Contents
1 Synthesis and Characterization S3
2 Spectroscopy and Molecular Dynamics S15
2.1 Experimental Details . . . S15 2.1.1 Chemicals . . . S15 2.1.2 Steady State Measurements . . . S15 2.1.3 Transient Absorption (TA) . . . S15 2.2 Molecular Dynamics Simulations . . . S16 2.2.1 Parameterization and Initial Equilibration . . . S16 2.2.2 Umbrella Sampling . . . S17 2.2.3 Special Treatment of18C6⊂⊂⊂Ba2+. . . S17 2.3 Photophysical properties ofRef . . . S17 2.3.1 Comparison betweenRefandPtc . . . S17 2.3.2 Solvatochromism ofRef. . . S18 2.3.3 Internal conversion ofReffollows the energy-gap law . . . S19 2.4 Transient absorption spectroscopy ofRef . . . S20 2.4.1 Solvent dependence of fs-ps transient absorption spectroscopy . . . S20 2.4.2 Spectral signatures and comparison of Sn←S0and Sn←S1absorption . . . S21 2.4.3 ps -µs transient absorption ofRefand triplet sensitization experiments . . . S22 2.5 Photophysical properties of18C6 . . . S23 2.5.1 Steady-state absorption of18C6 . . . S23 2.5.2 Absorption spectra of18C6in linear alkanes . . . S23 2.5.3 Fluorescence quantum yields of18C6 . . . S24 2.5.4 Time-correlated single photon counting of18C6 . . . S25 2.5.5 Solvent dependence of absorption transition dipole moment ofRefand18C6 . . . S26 2.6 Femtosecond transient absorption spectroscopy of18C6. . . S27 2.6.1 Visualising triplet formation usingRT/S(t) . . . S27 2.6.2 Solvation dynamics and singlet fission dynamics in ACN . . . S27 2.6.3 Polarity dependence of singlet fission in18C6 . . . S29 2.6.4 Viscosity dependence of singlet fission in18C6 . . . S30 2.7 Nanosecond transient absorption spectroscopy of18C6 . . . S31 2.7.1 Comparison of fs -µs transient absorption spectra ofRef, 18C6and18C6⊂⊂⊂Ba2+in ACN S31 2.7.2 Comparison of the late triplet decay of 18C6 and the triplet decay of Ref in ACN . . . S31 2.7.3 Viscosity dependence of the triplet decay of18C6 . . . S32 2.7.4 Extracting the concentration profiles from ps -µs transient absorption . . . S33 2.8 Estimation of the singlet fission yield ΦSF . . . S40
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1
Synthesis and Characterization
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Synthesis and characterization of organic compounds
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General information
All reactions were carried out under N2 in oven-dried glassware, using dry solvents (CH2Cl2, tetrahydrofuran, and Et3N). Where specified for reactions involving oxygen sensitive compounds, the solvents were degassed by N2 bubbling. Unless otherwise stated, all reagents were purchased from commercial sources and used without further purification. Analytical thin layer chromatography (TLC) was performed with silica gel 60 F254 aluminium sheets from Merck. Flash column chromatography was performed using silica SiliaFlash P60, 40-63 μm (230-400 mesh), where specified the silica was neutralized with Et3N prior to sample loading. NMR spectra were recorded on a Bruker AVANCE III HD-NanoBay 400 MHz spectrometer, equipped with a 5 mm CryoProbeProdigy, or on a Bruker II 500 MHz spectrometer, equipped with a 5 mm Cryogenic DCH (1H/13C) probe at 298 K. 1H-NMR chemical shifts are given in ppm relative to Me4Si using solvent resonances as internal standards (CD2Cl2 δ= 5.32 ppm). NMR peaks are reported as follows: chemical shift (δ) in ppm, multiplicity (s = singlet, bs = broad singlet, d =doublet, t = triplet, dd = doublet of doublet, m =multiplet), coupling constant (Hz) and integration. 13C-NMR chemicals shifts were given in ppm relative to Me4Si with solvent resonances used as internal standards (CD2Cl2 δ= 53.84 ppm. Melting points (m.p.) were measured in open capillary tubes with a Büchi B-550 melting point apparatus and were uncorrected. High resolution mass spectra (HR-MS) were recorded on a QSTAR Pulsar (AB/MDS Sciex) spectrometer by the Department of Mass Spectroscopy at the University of Geneva.
