Effects of Peripheral and Axial Substitutions on Electronic Transitions

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Effects of Peripheral and Axial Substitutions on Electronic Transitions of Tin Naphthalocyanines Elena Jakubikova,† Ian H. Campbell,‡ and Richard L. Martin*,‡ † ‡

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

bS Supporting Information ABSTRACT: Tin naphthalocyanine molecules display strong absorption in the infrared region (IR), making them ideal as components of organic photodiodes and solar cells. We use density functional theory and time-dependent density functional theory (TD-DFT) at the B3LYP level to study the influence of axial and peripheral ligands on the absorption wavelength of tin naphthalocyanines. We find that TD-DFT is successful at reproducing the experimental absorption spectra of free base naphthalocyanine and tin naphthalocyanine molecules and can be used as a reliable tool to predict absorption spectra of substituted naphthalocyanines. Functional groups attached axially to tin (-F, -Cl, -Br, -I) and peripherally to the inner ring (-F, -Cl, -Br, -Ph, -OH, -COCH3, -O(CH2)3CH3) of the tin naphthalocyanine molecule tune the excitation wavelength in the nearinfrared region between 770 and 940 nm. While substituents to the outer naphthalocyanine ring (-Cl, -Br) affect the intensity of the absorption peaks in the NIR region, they do not influence their absorption wavelength. Asymmetric substitution of naphthalocyanine pendant arms can be employed to decrease intensity of the absorption peaks in the visible region with respect to the intensity of the peaks in the NIR.

’ INTRODUCTION Organic photodiodes are the focus of much current research as promising components of solar cells and for large area applications.1 6 Most organic semiconductors have large energy gaps (i.e., energy difference between the HOMO and LUMO energy levels: HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital) and do not have strong absorption in the infrared (IR) region.7 It is desirable to extend organic photodiode sensitivity into the NIR for both solar energy and communications applications. Additionally, due to the molecular electronic structure of organic semiconductors, infrared absorbing organic photodiodes can be highly transparent in the visible spectrum. This is in contrast to conventional inorganic semiconductors, such as Si and GaAs, which have wide electronic energy bands so that if they absorb in the infrared region, then they also absorb in the visible. Transparent photodiodes are of potential interest for building integrated solar cells,8 as well as optical alignment and control systems.9 The wavelength of optical transitions, their extinction coefficients, and the absolute alignment of the HOMO and LUMO energy levels are among the properties that determine the overall functionality and efficiency of an organic semiconductor. Transparency in the visible region can also be important, depending on the intended application. For solar cell applications, one can determine optimal semiconductor energy gaps for use in one, two, and three material based solar cell designs.10 These r 2011 American Chemical Society

optimum energy gaps correspond to optical absorption transitions of about 920 nm (one material), 800 and 1320 nm (two materials), and 710, 1050, and 1650 nm (three materials). To make an organic photodiode or solar cell with high quantum efficiency, it is desirable to have strong optical absorption such that the ratio of the exciton diffusion length to the optical absorption depth can approach unity.7 Exciton diffusion lengths in organic materials can vary from about 5 to 40 nm,7 depending on the degree of order in the organic film. Organic thin film materials have molecular densities of ∼10 3 mol/cm3 so an extinction coefficient of 3 105 M 1 cm 1 corresponds to an optical absorption depth of ∼30 nm. The range of extinction coefficients required for an efficient photodetector is therefore 2.5 20 105 M 1 cm 1 (40 5 nm exciton diffusion lengths). The absolute HOMO and LUMO energy levels of organic materials are also important for device applications. For example, in many applications, the HOMO energy should be in a range that can be electrically contacted by typical transparent electrodes. This corresponds to an energy range of about 4.5 to 6 eV below vacuum.11,12 Similarly, the LUMO should be able to be electrically contacted by low work function metallic electrodes. This corresponds to an energy range of about 3 4.3 eV below vacuum.13 Received: April 1, 2011 Published: August 01, 2011 9265

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Figure 3. Calculated absorption spectra of naphthalocyanine molecules in dichlorobenzene at B3LYP level of theory.

