Conformational Impact on Energy Storage Efficiency of

Article pubs.acs.org/JPCA

Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

Conformational Impact on Energy Storage Efficiency of Subphthalocyanine−Fullerene Hybrids Jonas Sandby Lissau,‡ Alberto Viñas Muñoz, Henrik Gotfredsen, Martyn Jevric,§ Mogens Brøndsted Nielsen, and Theis I. Sølling* Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark

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S Supporting Information *

ABSTRACT: Hybrid molecules involving subphthalocyanine and Buckminsterfullerene derivatives are interesting candidates as heavy metal free triplet sensitizers. Subphthalocyanine efficiently absorbs visible photons and transfer the singlet excited state energy to the Buckminsterfullerene where intersystem crossing produces triplet states in high yield. Thus, far the efficiency of the triplet-generating photophysics in these systems has been hampered by back energy transfer to the subphthalocyanine triplet state resulting in loss of excitation energy. Herein an efficient strategy is realized to avoid loss of triplet energy by back energy transfer. A hybrid molecule based on subphthalocyanine and Buckminsterfullerene is presented in which dispersion-induced π−π interactions result in a molecular geometry where highly efficient through-space singlet excited state energy transfer takes place in one direction, whereas energy flow in the opposite direction via the triplet manifold is blocked by lack of orbital overlap. The approach opens for a new class of heavy-metal-free triplet sensitizers of particular relevance to the fields of photodynamic therapy and noncoherent photon upconversion.

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INTRODUCTION The discovery of Buckminsterfullerene, C60, marked the beginning of a new era of intriguing nonplanar aromatic molecules.1 In addition to their importance in fundamental research, Buckminsterfullerene derivatives have served as favorite electron acceptor materials in organic photovoltaics (OPV)2 and electron transport layers in perovskite solar cells.3 While subphthalocyanines were discovered already in 1972 by Meller and Ossko,4 it is first in recent years that this additional rare class of nonplanar aromatic compounds5 have emerged as favorable electron donating partners to fullerenes in OPVs.6 The advantageous donor−acceptor properties of this molecular couple have prompted a large number of photophysical studies on covalently linked hybrid molecules incorporating derivatives of the Buckminsterfullerene/subphthalocyanine couple as potential model systems for photoinduced electron transfer (PET) events in photosynthesis.7−16 The high triplet quantum yield of Buckminsterfullerene, as a fully carbon based molecule, is another rather unique property of this species. The triplet state sensitizes singlet oxygen with a quantum yield of 0.96 in benzene, which is the lower limit of the triplet quantum yield.17 As an efficient heavy-metal-free triplet state sensitizer,18,19 Buckminsterfullerene is thus of interest also to the fields of photodynamic therapy (PDT)20−23 and noncoherent photon upconversion via triplet fusion. The latter offers a promising route to improve the efficiency of conventional single-threshold solar cells by absorption and upconversion of low-energy photons transmitted by the solar cell.24,25 In solar energy applications Buckminsterfullerene is © XXXX American Chemical Society

not an optimal triplet sensitizer due to its low absorbance in the visible spectrum.26 Instead, a number of hybrid molecules have been tested in which Buckminsterfullerene is attached to various antenna light-harvesting chromophores from which excitation energy is transferred to the fullerene.27−31 Subphthalocyanines have favorable light harvesting properties such as strong and easily tunable absorption in the visible light spectrum and the aforementioned nonplanar geometry, which is helpful in avoiding aggregate formation, a well-known issue in solar harvesting systems based on closely related planar chromophores like phthalocyanines and porphyrins. The importance of energy transfer reactions has been recognized in OPVs on the basis of the subphthalocyanine/Buckminsterfullerene couple,32 but surprisingly hybrids based on this molecular couple have not yet been proposed as efficient heavy-metal-free triplet state sensitizers in the aforementioned applications. In fact, the majority of such molecular hybrids proposed as PET model systems has been shown to efficiently quench the singlet excited state of the subphthalocyanine subunit via excitation energy transfer (EET) to the Buckminsterfullerene part of the molecule.9−16 However, the produced triplet excited states in the fullerene subunits of these systems are not useful for triplet sensitization since they are quenched by fast back triplet energy transfer (TET) to the subphthalocyanine with intersystem crossing (ISC) on the Buckminsterfullerene moiety being the rate-determining step Received: June 25, 2018 Published: July 24, 2018 A

DOI: 10.1021/acs.jpca.8b06064 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Chart 1. Chemical Structures of the Compounds Studied in This Work

of the process.10 In the back-TET process excitation energy is lost and the process is therefore undesired in solar energy applications. A rare exception to the rule of back-TET occurring in subphthalocyanine/Buckminsterfullerene hybrids is a supramolecular (noncovalent) complexation of the C60 fullerene trapped between two subphthalocyanines.33 In this complex, unidirectional EET was responsible for the production of a fullerene triplet state after excitation of the surrounding subphthalocyanines. This supramolecular complex is not an optimal triplet sensitizer since its triplet state is hardly accessible from the surroundings due to the caging subphthalocyanines. The system is interesting though, since the lack of back-TET to the subphthalocyanines appears, in our view, to be caused by the noncovalent nature of the interaction, which minimizes the orbital overlap and therefore the efficiency of back-TET taking place through electron exchange (Dexter-type EET). In covalently linked subphthalocyanine/Buckminsterfullerene dyads, back-TET has been shown to occur primarily via a through-bond mechanism,14−16 and it is therefore plausible to suggest that the lack of backTET in the noncovalent complex studied by Sanchez-Molina and co-workers33 is caused by the absence of covalent bonds between donor and acceptor units. Following the reasoning given above, we designed a covalently linked subphthalocyanine/Buckminsterfullerene hybrid that would allow for efficient singlet EET from subphthalocyanine to Buckminsterfullerene but at the same time efficiently block back-TET to the subphthalocyanine. The structure of the resulting molecular triad based on SubPc, azulene (Azu), and C60 (SubPc−Azu−C60) is shown in Chart 1. The basic idea of the molecular design is to place the Buckminsterfullerene in a relatively well-defined position with respect to subphthalocyanine without direct covalent linkage between the two subunits and maintaining the accessibility of the Buckminsterfullerene from the surroundings. The proposed structure involves a linker subunit, cyanoazulene, chosen to efficiently block through-bond communication between donor and acceptor subunits. The linker subunit

also provides the right geometry where the fullerene (energy acceptor) is allowed to rotate around its attachment to cyanoazulene to interact through π-stacking with the energy donor subunit. The aim is to obtain the right geometry for efficient “through-space” EET while blocking through-bond back-TET, thus producing a triplet state energy reservoir on the fullerene subunit accessible for exploitation in a variety of applications. By probing the excited state dynamics at the ultrafast time scale, we clarified the competition between these initial photophysical pathways and determined the excited state quenching mechanism and end-product. A number of additional, related compounds shown in Chart 1 were included in the study for reference.

