Tunable Photochemical / Redox Properties of ( Phenylthio ) ncorannulenes : Application to a Photovoltaic Device

In a similar fashion to fullerenes, curved aromatic hydrocarbons, like corannulene, 1, behave as electrondeficient π-systems, the redox properties of which can be tuned by derivatization (Figure 1). Curved aromatic hydrocarbons related to corannulene have been used in various materials applications including the development of fluorescent chemosensors, organic field-effect transistors, and heterojunction photovoltaics. In each case, specific photochemical and redox properties have been important to the success of the application. Tuning the photochemical and redox properties of corannulene cognates corresponds to influencing the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as well as the HOMOLUMO gap. Whereas simple alkyl substituents raise HOMO and LUMO levels, fluorinated alkyl groups lower both; in neither case is there a substantial effect on the HOMO-LUMO gap as gauged from absorption spectroscopy. Organothio-substituents raise HOMO and lower LUMO levels such that one can obtain good electron acceptors with smaller HOMO-LUMO gaps. For example, compared to 1, a pale yellow material with first reduction at −2.33 eV in acetonitrile, decasubstitution of 1 with thiophenol (SPh) produces a deep red material with the first reduction potential at −1.22 eV, which is similar to the reported value of −1.17 volts for [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Given that many high-performing organic photovoltaic (OPV) devices rely on C60 and its derivatives to act as electron acceptors, this similarity in electrochemistry between PCBM and decakisphenylmercaptocorannulene motivates a study into the nature of tuning the physical properties (electrochemistry, ultraviolet-visible (UV-Vis) absorption, fluorescence, and phosphorescence) of corannulene by phenylmercapto substitution and the photovoltaic characterization of decakis(phenylthio-)corannulene as an acceptor in an OPV device. Furthermore, the optoelectronic tunability of corannulenes makes them potentially useful as organic light emitting diodes (OLEDs), as organic conductors, or in high density carbon electrodes.


Introduction
In a similar fashion to fullerenes, 1 curved aromatic hydrocarbons, like corannulene, 1, behave as electrondeficient π-systems, 2 the redox properties of which can be tuned by derivatization (Figure 1). 3 Curved aromatic hydrocarbons related to corannulene have been used in various materials applications including the development of fluorescent chemosensors, 4 organic field-effect transistors, 5 and heterojunction photovoltaics. 6In each case, specific photochemical and redox properties have been important to the success of the application.Tuning the photochemical and redox properties of corannulene cognates corresponds to influencing the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels as well as the HOMO-LUMO gap.Whereas simple alkyl substituents raise HOMO and LUMO levels, 7 fluorinated alkyl groups lower both; 8 in neither case is there a substantial effect on the HOMO-LUMO gap as gauged from absorption spectroscopy.Organothio-substituents raise HOMO and lower LUMO levels such that one can obtain good electron acceptors with smaller HOMO-LUMO gaps. 9or example, compared to 1, a pale yellow material with first reduction at −2.33 eV in acetonitrile, decasubstitution of 1 with thiophenol (SPh) produces a deep red material with the first reduction potential at −1.22 eV, 10 which is similar to the reported value of −1.17 volts for [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM).Given that many high-performing organic photovoltaic (OPV) devices rely on C 60 and its derivatives to act as electron acceptors, 11 this similarity in electrochemistry between PCBM and decakisphenylmercaptocorannulene motivates a study into the nature of tuning the physical properties (electrochemistry, ultraviolet-visible (UV-Vis) absorption, fluorescence, and phosphorescence) of corannulene by phenylmercapto substitution and the photovoltaic characterization of decakis(phenylthio-)corannulene as an acceptor in an OPV device.Furthermore, the optoelectronic tunability of corannulenes makes them potentially useful as organic light emitting diodes (OLEDs), 12 as organic conductors, 13 or in high density carbon electrodes. 14

Experimental
General synthesis 1,3-Dimethyl-2-imidazolidinone (DMI; 5-10 mL, dried over molecular sieve) was added to a round bottom flask equipped with a reflux condenser.Thiophenol (1.5 equiv./reaction site) and sodium hydride (1.2 equiv./reactionsite, 60% in mineral oil) were added and stirred at room temperature for 10 min.The halogenated corannulene derivative (2a-7a, 1 equiv.)was added and the solution was heated at 60 °C for 18 h.The solution was cooled and extracted three times with dichloromethane (DCM; 15-20 mL).The combined organic layers were washed with water, dried over magnesium sulfate, filtered, and the solvents were removed under reduced pressure.The product was purified by column chromatography (silica gel) with a hexane/DCM mixture as eluent (0-25% DCM, depending on the compound).Compound 5b was further purified by high-performance liquid chromatography (HPLC).The yield of a light yellow to dark red solid (2b-7c) was 28-89%.Further experimental details can be viewed in the Supplementary Information.

