Boron Subphthalocyanine Based Molecular Triad Systems for the

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Boron Subphthalocyanine Based Molecular Triad Systems for the Capture of Solar Energy Freja Eilsø Storm, Stine T. Olsen, Thorsten Hansen, Luca De Vico, Nicholas E. Jackson, Mark A. Ratner, and Kurt V. Mikkelsen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05518 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

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

The Journal of Physical Chemistry

Boron Subphthalocyanine Based Molecular Triad Systems for the Capture of Solar Energy Freja E. Storm,† Stine T. Olsen,∗,† Thorsten Hansen,† Luca De Vico,† Nicholas E. Jackson,‡ Mark A. Ratner,‡ and Kurt V. Mikkelsen† †Department of Chemistry, H. C. Ørsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen,Denmark ‡Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL E-mail: [email protected]

1

ACS Paragon Plus Environment


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

Abstract In this study a number of chromophores based on boron subphthalocyanines are investigated for use in future design of organic photo voltaic devices based on molecular triad systems. The computational study is performed at the TD-DFT CAM-B3LYP/6311G(d) level of theory. The absorption spectra of these chromophores are simulated using TD-DFT and compared to experimental results. All investigated chromophores absorb light in the visible range, and thus are suitable for absorption of sunlight in solar cell applications. Based on energy-level alignments, suitable combinations of moieties for a molecular triad system are proposed. The molecular triads will be used in future work as the functional part of organic photo voltaic devices, where the chromophore will be used both to absorb the incoming solar radiation and to increase the distance between the separated charges on donor and acceptor units in order to increase the lifetime of the charge separated state.

Introduction An increase in the global power consumption and awareness of the effects related to the use of fossil fuels to produce this energy have lead to an enhanced interest in alternatives for energy production. The capture of solar energy via organic molecules is a topic where many different approaches have been pursued. 1–7 Many parameters related to the efficiency and usefulness of organic solar cells have been identified. 1,8,9 Some relate to the actual capture of the energy, while others relate to cost and toxicity of the finished product. 9 One advantage of organic solar cells, compared to traditional solar cells, is the potentially cheaper production cost. 10,11 A disadvantage in, for example, dye-sensitized solar cells (DSSCs) is the use of rutheniumbased sensitizers. The cost and toxicity of these complexes make them undesirable for large scale production. 12 Several alternative sensitizers have been investigated in recent years. 6,7 Common to all the organic molecules used in solar cells, DSSCs or other types, is that a number of physical 2


Page 2 of 33

Page 3 of 33


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 ACS Paragon Plus Environment


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

based on these energy alignments will be synthesized and spectroscopically characterized. Furthermore, the charge states of the full triads will be calculated using constrained Density Functional Theory 13–16 in order to find parameters relevant for a Marcus transfer rate expression. The first moiety is a central chromophore unit (C) where the absorption of sunlight takes place. The C is responsible for the absorption of energy from the sun, and therefore should be picked with focus on absorption in the highest intensity part of the AM 1.5 solar spectrum. The second moiety is the electron donor (D), which will need to donate an electron to C after photo excitation. The photo excitation of C will leave a hole in the highest occupied molecular orbital (HOMO) of C. This hole can then be filled with an electron from D if the electronic properties of D and C are suitably tuned to each other. D needs to be tailored so that there is an energy gain in the electron transfer (ET) process between D and C, in other words, the electron in D-HOMO should be at a higher energy than in the C-HOMO. The third moiety, is the acceptor (A). After C has been photo excited an electron can move from the excited state in C (C*) to the lowest unoccupied molecular orbital (LUMO) in A, in an ET reaction, instead of relaxing back to C-HOMO. If this happens in conjunction with the D to C ET reaction an effective charge separation has occurred. This can be visualized as in Figure 1, where a schematic representation of the relevant moieties and energy levels can be seen. The advantage of a D-C-A triad system compared to a D-A dyad is the increased charge separation distance. 17,18 In a dyad system, charge separation occurs directly from D to A and this ET reaction can be very efficient, but unfortunately the rate of the reverse recombination event is also fast. 17 In other words, the charge separated state (CSS) can be short lived, while longer lifetimes are desirable for solar cell applications. It is possible to enhance the lifetime of a CSS by extending the system and obtaining charge separation through multiple ET reactions, and these charges can then be used to produce electricity in the solar cell. 18 The use of small molecules also has other desirable properties. Once the synthesis of the small

4


Page 4 of 33

Page 5 of 33

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


molecule have been established it is relatively easy to reproduce the product, and high purity can be reached. The electronic structure properties of the triad molecules can be tuned via functionalization with suitable substituents, so as to increase the absorption in the AM 1.5 solar spectrum range, 19 and enhanced desirable ET reactions. Multi-step electron transfer relies on π-conjugation and units with mutually suitable electronic properties. These units should be arranged so that the transfer process can follow a redox gradient, and should lead to spatially separated charges. 20,21 Via a multi-step transfer reaction, the spatial distance between electron and hole and the lifetime of the CSS is increased compared to a single step process. Unfortunately, since each electron transfer must follow the redox gradient, it is also related to a loss in energy. This means that a triad system must be designed with these contrasting effects in mind.