Figure 1. Optimized geometries of naphthalocyanines considered in this study.

have infrared transitions ranging from 770 to 950 nm with extinction coefficients between 2.5 and 4.5 105 M 1 cm 1 and are thus well suited for use as a component in one or two material based solar cell designs.

’ METHODOLOGY

Figure 2. Ground state frontier orbitals of bare naphthalocyanine in vacuum.

We have recently used naphthalocyanines to demonstrate two types of IR organic photodiodes: (1) diodes with photoresponse out to ∼1100 nm with a photoconductive gain of ∼10 at 1000 nm under 5 V reverse bias14 and (2) diodes with a transparency of ∼80% throughout the visible spectrum with ∼80% external quantum efficiency in the IR.15 An attractive feature of naphthalocyanine complexes is the tunability of their electronic properties either by insertion of a central metal atom into the inner porphyrazine ring,16 axial substitutions to the central metal atom,17 19 or substitutions of various functional groups onto the pendant arms of the naphthalocyanine.20 23 Naphthalocyanines are very closely related to the phthalocyanine compounds that exhibit great tunability of the near-IR absorption peak wavelength via modifications of their macrocycle through substitutions with various groups.24 29 Therefore, the general trends in the IR absorption wavelength shifts displayed by the substituted phthalocyanines hold for the naphthalocyanines as well.24 In this work, we use density functional theory (DFT) to investigate the influence of various substituents on tuning the wavelength and intensity of optical absorption transitions of tin naphthalocyanines in the infrared region, as well as suppressing transitions in the visible region to increase the transparency. We find that the substituted tin naphthalocyanines discussed here

Experimental Section. The naphthalocyanines were obtained from Sigma-Aldrich (octabutoxy naphthalocyanine, [Nc(OBu)8]2+, and tin naphthalocyanine, [SnNc]0) and Frontier Scientific (octabutoxy tin naphthalocyanine dichloride, [SnNcCl2(OBu)8]0) and used as received. They were processed in an argon atmosphere glovebox with 1 ppm O2 and H2O. Thin films on quartz substrates were formed either by spin-casting from chlorobenzene solutions (5 mg/mL) or thermal evaporation in vacuum. A spectroscopic ellipsometer (J.A. Woollam Co., M-2000) was used to measure the spectrum of thin films. Optical absorption measurements of naphthalocyanine molecules were made using dilute solutions (few μg/mL) in chlorobenzene on Varian Cary 50 Bio UV vis spectrophotometer. It is important to note that tin naphthalocyanine has a propensity to aggregate in solution. To avoid the aggregation, it was first dissolved in 1-bromonaphthalene and stirred at 130 °C for 12 h. The resulting solution was then dissolved in chlorobenzene before measuring the UV vis spectrum. Computational Section. We have performed geometry optimization of all molecules discussed in this paper at the B3LYP level of theory30,31 using the LANL08+d basis set and effective core potential32,33 on Sn and 6-31G* basis set34,35 for all other atoms. The Gaussian 09 computational package36 was used to carry out all calculations. All ground state optimizations were performed in vacuum. Time-dependent density functional theory37 39 (TD-DFT) was applied to obtain the absorption spectra of all naphthalocyanine molecules in dichloromethane. Solvent effects were included in all TD-DFT calculations via the polarizable continuum model40 (PCM). Absorption spectra were simulated using the Lorentzian line shape by convoluting the spectrum composed of the δ-functions at the excitation energies times the oscillator strengths with the half width at halfmaximum (hwhm) equal to 0.04 eV. The resulting spectra were scaled by a factor of 1/4.33 109 to obtain the intensities measured in the molar extinction coefficient ε.41 9266

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Figure 5. Ground state molecular orbitals involved in the Q-band electronic transitions of octabutoxy naphthalocyanine.