Computational methods
The conformational analyses of the molecular systems described in this study, including structural and orbital arrangements, were carried out with the dispersionenabled functional, B97-D, 15 using an ultrafine grid for evaluation of integrals together with the def2-TZVPP basis set. 16Full geometry optimizations were performed and uniquely characterized via second derivatives (Hessian) analysis to determine the number of imaginary frequencies (0 = minimum; 1 = transition state) as well as zero point and thermal corrections.Effects of solvation were taken into account with the COSab method, 17 using the dielectric for tetrahydrofuran (THF) and solvent radii from Klamt et al. 18 Ionization potentials were determined using ∆SCF/ CBS-4M 19 //B97D/Def2-TZVPP.Reduction potentials were determined using E° = −∆G/nF, where n = 1, and F = 1 eV, corrected to the Ag/AgCl electrode (4.52 eV).Molecular orbital contour plots, used as an aid in the analysis of results, were generated and depicted using the program Avogadro. 20

Photophysical and redox properties
The UV-Vis absorption spectra of 2b-7b and 7c are shown in Figure 3.Each SPh addition shifts the longest wavelength absorption [λ(2)] bathochromically 10-15 nm while leaving the maximum absorption wavelength [λ(1)] relatively unshifted (Table 1).The linearity of the bathochromic shift is shown in the inset of Figure 3.This shift results in a change from colorless 1 to orange-red 7b (Figure 4).These data suggest a diminution of the HOMO-LUMO gap.This is also evidenced by the difference in reduction and oxidation potentials (Table 2).For 7b, the reduction and oxidation potentials are each ca. 1 eV easier to achieve than for 1 (D redox 7b = 2.05 eV vs. D redox 1 = 4.02 eV).Linear regression of the first reduction potential vs. the number of SPh groups suggests a stabilization effect of the radical anion by ca.+0.1 V/SPh group (Figure 5).The opposite trend is seen for the methyl series 1-7c (ca.-0.02 V).     2). 7ol.27, No. 10, 2016   Although oxidations were largely irreversible, except for 7b, a crude estimation of the ionization potentials was possible from the oxidation waves by using Cavalieri's method (Table 2). 26Whereas isolated SPh groups provide small stabilization of the radical cation, as in 6b vs. 1, peri-substitution provides a substantial stabilization effect, as in 3b.
Emission spectra also shift to longer wavelengths with increasing SPh-substitution of 1 (Table 1).Fluorescence quantum yields from 1 up to 20% were observed and lifetimes were between 1-10 ns across the series and can be seen in Figure 6.The difference between the longest absorption and shortest emission ranges from 50-70 nm suggesting little structural change in the fluorescent state.At 77 K, phosphorescence can be observed at about 100-120 nm longer wavelength than the fluorescent emission.Again, higher substitution leads to longer wavelength phosphorescence.Understanding this behavior may be a key to the development of cognates as photonic materials.
Charge-transfer absorptions between tetracyanoethylene (TCNE) and the (SPh) n series were measured using the Kuroda method in order to deduce ionization potential (IP) values. 29Weak charge-transfer (CT) bands appear at 512-632 nm; however, the spectra are not precise enough to calculate useful IP values from CT absorptions (IP CT ).In the case of 7•TCNE, no CT band was observed, which may be due to the increased steric hindrance.In general, steric hindrance of the π system in these systems may preclude the use of a classical CT band to IP correlation model; the change in the geometry of the complex having a greater effect than the change in electronic π character.

Photovoltaic device performance
Because the reduction potentials are close to those of active fullerene derivatives, 7b was chosen to test the acceptor capabilities of these compounds in organic photovoltaic devices.The results are shown in Figure 7, and the cell parameters are summarized in Table 3 The open-circuit voltage for the highest efficiency cell was 0.644 V, which is comparable to values observed for P3HT:PCBM devices.However, the short-circuit current densities were very low.The reduction potential of 7b is higher than that for PCBM (−1.05 V vs. Ag/Ag + in THF). 30n fact, the value is much closer to the second reduction of PCBM (−1.46 V).The energy level difference could account for the relatively high open-circuit voltages.Because the reduction potential is considerably higher, it is possible that a more stable P3HT state provides a pathway for recombination.The electron in the 7b-anion could relax to a lower energy P3HT triplet state or polaron state, thus reducing overall efficiency. 31lternatively, the morphology of the solar cell device could be the cause of the observed low currents.The lack of stacked domains may mean that intermolecular electron transfer is limited, which would lead to the observed low currents.Devices were fabricated using procedures optimized for PCBM devices for comparison.Efficiencies may be improved by optimizing fabrication procedures and/or active layer compositions for these functionalized corannulenes.

Conclusions
In short, a series of (phenylthio) n corannulenes was synthesized and their properties investigated.There is a rough linear dependence between the HOMO-LUMO gaps and the number of substituents.This is demonstrated in the results found for both redox potentials and UV-Vis absorption data.Fabrication of a 7b:P3HT photovoltaic device resulted in a 0.3% PCE.

Figure 2 .
Figure 2. Molecular structure of the two independent molecules of 6b from X-ray diffraction analysis.Top and side views.