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RESULTS Steady-State Photophysical Characterization. Steadystate UV−vis absorption spectra of SubPc, SubPc−Azu, and SubPc−Azu−C60 in toluene are shown in Figure 1.34 The absorption spectrum of SubPc (black solid line in Figure 1, Amax = 568 nm) resembles that of the closely related boron subphthalocyanine chloride. This resemblance is in line with previous observations that the nature of the axial substituent does not substantially affect the position of the Q band (S1 ←

Figure 1. Normalized absorption and photoluminescence spectra (λexc = 532 nm) of SubPc (absorbance, black solid line; photoluminescence, black dashed line), SubPc−Azu (green), and SubPc− Azu−C60 (red line) in toluene. B


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The Journal of Physical Chemistry A S0) absorption.5 Inclusion of the cyanoazulene subunit in SubPc−Azu (green line) introduces the characteristic S2 ← S0 absorption of azulene in the spectrum around 400 nm,35 while the spectral range around the SubPc Q band appears largely unaltered. It should be noted, though, that the Azu moiety has a weak but distinct absorption feature in the 450−700 nm region with maximum absorbance at 587 nm (ε587 nm(Azu) = 638 M−1 cm−1) arising from its S1 ← S0 transition (Figure S20 in the Supporting Information). In SubPc−Azu this low-energy Azu absorption band is covered by the much stronger SubPc Q band. Also for SubPc−Azu−C60 the SubPc Q band is the dominant absorption feature at longer wavelengths while the presence of the C60 moiety becomes clearly visible at wavelengths shorter than 450 nm (red line in Figure 1). Like Azu, C60 has a broad absorption band from 450 to 750 nm (Figures S19 and S21 in the Supporting Information), which is too weak (ε705 nm(Azu−C60) = 363 M−1 cm−1) to discern from the strong SubPc Q band absorption in SubPc−Azu−C60. Fluorescence from the S1 excited state in SubPc follows the mirror image rule and has a small Stokes shift with maximum fluorescence intensity at 576 nm (black dashed line in Figure 1). The fluorescence quantum yield of SubPc in toluene was determined to be 0.23 for excitation at 532 nm (Figure S23 in the Supporting Information). The photoluminescence spectra of SubPc−Azu and SubPc−Azu−C60 have shapes very similar to the fluorescence spectrum of SubPc (Figure S22 in the Supporting Information), but the photoluminescence intensity is quenched to a different extent. The ratio of the maximum photoluminescence intensity of SubPc−Azu corrected for the light harvesting efficiency (LHE = 1−10−A) at the excitation wavelength to the same quantity for SubPc is 1.07 × 10−1. The corresponding ratio for SubPc−Azu−C60 vs SubPc is 1.22 × 10−2. Photoluminescence spectra corrected for LHE can be found in the inset of Figure S22 in the Supporting Information. In addition to the SubPc fluorescence, the photoluminescence spectrum of SubPc−Azu−C60 has a low-intensity emission band around 715 nm (inset of Figure 2). The

Time-Resolved Photophysical Characterization. The fluorescence lifetime of SubPc in toluene was determined by time-correlated single photon counting (TCSPC) to be 1.86 ns for excitation at 560 nm (Figure S24 in the Supporting Information). The SubPc singlet excited states of SubPc−Azu and SubPc−Azu−C60 are much shorter lived than in the isolated SubPc. Hence the dynamics of these states were followed exclusively by ultrafast transient absorption spectroscopy. The transient absorption spectral shape of SubPc, SubPc−Azu, and SubPc−Azu−C60 all have similar features, including a strong ground state bleach (GB) in the spectral range from 500 to 600 nm and much less intense excited state absorption (ESA) bands for wavelengths 600 nm (Figure 3a). The dynamics of these features, however, vary

Figure 3. (a) Transient absorption spectra of SubPc−Azu−C60 in toluene measured at 2 ps (orange circles) and 12 ps (cyan bullets) after excitation (λexc = 540 nm). A scatter signal from the pump is not shown in this plot. (b) Kinetic traces of the same solution probed at 462 (orange squares), 568 (cyan bullets), and 610 nm (green circles). The traces were fitted globally to a sum of two exponentials (solid lines), and the residuals are plotted on the upper vertical axis. Fit constants are given in Table 1.

significantly throughout the series. For SubPc only a slight change in transient absorption signal is seen between delay times of 5 and 700 ps with respect to the pump pulse excitation (Figure S27 in the Supporting Information), which is consistent with the long-lived fluorescence signal observed by TCSPC for this compound. For SubPc−Azu (Figure S25 in the Supporting Information) the majority (≥90%) of the excited state population decays with a time constant of 203 ps, while at certain wavelengths including the GB interval, a longer lived component is also present with a small amplitude (≤10%, Table 1). For SubPc− Azu−C60 (Figure 3) the time constant for the fast decay component is further reduced to 16.1 ps (Table 1). In addition, a longer decay component is strongly represented (≥20%) at most wavelengths, except at the GB band where 100% of the signal decays with a time constant of 16.1 ps. The long-lived component is especially strongly present at 720 nm where it has a relative magnitude of 91% (Figure S26 in the Supporting Information).

Figure 2. Normalized absorbance (red line) and excitation spectra (λem = 640 nm, dashed black line; λem = 715 nm, green line) of SubPc−Azu−C60 in toluene. Inset: far-red photoluminescence (PL) band (λexc = 532 nm) of SubPc−Azu−C60 in toluene.

position and shape of this band is characteristic of the fluorescence from pyrrolidino[60]fullerene.36 The excitation spectrum of Sub−Azu−C60 measured at 715 nm has a shape similar to the absorption spectrum of the compound (Figure 2, green and red solid lines, respectively). The similarity suggests that the 715 nm photoluminescence is induced by excitation at both the C60 and the SubPc subunits of the triad. In contrast, an excitation spectrum measured at 640 nm only show contribution from excitation of the SubPc subunit (black dashed line in Figure 2). C


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The Journal of Physical Chemistry A

Computational Results. The structures of SubPc−Azu and SubPc−Azu−C60 were optimized using the density functional theory (DFT) functionals CAM-B3LYP and ωB97XD in combination with the 3-21G basis set either in the gas phase or with an IEFPCM model of toluene. ωB97XD/ 3-21G optimized structures of SubPc−Azu and SubPc−Azu− C60 with the IEFPCM toluene model are shown in Figure 4.