Figure 2: 3D structure of SubPc functionalized at the axial position with benzene, and schematic of the SubPc chromophore illustrating possible peripheral and axial functionalization positions. We investigate a number of chromophore systems based on boron subphthalocyanine (SubPc) derivatives. The SubPc molecule is aromatic, but adopts a non-planar cone-like structure as seen in Figure 2.

The boron atom in the SubPc structure is surrounded

by three coupled benzoisoindole units giving in total a 14 π electron aromatic macro cycle. 19,22,23 This leads to interesting electronic and spectroscopic properties. 23 Furthermore, SubPcs have high solubility and low tendency to aggregate because of their cone-shaped structure. 19 The synthesis of SubPc and SubPc derivatives has been developed over the last two decades. 22,24–27 The molecular structure of SubPcs can be varied to a high degree with 5




Page 6 of 33

Page 7 of 33




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

Page 8 of 33

a dielectric medium based on chloroform (CHCl3 ) using the integral equation formalism variant of the Polarizable Continuum Model (IEF-PCM) 43,44 module in Gaussian09. In order to check that the structure is indeed a minimum energy structure, it is confirmed that all eigenvalues of the Hessian are positive. In the IEF-PCM model, the solvent is modeled as an isotropic continuous medium, where the dielectric constant ǫs of the medium is based on the solvent molecule to be simulated, in this case CHCl3 with ǫs = 4.7113. 45 The UV-Vis spectra of the optimized Cs are then calculated, where the spectra in vacuum are based on the geometries optimized in vacuum and the CHCl3 spectra are based on the structures optimized in the continuum model CHCl3 . In order to construct the UV-Vis spectra the calculated transition energies from the time dependent DFT (TD-DFT) are used as the center of peaks around which a Gaussian band shape is assumed, and where the maximum is related to the oscillator strengths. In the case of the PCM CHCl3 spectra, the PCM module was also used in the TD-DFT calculation. The standard deviation been used in the spectra presented here is 0.4 eV. Since the spectrum in most cases will be constituted by more than one excitation, the collected spectrum must be investigated for all wavelengths of interest, say from λ = 250 to λ = 650nm, and summed over all the transitions (n) found using TD-DFT in Gaussian09. The collected absorption at λ, ǫ(˜ ν ), from all transitions is then calculated following Eq. (1), where the sum is transition in the TD-DFT calculation, and in this case n = 38. n X

f j NA e 2 ǫ(˜ ν) = ǫj (˜ ν) = 2 ln(10)me c2 ǫ0 σ j=1

r

2 ln(2) j exp −4 ln(2) (˜ ν − ν˜if )/σ π

(1)

where e is the elemental charge, NA is Avogadro’s number, c is the speed of light, me j are the oscillator strength and wave number calculated is the electron mass and f j and ν˜if

using linear response and finally ν˜ is the independent variable over which the spectra are simulated. 46–48 This relationship is found by relating the integrated absorption and the oscillator strength as 48

8


Page 9 of 33

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


4me c2 ǫ0 f= ln(10) NA e 2

Z

ν2

ǫ d˜ ν

(2)

ν1

After calculating the UV-Vis spectra of the chromophores, the energy alignment of selected D and A units are investigated. In order to make a simple model of a triad system, and to identify relevant electron donor/acceptor units in relation to the investigated SubPcs, a number of candidates are investigated, see Figure 4, at the CAM-B3LYP/6-311G(d) level of theory, In Figure 1, the energy levels to be characterized for a simple model of the triad system can be seen. In a triad system multiple ET reactions will need to occur to cause charge separation, one ET from D to to SubPc and one ET from SubPc* to A. The order in which these reactions occur will influence the character of the three moieties D, A and C. It is assumed that the ET is photo induced, so that before any ET reaction occurs the SubPc has been photo excited. Depending on the relative time scales for the two ET reactions, three possible situations may arise: (i) ET between D and C is faster than ET between C* and A (ii) ET between C* and A is faster than ET between D and C (iii) ET between D and C occurs simultaneously with ET between C* and A In (i) the charge separation process between D and A can be described via the reactions D + C* −−→ D+ + C – and C – + A −−→ C + A – . In (ii) the process can be described via C* + A −−→ C+ + A – and C+ + D −−→ C + D+ . Finally, in (iii) the reaction becomes D+C* +A −−→ D+ +C+A – . Based on this, the energies of charged and neutral states of the moieties are needed, as well as the excited energy level of the SubPc in order to investigate the relevant energy alignment. We assume that the Franck Condon approximation is valid in these systems. This means that the electron transition is vertical between ground state and electronically excited state 9



