MP (47%) > Ph (44%). In addition, as shown in Figure 4.10, both DD-DPP- DMP and bTPA-DPP-DMP exhibited an overlapping PDOS for the LUMO and LUMO+1 because of the similar energy levels of LUMO and LUMO+1 (Figure 4.7). Based on these results, all DPP- sensitizers could undergo an efficient charge injection process in DSCs. In addition, from time- dependent density-functional theory (TD-DFT) calculations, we obtained the UV–vis spectra of DPP- sensitizers (Figure 4.12) and MO compositions in the electronic transition (Table 4.2).

Figure 4.7. DFT-calculated energy levels of the DPP-based sensitizers and MOs.

Figure 4.8. LUMO of the DPP-based sensitizers. The dashed boxes indicate the π-spacer and anchoring group.

Figure 4.9. MO contribution profiles. Percentage contributions of each fragment (donor, DPP, and π- spacers) to MO density in HOMO, LUMO, and LUMO+1 of the DPP-sensitizers.

Figure 4.10. PDOS profiles. Total PDOS are shown in black solid lines.

Figure 4.11. DFT calculated dipole moments in ground and excited state. (a) donor-absent fragment structures and (b) the DPP-based sensitizers. Blue arrows (µG) and red arrows (µE) correspond to the ground-state and excited-state dipole moment (Debye, D), respectively. µE was evaluated on the optimized geometry structure in the excited state using TD-DFT calculation with CAM-B3LYP, 6- 311G(d,p) basis set and conductor-like polarisable continuum model (C-PCM, CH3CN). µE is related to the propensity to ICT in the excited-state.²⁸ All DPP-sensitizers exhibit higher µE of 14–21 D than those of donor-absent fragment structures.

Table 4.2. DFT-calculated photophysical parameters.

Sensitizer HOMO

(eV)

LUMO (eV)

E0–0

(eV) ^a) States λmax, DFT

(nm)

Oscillator

strength (f) Composition ^b)

DD-DPP-Ph −5.00 −3.25 1.75

S1 604.83 2.2331

H−1 → L (16%) H → L (68%) H → L+1 (10%) S3 393.96 0.4116

H−1 → L (11%) H−1 → L+1 (30%) H → L (14%) H → L+1 (28%) S4 353.19 1.0237

H−7 → L (20%) H−3 → L (33%) H → L+2 (13%)

DD-DPP-MP −4.99 −3.15 1.84

S1 583.29 2.0941

H−1 → L (16%) H → L (67%) H → L+1 (12%) S3 384.70 0.3839

H−1 → L+1 (28%) H → L (14%) H → L+1 (34%) S5 341.93 1.1818 H−3 → L (12%) H → L+2 (33%)

DD-DPP-DMP −4.99 −2.92 2.07

S1 551.78 1.7932 H−1 → L (19%) H → L (74%) S2 397.77 0.5672

H−2 → L (11%) H−1 → L (51%) H → L+2 (16%) S7 308.78 1.0760 H−3 → L (13%) H−3 → L+1 (63%)

bTPA-DPP-DMP −5.02 −2.92 2.10

S1 546.88 1.6503 H−1 → L (20%) H → L (73%) S2 390.59 0.3342

H−3 → L (10%) H−1 → L (53%) H → L (11%) H → L+2 (14%) S6 308.84 1.3727 H−7 → L+1 (36%) H−6 → L+1 (32%) S7 304.67 0.8904 H−1 → L+3 (32%) H → L+3 (49%)

a) The energy gaps (E0–0) were calculated by E0–0 = LUMO (eV) – HOMO (eV). ^b) H = HOMO and L = LUMO.

Figure 4.13. TD-DFT calculated UV–vis–NIR spectra of the oxidized-state DPP-based sensitizers.

The bars in the panel indicate the oscillator strengths of the first excitation wavelength.

4.2.3. Electronic Coupling and Spin Density

The presented electronic coupling engineering was confirmed by measuring the electronic coupling of the photosensitizers. Furthermore, electronic couplings are important parameters for understanding the photophysical of the DPP-based sensitizers. For evaluating electronic coupling values between the donor and DPP (HD|DPP) and between the DPP unit and π-spacer (HDPP|π, π = Ph, MP, DMP), we measured the electronic absorption spectra of one-electron oxidized DPP-based sensitizers in solution, which was titrated with a NOBF4 stock solution (10 mM in CH3CN) as an oxidizing agent. In Figure 4.14, with an increase in the equivalent of NOBF4, the ICT bands of the ground-state DPP- sensitizers gradually decrease, and the new absorption bands corresponding to IVCT transitions appear at 750–1000 nm and 1200–2100 nm. However, the distinct IVCT absorption peak for bTPA-DPP-DMP in the NIR region (1200–2100 nm) did not appear, as shown in Figure 4.15.

In the oxidized state (Figure 4.16), two characteristic absorption bands appear in the 1200–2100 region. The NIR absorption bands are assigned to the IVCT of the DPP-sensitizers as supported by the DFT calculated UV–vis–NIR spectra (Figure 4.13). Figure 4.16 shows two distinct IVCT bands in the NIR region. These bands were separated into two Gaussian-shaped fit curves (IVCT 1; 1600–2100 nm and IVCT 2; 1200–1600 nm). The electronic coupling values (HD|DPP and HDPP|π) were calculated using following Mulliken–Hush equation:¹³

H_A|B = ^j4.2×10

–4 · ε_max · Δv_1/2 · E_abs

d (4.1) where HA|B (cm⁻¹) is the electronic coupling between A and B, εmax (cm⁻¹ M⁻¹) is the molar extinction coefficient of IVCT bands, ∆v1/2 (cm⁻¹) is full-width at half-maximum of the Gaussian-shaped band, Eabs (cm⁻¹) is energy of the transition maximum, and d (Å) is distance between A and B. The detailed parameters, HD|DPP, and HDPP|π are summarized in Table 4.3. DPP-sensitizers have similar electronic coupling values (560–650 cm⁻¹) evaluated from IVCT 1, indicating IVCT 1 can be assigned to the electronic coupling between the DPP and donor (HD|DPP). However, the electronic coupling values evaluated from IVCT 2 decrease in the following order: 610 ± 10 cm⁻¹ for DD-DPP-Ph > 460 ± 20 cm⁻¹ for DD-DPP-MP > 310 ± 20 cm⁻¹ for DD-DPP-DMP, which are in good agreement with the dihedral angle between the DPP and π-spacer (high dihedral angle decrease the HDPP|π). Therefore, we assign IVCT 2 to the electronic coupling between the DPP and π-spacer (HDPP|π).

문서에서 Deok-Ho Roh (페이지 83-89)