4-bromo-1,2-bis(dibromomethyl)benzene (2)
Following literature procedures,1-2 4-bromo-1,2-dimethylbenzene (1) (10.0 g, 53.8 mmol) was dissolved in 130 mL of degassed CHCl3, then N-bromosuccinimide (40.0 g, 225.0 mmol) and benzoylperoxide (1.25 g , 5.0 mmol) were added. The mixture was heated to reflux under stirring for 24 h. The organic phases were washed with H2O (2x200 mL) and brine (2x200 mL), dried over Na2SO4 and concentrated under reduced pressure.
The crude was crystallized from hot diethylether upon addition of pentane (~200 mL). The precipitated was filtered and washed with cold pentane. 9.38 g of a pale yellow powder were recovered. The compound was obtained as an inseparable mixture of brominated compounds and its spectral data were in agreement with that previously reported in the literature.1-2
Anthracene-1,4-dione (4)
Following a literature procedure,2 quinizarine (3) (15.0 g, 62.5 mmol) was dissolved in 300 mL of methanol, the mixture was then cooled to 0 °C and NaBH4 (9.7 g, 25 mmol) was added portion-wise. The mixture was stirred for 2 h, then 110 mL of a 6 N aqueous solution of HCl were slowly added. The precipitated was filtered and washed with water and cold acetone. 11.6 g (yield 89%) of a red-orange solid were obtained. The spectral data were in agreement with that previously reported in the literature.2
2-bromopentacene-6,13(5aH,13aH)-dione (5)
Following literature procedures,2-3 compound 2 (9.0 g, 18.0 mmol), compound 4 (3.6 g, 17.0 mmol) and NaI (12.6 g, 84 mmol) were dissolved in 70 mL of dimethylformamide. The mixture was stirred at 110 °C for 24 h. Then the reaction was cooled to 0 °C and filtered, the collected solid was washed thoroughly with H2O, methanol, acetone and diethylether. 4.34 g (yield 66%) of a gold-like insoluble solid were recovered. The spectrometric mass data are in agreement with that previously reported in the literature.2
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((2-bromopentacene-6,13-diyl)bis(ethyne-2,1-diyl))bis(triisopropylsilane) (6)
Following literature procedures,2-3 triisopropylsilylacetylene (10.0 g, 55.0 mmol) was dissolved in 160 mL of dry THF, the solution was cooled down to 0 °C and 26 mL (40.3 mmol) of a nBuLi solution (1.55 M in hexanes) were slowly added. The ice bath was removed and the mixture was stirred at room temperature for 1 h. Then compound 5 (4.15 g, 10.7 mmol) was added and the mixture was stirred at 65 °C for 18 h. The end of the reaction was indicated by the complete dissolution of the precipitate. The reaction was cooled to room temperature and a solution containing 20 g (10.6 mmol) of SnCl2·H2O in 20 mL of 10% (v/v) aqueous HCl was slowly added. The reaction was left under stirring at 60 °C for 1 h, during which the solution turned into an intense blue color. The reaction mixture was concentrated under reduced pressure to remove THF, then the aqueous phase was extracted with CH2Cl2 (2x150 mL). The combined organic phases were washed with water several times until neutrality and then with brine. The organic phases were dried over Na2SO4 and concentrated under reduced pressure. The crude was purified by flash chromatography over silica gel (eluent: pentane). 3.33 g (yield 43%) of a blue solid were obtained. The physical and spectral data are in agreement with that previously reported in the literature.2
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N-(diphenylmethylene)-6,13-bis((triisopropylsilyl)ethynyl)pentacen-2-amine (7)
Following a literature procedure,3 Pd2(dba)3 (0.90 g, 0.98 mmol) and BINAP (1.22 g, 1.96 mmol) were mixed in 150 mL of dry and degassed toluene. The solution was refluxed for 30 min. Then the solution was cooled down to room temperature and diphenylmethanimine (2.95 mL, 17.58 mmol), tBuONa (1.69 g, 17.58) and compound 6 (3.00 g, 4.22 mmol) were added at once. The mixture was refluxed for 15 h. The solvent was evaporated under reduced pressure and the compound purified by flash chromatography over neutralized silica gel (eluent: pentane/CH2Cl2, 2:1). 3.32 g of a green solid (yield 96 %) were recovered.