Table 1. Decay Time Constants (τ) and Associated Relative Amplitudes (a) for Multiexponential Global Fits to Transient Absorption Data of SubPc−Azu and SubPc− Azu−C60 in Toluene compound

λprobe/nm

signala

τ1/ps (a1/%)b

τ2/ns (a2/%)b

462 568 610 462 568 610 720

ESA GB ESA ESA GB ESA ESA

203 (90) 203 (−91) 203 (100) 16.1 (47) 16.1 (−100) 16.1 (80) 16.1 (9)

>2 (10) >2 (−9)

SubPc−Azu

SubPc−Azu−C60

>2 (53) >2 (20) >2 (91)

a ESA = excited state absorption, GB = ground state bleach. bNegative amplitudes represent rising components.

Electrochemistry. The redox properties of the selected SubPc derivatives were investigated by cyclic voltammetry (CH2Cl2, 0.1 M Bu4NPF6). The cyclic voltammograms of SubPc, Azu, Buckminsterfullerene (C60), SubPc−Azu, and SubPc−Azu−C60 are shown in Figures S28−S32 in the Supporting Information. The formal potentials, E°′, are reported for reversible electron transfers with reverse current being observed during the back scan (Table 2). When followup reactions are so fast that reverse currents are not clearly observed, and the potentials for that reason do not necessarily have a thermodynamic significance, the oxidation/reduction potentials are estimated by subtracting half the value of the typically observed peak separation for this type of compounds. Compound SubPc shows the typical electrochemistry of axially substituted subphthalocyanines.37 A two-step reduction with a first reversible one-electron transfer to form the radical anion at −1.51 V followed by further reduction to the reactive radical dianion at −2.11 V (Table 2 and Figure S28 in the Supporting Information). A one-electron oxidation at 0.50 V forms a reactive radical cation that shows some reversibility on the time scale of the experiment. The incorporation of the cyanoazulene (SubPc−Azu) moiety has little effect on the oxidation behavior of the subphthalocyanine ring, only causing a small shift of −7 mV for the first oxidation (Table 2 and Figure S31 in the Supporting Information). This is in line with previous studies showing that redox properties of subphthalocyanines are largely unaltered by axial substitution.5 In contrast, for the case where C60 is incorporated, i.e., SubPc− Azu−C60, the first oxidation is shifted by +59 mV as compared to that for SubPc−Azu, which is not expected from the position where C60 is attached to the molecule, on the cyanoazulene, separated from the SubPc subunit (Table 2 and Figure S32 in the Supporting Information).

Figure 4. ωB97XD/3-21G optimized structure of SubPc−Azu (left) and SubPc−Azu−C60 (right) using the IEFPCM model for toluene.

The remaining optimized molecular geometries can be found in Figures S33−S40 and Tables S1−S8 in the Supporting Information. In general, the inclusion of the solvent model does not significantly alter the optimized geometries. In contrast, the type of functional, CAM-B3LYP vs ωB97XD, is seen to play an important role for the “secondary” structure of SubPc−Azu−C60. For example, the center-to-center distance (RCC) from the SubPc to the C60 subunits of SubPc−Azu−C60 is 5.83 Å in the CAM-B3LYP/3-21G optimized structure while the same distance is only 4.56 Å in the geometry resulting from ωB97XD/3-21G optimization, both using IEFPCM for toluene. Single-point energy calculations were performed on SubPc− Azu and SubPc−Azu−C60 on the CAM-B3LYP/6-311+G(d) and ωB97XD/6-311+G(d) levels with and without inclusion of IEFPCM for toluene. Representative molecular orbital visualizations from the ωB97XD/6-311+G(d) (IEFPCM, toluene) calculation on SubPc−Azu−C60 are shown in Figure 5 and additional visualizations and corresponding eigenvalues can be found in Tables S9−S15 in the Supporting Information. Natural transition orbitals (NTOs) of singlet and triplet excited states were formed from the results of time-dependent (TD) DFT calculations on SubPc−Azu and SubPc−Azu−C60, while using the Tamm−Dancoff approximation (TDA) for the latter.38 The NTO visualizations of SubPc−Azu show that the excited states are localized on one of the two subunits and not

Table 2. Formal Potentials (E°′) or Peak Potentials (Ep) for the Electron Transfer Reactions Observed by Cyclic Voltammetrya compound SubPc Azu C60 SubPc−Azu SubPc−Azu−C60

Eox2 °′

1.053c

Eox1 °′

Ered1 °′

Ered2 °′

0.504 1.070c

−1.514 −1.619b −1.004 −1.559 −1.140d

−2.114b

0.497 0.556c

Ered3 °′

−1.390

−1.790

−1.622d

−2.087d

a

Electrochemical potentials are reported in units of V vs Fc/Fc+. CH2Cl2 was the solvent and Bu4NPF6 (0.1 M) the supporting electrolyte. The voltage sweep rate was 0.1 V s−1. bEstimated from the potential for the reduction peak by addition of 40 mV (=half the value of the peak separation typically observed for this series of compounds). cEstimated from the potential for the oxidation peak by subtraction of 40 mV (=half the value of the peak separation typically observed for this series of compounds). dMultielectron process. D


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results show a strong dependence on the presence/absence of the IEFPCM rather than the type of functional used (Figures S41 and S42 in the Supporting Information).

