Triplet-State and Singlet Oxygen Formation in Fluorene-Based Alternating Copolymers

Abstract

Data are reported on the triplet states of a series of fluorene-based A-alt-B type alternating copolymers based on pulse radiolysis-energy transfer and flash photolysis experiments. From the pulse radiolysis experiments, spectra are given for eight copolymers involving phenylene, thiophene, benzothiadiazole, and oligothienylenevinylene groups. Quantum yields for triplet-state formation (PhiT) have been obtained by flash photolysis following laser excitation and in one case by photoacoustic calorimetry. In addition, yields of sensitized formation of singlet oxygen have been determined by time-resolved phosphorescence and are, in general, in excellent agreement with the PhiT values. In all cases, the presence of thiophene units is seen to increase intersystem-crossing quantum yields, probably because of the presence of the heavy sulfur atom. However, with the poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-1,4-phenylene] (PFP), thiophene S,S-dioxide (PFTSO2) and benzothiadiazole (F8BT) copolymers, low yields of triplet formation are observed. With three of the copolymers, the energies of the triplet states have been determined. With PFP, the triplet energy is virtually identical to that of poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)]. In contrast, with fluorene-thiophene copolymers PFaT and PF3T, the triplet energies are closer to those of thiophene oligomers, indicating that there is significant conjugation between fluorene and thiophene units but also that there is a more localized triplet state than with the homopolymers.

Side‐Chain Degradation of Ultrapure π‐Conjugated Oligomers: Implications for Organic Electronics

“…Our data is consistent with the hypothesis that F5 acts as a sensitizer in the formation of singlet oxygen ( 1 O 2 ), [34] which then attacks the aliphatic side chains, initially creating hydroperoxide intermediates that degrade further via a variety of steps. Since 1 O 2 can attack any arbitrary hydrogen atom on the alkyl chain, albeit most easily at tertiary C-H, a plurality of compounds results.…”

“…S6 [30] ), possibly indicative of an envelope containing several vibrations. Consequently, we were not able to distinguish this band, which is usually assigned to the C --O streching vibration of 9-fluorenone, [19,20,34] from the one caused by alternative oxidation products, such as ketones in the side chains or carboxylic acids, using a simple infrared (IR) spectrum. When a degraded film (6 h at 150 o C in air and daylight) was redissolved, its fluorescence emission strongly resembled that of the pristine compound in solution.…”

A New Nonconjugated Naphthalene Derivative ofMeso‐tetra‐(3‐hydroxy)‐phenyl‐porphyrin as a Potential Sensitizer for Photodynamic Therapy

“…3). The triplet molar absorption coefficient ( ε T ) was calculated by the singlet depletion method (26,27). The triplet formation quantum yield ( Φ T ) was obtained by the comparative technique using TPP as standard ( ε T = 6.6 × 10 4 m −1 cm −1 at 440 nm, Φ T = 0.82) (28,29).…”

Singlet Excitation Energy Harvesting and Triplet Emission in the Self‐Assembled System Poly{1,4‐phenylene‐[9,9‐bis (4‐phenoxy‐butylsulfonate)]fluorene‐2,7‐diyl} copolymer/tris(bipyridyl)ruthenium(II)in Aqueous Solution

“…Data on the related poly[2,7-(9,9-bis(2'-ethylhexyl)fluorene)-alt-1,4-pheneylene] give a triplet energy of 2.32 eV. [18] This is higher then that of the 3 MLCT state of Ru(bpy) 3 2þ (2.12 eV), [13a] such that back electronic energy transfer from the metal complex is not possible in this system. In addition, although the excited ruthenium(II) complex is also known to be a relatively strong electron donor (E8 À0.86 V in water) [13a] and acceptor (E8 þ 0.84 eV in water), [13a] the lack of quenching of its luminescence by the polymer PBS-PFP is in agreement with the reported values for the oxidation (E8 ox 1.08 V vs. Ag/AgCl) [25] and reduction (E8 red À2.28 V vs. Ag/AgCl) [26] potentials of PFP derivatives, forbidding oxidation or reduction of the polymer by the excited metal complex.…”

“…Energies are taken from ref. [13a] and [18]. conjugated polyelectrolyte, [21] and with our own unpublished work on the quenching of PBS-PFP fluorescence by Ca(II) in dioxane-water mixtures, where the divalent cation induces aggregation of the CPE.…”