Figure 4. Experimental and calculated (B3LYP level of theory) spectra of octabutoxy naphthalocyanine. The y-axis shows intensities obtained from TD-DFT calculations. The intensity of the experimental spectrum is shown in arbitrary units in which the intensity of the IR peak was set equal to the intensity of the IR peak obtained from the TD-DFT calculations.

’ RESULTS AND DISCUSSION In this section we first present experimental and theoretical results for naphthalocyanine and tin naphthalocyanine molecules that allow us to benchmark our computational approach. Next, we explore the effects of various substituents on the absorption spectrum of a tin naphthalocyanine molecule. Naphthalocyanines. First, we have studied a series of bare and functionalized naphthalocyanine molecules, including a bare naphthalocyanine, octamethoxy naphthalocyanine, and octabutoxy naphthalocyanine molecules. Because the butoxy and methoxy side chains are quite flexible, there are many conformational isomers of substituted naphthalocyanines differing by the side chain orientation only. These conformations are very close in energy (∼0.5 kcal/mol) and their ground and excited state properties are virtually identical. We have explored substituted naphthalocyanines with two different conformations in our calculations. Figure 1 shows the optimized geometries of the naphthalocyanine molecules considered in this work. The ground state geometry of the bare unsubstituted naphthalocyanine is completely flat, while the introduction of bulky substituent groups in methoxy and butoxy naphthalocyanines results in the deformation of the naphthalocyanine ring. All naphthalocyanines were optimized in their ground singlet state in vacuum. Frontier orbitals (HOMO-3 LUMO+4) of the bare naphthalocyanine in vacuum are shown in Figure 2. Frontier orbitals of substituted naphthalocyanines are very similar to those shown in Figure 2, but they are shifted in energy due to the presence of substituent groups; methoxy and butoxy groups destabilize HOMO-3 HOMO orbitals due to their electron donating character (see Supporting Information). The calculated absorption spectra in chlorobenzene of the three naphthalocyanine molecules are shown in Figure 3. The presence of substituent groups induces a red shift of the Q-band from ∼750 nm (bare naphthalocyanine) to ∼850 nm (butoxy naphthalocyanine). We measured the absorption spectrum of octabutoxy naphthalocyanine in chlorobenzene, as well as the absorption spectrum of an octabutoxy naphthalocyanine film. The measured spectra are shown in Figure 4 along with the calculated spectrum of octabutoxy naphthalocyanine in chlorobenzene. Overall, there

is a good agreement between the experimental and theoretical spectrum and the positions of the major peaks are reproduced within 0.1 eV. The shoulder at ∼780 nm in the experimental spectrum arises from the vibronic transitions associated with the Q-band that are not described by the TD-DFT, so we do not expect to reproduce it in the calculated spectrum. The main disagreement arises from the presence of the infrared band (or Q-band) splitting in the theoretical spectrum, which is not present in the experimental spectrum. The electronic transitions that give rise to the IR band can be described as HOMO f LUMO and HOMO f LUMO+1 transitions, with a small contribution from transitions involving other orbitals. Both LUMO and LUMO+1 orbitals have π* character and are localized on the two opposing naphthalocyanine pendant arms (see Figure 5). Only two nitrogen atoms out of four in the inner porphyrazine ring coordinate a proton, which results in a small energy difference between the LUMO and LUMO+1 orbitals and a split in the theoretical spectrum. Interestingly, the Q-band splitting is observed in the experimental spectra of both porphyrazine and phthalocyanine compounds, while it is absent in the spectrum of naphthalocyanines.23 In an attempt to understand this phenomenon better, we have calculated the absorption spectrum of bare porphyrazine and phthalocyanine molecules. While our TD-DFT calculations correctly predict the Q-band splitting for a porphyrazine molecule, they disagree with the experimental result in the case of phthalocyanine by predicting a Q-band with only a single peak. This suggests that effects that contribute to the Q-band splitting are quite subtle and go beyond the LUMO/LUMO+1 energy difference. The level of theory used in this study is insufficient to describe the Q-band splitting for these compounds correctly, especially when the splitting is very small (at the order of hundredths of eV). The absorption spectrum of octabutoxy naphthalocyanine film is broader, with the IR peak shifted to lower wavelengths. We attribute these changes to the intermolecular interactions between the naphthalocyanine molecules, which are present when they are closely packed in the film. The overall shape of the spectrum in the IR and visible regions is very similar to the absorption spectrum of a single naphthalocyanine molecule in solution. These trends hold for [SnNc]0 and [SnNcCl2(OBu)8]0 compounds as well (see Figures 7 and 10), albeit the IR peak of the [SnNc]0 film is significantly broader. This is due to the absence of bulky substituents on the naphthalocyanine ring of [SnNc]0, which facilitates the intermolecular interaction between the molecules. The similarities between the absorption spectra of naphthalocyanines in solution and naphthalocyanine films suggest that the investigation of electronic structure and excited state properties of single molecules in solution is relevant to the photophysical properties of naphthalocyanine films. Tin Naphthalocyanines. We studied tin naphthalocyanine ([SnNc]0) and octabutoxy-substituted tin naphthalocyanine 9267