Due to instability the compound was characterized only by 1H-NMR spectroscopy and used immediately for the next step.
Rf = 0.7 (SiO2, pentane/CH2Cl2 2:1)
1H-NMR (500 MHz, CD2Cl2): δ/ppm = 1.29-1.45 (m, 42H, -CH- + -CH3i
Pr), 6.97 (dd, 1H, J = 7.0 Hz, aromatic), 7.20 (s, 1H, aromatic), 7.25-7.29 (m, 5H, aromatic), 7.41-7.55 (m, 5H, aromatic), 7.79-7.84 (m, 3H, aromatic), 7.98 (m, 2H, aromatic), 9.04 (s, 1H, aromatic), 9.18 (s, 1H, aromatic), 9.27 (d, 2H, J = 7.0 Hz, aromatic).
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6,13-bis((triisopropylsilyl)ethynyl)pentacen-2-amine (8)
Compound 7 (3.30 g, 4.00 mmol) was added to 30 mL of pentane, 30 mL of ethanol and 30 mL of 10% HCl (v/v) aqueous solution and the mixture was stirred for 3 h at room temperature. The reaction was quenched by addition of a saturated solution of NaHCO3 and the aqueous phases were extracted with pentane (3x150 mL), then the organic phases combined were washed with brine (3x150 mL), dried over Na2SO4 and concentrated under reduced pressure. The crude was purified by flash chromatography over neutralized silica gel (eluent:
pentane/CH2Cl2, 2:1). 1.93 g of a green solid (yield = 73%) were recovered.
Rf = 0.6 (SiO2, pentane/CH2Cl2 2:1) m.p.: >300 °C
HR MS (ESI) [M+H]+ m/z calculated for [C44H55NSi2 + H]+ 654.3946, observed 654.3951 (+0.7 ppm).
1H-NMR (500 MHz, CD2Cl2): δ/ppm = 1.36-1.45 (m, 42H, -CH,-CH3), 4.14 (bs, 2H, -NH2), 7.00-7.02 (m, 2H, aromatic), 7.41 (m, 2H, aromatics), 7.87 (m, 1H, aromatic), 7. 89 (m, 2H, aromatic), 8.94 (s, 1H, aromatic), 9.17 (s, 1H, aromatic), 9.26 (d, 2H, J = 5Hz, aromatic).
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13C-NMR (126 MHz, CD2Cl2): δ/ppm = 11.7 (6 CH iPr), 18.8 (12 CH3iPr), 104.3 (CH, aromatic), 104.5 (≡C), 104.9 (≡C), 106.7 (≡C), 106.9 (≡C), 116.5 (C aromatic), 118.4 (C aromatic), 121.1 (CH aromatic), 121.9 (CH aromatic), 125.7 (CH aromatic), 125.8 (CH aromatic), 126.0 (C+CH aromatic), 126.2 (2 CH aromatic), 128.5 (2 CH aromatic), 129.0 (C aromatic), 129.3 (C aromatic), 129.8 (C aromatic), 130.2 (CH aromatic), 130.7 (C aromatic), 131.8 (C aromatic), 132.2 (C aromatic), 134.0 (C aromatic), 144.3 (C-N aromatic).