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DISCUSSION Photoinduced Dynamics of SubPc−Azu. The strong reduction of the fluorescence intensity and excited state lifetime of SubPc−Azu as compared to that for SubPc strongly suggests that the Azu subunit acts as a quencher of the SubPc singlet excited state. The small contribution to the transient absorption signal from a long-lived component (>2 ns) may result from a small population of SubPc triplet states generated by intersystem crossing (ISC) prior to singlet state quenching. Mechanistically, there are two candidates for the quenching to consider; photoinduced electron transfer (PET) and excitation energy transfer (EET). Concerning PET, this mechanism requires that the Gibbs free energy of the process, ΔGPET, is negative. As shown in Table 3, the Gibbs free energy for reductive intramolecular PET following excitation of the SubPc moiety is 1.73 eV, and the process is therefore highly unlikely to take place. Furthermore, there is no indication in transient absorption spectroscopy data that PET takes place. The GB signal decays at the same rate as the observed ESA signals and no new ESA signals grow in with time, which would be expected in a PET reaction where radical ions with distinct transient absorption signatures are generated. Intramolecular nonradiative EET can take place through a Coulombic interaction or via an electron exchange mechanism (Dexter-type EET). In general, electron exchange can contribute significantly to EET when the donor and acceptor units are placed at a short distance, allowing for significant overlap between their orbitals. Hence from this requirement alone electron exchange should be considered for SubPc−Azu. However, the energetics of the Dexter EET appears to be unfavorable. The cyclic voltammetry results (Table 2) show that the first oxidation of Azu is at a potential more positive than the first oxidation of SubPc, and the first reduction of Azu is at a more negative potential than the first reduction of SubPc. The rate of the EET process as observed by transient absorption suggests that the involved donor and acceptor states have had time to equilibrate before energy transfer, and together with the CV data, this suggests that electron exchange is unlikely. This view finds further support from TDDFT calculations, where the natural transition orbitals (NTOs) describing the singlet excited states of lowest energy are localized on the SubPc unit, while Azu localized NTOs have higher energies. From the TCSPC data the singlet excited state decay rate (k0 = 1/τ0) of SubPc in toluene is determined to be

Figure 5. Molecular orbitals of SubPc−Azu−C60 calculated at the ωB97XD/6-311+G(d) level using IEFPCM for toluene.

delocalized over the entire molecule (Table S16 in the Supporting Information). For SubPc−Azu−C60 the NTOs corresponding to S1−S3 excited states are located exclusively on the SubPc and C60 subunits, while the Azu subunit is only involved in higher lying NTOs ((Table S17 in the Supporting Information). The NTOs corresponding to the triplet excited states of lowest energy, T1 and T2, are localized entirely on the SubPc, and the T3 NTO is localized on C60. The TDDFT

Table 3. Gibbs Free Energies of Photoinduced Intramolecular Electron Transfers (ΔGPET) in SubPc−Azu and SubPc−Azu− C60 in Toluenea product state −•

SubPc −Azu SubPc+•−Azu−C60−• +•

R+/Åb

R−/Åb

RCC/Åb

ΔGS/eV

Eox/Vc

Ered/Vc

ΔE00/eVd

−ΔGPET/eV

2.05 3.23

3.20 1.95

6.99 4.56

1.28 1.84

1.053 0.556

−1.559 −1.140

2.17 2.17

−1.73 −1.37

−ΔGPET = ΔE00 − {e(Eox − Ered) + ΔGS}, where ΔE00 is the 0−0 transition of 1SubPc* and Eox and Ered are the oxidation and reduction potential, respectively, of the indicated molecular unit (Table 2). ΔGS is the ion pair solvation energy, which is calculated from ΔGS = −e2/(4πε0){[(2R+)−1 + (2R−)−1 − RCC−1]εS−1 − [(2R+)−1 + (2R−)−1]εR−1}, where R+ and R− are the radii of the radical cation and radical anion, respectively, RCC is the center-to-center distance between donor and acceptor subunit, and εR and εS are the dielectric constants for solvents used in electrochemical and photophysical measurements, respectively. bEstimated from DFT optimized structures at the ωB97XD/3-21G level using the IEFPCM model for solvation in toluene. cObtained from cyclic voltammetry measurements in dichloromethane (Table 2). dObtained from the intersection point of the normalized absorption and fluorescence spectra of SubPc in toluene. a

E


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The Journal of Physical Chemistry A Table 4. Experimental Rates of EET (kexp EET) in Toluene Compared to Modeled Rates product state

J/(M−1 cm−1 nm4)a

SubPc−{Azu}* SubPc−Azu−{C60}*

6.00 × 10 8.22 × 1013 13

RF/Åb 23.4 24.6

RCC/Åc

−1 d kdd EET/s

6.99 4.56

7.54 × 10 1.34 × 1013 11

l/Åe

−1 f kemp EET/s

−1 g kexp EET/s

8.90 9.30

5.47 × 10 1.70 × 1010

4.39 × 109 5.72 × 1010

9

4 The overlap integral J is given by ∫ ∞ 0 ID(λ) εA(λ)λ dλ, where ID is the fluorescence spectrum of SubPc (donor subunit) normalized so that I (λ) dλ = 1 and ε (λ) is the molar attenuation coefficient of the acceptor subunit. For C60 the εA(λ) values were estimated by subtraction of ∫∞ 0 D A the Azu spectrum from the Azu−C60 spectrum (Figure S21 in the Supporting Information). bThe Förster radius is given by RF = 0.2108[κ2ΦDn−4J]1/6, where κ is the orientational factor, taken to be 2/3 , ΦD is the fluorescence quantum yield of SubPc (0.23), and n is the refractive index of toluene. cEstimated from DFT optimized structures at the ωB97XD/3-21G level using the IEFPCM model for solvation in e −1 6 toluene. dkdd EET = τD (RF/RCC) , where τD is the excited state lifetime of SubPc (1.86 ns). l is an approximate measure of the conjugation length of the involved donor and acceptor subunits, estimated here as the sum of the longest dimensions of the subunits in the ωB97XD/3-21G optimized −1 6 g structures. fThe empirical rate model put forward by Wong et al.39 is given by kemp EET = τD {RF/(l + RCC} . Derived from transient absorption spectroscopy and TCSPC data. See Discussion in the main text. a

5.38 × 108 s−1. The decay rate of the singlet excited state of the SubPc subunit of SubPc−Azu in toluene can be approximated by k1 = τ1−1 = k 0 + kEET(Azu)

SubPc−Azu. In addition, the long-lived transient absorption component seen for the latter is not seen in the ground state bleach (GB) signal of SubPc−Azu−C60, which indicates that the faster quenching process introduced in SubPc−Azu−C60 outcompetes ISC to the SubPc triplet manifold. Photoinduced electron transfer is excluded as a mechanism of quenching for the reasons given for SubPc−Azu (Table 3). Following the approach used for SubPc−Azu, the rate of energy transfer from SubPc to C60 in the triad, kEET(C60), can be approximated by

