Tetrathienothiophene Porphyrin as a Metal-Free Sensitizer for Room-Temperature Triplet-Triplet Annihilation Upconversion

Abstract

Optically excited triplet states of organic molecules serve as an energy pool for the subsequent processes, either photon energy downhill, such as room temperature phosphorescence, or photon energy uphill process-the triplet-triplet annihilation upconversion (TTA-UC). Manifestation of a high intersystem crossing coefficient is an unavoidable requirement for triplet state formation, following the absorption of a single photon. This requirement is even more inevitable if the excitation light is non-coherent, with moderate intensity and extremely low spectral power density, when compared with the light parameters of 1 Sun (1.5 AM). Coordination of a heavy atom increases substantially the probability of intersystem crossing. Nevertheless, having in mind the global shortage in precious and rare-earth metals, identification of metal-free organic moieties able to form triplet states becomes a prerequisite for environmental friendly optoelectronic technologies. This motivates us to synthesize a metal-free thienothiophene containing porphyrin, based on a condensation reaction between thienothiophene-2-carbaldehyde and pyrrole in an acidic medium by modified synthetic protocol. The upconversion couple tetrathienothiophene porphyrin/rubrene when excited at λ = 658 nm demonstrates bright, delayed fluorescence with a maximum emission at λ = 555 nm. This verifies our hypothesis that the ISC coefficient in thienothiophene porphyrin is efficient in order to create even at room temperature and low-intensity optical excitation densely populated organic triplet ensemble and is suitable for photon energy uphill processes, which makes this type of metal-free sensitizers even more important for optoelectronic applications.

Keywords: acidification; inter system crossing; metal-free triplet–triplet annihilation; room temperature; triplet state formation.

FIGURE 1

Simplified energetic scheme of the TTA−UC process in the multicomponent organic system in a molecular oxygen-free environment. Left—structures of the sensitizer (TTP, meso-5,10,15,20-tetrathieno[3,2-b]thienyl porphyrin) and right—structure of the emitter (rubrene).

SCHEME 1

Synthetic methods and the general structure of substituted thienyl-porphyrins.

SCHEME 2

Reaction conditions and yields: (A) Preparation of intermediate 2: 1, DMF, POCl3, 80°C, 2 h, and Yield: 81%; (B) Adler’s method procedure A: 2, 3, acetic acid, 60°C, 2 h, and Yield: 2%; (C) Adler’s method procedure B: 2, 3, acetic acid, reflux 30 min, and Yield: 2.4%; (D) Adler’s method procedure C: 2, 3, Propionic acid, reflux 1 h, 3.3%; and (E) Lindsey’s method procedure D: 2, 3, r. t., dry DCM, TFA, DDQ, TEA, and Yield: 5.1%.

FIGURE 2

Absorption spectra of TTP in ethanol neat (the dark cyan line) and in the presence of 3 × 10^–4 M HCl (the wine line) and in the presence of 5 × 10^–3 M HCl (the violet line). For clarity, all intermediate absorption spectra are in light gray.

FIGURE 3

Normalized Q-band absorption spectrum of the meso-5,10,15,20-tetrathieno[3,2-b]thienyl porphyrin, dissolved in 98 toluene/2 vol% squalene (the dark red line); normalized delayed fluorescence of rubrene (the cyan curve) excited in UC regime, whose excitation wavelength λ _exc = 658 nm. The excitation wavelength is suppressed by using a super broadband notch filter (rejection: more than 10⁴ for the wavelength range centered at Δλ = 705 nm, with FWHM = 175 nm, and transparency, for all other wavelengths, better than 0.99).

FIGURE 4

(A)–Dependence of the UC intensity on the excitation intensity. Experimental conditions: UC couple, TTP (2 × 10^-5 M)/rubrene (4 × 10^-4 M); ratio sensitizer/emitter - C_S/C_E = 1/20; room temperature; sample thickness – 1000 μm; single-mode laser diode, λ _exc = 658 nm; optical registration–via fiber–spectrometer (C10083CA, Hamamatsu Inc.) with absolute wavelength calibration and corrected spectral response; integration time, 100 ms; excitation beam diameter, d_EXC = 1600 µm; Vitrotube^® glass sample, sealed in a nitrogen-filled glove-box, residual oxygen < 2 ppm; each measurement is performed on a neat sample spot; solvent, 98 vol% toluene/2 vol% squalene; Q.Y._TTA-UC = 0.018 (following the IUPAC definition). (B)–Photograph of a working metal-free TTA-UC sample. In order to suppress the diffuse scattered excitation light, a super broadband notch filter was used.