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Figure 8. Ground state molecular orbitals involved in the Q-band electronic transitions of [SnNc]0.

Figure 9. Optimized geometry of [SnNcCl2(OBu)8]0.

Figure 6. Optimized structures of tin naphthalocyanines.

Figure 10. Experimental and calculated (B3LYP level of theory) absorption spectrum of [SnNcCl2(OBu)8]0. The intensity of the experimental spectrum is shown in arbitrary units, in which the intensity of the IR peak was set equal to the intensity of the IR peak obtained from the TD-DFT calculations.

Figure 7. Experimental and calculated (B3LYP level of theory) absorption spectra of tin naphthalocyanine in chlorobenzene. The y-axis shows the intensities obtained from TD-DFT calculations. The intensity of the experimental spectrum is shown in arbitrary units, in which the intensity of the IR peak was set equal to the intensity of the IR peak obtained from the TD-DFT calculations.

dichloride ([SnNcCl2(OBu)8]0). The tin naphthalocyanine molecule has two possible conformations: flat, in which the tin atom occupies the same plane as the inner porphyrazine ring, and

puckered, in which the tin atom is out of plane relative to the naphthalocyanine ring that is also puckered (see Figure 6). DFT calculations predict the puckered conformation to be more stable by 59 kcal/mol. We have also explicitly included the first solvation shell, which consists of two molecules of chlorobenzene (CLB) in our calculations and found that chlorobenzene coordinates to the tin atom. The energy difference between the flat and puckered conformation is still relatively large (46 kcal/mol), with the puckered conformation predicted to be more stable. The calculated and measured absorption spectrum of the tin naphthalocyanine is shown in Figure 7. We have calculated absorption spectra of puckered conformation of naphthalocyanine molecule, with and without the presence of the first solvation shell. The calculated absorption spectrum matches the IR peak (Q-band) of the experimental tin naphthalocyanine spectrum 9268



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Figure 11. Ground state molecular orbitals involved in the Q-band electronic transitions of [SnNcCl2(OBu)8]0.

Figure 13. Absorption spectra of axially substituted naphthalocyanines obtained at B3LYP level of theory.

Figure 12. Tin naphthalocyanine molecule showing various substitution sites (R1 R3). Both octa- and tetra-substituted naphthalocyanines were considered on sites R2 and R3. Green boxes highlight sites substituted in tetra-substituted naphthalocyanines.