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

of the SubPc units. Spectroscopic investigations of SubPc derivatives have shown that the Stokes-shift in these structures is small., 23,49,50 hence dipole moments of the ground and excited states are practically the same. Therefore, little to no reorganization occurs upon photo excitations, and thus the ground and excited state geometries are expected to be nearly identical. In order to calculate the energies of the cat- and anion of the chromophores the timescale of ET vs. reorganization of the ion needs to be considered. If the ET reaction is fast relative to relaxation of the ion, then the ion will have the same geometry as the neutral chromophore during the ET. Therefore, the optimized neutral geometries including PCM solvent effects are used in single point calculations where the charge is changed to +1 and -1. If on the other hand relaxation is faster than ET then the geometry of the ion should be relaxed. This is done by optimizing the charged structures, and again PCM solvent effects were included. In the ET processes evaluated in this article a number of physical properties are included. Therefore, one needs to carefully consider the difference between the optical transition taking place in the chromophore compared to the reduction/oxidation of acceptor/donor. In other words, one needs to consider the difference between optical and electrochemical band gap. In our studied systems, the chromophore is photo excited before ET can occur, and therefore the band gap of interest in the chromophore is the optical band gap, which is modeled via the energy of the lowest energy allowed transition obtained through TD-DFT calculations. This energy is added to the Kohn-Sham HOMO orbital energy (KS-HOMO) giving a representation of the HOMO-LUMO of the chromophore moieties. On the contrary, donor and acceptor are not assumed to interact with the electromagnetic field, and therefore the only occupation of the virtual levels of the acceptor is via ET from the chromophore to the acceptor. This is a fundamentally different process and therefore the energy gap of interest to find the virtual level in the acceptor is the electrochemical energy gap. This energy gap is calculated as the difference between the ionization potential (IP) and electron affinity (EA),

10


Page 10 of 33

Page 11 of 33

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


which in turn are calculated as

EA = E0 − E0−

(3)

IP = E0+ − E0

(4)

where E0+ is the total energy of the neutral geometry with a positive charge and the same for −. According to Koopmans theorem, 51 the IP and the HOMO are related through IP = −ǫHOM O where ǫHOM O is the orbital energy of the HOMO level. Unfortunately this relation only holds for exact DFT. 8,52–54 As the approximation of DFT introduced an error in the calculation on both the optical and the electrochemical transition, a choice has to be made. In order to compare the energy levels of the photo excited donor with the energy levels of the electrochemically reduced acceptor the error in the calculation of the energy levels should be the same. We choose in this article to use the KS-HOMO as HOMO for all structures and the HOMO+TD-DFT energy of the lowest energy transition as LUMO for both Cs and As. The relationship between the Kohn Sham HOMO (KS-HOMO) and -IP of the chromophores as well as energy alignments can be seen in section ’Energy alignments’. In the calculations presented here D, C and A moieties are considered as three single molecules and the molecular orbital energies of the different charge states of these molecules are then compared. This leads to a computationally cheaper approach, and based on the results presented here, an initial guess for a full triad for future work can be made.

Results and discussion UV-Vis spectra Considering the spectra in the top plot of Figure 5, it is clear that the smaller Cs based on a single SubPc unit have two pronounced absorptions.

11




Page 12 of 33

Page 13 of 33

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


axial position, but that the bridging unit between SubPc and the substituent is increased from C3 to C5. Looking at the top plot in Figure 5 the effect on the absorption peak can be seen as a red-shift of the maximum absorption, but this effect is again in the UV-region, and is not expected to greatly influence the effectivness in solar cell application. All of the Cs from C1-C7 also have a absorption peak at ≈ 480 nm. Looking at the structure of C1 it can be seen that this SubPc is functionalized with only a phenyl-group in the axial position. This give rise to part of the absorption around 250nm, seen as a peak in the upper plot of figure 5. Investigation of the contributing molecular orbitals from the TD-DFT calculations shows that the orbitals related to the long wavelength transition is completely localized to the SubPc unit, and therefore the absorption band at 480 nm can be related to the SubPc unit. As seen from the plots in figure 5, all chromophores show absorption in the region close to 260 nm, but shifted according to the functionalization. This absorption in the UV involves multiple molecular orbitals as seen in table 1-22 of the Supporting Information (SI). Part of these transitions are still from orbitals localized on the SubPc unit, but part of the density is also on the functionalization group. Therefore, functionalization of the axial position influences the absorption in UV region. On the contrary, functionalization on the axial position does not influence the long wavelength absorption to any great extent, and it is therefore possible to take advantage of this position in furture work, for example by including anchoring without greatly influencing the absorption in the visible region. The bottom plot of Figure 5 contains the calculated absorption spectra of the Cs with two SubPc units. These Cs have an absorption peak at 480, 325 and at 260 nm. A few of these structures have an additional peak at 390 nm. Looking at the data in Table 1 the oscillator strength of C1 and these larger Cs for the 480 nm transition can be compared. It can be seen that all the Cs with two SubPcs have oscillator strengths of almost twice the size of the oscillator strength of C1. This indicates that this absorption is related to just the two SubPcs and that the bridging units incorporated into the SubPc dimers have only a very limited influence on this absorption. The complete overview of the oscillator strengths