(1)

where τ1 is the SubPc singlet excited state lifetime of SubPc− Azu, determined to be 203 ps by transient absorption spectroscopy, and kEET(Azu) is the energy transfer rate from the SubPc donor to the Azu acceptor subunits. Assuming k0 of the dyad to be similar to k0 of the isolated SubPc, this gives kEET(Azu) = 4.39 × 109 s−1. Due to the short donor−acceptor distance in the SubPc− Azu system, a simple dipole−dipole approximation fails to describe the observed energy transfer and overestimates the rate constant by more than 2 orders of magnitude (see kdd EET in Table 4). Similar observations have been done previously by Wong et al.39 in a study of the distance dependence of the EET rate constant in conjugated systems. They note the large discrepancy of the rate from the dipole−dipole approximation at short distances and propose an empirical modification of Förster’s model taking into account the conjugation length l for a better description of the rate at short distances. Using this model for SubPc−Azu predicts an EET rate constant of 5.47 × 109 s−1, which is very close to the experimentally observed rate of 4.39· 109 s−1 (Table 4). In summary, the data suggest that the excited singlet state located on the SubPc subunit of SubPc−Azu is quenched by EET to the Azu subunit with a time constant of τEET = 228 ps. The EET process is a result of a Coulombic interaction involving significant contributions from terms different from the dipole−dipole coupling. The 1Azu* excited subunit resulting from EET decays by a time constant too small for the state to be observable in the transient absorption data. The first excited singlet state (S1) of azulene decays with a time constant of ca. 1 ps,35 and it is reasonable to expect cyanoazulene to exhibit similar photophysics. The small population of triplet states generated prior to EET from the singlet excited SubPc chromophore remains trapped on the SubPc due to a positive Gibbs free energy for triplet energy transfer (TET) preventing electron exchange with the Azu moiety. According to TDωB97XD/6-311+G(d) calculations (including IEFPCM for toluene), the barrier for TET from SubPc to Azu is 0.70 eV as compared to 0.17 for singlet EET (Table S16 in the Supporting Information). Photoinduced Dynamics of SubPc−Azu−C60. By inclusion of the C60 subunit in SubPc−Azu−C60, a further strong reduction in SubPc fluorescence intensity and excited state lifetime is observed as compared to the data for

k 2 = k1 + kEET(C60)

(2)

where k2 is the decay rate of the singlet excited state of the SubPc subunit of SubPc−Azu−C60, which is determined to be 6.21 × 1010 s−1 (1/16.1 ps) by transient absorption spectroscopy. Using this approximation, it follows that kEET(C60) = 5.72 × 1010 s−1. The quantum yield of EET to C60 can now be estimated to be ΦEET(C60) = kEET(C60)/k2 = 0.92. The ratios k0/k1 = 1.09 × 10−1 and k0/k2 = 8.65 × 10−3 are similar to the corresponding ratios found for the steady state fluorescence signals (Results section), which supports the notion that k1 and k2 are the decay rates of the singlet excited SubPc subunits of SubPc−Azu and SubPc−Azu−C 60 , respectively. In contrast to SubPc−Azu the optimized SubPc−Azu−C60 geometry is strongly dependent on the DFT functional being used. Especially the center-to-center distance (RCC) from SubPc to C60 with IEFPCM for toluene goes from 5.83 Å with a CAM-B3LYP/3-21G optimization to 4.56 Å with a ωB97XD/3-21G optimization. This difference in distance is large enough to have important consequences for the modeling of EET. The large nonplanar conjugated systems of SubPc and C60 are known to participate in strong π-stacking,1,33,40 and including dispersion terms in the description the ωB97XD functional is expected to account better for a π−π interaction of this type than the CAM-B3LYP functional does. The assumption that the π systems of SubPc and C60 are interacting in SubPc−Azu−C60 finds further support in the cyclic voltammetry data where a large shift (+59 mV) in the first oxidation potential of SubPc−Azu−C60 is observed with respect to SubPc−Azu. Since this oxidation can be thought of as being localized on the SubPc subunit and through-bond interaction between the SubPc and the C60 subunits can be ruled out, a through-space interaction appears plausible. The close proximity between these two moieties found in the ωB97XD geometry calculations further supports this picture. Applying the donor−acceptor distance from the ωB97XD optimized SubPc−Azu−C60 geometry in the modified FRET F


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The Journal of Physical Chemistry A 10 −1 model by Wong et al.39 gives a kemp for EET value of 1.70 × 10 s energy transfer from SubPc to C60, which is on the same order of magnitude as the experimental EET rate constant (Table 4). The transient absorption data support EET to C60 by showing strong long-lived components at all probed wavelengths except for at the GB signal, which is expected from an EET process where the donor ground state is regenerated. In particular, the ESA signal at 720 nm which has >90% amplitude on the >2 ns component (Table 1 and Figure S26 in the Supporting Information) has been identified as a pronounced signature of the pyrrolidino[60]fullerene triplet state which develops via ISC in a few picoseconds.27 Finally, the photoluminescence band observed for SubPc− Azu−C60 around 715 nm in the steady state spectroscopic data is also a clear indicator of EET to C60 because the Nmethylpyrrolidine substituted C60 molecule has a fluorescence band around 715 nm36 with a quantum yield of 6.0 × 10−4.41 The shape of the excitation spectrum measured at λem = 715 nm shows not only that the photoluminescence signal can be induced by directly exciting the C60 subunit but also that excitation in the wavelength interval of the SubPc Q band induces the emission signal nearly exclusively through excitation of the SubPc part of the molecule (see the similarity in the spectra of the green and dashed black lines in Figure 2). This suggests that part of the SubPc singlet excited state energy is transferred to C60 where it produces fluorescence. The dominant photoinduced dynamics of SubPc−Azu−C60 are outlined in Scheme 1. Excitation of the SubPc produces a

which would have been a signature of back-TET to the SubPc moiety. This suggests that any back-TET is significantly slower than the temporal window of the experiment. The photophysics outlined for SubPc−Azu−C60 shows that a bended molecular conformation caused by dispersioninduced π−π interactions facilitates a high rate of forward singlet EET from SubPc to C60 via a Coulombic interaction, while the back-TET via electron exchange is efficiently blocked due to lack of orbital overlap. Interestingly, SubPc−Azu−C60 might not be the first system studied in which such a mechanism is active. The subphthalocyanine−azobenzene− fullerene (SubPc−PhNNPh−C60) triad studied by Kim et al.42 showed efficient energy transfer from SubPc to C60. The authors suggest that the EET process is mediated by the azobenzene linker subunit. However, as they also note, the azobenzene linkage does not play an active role in EET since the absorption spectrum of azobenzene does not have an overlap with the SubPc fluorescence spectrum.43 Therefore, an alternative interpretation of the photophysics in this system could be that the azobenzene subunit rather than being a mediater acts as a blocker of ET and EET, much like cyanoazulene in SubPc−Azu−C60, which opens a pathway of through-space EET to C60. As in the case of SubPc−Azu−C60, back-TET from C60 to SubPc in SubPc−PhNNPh−C60 would be blocked by the absence of an orbital overlap, which could explain the long-lived C60 triplet state transient absorption features (>1.5 μs) measured by Kim and co‑workers.42 A geometry optimization of the SubPc−PhN NPh−C60 structure including dispersion force contributions and solvent modeling could provide valuable insight to shed light on the active mechanism in this system. We expect a wide range of potential linker units to have similar desirable structural and electronic properties that may be exploited in fine-tuning of the mechanism.