quite well. The addition of the first solvation shell explicitly has no influence on the calculated spectrum. The Q-band is due to two excitations, HOMO-3 f LUMO and HOMO-2 f LUMO. The corresponding molecular orbitals are shown in Figure 8. Octabutoxy-substituted tin naphthalocyanine dichloride, like its octabutoxy-substituted naphthalocyanine analogue, can have a number of conformations due to the conformational flexibility of its butoxy substituents. As these various conformers have very similar ground and excited state properties, we have performed the calculations on one conformer only. The ground state optimized geometry in vacuum is shown in Figure 9. Figure 10 shows the experimental and calculated absorption spectrum of this molecule in chlorobenzene. The Q-band in the absorption spectrum of this molecule is again due to two transitions, corresponding to HOMO f LUMO and HOMO f LUMO +1 transitions (see Figure 11). Overall, there is a good agreement between the calculated and measured experimental spectrum in chlorobenzene, suggesting that density functional theory at the B3LYP level is suitable for calculations of the ground and excited state properties of tin naphthalocyanine compounds. Substitution Effects on the Absorption Spectrum of Tin Naphthalocyanines. In the previous section we showed that TD-DFT calculations at the B3LYP level are successful in reproducing the experimental absorption spectrum of various naphthalocyanine molecules. Therefore, we can use them as a tool in exploring the effects various substituents have on the absorption spectrum of the tin naphthalocyanine molecule. Knowledge obtained from such calculations can be then used to guide design of naphthalocyanine molecules with the capability to absorb light of desired wavelengths.

Figure 14. Frontier molecular orbitals of [SnNcCl2]0. MOs of tin naphthtalocyanines substituted with F, Br, and I are very similar and are shown in the Supporting Information.

Figure 15. Calculated absorption spectra at the B3LYP level of theory in the infrared region for tetra- and octa-substituted tin naphthalocyanine dichloride.

Figure 12 shows a tin naphthalocyanine molecule indicating three substitution sites considered in this study. R1 corresponds to the axial substitution of tin atom. A series of halogens (F, Cl, Br, and I) were used as substituents in this position. R2 denotes substitution sites in the inner naphthalocyanine ring. Here we explored the effect of halogen substituents (F, Cl, Br), as well as phenyl (-Ph), hydroxyl (-OH), acyl (-COCH3), and butoxy (-O(CH2)3CH3). Both octa-substituted and tetra-substituted naphthalocyanines at R2 positions were investigated. Finally, R3 labels the substitution sites in the outer naphthalocyanine ring, where we explored effects of halogen substituents only (F, Cl). Both octa- and tetra-substituted naphthalocyanines at R3 positions were investigated. 9269



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Figure 16. Structures of asymmetrically substituted tin naphthalocyanines.

Figure 18. Frontier orbitals of asymmetrically substituted tin naphthalocyanine dichloride with five butoxy groups.

Figure 17. Calculated absorption spectra at the B3LYP level of theory for symmetrically and asymmetrically substituted tin naphthalocyanine dichloride. The plots with solid lines depict spectra of the asymmetrically substituted compounds, while the dotted lines represent symmetrically substituted compounds.

Comparison of the Halogen Substituent Effects in R1, R2, and R3 Positions. First, we explored the effects that axially

substituted halogens (F, Cl, Br, I) at R1 positions have on the absorption spectrum of tin naphthalocyanine. No other substituents were present at R2 or R3 positions. Their calculated absorption spectra are shown in Figure 13. For all four molecules, the IR peak is due to two excitations that can be described as HOMO f LUMO and HOMO f LUMO+1. Smaller peaks between 470 and 500 nm are due to the HOMO-1 f LUMO and HOMO-1 f LUMO+1 excitations. Molecular orbitals involved in these excitations are shown in Figure 14. Both the IR peak and the peak in the visible region shift to higher wavelengths (lower excitation energies) with increasing electronegativity of the axial ligand. Next, we explored the effects of chlorine, bromine, and fluorine substituted at the R2 positions of the tin naphthalocyanine dichloride molecule. The presence of these substituents shifts the IR peak from 800 nm for unsubstituted tin naphthalocyanine dichloride to approximately 850 nm for substituted tin naphthalocyanine dichloride. Substitution of octabutoxy tin naphthalocyanine dichloride at the outer naphthalocyanine ring positions (R3) with either eight or four bromine or chlorine atoms did not have a significant influence on the position of the