13




Page 14 of 33

Page 15 of 33

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


In the SI of Ref. 27 the absorption of C2-C5, C7-C9 and C11 are reported. In that article it can be seen that the measured maximum absorption wavelength is 563 nm for all of these structures. Comparing with the longest wavelength transitions calculated using TD-DFT, see Table 2a, it can be seen that there is a difference of approximately 0.38 eV (60 nm), where the calculated transitions are blue shifted relative to the measured ones. As in the measured absorption spectra, it can be seen that functionalizing the SubPc(s) does not shift the maximum absorption wavelength, but it does influence the intensity of the transition. In the measured data, all of the chromophores have two absorption bands, one in the 500 nm regime and one close to 300 nm. This is also reproduced in the calculated absorption spectra. As can be seen from Table 2b, the intensities of the single SubPc chromophores are smaller than for the chromophores with two SubPc units, as seen in the TDDFT results. In Table 2b it can be seen that all of the measured spectra had highest intensity at the long wavelength absorption, which was not captured by the TD-DFT calculations, however given the scope of the article, we focus mainly on the position of the peaks not on the intensites. Based on these comparisons, the simulated spectra are blue shifted compared to the experimental ones, but the trends in the absorption can be reproduced using CAM-B3LYP/6311G(d). Finally, since the simulated UV-Vis spectra are calculated both in vacuum and in polar solvent, the effects of adding a PCM solvent can be discussed as well. A combined plot of the absorption in vacuum and PCM-CH3 Cl can bee seen in figure 1S in the SI. It can be seen from figure 5, and 6 that, the calculated absorption peaks in the (near)-visible range are red-shifted when the chromophores are placed in PCM CHCl3 , a polar medium . Tables 1 and 2a confirmed this, as the lowest energy absorption is seen to change from ≈ 480 nm in vacuum to ≈ 500 nm in CHCl3 , a shift of 0.10 eV. The oscillator strengths of these transitions are also increased from around 0.7 for the strongest transition in vacuum to 0.9 for the transition in CHCl3 . Based on Eq. (1) this gives rise to a shift in

15



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

Page 16 of 33

intensity from 1.0 × 105 M −1 cm−1 for the monomers to 1.3 × 105 M −1 cm−1 for dimers. Table 2: Calculated and measured transitions in C2-C5, C7-C9 and C11 in CHCl3 (a) Calculated wavelength and oscillator strength of the two transitions with higehst oscillator strength in PCM modelled CHCl3 in the region 250-650 nm

Name

C2

C4

C7

C9

Wavelength (nm)

Oscillator Strength

497.6

0.479

261.7

0.818

496.0

0.478

261.8

0.810

495.3

0.475

261.7

0.849

500.5

1.039

263.2

2.0305

Name

Wavelength (nm)

Oscillator Strength

497.7

0.477

267.8

0.867

496.2

0.481

305.4

0.832

495.9

0.949

261.5

1.5977

496.2

0.885

261.8

1.638

C3

C5

C8

C11

(b) Measured wavelength and intensity of the two absorption bands in CHCl3 from SI Ref. 27

Name

C2

C4

C7 C9

Wavelength Intensity Name (nm) (105 M−1 cm−1 ) 563.0

0.750

310.0

0.370

563.0

1.300

310.0

0.680

563.0

0.700

310.0

0.350

563.0

1.900

310.0

1.100

Wavelength Intensity (nm) (105 M−1 cm−1 )