Scheme 1. Jabłoński Diagram Showing the Dominant Photoinduced Dynamics in SubPc−Azu−C60a

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CONCLUSIONS A new approach has been laid out for controlling directionality of excitation energy transfer flow in molecular hybrids. The molecular design takes advantage of dispersion-induced intramolecular π−π interactions to facilitate highly efficient through-space singlet-EET (ΦEET = 0.92) while back-TET is efficiently blocked in the time scale of the experiment. This result stands in stark contrast to energy transfer dynamics observed in most similar subphthalocyanine/Buckminsterfullerene hybrid systems that have been studied previously, in which back-TET to the subphthalocyanine subunit occurs with a rate constant limited by intersystem crossing to the triplet manifold in the fullerene. We envision this design strategy to be useful for future considerations of excited state energy flow in hybrid and supramolecular systems for photon energy conversion. The presented design strategy can also be further refined by actively involving the choice of solvent in the finetuning of interactions and energy flow. The specific system of this study, SubPc−Azu−C60, has properties such as strong and easily tunable visible light absorption and nonplanar geometry of the light harvesting antenna unit. These properties render the hybrid interesting for applications in the fields of photodynamic therapy and noncoherent photon upconversion, where efficient conversion of incident photons to triplet excited states by a non-aggregating heavy-atom free compound is desirable.

a

EET is excitation energy transfer, ISC is intersystem crossing, and TET is triplet energy transfer. Triplet state energies were adopted from ref 10.

singlet excited state that is rapidly quenched (τ = 17.5 ps) by excitation energy transfer to the C60 subunit via a Coulombic “through-space” interaction. No significant contribution from electron exchange is expected in the EET process due to the absence of orbital overlap between donor and acceptor parts. After intersystem crossing to the C60 localized triplet manifold, back-EET to the SubPc triplet states is highly inefficient due to the exponential dependence on donor−acceptor distance of the rate constant of a Dexter-type EET mechanism, which is required for triplet energy transfer to take place. On the time scale of the transient absorption experiments in this study, no regain of the SubPc ground state bleach signal is observed, G


Article


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EXPERIMENTAL SECTION Steady-State Spectroscopy. Steady-state absorption measurements were performed using a Cary 300 spectrophotometer (Agilent Technologies) or a Lambda 1050 spectrophotometer (PerkinElmer). Steady-state emission spectra were measured using a Fluotime spectrofluorometer (PicoQuant) with a 532 nm laser as the excitation source, while excitation spectral measurements were done using a QuantaMaster 400 spectrofluorometer (Photon Technology International) equipped with a xenon arc lamp as the excitation source. For fluorometric measurements the absorbance was kept below 0.1 to minimize inner-filter effects, and all fluorescence spectra were corrected for the wavelength dependency of the detector system. Fluorescence quantum yields were derermined using the comparative method.44 Time-Resolved Spectroscopy. For time-correlated single photon counting (TCSPC) measurements a Fluotime spectrofluorometer (PicoQuant) with a 560 nm laser was applied. A scattering solution was used to record an instrument response function. Deconvolution of the emission decay data with the instrument response function was done using a deconvolution procedure implemented in the Igor Pro software (WaveMetrics). The laser system applied for ultrafast transient absorption measurements has been described elsewhere.45,46 Briefly, the 800 nm output of 110 fs laser pulses at 1 kHz from a Spitfire regenerative amplifier (Spectra Physics) was split into pump and probe beams. A mechanical chopper synchronized with the laser system blocked every second pump pulse. The pump beam was directed through an optical parametric amplifier (TOPAS, Light Conversion) to generate 540 nm pulses that was used for sample excitation. The intensity of the pump was kept below 4 μJ/pulse. A translation stage (Standa) was used to delay probe with respect to pump pulses before conversion of probe pulses to a white light continuum in a fused silica crystal. The polarization of the probe pulses was set to magic angle, 54.7°, with respect to the pump beam polarization. The pump and probe beams were overlapped in the sample contained in a 1 mm fused silica cuvette placed in a motorized sample holder generating a continuous vertical movement of the cuvette during measurements. The probe beam was split into signal and reference fractions with the reference directed around the sample. Both beams were detected at individual photodiodes after passing through a monochromator. The output from the photodiodes was collected by gated boxcar integrators and processed by an analog processor before phase sensitive detection in a lock-in amplifier. Electrochemical Methods. Cyclic voltammetry was carried out at room temperature in CH2Cl2 containing Bu4NPF6 (0.1 M) as the supporting electrolyte using an Autolab PGSTAT12 instrument driven by the Nova 1.11 software. The working electrode was a circular glassy carbon disk (d = 3 mm), the counter electrode was a thin platinum wire, and the reference electrode was a silver wire immersed in the solvent-supporting electrolyte mixture and physically separated from the solution containing the substrate by a ceramic frit. The potential of the reference electrode was determined vs the ferrocene/ferrocenium (Fc/Fc+) redox system in separate experiments. The voltage sweep rate was 0.1 V s−1. iR compensation was used in all experiments. Solutions were purged with argon saturated with CH2Cl2 for at

least 10 min before the measurements were made, after which a stream of argon was maintained over the solutions. Computational Methods. Density functional theory (DFT) calculations were performed using the Gaussian 16 quantum chemical package.47 The geometries were optimized on CAM-B3LYP/3-21G and ωB97XD/3-21G levels both in the gas phase and with a self-consistent reaction field (SCRF) solvent model with an integral equation formalism polarizable continuum model (IEFPCM).48 For single-point energy calculations, time-dependent (TD) DFT and natural transition orbital (NTO)49 calculations the 6-311+G(d) basis set were used in combination with the above functionals, with or without application of the Tamm−Dancoff approximation (TDA).38

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b06064.

