Letter pubs.acs.org/JPCL
Prolonged Charge Separated States in Twisted Stacks of All-Carbon Donor and Acceptor Chromophores Abbey M. Philip, Ajith R. Mallia, and Mahesh Hariharan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, CET Campus, Sreekaryam, Thiruvananthapuram, Kerala, India 695016 S Supporting Information *
ABSTRACT: Twisted donor-on-donor and acceptor-on-acceptor bicontinuous assembly in all-carbon pyren-1-ylaceanthrylene (PA) dyad extends the survival time of the photoinduced radical ion-pair intermediates. Aceanthrylene, a functional analog of C70, acts as a versatile electron acceptor owing to its high electron affinity and visible light absorption. Antithetical trajectories of the excitons in the nonparallel π-ways led to persistent radical ion-pair intermediates in aggregated (τAcr ∼ 1.28 ns) vs monomeric (τM cr ≤ 110 fs) PA dyad as observed using femtosecond transient absorption spectroscopy. Marcus theory of charge transfer rates predicts an ambipolar transport characteristic in crystalline PA, thereby endorsing PA as an all-carbon DA hybrid for nonfullerene photovoltaic applications.
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the forward and back (geminate) electron transfer dynamics.22−24 The forward electron transfer characteristics (kCS) of a DA hybrid can be partially regulated by enhancing the donor and acceptor electronic coupling.25,26 Photogenerated excitons at DA interface can instantaneously annihilate due to the Coulomb interactions and further improvement in CS efficiency demand attention toward delaying the geminate recombination.26,27 Bioinspired self-assembly approach of constructing organized covalently linked DA systems has resulted in improving the CS efficiency.28−30 Seminal supramolecular strategies demonstrated by several groups have proved fruitful in decelerating the instantaneous recombination of radical ion-pair intermediates.9,23,31−37 Thus, the concept of “emergence upon assembly” promotes the design of innovative DA architectures for solar light harvesting.37 Our concomitant efforts in developing nonplanar30 and orbitally decoupled near-orthogonal38−40 bichromophoric assembly prompted us to explore the twisted all-carbon pyren-1-ylaceanthrylene (PA, Figure 1A) dyad. The chromophore A with significant electron affinity41 (∼2.70 eV, see Supporting Information [SI] for details), visible region absorption (up to 610 nm) and structural resemblance to fullerene fragment, formulate an excellent all-carbon electron acceptor. Steric block, amidst the donor and acceptor units coupled through a single bond, offers a twisted (θ = 49°) geometry in PA dyad. Prevalent π−π and C−H···π interactions guide the segregated donor-on-donor (D−D) and acceptor-onacceptor (A−A) bicontinuous stacks thereby crafting “π-
anoallotropic forms of carbon such as fullerenes (C60, C70, etc.), carbon nanotubes (CNT), graphenes (G) and carbon nanodots (CND) hold promising optical and photovoltaic properties.1−5 By virtue of degenerate low-lying excited states, fullerenes act as an ideal electron sink to mimic the charge separation in natural photosynthesis.5−7 Fullerene-based all-carbon donor−acceptor (DA) hybrids, e.g., CNT-C60,8 GC60,9 carbon nanohorn-C60,10 G-CNT-C60,11 etc., exhibit emergent behavior to convert light into electrical energy.3,4 Hierarchical interpenetrating C60 domains with controlled molecular-scale morphology can endorse superior performance in organic photovoltaics (OPVs).12 Nevertheless, photodimerization of fullerenes in stacked assemblies can lead to performance losses in OPVs.13 Further enhancement in efficiency of fullerene-based materials can be achieved by improving (i) solubility,14 (ii) range/extinction coefficient of visible light absorption,14,15 (iii) chemical stability13 and (iv) the economy of synthesis.14 Alternatively, planar analogs of fullerenes possessing similar electronic and structural features could emerge as novel photofunctional materials.16−18 GarciaGaribay and co-workers reported a simple and elegant synthesis of planar aceanthrylene (A) and cyclopenta[hi]aceanthrylene that possess fullerene-like redox characteristics.16,17 Inclusion of planar fullerene analog in DA constructs could offer precise control over the hierarchical structure and exciton trajectories in molecular wire/junction.19 Understanding the photoexcited processes of DA hybrids comprising cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAHs) and electronically complementary polyarenes could offer new insights to develop nonfullerene DA systems for device application.20,21 Moderating the charge transfer (CT) and separation (CS) efficiencies of DA ensemble demands subtle balance between © XXXX American Chemical Society
Received: October 7, 2016 Accepted: November 8, 2016 Published: November 8, 2016 4751
DOI: 10.1021/acs.jpclett.6b02310 J. Phys. Chem. Lett. 2016, 7, 4751−4756
Letter
The Journal of Physical Chemistry Letters
Molecular packing of PA exhibits a one-dimensional (1D) columnar stacks of the dyad, progressing in a segregated D−D (50% overlap) and A−A (45% overlap) bicontinuous manner with an average π−π distance of 3.54−3.59 Å along b-axis (Figures S8A, C & S9, SI). A C−H···π (dC−H···π = 2.87−2.89 Å) interaction between stacks drive the propagation of assembly along c-axis, leading to a 2D β-motif (ρ = 0.81) arrangement (Figures S8B, S10 & S11, SI). To evaluate the preliminary photophysical properties, steadystate UV−vis spectra of pyrene (P), 1a and PA were recorded in THF at ambient temperatures (Figure 2A). The UV−vis
Figure 1. (A) Synthesis of dyad PA with a pictorial resemblance of A to C70; (B) top view and (C) graphic representation of segregated D− D and A−A bicontinuous assembly in PA. Figure 2. (A) Steady-state UV−vis absorption spectra of P, 1a, and PA recorded in THF and (B) cyclic voltammetric analysis of 1a and PA in CHCl3 vs Ag/AgCl.