IR and visible peaks in comparison to the unsubstituted molecule. They did, however, influence the IR peak intensities, which varied between 2 105 M 1 cm 1 (tetra-substituted tin naphthalocyanine with Br) and 3.4 105 M 1 cm 1 (tetra-substituted tin naphthalocyanine with Cl). Because the R3 position does not significantly tune the IR peak excitation energy, we did not investigate the influence of other substituents at these positions. In all cases, the IR peak is due to the HOMO f LUMO and HOMO f LUMO+1 excitations, while the peak in the visible region at 480 500 nm corresponds to the HOMO-1 f LUMO and HOMO-1 f LUMO+1 excitations. Effects of Inner Ring Substituents. Next, we examined the influence of various substituents attached to the inner naphthalocyanine ring at R2 positions. In addition to the halogens (F, Cl, Br) mentioned in the previous section, we investigated both octaand tetra-substituted phenyl (-Ph), hydroxyl (-OH), butoxy (-O(CH2)3CH3, -OBu), and acetyl (-COCH3) functional groups. As can be seen in Figure 15, the position of the IR peak is significantly influenced by these substituents and varies between 795 and 940 nm. The substitutions have a similar effect on the peaks in the visible region (480 540 nm). The IR peaks are due to the HOMO f LUMO and HOMO f LUMO+1 transitions, while the peaks in the visible region are due mostly to HOMO-1 f LUMO and HOMO-1 f LUMO +1 transitions. Tuning Absorption in the Visible Region. An interesting question regarding the absorption spectrum of the tin naphthalocyanine molecules is the possibility of eliminating their absorption in the visible region. This would be a step toward preparation of a completely transparent solar cell, which could be used in various applications, such as transparent solar windows.42 The absorption in the visible region is mainly due to two electronic transitions, HOMO-1 f LUMO and HOMO-1 f LUMO+1. Note that the IR peak is due to the HOMO f LUMO and HOMO f LUMO+1 transitions (see Figure 14). Decreasing the intensity of the transitions in the visible region, while keeping the IR peak unchanged is a challenging problem, because the substituents 9270


The Journal of Physical Chemistry A on all three positions influence the HOMO and HOMO-1 equally. Both HOMO and HOMO-1 are symmetric and delocalized over all four naphthalocyanine pendant arms. Therefore, a prerequisite for tuning the IR transitions separately from the transitions in the visible region would be to induce different localization of the HOMO and HOMO-1 orbitals. One way to do this could be to break the symmetry of naphthalocyanine ring by substituting it in an asymmetric way. To test the above hypothesis, we have looked at the absorption spectrum of three asymmetrically substituted tin naphthalocyanine molecules (see Figure 16) and compared them with the spectra of symmetrically substituted and unsubstituted tin naphthalocyanine dichlorides. Figure 17 shows absorption spectra of symmetrically as well as asymmetrically substituted tin naphthalocyanine dichloride ([SnNcCl2]0). For each spectrum, we have calculated the ratio of the intensity of the highest IR peak versus the intensity of the highest Vis peak. For the unsubstituted and symmetrically substituted [SnNcCl2]0, the ratio is between 1.56 2.6, with the octabutoxy-substituted tin naphthalocyanine dichloride displaying the smallest ratio (1.56) and relatively highest absorption peak in the visible region. The asymmetrically substituted tin naphthalocyanines display a decrease in the intensity of the visible absorption peak relative to the intensity of the IR peak, with the calculated ratios between 3.17 and 3.21. Figure 18 shows the molecular orbitals of asymmetrically substituted tin naphthalocyanine dichloride with five butoxy groups. As can be seen in this figure, asymmetric substitution breaks the symmetry of the HOMO-1 molecular orbital, thus decreasing its overlap with the LUMO and LUMO+1 orbitals and thereby decreasing the oscillator strength of the transitions in the visible region.