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Synthetic procedures and characterization data, NMR spectra, photophysical characterization (absorption spectra, extinction coefficient spectra, PL spectra, TCSPC data, transient absorption spectra), cyclic voltammograms, and computational results (optimized structures, atomic coordinates, molecular orbitals, TDDFT calculations, NTOs) (PDF)

AUTHOR INFORMATION

Corresponding Author

*T. I. Sølling. E-mail: [email protected]. ORCID

Mogens Brøndsted Nielsen: 0000-0001-8377-0788 Theis I. Sølling: 0000-0003-1710-9072 Present Addresses

‡ (J.S.L.) SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark. § (M.J.) Department of Chemistry and Chemical Engineering Division of Applied Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Sidsel Ammitzbøll Bogh (University of Copenhagen) for her help with fluorescence measurements. This work was supported by the Villum Foundation.

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REFERENCES

(1) Bühl, M.; Hirsch, A. Spherical Aromaticity of Fullerenes. Chem. Rev. 2001, 101, 1153−1184. (2) Cui, C.; Li, Y.; Li, Y. Fullerene Derivatives for the Applications As Acceptor and Cathode Buffer Layer Materials for Organic and Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1601251. (3) Gatti, T.; Menna, E.; Meneghetti, M.; Maggini, M.; Petrozza, A.; Lamberti, F. The Renaissance of Fullerenes with Perovskite Solar Cells. Nano Energy 2017, 41, 84−100. (4) Meller, A.; Ossko, A. Phthalocyaninartige Bor-Komplexe. Monatsh. Chem. 1972, 103, 150−155. (5) Claessens, C. G.; González-Rodríguez, D.; Rodríguez-Morgade, M. S.; Medina, A.; Torres, T. Subphthalocyanines, Subporphyrazines, H


Article


(22) Ü nlü, H.; Okutan, E. Preparation of BODIPY- Fullerene and Monostyryl BODIPY-Fullerene Dyads As Heavy Atom Free Singlet Oxygen Generators. Dyes Pigm. 2017, 142, 340−349. (23) Ü nlü, H.; Okutan, E. Novel Distyryl BODIPY-Fullerene Dyads: Preparation, Characterization and Photosensitized Singlet Oxygen Generation Efficiency. New J. Chem. 2017, 41, 10424−10431. (24) Lissau, J. S.; Nauroozi, D.; Santoni, M.-P.; Ott, S.; Gardner, J. M.; Morandeira, A. Photon Upconversion from Chemically Bound Triplet Sensitizers and Emitters on Mesoporous ZrO2: Implications for Solar Energy Conversion. J. Phys. Chem. C 2015, 119, 25792− 25806. (25) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395−465. (26) Sugunan, S. K.; Greenwald, C.; Paige, M. F.; Steer, R. P. Efficiency of Noncoherent Photon Upconversion by Triplet-Triplet Annihilation: The C60 Plus Anthanthrene System and the Importance of Tuning the Triplet Energies. J. Phys. Chem. A 2013, 117, 5419− 5427. (27) Wei, Y.; Zhou, M.; Zhou, Q.; Zhou, X.; Liu, S.; Zhang, S.; Zhang, B. Triplet-Triplet Annihilation Upconversion Kinetics of C60Bodipy Dyads As Organic Triplet Photosensitizers. Phys. Chem. Chem. Phys. 2017, 19, 22049−22060. (28) Guo, S.; Sun, J.; Ma, L.; You, W.; Yang, P.; Zhao, J. Visible Light-Harvesting Naphthalenediimide (NDI)-C60 Dyads As HeavyAtom-Free Organic Triplet Photosensitizers for Triplet-Triplet Annihilation Based Upconversion. Dyes Pigm. 2013, 96, 449−458. (29) Huang, D.; Zhao, J.; Wu, W.; Yi, X.; Yang, P.; Ma, J. VisibleLight-Harvesting Triphenylamine Ethynyl C60-BODIPY Dyads As Heavy-Atom-Free Organic Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversion. Asian J. Org. Chem. 2012, 1, 264−273. (30) Wu, W.; Zhao, J.; Sun, J.; Guo, S. Light-Harvesting Fullerene Dyads As Organic Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversions. J. Org. Chem. 2012, 77, 5305−5312. (31) Yang, P.; Wu, W.; Zhao, J.; Huang, D.; Yi, X. Using C60-Bodipy Dyads That Show Strong Absorption of Visible Light and Long-Lived Triplet Excited States As Organic Triplet Photosensitizers for TripletTriplet Annihilation Upconversion. J. Mater. Chem. 2012, 22, 20273− 20283. (32) Luhman, W. A.; Holmes, R. J. Investigation of Energy Transfer in Organic Photovoltaic Cells and Impact on Exciton Diffusion Length Measurements. Adv. Funct. Mater. 2011, 21, 764−771. (33) Sanchez-Molina, I.; Claessens, C. G.; Grimm, B.; Guldi, D. M.; Torres, T. Trapping Fullerenes with Jellyfish-like Subphthalocyanines. Chem. Sci. 2013, 4, 1338−1344. (34) The detailed synthetic procedures for making these novel compounds are included in the Supporting Information. A key synthetic step is introduction of a substituent at the axial boron position by a gentle alkynylation reaction where a SubPc boron chloride is treated with a trimethylsilyl-protected alkyne and AlCl3 at room temperature, as previously described by some of us.50 (35) Foggi, P.; Neuwahl, F. V. R.; Moroni, L.; Salvi, P. R. S1 → Sn and S2 →Sn Absorption of Azulene: Femtosecond Transient Spectra and Excited State Calculations. J. Phys. Chem. A 2003, 107, 1689− 1696. (36) Du, C.; Xu, J.; Li, Y.; Xu, W.; Zhu, D. Influence of Organic Acids on UV-Vis Spectra of Pyrrolidino-[60]fullerene Derivatives. Chin. Sci. Bull. 2001, 46, 1156−1159. (37) Gotfredsen, H.; Jevric, M; Kadziola, A.; Nielsen, M. B. Acetylenic Scaffolding with Subphthalocyanines. Eur. J. Org. Chem. 2016, 2016, 17−21. (38) Hirata, S.; Head-Gordon, M. Time-Dependent Density Functional Theory Within the Tamm-Dancoff Approximation. Chem. Phys. Lett. 1999, 314, 291−299. (39) Wong, K. F.; Bagchi, B.; Rossky, P. J. Distance and Orientation Dependence of Excitation Transfer Rates in Conjugated Systems: Beyond the Förster Theory. J. Phys. Chem. A 2004, 108, 5752−5763.