channels” for the transport of photodissociated charge carriers (Figure 1B and C).42 The photoexcitation of aggregated PA in tetrahydrofuran (THF) leads to persistent (τAcr ∼ 1.28 ns) radical ion-pair intermediates as revealed by femtosecond transient absorption (fTA) spectroscopy. A state-of-the-art computational method based on Marcus theory of charge transfer rates estimates PA as an ambipolar dyad with μh = 0.20−0.24 and μe = 0.16−0.17 cm2 V−1 s−1, respectively. Palladium-catalyzed Suzuki−Miyaura cross-coupling of 2bromoaceanthrylene (1a) with pyrene-1-boronic acid afforded the all-carbon donor−acceptor dyad PA (see SI). Slow evaporation of a solution of PA (in ethyl acetate/n-hexane [1:1] mixture) and 1a (in ethanol) offered good quality crystals for X-ray diffraction measurements (Figure S1, Table S1, SI). The derivative 1a provided dark brown needles in orthorhombic, enantiomorphic P212121 space group. Dyad PA afforded brownish black crystals in monoclinic, P21 /c centrosymmetric space group. The structure of dyad PA displays a twisted (θ = 49°) geometry with a C−C bond length of 1.48 Å, depicting a single bond with 4.6 kJmol−1 rotational barrier (Figure S2, SI). The thermal stability of PA and 1a, investigated employing thermogravimetric and differential scanning calorimetric analyses reveals stability up to 320 and 142 °C respectively (Figure S3, SI). Dyad PA (Tmp = 195.5 °C, ΔH = 35.5 kJmol−1) demonstrates a high thermal stability compared to 1a (Tmp = 131.2 °C, ΔH = 18.8 kJmol−1), as the latter showed an instant decomposition (exothermic peak) after the melting process (Figure S4, SI). A comprehensive inspection of crystal packing in 1a reveals a slip-stack motif progressing mutually in a skewed fashion to emerge as a two-dimensional (2D) network. The intermolecular π−π interactions at a distance of 3.47−3.50 Å leads to a slipped columnar arrangement along a-axis (Figure S5A,B,D, SI). Quantum theory of atoms in molecules30 (QTAIM) reveals a C···Br interaction (dC···Br = 3.53 Å) between stacks which direct the skew columns along the c-axis (Figures S5C & S6, Table S2, SI). Hirshfeld surface analysis43,44 of 1a demonstrates a marginally dominant C−H···π vs π−π interaction, which establishes a γ-motif (ρ45 = 1.47) arrangement with four molecules in the unit cell (Figure S7A-C, Table S3, SI).