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N-(6,13-bis((triisopropylsilyl)ethynyl)pentacen-2-yl)acetamide (Ref)
Compound 8 (50 mg, 0.076 mmol) and Et3N (0.032 mL, 0.229 mmol) were dissolved in 2 mL of dry CH2Cl2. The mixture was cooled to 0 °C and acetyl chloride (0.011 mL, 0.153 mmol) was added. The ice bath was removed, the mixture was allowed to warm up to room temperature and it was left under stirring for 3 h. The reaction was then quenched by H2O addition. The organic phase was recovered, washed with water (3x50 mL), dried over Na2SO4 and concentrated under reduced pressure. The crude was purified by flash chromatography over silica gel (eluent: CH2Cl2 then CH2Cl2/methanol gradient up to 98:2). 51 mg (yield 96%) of a blue solid were recovered.
Rf = 0.3 (SiO2, CH2Cl2/methanol 98:2).
m.p.: 216-220 °C
HR MS (ESI) [M+H]+ m/z calculated for [C46H57NOSi2+ H]+ 696.4052, observed 696.4035 (-2.4 ppm).
1H-NMR (500 MHz, CD2Cl2): δ/ppm = 1.36-1.47 (m, 42H, -CH- + -CH3iPr), 2.24 (s, 3H, CH3CO), 7.43-7.48 (m, 4H, aromatic + NH), 7.96-8.01 (m, 3H, aromatic), 8.30 (s, 1H, aromatic), 9.22 (s, 1H, aromatic), 9.26 (s, 1H, aromatic), 9.30 (s, 2H, aromatic).
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13C-NMR (126 MHz, CD2Cl2): δ/ppm = 12.1 (6 -CH- iPr), 19.2 (12 -CH3iPr), 25.0 (CH3CO), 104.8 (≡C), 104.9 (≡C), 107.9 (≡C), 108.0 (≡C), 115.0 (CH aromatic), 118.2 (C aromatic), 118.9 (C aromatic), 122.4 (CH aromatic), 125.6 (CH aromatic), 126.5 (CH aromatic), 126.6 (4 CH aromatic), 128.9 (2 CH aromatic), 130.1 (CH aromatic), 130.5 (C aromatic), 130.6 (C aromatic), 130.7 (C aromatic), 131.0 (C aromatic), 131.3 (C aromatic), 132.6 (C aromatic), 132.8 (C aromatic), 132.9 (C aromatic), 136.1 (C aromatic), 168.9 (C=O).
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Macrocycle 9 was synthesized following a literature procedure.4
18C6
Following literature procedures,5-6 compound 8 (235 mg, 0.37 mmol) and macrocycle 9 (50 mg, 0.12 mmol) were mixed in 2 mL of dry and degassed THF. The mixture was cooled to −100 °C (EtOH/liquid nitrogen bath) and freshly sublimed tBuOK (54 mg, 0.48 mmol) was added in one portion. Then the cooling bath was removed and the mixture was allowed to warm up to room temperature and it was left under stirring for 4 h.
The reaction was quenched by the addition of 4-5 drops of MeOH and the reaction mixture was directly loaded onto a flash chromatography column (silica gel; eluent: CH2Cl2 then CH2Cl2/methanol gradient up to 95:5).
The compound was further purified by semipreparative HPLC (NUCLEOSIL column, CH2Cl2/methanol 97:3, flow 4 mL/min). 51 mg of a green solid (yield 26 %) were obtained.
Rf = 0.4 (SiO2, CH2Cl2/methanol 95:5).
m.p.: >300 °C
HR MS (ESI) [M+NH4]+ m/z calculated for [C104H130N2O8Si4 + NH4]+ 1664.9243, observed 1664.9239 (-0.2 ppm).