C3

C5

C8 C11

16


563.0

0.820

300.0

0.610

563.0

1.200

310.0

0.800

563.0

1.900

310.0

0.900

563.0

2.300

310.0

1.600

Page 17 of 33





Page 18 of 33

Page 19 of 33

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


one would not expect them to. These investigations present fundamental problems of the identification of the molecular orbitals used in this paper. The difference in energy of the IP-EA gap relative to the TD-DFT predicted transition is on the order of 10 kcal/mol ≈ 0.4 eV for the monomer, where experimental results show that they should be identical. The IP and KS-HOMO follow almost the same trend for the investigated systems, but here there is a constant error in the order of 19 kcal/mol ≈ 0.8 eV, as seen in figure 7. It is expected that the size of similar errors will differ substantially based on the structure of the investigated system. Therefore, a comparison where KS-HOMO and TD-DFT results are used on the Cs and -IP/-EA are used on the Ds and As would not be a suitable choice. Instead we choose to use KS-HOMO for all moieties. The LUMO for both Cs and As are made by adding the TD-DFT lowest energy transition to the KS-HOMO. The smaller Cs are collectively represented by C1, since the HOMO energies of C1-C7 are very close in energy, see figure 7. The three possible situations (Scheme (i) - (iii)) as described in the Methods section will be analyzed one by one. Since the ET reactions can occur with different rates, the HOMO orbital energy of C + s and C − s may also be needed. If the second ET reaction is sufficiently fast, C does not undergo reorganization, in spite of the extra charge, and then the geometry of the cat- and anion should be identical to the neutral C. Else, if the ET is slower than reorganization, the ions should be considered in their respective relaxed geometries.

Scheme (i) In Scheme (i) the D to C* ET happens first, thus we focus on the reactions D + C* −−→ D+ + C – and C – + A −−→ C + A – . The upper plot of Figure 9 shows ǫhomo of the investigated D’s and C1, C8-C11. In this plot it is assumed that C has been photo excited so that an electron can be thought of as having left the HOMO, leaving behind a hole to be occupied by the electron of D. As reported in table 1S - 22S of the SI, the orbitals involved in the longest wave length transition 19




Page 20 of 33

Page 21 of 33




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

In Figure 10 energy alignments related to scheme(ii) can be seen. In this scheme the ET between C* and A is faster. In all Cs LUMO and LUMO+1 are at the same energy. Again the LUMO of all the As is lower in energy than the LUMO of the Cs and based on these all investigated As could therefore be used. The picture of the D-C ET on the other hand is changed. Any D can perform an ET to C1+ . However, too large an energy drop is also not sought after because this leads to a larger energy lost, and thus C1 may not be the best candidate. Only D5 and D6 can achieve an ET to C8+ , C10+ and C11+ . Finally, only D5 can confidently perform an ET to C9 in both the neutral and relaxed geometry of the cation. If the ET is sufficently fast that C9 does not relax all donors but D1 can be used. By considering those C+ s that can allow a definite ET, and the choices already made in scheme (i), a triad system constituted by D5/D6 - C8/C10/C11- A2/A3/A4 should represent viable options.

Scheme (iii) In the last scheme the two ET reactions occur simultaneously. For clarity Figure 2S in the SI shows the two relevant plots again. In this scheme the triad D5/D6 - C8/C10/C11A2/A3/A4 would also have suitable energy aliments.

Conclusions In this paper 11 chromophore structures based on SubPc are investigated as well as 6 electron donors and 4 electron acceptors, see figures 3, 4. The SubPc units are to be used as central chromophore moieties in molecular triads. TD-DFT results showed that the absorption of all the investigated chromophores is within the visible range of the solar spectrum, see figures 5,6 and therefore these units will be suitable as the solar absorber in solar cell applications. Combining this with the relative ease with which these compounds can be synthesized as well as their low tendency to aggregate, the SubPcs are expected to be good candidates. By

22


Page 22 of 33

Page 23 of 33

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


creating a molecular triad it is expected that the lifetime of the charge separated state will be increased, and therefore the efficiency of a future solar cell. Among the investigated chromophores, the compounds C8-C11 have the largest simulated intensities in the visible part of the spectrum. The lowest energy absorption is at approximately the same wavelength for all the investigated Cs, indicating that the higher absorption intensity of the larger chromophores is the result of additive effects by combining two SubPc units. When comparing the simulated spectra with experimental ones it was shown that the same trends could be identified, but that all the simulated spectra are blue shifted by 0.38 eV (60 nm) compared to the experimental ones, see table 2a and 2b. Repeating the spectra simulations, including solvent effects from CHCl3 by using a PCM modeling, showed that a polar solvent induces an increase in the absorption intensity and red shift of ca. 0.1 eV (20 nm), table 1 and 2a. Computational work further showed that functionalization at the axial position had little or no effect on the longest wave length absorption. This can be used for future design of chromophore units, where either multiple chromophores can be coupled via the axial position, or the solubility of the molecular triad can be controlled via suitable side-groups in this position. In order to further characterize the potential moieties to be used in triad systems an investigation of relevant energy levels is performed. Since the molecular orbitals of interest here are the HOMO/LUMO of donor/acceptor and both for the chromophores, identification of these levels had to be clarified. Investigation of the Cs showed that the Kohn-Sham HOMO and the ionization potential of these structures have the same trends, but that a constant error of approximately 19 kcal/mol can be identified, where the IP is higher in energy. Based on experimental results 23,55,56 the electrochemical gap IP-EA and the optical gap of the SubPC monomers should be identical, but here an error of 10 kcal/mol can be found, with a large electrochemical gap relative to the optical, see figure 7. Therefore, the authors chose to use the optical gap, computed as the lowest energy transition in the TDDFT calculation, as the gap from HOMO to LUMO in all structures. KS-HOMO was used