’ CONCLUSIONS In this work, we applied DFT and TD-DFT calculations at the B3LYP level of theory to study the ground and excited state properties of naphthalocyanine and tin naphthalocyanine compounds. We established that DFT at the B3LYP level is successful at describing the experimentally measured absorption spectra of naphthalocyanines and tin naphthalocyanines. We also found that individual solvent molecules can directly coordinate to the central metal in tin naphthalocyanines. We have computationally explored the influence that various functional groups substituted onto the tin naphthalocyanine molecule have on its absorption spectrum. We found that functional groups attached axially to tin (-F, -Cl, -Br, -I) and to the inner naphthalocyanine ring (-F, -Cl, -Br, -Ph, -OH, -COCH3, -O(CH2)3CH3) are capable of tuning the excitation energy of the IR absorption peak between 770 and 940 nm. Substituents to the outer naphthalocyanine ring (-Cl, -Br) do not significantly influence the excitation energy of the tin naphthalocyanine in the infrared region, while they do influence the intensity of the IR peaks. Finally, we have investigated the possibility of tuning absorption peaks in the visible region with the aim to decrease their intensity with respect to the intensity of the IR peak. We found that this can be achieved by asymmetric substitutions of the naphthalocyanine pendant arms. While it might be difficult to actually synthesize the asymmetrically substituted compounds explored here, our computational studies suggest a strategy for tuning the visible transitions independently of the IR

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transitions, which is a prerequisite to the preparation of transparent solar cells.

’ ASSOCIATED CONTENT

bS

Supporting Information. Ground state frontier orbitals of octamethoxy and octabutoxy naphthalocyanines, calculated absorption spectra of bare porphyrazine, phthalocyanine, and naphthalocyanine molecules, and calculated absorption spectra of peripherally and axially substituted tin naphthalocyanines. List of Cartesian coordinates in Å for bare naphthalocyanine, two conformations of octamethoxy naphthalocyanine, two conformations of octabutoxy naphthalocyanine, puckered and flat conformations of tin naphthalocyanine, puckered and flat conformations of tin naphthalocyanine with two molecules of chlorobenzene, octabutoxy tin naphthalocyanine dichloride, and tin naphthalocyanine dichloride. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Authors thank Dr. Reza Ghiladi for his generous help with the measurement of UV vis absorption spectra in solution. This work was supported by the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract DE-AC52-06NA25396. ’ REFERENCES (1) Lee, J.; Jadhav, P.; Baldo, M. A. Appl. Phys. Lett. 2009, 95, 033301. (2) Xu, X.; Mihnev, M.; Taylor, A.; Forrest, S. R. Appl. Phys. Lett. 2009, 94, 043313. (3) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3, 297. (4) Kippelen, B.; Bredas, J.-L. Energy Environ. Sci. 2009, 2, 251. (5) Ameri, T.; Dennler, G.; Lungenschmied, C.; Brabec, C. J. Energy Environ. Sci. 2009, 2, 347. (6) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (7) Forrest, S. R. MRS Bull. 2005, 30, 28. (8) Wong, P. W.; Shimoda, Y.; Nonaka, M.; Inoue, M.; Mizuno, M. Renew. Energy 2008, 33, 1024. (9) Morimune, T.; Kajii, T.; Ohmori, Y. IEEE Photonics Technol. Lett. 2005, 18, 2662. (10) Sze, S. M. Physics of Semiconductor Devices, 2nd ed.; John Wiley and Sons: New York, NY, 1981. (11) Cui, J.; Wang, A.; Edleman, N. L.; Ni, J.; Lee, P.; Armstrong, N. R.; Marks, T. J. Adv. Mater. 2001, 13, 1476. (12) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880. (13) Michaelson, H. B. In Handbook of Chemistry and Physics; 62nd ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1981, p E. (14) Campbell, I. H.; Crone, B. K. Appl. Phys. Lett. 2009, 95, 263302. (15) Campbell, I. H. Appl. Phys. Lett. 2010, 97, 033303. (16) Vagin, S.; Hanack, M. Eur. J. Org. Chem. 2003, 2003, 2661. (17) Chen, Y.; O’Flaherty, S.; Fujitsuka, M.; Hanack, M.; Subramanian, L. R.; Ito, O.; Blau, W. J. Chem. Mater. 2002, 14, 5163. 9271



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Effects of Peripheral and Axial Substitutions on Electronic Transitions

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