1H-NMR (500 MHz, CD2Cl2): δ/ppm = 1.15-1.32 (m, 84H, -CH- + -CH3iPr), 3.65-3.7 (m, 4H), 3.74-3.78 (m, 2H, -CH2-), 3.84-3.87 (m, 2H, -CH2-), 3.91-4.0 (m, 6H, -CH2-), 4.04-4.07 (m, 2H, -CH2-), 4.40 (d, 2H, J = 4.4 Hz, =CH2), 4.48 (s, 2H, -CH-), 4.51 (d, 2H, J = 4.4 Hz, =CH2), 7.38-7.42 (m, 4H, aromatic), 7. 59 (d, 2H, J = 7.7 Hz, aromatic), 7.71 (d, 2H, J = 7.7 Hz, aromatic), 7.89-7.94 (m, 4H, aromatic), 8.51 (s, 2H, aromatic), 9.00 (s, 2H, aromatic), 9.07 (s, 2H, aromatic), 9.13 (s, 2H, aromatic), 9.18 (s, 2H, aromatic), 9.64 (s, 2H, NH).
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13C-NMR (126 MHz, CD2Cl2): δ/ppm = 11.9 (6 -CH- iPr), 12.1 (6 -CH- iPr), 19.0 (12 -CH3iPr), 19.2 (12 -CH3 iPr), 67.6 (2 -CH2-), 69.2 (2 -CH2-), 69.6 (2 -CH2-), 70.8 (2 -CH2-), 83.7 (2 -CH-), 88.7 (2 =CH2), 104.8 (2
≡C), 105.0 (2 ≡C), 107.6 (2 ≡C), 107.8 (2 ≡C), 115.4 (2 CH aromatic) , 117.9 (2 C aromatic), 118.7 (2 C aromatic), 122.8 (2 CH aromatic), 125.4 (2 CH aromatic), 126.3 (2 CH aromatic), 126.4 (8 CH aromatic), 126.5 (2 CH aromatic), 128.9 (2 CH aromatic), 129.9 (2 CH aromatic), 130.4 (2 C aromatic), 130.5 (4 C aromatic), 130.9 (2 C aromatic), 131.2 (2 C aromatic), 132.4 (2 C aromatic), 132.6 (2 C aromatic), 133.0 (2 C aromatic), 135.9 (2 C aromatic), 157.1 (2 =C), 168.0 (2 C=O).
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References
1. Kaitz, J. A.; Diesendruck, C. E.; Moore, J. S., Dynamic covalent macrocyclic poly (phthalaldehyde) s: scrambling cyclic homopolymer mixtures produces multi-block and random cyclic copolymers.
Macromolecules 2013, 46 (20), 8121-8128.
2. Kato, D.; Sakai, H.; Tkachenko, N. V.; Hasobe, T., High‐Yield Excited Triplet States in Pentacene Self‐Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission. Angew. Chem. Int. Ed.
2016, 55 (17), 5230-5234.
3. Xia, D.; Guo, X.; Chen, L.; Baumgarten, M.; Keerthi, A.; Müllen, K., Layered Electron Acceptors by Dimerization of Acenes End‐Capped with 1, 2, 5‐Thiadiazoles. Angew. Chem. 2016, 128 (3), 953-956.
4. Poggiali, D.; Homberg, A.; Lathion, T.; Piguet, C.; Lacour, J., Kinetics of Rh (II)-Catalyzed α-Diazo-β-ketoester Decomposition and Application to the [3+ 6+ 3+ 6] Synthesis of Macrocycles on a Large Scale and at Low Catalyst Loadings. ACS Catalysis 2016, 6 (8), 4877-4881.
5. Jarolímová, Z.; Vishe, M.; Lacour, J.; Bakker, E., Potassium ion-selective fluorescent and pH independent nanosensors based on functionalized polyether macrocycles. Chem. Sci. 2016, 7 (1), 525-533.
6. Homberg, A.; Brun, E.; Zinna, F.; Pascal, S.; Górecki, M.; Monnier, L.; Besnard, C.; Pescitelli, G.; Di Bari, L.; Lacour, J., Combined reversible switching of ECD and quenching of CPL with chiral fluorescent macrocycles. Chem. Sci. 2018, 9 (35), 7043-7052.