23




Page 24 of 33

Page 25 of 33

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


Supporting Information Available In the Supporting information the absorption wavelength, oscillator strength, and contributing orbitals of all transistion in C1 to C11 in vacuum as well as PCM solvent CHCl3 is given i table S1-S22. Table S23-S26 gives the linear response information of A1 to A4 in PCM solvent CHCl3 . Figure 1S shows the absorption of C1 and C8-C11 in both vacuum and PCM CHCl3 in order to clarify the solvent effect on absorption. This material is available free of charge via the Internet at http://pubs.acs.org/.

25



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

References (1) Gautam, P.; Misra, R.; Siddiqui, S.; Sharma, G. Donor–acceptor–π–acceptor Based Charge Transfer Chromophore as Electron Donors for Solution Processed Small Molecule Organic Bulk Heterojunction Solar Cells. Org. Elec. 2015, 19, 76–82. (2) G¨ unes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated Polymer-based Organic Solar Cells. Chem. Rev. 2007, 107, 1324–1338. (3) Hoppe, H.; Sariciftci, N. S. Organic Solar Cells: An Overview. J. Mater Res 2004, 19, 1924–1945. (4) Roncali, J.; Leriche, P.; Blanchard, P. Molecular Materials for Organic Photovoltaics: Small is Beautiful. Adv.Mater. 2014, 26, 3821–3838. (5) Cao, W.; Xue, J. Recent Progress in Organic Photovoltaics: Device Architecture and Optical Design. Energy Environ. Sci. 2014, 7, 2123–2144. (6) Li, L.-L.; Diau, E. W.-G. Porphyrin-sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 291–304. (7) Grätzel, M. The Light and Shade of Perovskite Solar Cells. Nature Mater 2014, 13, 838–842. (8) Manna, A. K.; Balamurugan, D.; Cheung, M. S.; Dunietz, B. D. Unraveling the Mechanism of Photoinduced Charge Transfer in Carotenoid–Porphyrin–C60 Molecular Triad. JPCL 2015, 6, 1231–1237. (9) Abdulrazzaq, O. A.; Saini, V.; Bourdo, S.; Dervishi, E.; Biris, A. S. Organic Solar Cells: a Review of Materials, Limitations, and Possibilities for Improvement. Particul Sci Techno 2013, 31, 427–442. (10) Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, 6736–6767. 26


Page 26 of 33

Page 27 of 33

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


(11) Mazzio, K. A.; Luscombe, C. K. The Future of Organic Photovoltaics. Chem. Soc. Rev 2015, 44, 78. (12) Harlang, T. C.; Liu, Y.; Gordivska, O.; Fredin, L. A.; Ponseca Jr, C. S.; Huang, P.; Chábera, P.; Kjaer, K. S.; Mateos, H.; Uhlig, J. et al. Iron Sensitizer Converts Light to Electrons with 92% yield. Nature Chem. 2015, 883–889. (13) Wu, Q.; Van Voorhis, T. Constrained Density Functional Theory and its Application in Long-range Electron Transfer. J. Chem. Theory Comput. 2006, 2, 765–774. (14) Wu, Q.; Van Voorhis, T. Extracting Electron Transfer Coupling Elements from Constrained Density Functional Theory. J. Chem. Phys. 2006, 125, 164105. (15) Kaduk, B.; Kowalczyk, T.; Van Voorhis, T. Constrained Density Functional Theory. Chem. Rev 2012, 321–370. (16) Yeganeh, S.; Voorhis, T. V. Triplet Excitation Energy Transfer with Constrained Density Functional Theory. J. Phys. Chem. C 2010, 114, 20756–20763. (17) Lazorski, M. S.; Gest, R. H.; Elliott, C. M. Photoinduced Multistep Charge Separation in a Heteroleptic Cu (I) bis (phenanthroline)-based Donor–chromophore–acceptor Triad. J. Am. Chem. Soc 2012, 134, 17466–17469. (18) Wasielewski, M. R. Energy, Charge, and Spin Transport in Molecules and Selfassembled Nanostructures Inspired by Photosynthesis. J. Org. Chem. 2006, 71, 5051– 5066. (19) Mauldin, C. E.; Piliego, C.; Poulsen, D.; Unruh, D. A.; Woo, C.; Ma, B.; Mynar, J. L.; Frechet, J. M. Axial Thiophene- Boron (subphthalocyanine) Dyads and Their Application in Organic Photovoltaics. ACS Appl. Mater. Interfaces 2010, 2, 2833–2838. (20) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. Modulating Charge Separation and Charge Recombination Dynamics in 27