spectrum of 1a characterizes vibrational bands at 432, 408, 386, 367, and 350 nm, followed by a diffuse band that extends between 450 and 600 nm. The low energy band (450−600 nm, f = 0.0532) is attributed to the π−π* (HOMO → LUMO) transition and is responsible for the prominent reddish-orange color of solution (Table S4, SI).46 The HOMO−1 → LUMO (408 nm, f = 0.0569) and HOMO−2 → LUMO (367 nm, f = 0.0956) transitions further lead to a vibrationally resolved absorption in 1a. PA exhibits an absorption resembling the addition of P and 1a spectra in 300−400 nm region and has a marginally red-shifted broad absorption in 450−700 nm region when compared to 1a (Figure 2A). Negligible emission from PA in THF upon photoexcitation at 350 nm can be attributed to plausible photoinduced electron transfer from singlet excited state of pyrene (1P*) to the efficient electron acceptor A. Electrochemical studies of 1a and PA in CHCl3 were carried out at ambient temperatures to understand the redox properties (Figure 2B). Cyclic voltammogram (CV) and differential pulse voltammogram (DPV) of 1a reveals two well-resolved reduction waves with E11/2 = −1.16 V and E21/2 = −1.80 V vs Ag/AgCl consistent with earlier reports for substituted aceanthrylene derivative (Figures 2B and S12, SI).17 The first reduction wave of 1a is only slightly higher than that of C70, which further endorses its similarity to fullerenes.47 The dyad PA shows a reduction potential with E11/2 = −1.25 V; E21/2 = −1.81 V and E31/2 = −2.26 V vs Ag/AgCl. The first two reduction waves are similar to redox potentials of 1a, whereas the third resembles with the reduction potential of P.48 HOMO, LUMO and bandgap energies obtained from electrochemical and DFT calculations on PA dyad show a similar trend, thereby signifying a reasonable correlation between the experimental and DFT calculation (Table S5, SI). Steady-state concentration-dependent UV−vis absorption studies of PA demonstrate a deviation from linearity above 60 μM dyad concentration (Figure S13A-F, SI). Nonlinear increase in the absorbance as a function of dyad concentration signifies the ability of dyad PA to aggregate in solvents of wide4752
DOI: 10.1021/acs.jpclett.6b02310 J. Phys. Chem. Lett. 2016, 7, 4751−4756
Letter
The Journal of Physical Chemistry Letters ranging polarity beyond a critical concentration. The existence and nature of the PA aggregate were further probed using scanning electron (SEM), tunneling electron (TEM) and atomic force (AFM) microscopic analysis (Figure 3A−C).
Figure 4. (A) fTA spectrum and (B) DADS of 0.6 mM PA in THF. (C) UV−vis SEC analysis of PA in THF using 0.1 M n-Bu4NPF6, applied reduction potential: 0 to −1.5 V, and (D) CW-EPR spectrum of PA (2 mM in THF) at 77 K; smooth curve (red) is computer simulation for experimental spectra (black). Figure 3. (A) SEM (silicon wafer); (B) 3D AFM (silicon wafer); (C) TEM (carbon coated Cu grid) images, and (D) SAED pattern of PA (0.6 mM) in THF.
positive absorptions at 450 and 580 nm and ground state depletion below 420 nm. The singular value decomposition (SVD) followed by global analysis of fTA spectra yielded the decay associated difference spectra (DADS) as shown in Figure 4B. Three components with time constants of (a) 6 ps, (b) 1.28 ns and (c) long time (nondecaying within experimental time scale) were sufficient to get a reasonable fit to fTA data (Figure S18, SI). The initial short-lived component in the fTA spectra can be attributed to singlet excited state of pyrene (1P*), which undergoes charge separation to give the radical ion-pair intermediates. The first DADS (6 ps, orange) centered at 580 nm is attributed to A•− with the evidence from spectroelectrochemical (SEC) studies of PA and 1a in THF (Figures 4C and S19, SI). The UV−vis SEC analysis indicates the growth of a new band (570−600 nm) with increasing reduction potential (0 to −1.5 V), which depicts the formation of radical anion of aceanthrylene (A•−). The differential absorption spectrum of the UV−vis SEC data, before and after analysis, reveals a positive absorption at 580 nm and a diffuse band at 450 nm, which agrees with the first component of DADS (Figure S20, SI). A similar characteristic absorption spectrum was obtained by chemically reducing 1a and PA using fluoride reduction method (Figure S21, SI).50 The second DADS (1.28 ns, green) component at 450 nm can be ascribed to the radical cation of pyrene (P•+)51 generated as a result of electron transfer occurring from the photoexcited donor (1P*) to the acceptor in dyad PA. To confirm the radical cation of pyrene (P•+), UV−vis SEC analysis of PA was carried out in THF. The difference spectrum obtained from the UV− vis SEC analysis indicates the formation of a broad absorption in 400−500 nm region, which clearly denotes the presence of pyrene radical cation (Figure S22B, SI). The third DADS (long time, gray) component with ground state depletion at 450 nm can arise only from recombination of the radical ion-pair intermediates. The absence of radical ion-pair intermediates in the fTA analysis of monomeric PA (