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2
Spectroscopy and Molecular Dynamics
2.1 Experimental Details 2.1.1 Chemicals
Acetonitrile (ACN, Roth,≥99.9%), tetrahydrofuran (THF, Roth≥99%), n-hexane (HEX, Roth≥99%), dodecane (N10, Sigma-Aldrich≥99%), tetradecane (N14, Roth≥99 %) were used as received. Ba(ClO4)2(Sigma-Aldrich, 97%) was used as received and stored in a desiccator.
2.1.2 Steady State Measurements
Absorption
Absorption spectra were measured with a Cary 50 spectrometer.
Emission
Emission spectra were measured with a Horiba Scientific FluoroMax-4 fluorometer and corrected using a set of secondary emissive standards.1
2.1.3 Transient Absorption (TA)
General Remarks
The TA data presented in this work were recorded with three different experimental setups: a fs-ps visible (fs-VIS), a fs-ps near infrared (fs-NIR) and a ps-µs visible (ps-VIS) TA setup. A detailed description of the general principle of the fs-ps as well as ps-µs TA applying referenced detection using two spectrographs is presented elsewhere.2The fs-VIS and fs-NIR setups share the same pump path, whereas the layouts of the fs-VIS and the ps-VIS probe paths are identical. The absorbance of the sample at the excitation wavelength was 0.05-0.4 on 1 mm. The absorption spectra of all samples before and after the transient absorption experiments showed no sign of degradation. The samples were measured in 1 mm quartz cuvettes (Starna, model 1GS/Q/1) and bubbled with nitrogen during the measurements giving a wavelength dependent instrument response function (IRF) of about 80-350 fs (fwhm of optical Kerr effect (OKE)).
fs-ps Pump
Excitation was performed using 650, 675 of 690 nm pulses from a TOPAS-Prime parametric amplifier in combi-nation with a NirUVis frequency mixer (both from Light Conversion), themselves seeded with the output of a 1 kHz Ti:Sapphire amplified system (Spectra Physics, Solstice Ace). The transient absorption signal was checked prior to the measurements to scale linearly with the pump pulse energy. The polarization of the pump pulses was set to magic angle relative to the white-light pulses. In order to check for pump beam divergence and/or delay-line misalignment, the fs-VIS TA dynamics were cross checked by comparing the kinetics of a calibration sample (perylene in DMSO) measured with the TA setup and a time-correlated single photon counting (TCSPC) setup.
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ps-µsPump
The ps-µs pumping was described in detail in ref. 3. Excitation was performed at 532 of 355 nm using a passively Q-switched, frequency doubled Nd:YAG laser (Teem Photonics, Powerchip NanoUV) producing pulses with a 500 Hz repetition rate, approximately 20µJ per pulse, and 300 ps duration.
Visible Probe
Probing was achieved using white light pulses generated by focusing the 800 nm pulses of the Ti:Sapphire amplified system in a CaF2plate. The experimental setup was the same as that described earlier,4except that all lenses after white light generation were replaced by spherical mirrors to prevent chromatic aberration.
Near Infrared Probe
The white light was generated by focusing the 800 nm pulses in a YAG crystal. To balance the white light spectrum, the intense 800 nm light was removed by a beam stop after generation as well as by a 1 mm cuvette containing IR140 in DMSO. The probe light was then split into a reference and a sample beam using a reflective metallic neutral density filter. After passing the sample, the beam was dispersed in a home-built prism spectrometer and the intensity recorded with a InGaAs detector. To balance the white light spectrum, apodizing neutral density filters were placed directly before both detectors.
Data Treatment
The pixel to wavelength conversion was achieved using a standard containing rare earth metals (NIST 2065 for NIR and holmium oxide for VIS), which shows narrow bands from the UV to the NIR. All transient absorption spectra were corrected for background signals showing up before time zero (e. g. spontaneous emission).