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

Porphyrin-fullerene Linked Dyads and Triads: Marcus-normal Versus Inverted Region. J. Am. Chem. Soc 2001, 123, 2607–2617. (21) Imahori, H.; Mori, Y.; Matano, Y. Nanostructured Artificial Photosynthesis. J. Photochem. Photobiol. C 2003, 4, 51 – 83. (22) Claessens, C. G.; González-Rodr´ıguez, D.; Torres, T. Subphthalocyanines: Singular Nonplanar Aromatic Compounds Synthesis, Reactivity, and Physical Properties. Chem. Rev. 2002, 102, 835–854. (23) Claessens, C. G.; Gonzalez-Rodriguez, D.; Rodriguez-Morgade, M. S.; Medina, A.; Torres, T. Subphthalocyanines, Subporphyrazines, and Subporphyrins: Singular Nonplanar Aromatic Systems. Chem. Rev. 2013, 114, 2192–2277. (24) Kobayashi, N.; Kondo, R.; Nakajima, S.; Osa, T. New Route to Unsymmetrical Phthalocyanine Analogs by the use of Structurally Distorted Subphthalocyanines. J. Am. Chem. Soc 1990, 112, 9640–9641. (25) Sastre, A.; Torres, T.; Hanack, M. Synthesis of Novel Unsymmetrical Monoaminated Phthalocyanines. Tetrahedron Lett. 1995, 36, 8501–8504. (26) Sastre, A.; Torres, T.; Diaz-Garcia, M.; Agulló-López, F.; Dhenaut, C.; Brasselet, S.; Ledoux, I.; Zyss, J. Subphthalocyanines: Novel Targets for Remarkable Second-order Optical Nonlinearities. J. Am. Chem. Soc 1996, 118, 2746–2747. (27) Gotfredsen, H.; Jevric, M.; Kadziola, A.; Nielsen, M. B. Acetylenic Scaffolding with Subphthalocyanines. Eur J Org Chem 2016, 17–21. (28) Gonzalez-Rodriguez, D.; Carbonell, E.; de Miguel Rojas, G.; Castellanos, C. A.; Guldi, D. M.; Torres, T. Activating Multistep Charge-Transfer Processes in FullereneSubphthalocyanine-Ferrocene Molecular Hybrids as a Function of π - π Orbital Overlap. J. Am. Chem. Soc 2010, 132, 16488–16500. 28


Page 28 of 33

Page 29 of 33

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


(29) KC, C.; Lim, G.; Zandler, M.; D’Souza, F. Synthesis and Photoinduced Electron Transfer Studies of a Tri(Phenothiazine) – Subphthalocyanine – Fullerene Pentad. Org.Lett 2013, 15, 4612–4615. (30) Isaacs, E.; Sharifzadeh, S.; Ma, B.; Neaton, J. Relating Trends in First-Principles Electronic Structure and Open-Circuit Voltage in Organic Photovoltaics. J. Phys. Chem. Lett. 2011, 2, 2531–2537. (31) Gotfredsen, H.; Jevric, M.; Broman, S. L.; Petersen, A. U.; Nielsen, M. B. Aluminum Chloride Mediated Alkynylation of Boron Subphthalocyanine Chloride Using Trimethylsilyl-Capped Acetylenes. J. Org. Chem 2016, 81, 1–5. (32) Romero-Nieto, C.; Medina, A.; Molina-Ontoria, A.; Claessens, C. G.; Echegoyen, L.; Martin, N.; Torres, T.; Guldi, D. M. Towards Enhancing Light HarvestingSubphthalocyanines as Electron Acceptors. Chem. Commun. 2012, 48, 4953–4955. (33) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange–correlation Functional using the Coulomb–atttenuating Method CAM–B3LYP. Chem. Phys. Lett. 2004, 393, 51–57. (34) Tawada, Y.; Tsuneda, T.; Yanagisawa, S.; Yanai, T.; Hirao, K. A Long-range-corrected Time-dependent Density Functional Theory. J. Chem. Phys 2004, 120 . (35) Raghavachari, K.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. 20. Basis set for correlated wave-functions. J. Chem. Phys. 1980, 72, 650–54. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09 Revision D.01. Gaussian Inc. Wallingford CT 2009.