and Subporphyrins: Singular Nonplanar Aromatic Systems. Chem. Rev. 2014, 114, 2192−2277. (6) Torre, G. d. l.; Bottari, G.; Torres, T. Phthalocyanines and Subphthalocyanines: Perfect Partners for Fullerenes and Carbon Nanotubes in Molecular Photovoltaics. Adv. Energy Mater. 2017, 7, 1601700. (7) KC, C. B.; Lim, G. N.; Zandler, M. E.; D’Souza, F. Synthesis and Photoinduced Electron Transfer Studies of a Tri(Phenothiazine)Subphthalocyanine-Fullerene Pentad. Org. Lett. 2013, 15, 4612−4615. (8) El-Khouly, M. E.; Shim, S. H.; Araki, Y.; Ito, O.; Kay, K.-Y. Effect of Dual Fullerenes on Lifetimes of Charge-Separated States of Subphthalocyanine-Triphenylamine-Fullerene Molecular Systems. J. Phys. Chem. B 2008, 112, 3910−3917. (9) González-Rodríguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Echegoyen, L. Energy Transfer Processes in Novel Subphthalocyanine-Fullerene Ensembles. Org. Lett. 2002, 4, 335−338. (10) González-Rodríguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. A.; Echegoyen, L. Subphthalocyanines: Tuneable Molecular Scaffolds for Intramolecular Electron and Energy Transfer Processes. J. Am. Chem. Soc. 2004, 126, 6301−6313. (11) Rudolf, M.; Trukhina, O.; Perles, J.; Feng, L.; Akasaka, T.; Torres, T.; Guldi, D. M. Taming C60 Fullerene: Tuning Intramolecular Photoinduced Electron Transfer Process with Subphthalocyanines. Chem. Sci. 2015, 6, 4141−4147. (12) KC, C. B.; Lim, G. N.; D’Souza, F. Tuning Optical and Electron Donor Properties by Peripheral Thio-Aryl Substitution of Subphthalocyanine: A New Series of Donor-Acceptor Hybrids for Photoinduced Charge Separation. Chem. - Eur. J. 2016, 22, 13301− 13311. (13) KC, C. B.; Lim, G. N.; D’Souza, F. Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in SubphthalocyanineFullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process. Chem. - Asian J. 2016, 11, 1246−1256. (14) González-Rodríguez, D.; Torres, T.; Herranz, M.; Echegoyen, L.; Carbonell, E.; Guldi, D. Screening Electronic Communication Through ortho-, meta- and para-Substituted Linkers Separating Subphthalocyanines and C60. Chem. - Eur. J. 2008, 14, 7670−7679. (15) González-Rodríguez, D.; Carbonell, E.; Guldi, D.; Torres, T. Modulating Electronic Interactions Between Closely Spaced Complementary π Surfaces with Different Outcomes: Regio- and Diastereomerically Pure Subphthalocyanine-C60 Tris Adducts. Angew. Chem., Int. Ed. 2009, 48, 8032−8036. (16) González-Rodríguez, D.; Carbonell, E.; Rojas, G. d. M.; Castellanos, C. A.; Guldi, D. M.; Torres, T. Activating Multistep Charge-Transfer Processes in Fullerene-Subphthalocyanine-Ferrocene Molecular Hybrids As a Function of π-π Orbital Overlap. J. Am. Chem. Soc. 2010, 132, 16488−16500. (17) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. Photophysical Properties of C60. J. Phys. Chem. 1991, 95, 11−12. (18) Zhao, J.; Chen, K.; Hou, Y.; Che, Y.; Liu, L.; Jia, D. Recent Progress in Heavy Atom-Free Organic Compounds Showing Unexpected Intersystem Crossing (ISC) Ability. Org. Biomol. Chem. 2018, 16, 3692. (19) Manna, M. K.; Shokri, S.; Wiederrecht, G. P.; Gosztola, D. J.; Ayitou, A. J.-L. New Perspectives for Triplet-Triplet Annihilation Based Photon Upconversion Using All-Organic Energy Donor & Acceptor Chromophores. Chem. Commun. 2018, 54, 5809. (20) Chiang, L. Y.; Padmawar, P. A.; Rogers-Haley, J. E.; So, G.; Canteenwala, T.; Thota, S.; Tan, L.-S.; Pritzker, K.; Huang, Y.-Y.; Sharma, S. K.; et al. Synthesis and Characterization of Highly Photoresponsive Fullerenyl Dyads with a Close Chromophore Antenna-C60 Contact and Effective Photodynamic Potential. J. Mater. Chem. 2010, 20, 5280−5293. (21) Huang, L.; Yu, X.; Wu, W.; Zhao, J. Styryl Bodipy-C60 Dyads As Efficient Heavy-Atom-Free Organic Triplet Photosensitizers. Org. Lett. 2012, 14, 2594−2597. I


Article

The Journal of Physical Chemistry A (40) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.; Olmstead, M. M. A Double Concave Hydrocarbon Buckycatcher. J. Am. Chem. Soc. 2007, 129, 3842−3843. (41) Martín, N.; Sánchez, L.; Herranz, M. A.; Guldi, D. M. Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF). J. Phys. Chem. A 2000, 104, 4648−4657. (42) Kim, J.-H.; El-Khouly, M. E.; Araki, Y.; Ito, O.; Kay, K.-Y. Photoinduced Processes of Subphthalocyanine-Diazobenzene-Fullerene Triad As an Efficient Excited Energy Transfer System. Chem. Lett. 2008, 37, 544−545. (43) Vetráková, L.; Ladányi, V.; Al Anshori, J.; Dvořaḱ , P.; Wirz, J.; Heger, D. The Absorption Spectrum of Cis-Azobenzene. Photochem. Photobiol. Sci. 2017, 16, 1749−1756. (44) Crosby, G. A.; Demas, J. N. Measurement of Photoluminescence Quantum Yields. Review. J. Phys. Chem. 1971, 75, 991−1024. (45) Rusteika, N.; Møller, K. B.; Sølling, T. I. New Insights on the Photodynamics of Acetone Excited with 253−288nm Femtosecond Pulses. Chem. Phys. Lett. 2008, 461, 193−197. (46) Schalk, O.; Broman, S. L.; Petersen, M. Å.; Khakhulin, D. V.; Brogaard, R. Y.; Nielsen, M. B.; Boguslavskiy, A. E.; Stolow, A.; Sølling, T. I. On the Condensed Phase Ring-Closure of Vinylheptafulvalene and Ring-Opening of Gaseous Dihydroazulene. J. Phys. Chem. A 2013, 117, 3340−3347. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R., Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; et al. Gaussian 16, Revision A.03; Gaussian Inc.: Wallingford, CT, 2016. (48) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3093. (49) Martin, R. L. Natural Transition Orbitals. J. Chem. Phys. 2003, 118, 4775−4777. (50) 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.

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