The fs-ps spectra were corrected for the dispersion due to the optical chirp using the optical Kerr effect.5 For merging the fs-VIS and the fs-NIR spectra, the TA signals in the overlapping region between 690 and 740 nm were compared and one of the two datasets was multiplied with a constant factor accounting for the difference in pump intensity. The comparison of the kinetics recorded in the overlap region additionally serves as quality control, directly revealing erroneous kinetics due e.g. to deviation from the magic angle or poor alignment of the delay line.
2.2 Molecular Dynamics Simulations 2.2.1 Parameterization and Initial Equilibration
All molecular dynamics simulations were undertaken using GROMACS 2018.1.6 All visualizations were carried out using VMD 1.9.3.7 The OPLS-AA force-field8with full flexibility was used for the18C6solute and solvents.
Solvent parameters for tetrahydrofuran (THF) were taken directly from the force-field, whereas the acetonitrile (ACN) parameters were taken from later optimizations of the force-field,9and n-hexane (HEX) parameters were taken from the optimizations of Siuet. al.10
For parameterization of18C6, the dimer was broken into two principle components: the crown ether (CRET) and the TIPS pentacene (TIPS). The CRET component included all atoms of the ether ring up to the nitrogen atom of the amine bridges. Non-bonded parameters for this component were taken directly from the OPLS-AA force-field.
Non-bonded parameters for TIPS were taken from Steineret al.11Atomic charges were determined from a CHELPG fit of the electrostatic potential generated from a density functional theory geometry optimization calculation performed in Gaussian1612at the B3LYP/G-31G(d,p) level of theory.13 Charges of chemically equivalent atoms were averaged in order to symmeterize the molecule.
The simulations began by placing the optimized dimer at the center of an 8×8×8 nm cube. The box was filled with 2330 ACN molecules. Following a steepest-descent energy minimization procedure, the system was equilibrated using three consecutive 50 ps simulations in the NVT, NPT, and NVT ensembles. Integration was handled using the Verlet leap-frog algorithm with a timestep of 0.002 ps. Hydrogen-containing bonds were constrained using the P-LINCS algorithm.14Long-range electrostatics were handled using the particle mesh Ewald method, and both the Coulomb and Lennard-Jones potentials were cutoff at 1.4 nm. The temperature was set to 293.15 K and was kept constant using the modified Berendsen thermostat with a relaxation time of 0.5 ps. For the NPT simulations, a Berendsen barostat was employed with a 5 ps relaxation time and a reference pressure of 1.013 bar.
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2.2.2 Umbrella Sampling
Following equilibration, a production simulation of the dimer/ACN system in the NVT ensemble was carried out for a time sufficient to generate a configuration with a pentacene-pentacene center-of-mass separation,r, less than 0.40 nm. This dimer conformation was extracted and was used as the starting point for pulling simulations in ACN, THF (1600 molecules per box), and HEX (2300 molecules per box) in order to generate a range of conformations with differentr to act as starting points for umbrella sampling. Boxes were constructed and equilibrated as previously described, but withrfixed using a harmonic potential. Following this equilibration, a pulling simulation was carried out using a harmonic potential onrto generate a range of dimer conformations withrup to 2.75 nm. A total of 40 conformations linearly spaced inrwere then selected as starting points for umbrella sampling simulations.
The 40 umbrella sampling simulations for each solvent were carried out for 10 ns, with the first 0.5 ns discarded as equilibration. The reaction coordinate,r, was constrained using a harmonic potential with a 1000 kJ mol−1nm−2 spring constant. Potentials of mean force were then calculated using the WHAM procedure15as implemented in GROMACS.16
2.2.3 Special Treatment of 18C6⊂⊂⊂Ba2+
The18C6⊂⊂⊂Ba2+system had to be treated specially in order to ensure that crown ether was properly equilibrated around the Ba2+ion. Two Cl−ions were placed in the box at opposite corners and fixed in place in order to preserve charge balance, and the Lennard-Jones parameters for these ions were taken from the OPLS-AA force-field. An18C6conformation with a particularly ’open’ crown ether was selected from the THF simulations and the Ba2+ion was placed near this opening. The system was then equilibrated as previously described, but with the distance between one crown ether oxygen and the Ba2+fixed. This procedure allowed the crown ether to orient itself around the Ba2+and hold the Ba2+inside. Then, two pull simulations were carried out, one pulling the TIPS pentacenes together to generate the close-contactr, and one pulling them away to generate the long-distance r. Umbrella simulations and PMF calculations were then conducted as previously described.