29



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

(37) Tortorella, S.; Talamo, M.; Cardone, A.; Pastore, M.; De Angelis, F. Benchmarking DFT and Semi-empirical Methods for a Reliable and Cost-efficient Computational Screening of Benzofulvene Derivatives as Donor Materials for Small-Molecule Organic Solar Cells. J. Phys. Cond. Matt. 2016, 28, 074005. (38) Bhatta, R.; Pellicane, G.; Tsige, M. Tuning Range-separated DFT Functionals for Accurate Orbital Energy Modeling of Conjugated Molecules. Comp. Theo. Chem. 2015, 1070, 14–20. (39) Olsen, S.; Elm, J.; Storm, F.; Gejl, A.; Hansen, A.; Hansen, M.; Nikolajsen, J.; Nielsen, M.; Kjaergaard, H.; Mikkelsen, K. Computational Methodology Study of the Optical and Thermochemical Properties of a Molecular Photoswitch. J. Phys. Chem. 2015, 119, 896–904. (40) Limacher, P.; Mikkelsen, K.; Luthi, H. On the Accurate Calculation of Polarizabilities and Second Hyperpolarizabilities of Polyacetylene Oligomer Chains using the CAMB3LYP Density Functional. J. Chem. Phys. 2009, 130, 194114. (41) Dreuw, A.; Weisman, J. L.; Head-Gordon, M. Long-range Charge-transfer Excited States in Time-dependent Density Functional Theory Require Non-local Exchange. J. Chem. Phys. 2003, 119, 2943–2946. (42) Dreuw, A.; Head-Gordon, M. Failure of Time-dependent Density Functional Theory for Long-range Charge-transfer Excited States: the Zincbacteriochlorin-bacteriochlorin and Bacteriochlorophyll-spheroidene Complexes. J. Am. Chem. Soc 2004, 126, 4007– 4016. (43) Tomasi, J.; Mennucci, B.; Cances, E. The IEF Version of the PCM Solvation Method: an Overview of a New Method Addressed to Study Molecular Solutes at the QM ab initio level. J MOL STRUC-THEOCHEM 1999, 464, 211–226.

30


Page 30 of 33

Page 31 of 33

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


(44) Cossi, M.; Barone, V. Time-Dependent Density Functional Theory for Molecules in Liquid Solutions. J. Chem. Phys. 2001, 115, 4708–4717. (45) Gaussian09, List of Defined Solvents. 2014; http://www.gaussian.com/g_tech/g_ur/ k_scrf.htm, Accessed: 2016-05-13. (46) Atkins, P.; de Paula, J.; Friedman, R. Physical Chemistry Quanta, Matter and Charge, 2nd ed.; Oxford, 2014; pp 381 – 382. (47) Atkins, P.; Friedman, R. Molecular Quantum Mechanics, 5th ed.; Oxford, 2011; pp 432 – 433. (48) GaussSum, Convoluting UV-Vis spectra using oscillator strength. 2009; http:// gausssum.sourceforge.net/GaussSum_UVVis_Convolution.pdf, Accessed: 2016-0513. (49) Morse, G. E.; Bender, T. P. Boron subphthalocyanines as organic electronic materials. Acs. Appl. Mater. Inter. 2012, 4, 5055–5068. (50) Martin, G.; Rojo, G.; Lopez, F. A.; Ferro, V. R.; de la Vega, J. M. G.; MartinezDiaz, M. V.; Torres, T.; Ledoux, I.; Zyss, J. Subphthalocyanines and Subnaphthalocyanines: Nonlinear Quasi-Planar Octupolar Systems with Permanent Polarity. J. Phys. Chem. B 2002, 106, 13139–13145. ¨ (51) Koopmans, T. Uber die Zuordnung von Wellenfunktionen und Eigenwerten zu den Einzelnen Elektronen Eines Atoms. Physica 1934, 1, 104 – 113. (52) Kl¨ upfel, P.; Dinh, P. M.; Reinhard, P.; Suraud, E. Koopmans’ condition in selfinteraction-corrected density-functional theory. Phys. Rev. A 2013, 88, 052501. (53) Phillips, H.; Zheng, Z.; Geva, B.; Dunietz, B. D. Orbital gap predictions for rational design of organic photovoltaic materials. Org. Elec. 2014, 15, 1509 – 1520.

31



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

(54) Stein, T.; Eisenberg, H.; Kronik, L.; Baer, R. Fundamental Gaps in Finite Systems from Eigenvalues of a Generalized Kohn-Sham Method. Phys. Rev. Lett. 2010, 105, 266802. (55) Ebenhoch, B.; Prasetya, N. B. A.; Rotello, V. M.; Cooke, G.; Samuel, I. Solution-processed boron subphthalocyanine derivatives as acceptors for organic bulkheterojunction solar cells. J. Mat. Chem. A 2015, 3, 7345–7352. (56) Sullivan, P.; Duraud, A.; Hancox, l.; Beaumont, N.; Mirri, G.; Tucker, J. H.; Hatton, R. A.; Shipman, M.; Jones, T. S. Halogenated Boron Subphthalocyanines as Light Harvesting Electron Acceptors in Organic Photovoltaics. Adv. En. Mat. 2011, 1, 352– 355.

32


Page 32 of 33

Page 33 of 33



Boron Subphthalocyanine Based Molecular Triad Systems for the

Recommend Documents