Controlling Intramolecular Förster Resonance Energy Transfer and Singlet Fission in a Subporphyrazine–Pentacene Conjugate by Solvent Polarity

Abstract Due its complementary absorptions in the range of 450 and 600 nm, an energy‐donating hexaaryl‐subporphyrazine has been linked to a pentacene dimer, which acts primarily as an energy acceptor and secondarily as a singlet fission enabler. In the corresponding conjugate, efficient intramolecular Förster resonance energy transfer (i‐FRET) is the modus operandi to transfer energy from the subporphyrazine to the pentacene dimer. Upon energy transfer, the pentacene dimer undergoes intramolecular singlet fission (i‐SF), that is, converting the singlet excited state, via an intermediate state, into a pair of correlated triplet excited states. Solvatochromic fluorescence of the subporphyrazine is a key feature of this system and features a red‐shift as large as 20 nm in polar media. Solvent is thus used to modulate spectral overlap between the fluorescence of subporphyrazine and absorption of the pentacene dimer, which controls the Förster rate constant, on one hand, and the triplet quantum yield, on the other hand. The optimum spectral overlap is realized in xylene, leading to Förster rate constant of 3.52×1011 s−1 and a triplet quantum yield of 171 % ±10 %. In short, the solvent polarity dependence, which is a unique feature of subporphyrazines, is decisive in terms of adjusting spectral overlap, ensuring a sizable Förster rate constant, and maximizing triplet quantum yields. Uniquely, this optimization can be achieved without a need for synthetic modification of the subporphyrazine donor.


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to correct for fluctuations in the probe beam intensity, the probe and reference beam were detected independently. Furthermore the spectra were acquired shot-by-shot and averaged at each delay over ~1500 times, to ensure a reasonable signal-to-noise ratio.
FsTA and NsTA Data Evaluation. Results from fsTa and nsTA data have been analyzed via multiwavelength and GloTarAn target analysis. Target analysis was performed on the TA data sets using the proposed sequential model. The analytical solution to the coupled differential equations that describe the kinetic model is convoluted with a Gaussian instrument response function. After the least-squares fitting has converged, the raw data matrix is deconvoluted using the specific solution to the kinetic model and parameters from the fit to obtain the species-associated spectra (SAS) and their populations as a function of time. GloTarAn is a Java-based graphical user interface to the R-package TIMP, which was developed for global and target analysis of time-resolved spectroscopy data. The dispersion (chirp of the whitelight pulse) of the instrument response function (IRF) was modeled, via Surface Xplorer by Ultrafast Systems, and taken into account during the fitting procedure.
Triplet Quantum Yield (TQY) Determination. The determination of the TQY followed the same procedure as in our previous works. 4,5 In brief summary, the TQY was obtained by following the intensification of the ground state bleach (GSB) relating to Pnc2. The SAS were corrected, in order to account for residual SubPz centered contributions (e.g. GSB, fluorescence) of the Pnc2 centered GSB. Equation (1) was used for the TQY calculation of the Pnc2COOH reference compound: While equation (2) was used for the TQY calculation of the SubPzPnc2 conjugate:

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FRET Rate Constant Determination. The determination of the FRET rate also followed the same procedure from previous works, [4], [5] using the following equations (3)  The fluorescence spectra have to be normalized to fulfill equation (7); εA = extinction coefficient of the acceptor at wavelength λ The detailed procedure (including a description of a software-based algorithm to calculate the spectral overlap integral J) is described by Hink et al. [6] For κ 2 we used 2/3 (~0.67), [7] as often used for non-rigid systems ("dynamic isotropic limit"). [8] The refractive indices n were taken as 1.506, [9] 1.496, [10] 1.518, [11] and 1.529 [9] for xylene, toluene, anisole, and benzonitrile, respectively. See Table 3 for a summary of the parameters used for the FRET rate constant calculation as well as for an overview of the obtained values for the FRET constants.

Synthesis and Characterization
Scheme S1. Synthetic route and detailed steps for the preparation of SubPzOH and SubPzPnc2 conjugate. Then, 1-propanethiol (36 μL, 0.40 mmol, 0.2 equiv respect to bis(propylthio)maleonitrile) and freshly distilled triethylamine (56 μL, 0.40 mmol, 0.2 equiv respect to bis(propylthio)maleonitrile) were added via the syringe, and the resulting mixture was stirred heating from 0 °C to room temperature for 1 h. Once the reaction is completed (TLC showed just one red spot in n-hexane/ethyl acetate 20:1) the product was extracted with diethyl ether and washed three times with water, dried over MgSO4, and the solvent was evaporated under reduced pressure. The crude product was chromatographed on silica gel (7:3 mixture of nhexane/ethyl acetate) giving pure target compound 1 (65.0 mg, 12%) as a red viscous solid.      In an oven-dried 5 mL round-bottom Schlenk was charged with formyl-SubPz 2 (17.8 mg, 13.06

SubPzPnc2
To a solution of SubPzOH