2.3 Photophysical properties of Ref
In order to assess the impact of the amide linker to the optical properties of TIPS-pentacene (Ptc), steady-state absorption, emission and time-correlated single photon counting (TCSPC) measurements were performed in solvents of varying polarity.
2.3.1 Comparison between Ref and Ptc
The steady-state absorption and emission spectra of PtcandRef in low and medium polarity solvents are compared in Figure S1. The addition of the amide toPtcinduces a broadening of the absorption and emission bands as well as an increased emission solvatochromism.
A B
Figure S1: Steady-state absorption and emission spectra of (A)Ptc(A) and (B)Refin different solvents.
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2.3.2 Solvatochromism of Ref
Figure S2: (A) Absorption and emission solvatochromism ofRef and (B) solvent polarity dependence of the fluorescence lifetime.
The amide group also increases the solubility in polar solvents compared toPtc, enabling a systematic sol-vatochromic study in a set of solvents that differ in polarity but with a nearly identical refractive index. A bathochromic shift of the emission spectrum exceeding 1000 cm−1is accompanied by a significant decrease of the fluorescence lifetime (Figure S2).
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2.3.3 Internal conversion of Ref follows the energy-gap law
To determine the reason for the solvent dependence of the fluorescence lifetime, the fluorescence quantum yields, Φf, radiative (kR) and non-radiative (kNR) rate constants as well as the fluorescence transition dipole moments (|~µem|) were determined. The extracted photophysical parameters are correlated with the first moment (m1) of the emission spectrum in Figure S3. kNRchanges by a factor of 10 whereas thekRonly changes by a factor of 2, illustrating that the lifetime decrease is mainly due tokNR. kNRfollows the energy-gap law, which predicts an acceleration of the internal conversion with decreasing energy gap (Figure S3A). The substantial increase of internal conversion rate constant due to the solvatochromic shift is particularly pronounced in this system due to the small energy gap compared to usual solvatochromic probes such as coumarins.
A B
C D
E F
φf
Figure S3: Correlation between the first moment of the fluorescence spectra,m1, and (A,B) the non-radiative rate constantkNR, (C) the emission transition dipole moment,|~µem|, (D), the radiative rate constant,kR, (E), the fluorescence quantum yield, Φf, and (E) the fluorescence rate constant,kfl, in solvents of different polarity.
We will show in section 2.5.5 that the absorption transition dipole moment (|~µabs|) ofRefis independent of solvent polarity. The slight decrease of|~µem|in ACN could be due to a systematic error originating from the red shifted absorption band. Not only Φfbut also the sensitivity of the detector decreases drastically in this spectral region.
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2.4 Transient absorption spectroscopy of Ref
2.4.1 Solvent dependence of fs-ps transient absorption spectroscopy
0
10 Ref,NHX
0 50
A / 103
Ptc, NHX 0.2 ps
2.0 ps 1000 ps
0
20 Ref, ACN
8 10 12 14 16 18 20 22 24 26 28 30
/ 103 cm1 0
10 Ptc, ACN
1600 1000 800 700 600 500 400
350 / nm
Figure S4: Femtosecond transient absorption spectra ofRefandPtcin the apolar solvent n-hexane (HEX) and polar solvent acetonitrile (ACN).
The femtosecond transient absorption spectra from the UV-Vis to the NIR spectral regions measured withPtc andRefare shown in Figure S4. In the apolar HEX no spectral shift can be observed for both chromophores
The femtosecond transient absorption spectra from the UV-Vis to the NIR spectral regions measured withPtc andRefare shown in Figure S4. In the apolar HEX no spectral shift can be observed for both chromophores