Fluorescent Charge-Transfer Excited States in Acceptor Derivatized Thiophene Oligomers

Abstract

A family of thiophene oligomers with lengths of 3, 4, 5, 6, and 8 units were synthesized and end-capped with a strongly coupled naphthalimide acceptor (T n NIF) which produces an emissive intramolecular charge-transfer state. A thorough photophysical study was performed on the oligomers including UV−vis absorption, fluorescence, and picosecond transient absorption spectroscopy to investigate the effect of thiophene oligomer length/donor strength and solvent polarity on the intramolecular charge-transfer properties. In hexane, the T n NIF compounds behave in a manner similar to that of oligothiophenes as fluorescence from a local singlet excited state and intersystem crossing to the triplet state dominates the excited-state dynamics. Interestingly, the excited-state dynamics become much more complicated with increasing solvent polarity, from ether to acetone, where emission from a charge-transfer state ( δ+ T n NIF −δ ) and quenching from a charge-separated state ( •+ T n NIF −• ) become competitive. A mechanism is proposed that consists of a four-state diagram including a locally excited singlet state ( 1 T n NIF), a triplet state ( 3 T n NIF), an emissive charge-transfer state, and a nonemissive charge-separated state. The population of each of these states is highly dependent on both the thiophene oligomer length and solvent polarity which results in a mixture of excited states.

Photoinduced Intramolecular Electron Transfer in Phenylene Ethynylene Naphthalimide Oligomers

“…The NI acceptor was used in this investigation with the expectation that fluorescence from the OPE → NI CT excited state would be observed. This expectation was based on previous work on a variety of π-conjugated chromophores that are substituted with the NI acceptor, including oligo- and polyfluorenes and oligothiophenes. ,, …”

“…This expectation was based on previous work on a variety of π-conjugated chromophores that are substituted with the NI acceptor, including oligo-and polyfluorenes and oligothiophenes. 32,33,36 Linear Absorption and Photoluminescence Spectroscopy of OPEn and OPEn-NI Series. The photophysical properties of the OPEn and OPEn-NI compounds were studied in solvents with varying polarity and the results are summarized in Table 1.…”

“…The goal of the study was to examine photoinduced ET where the OPE acts as the donor and the NI moiety is the acceptor. Previous work has demonstrated that when electron-donating substituents are linked to the 4-position of the NI moiety, the compounds exhibit CT absorption and fluorescence. − Comparison of the absorption spectra of the OPE n -NI series with the parent OPEs reveals significant differences due to the NI acceptor (Figure ). In particular, OPE3-NI features a distinct, red-shifted CT absorption band that is solvatochromic.…”

“…Due to the weak coupling, direct radiative decay of the charge-separated state (via CT emission) is not observed. However, several recent reports have shown that using the 1,8-naphthaleneimide (NI) acceptor on oligofluorenes and oligothiphenes gives rise to donor–acceptor systems where charge transfer (CT) emission is observed. − …”

Solvent Effect on the Photoinduced Intramolecular Charge–Transfer and Redox Potentials of Three Difuranones

“…Except 2 , which is fluorescence inactive, the emission spectra of the compounds 1 and 3 show broad and structureless bands with remarkable bathochromic shift from low to high polar solvents (Figure ), which are characteristic features of intramolecular charge-transfer (ICT) transitions. ,, λ max fl for 1 and 3 shifts from 612 and 684 nm in DCM, respectively, to 626 and 710 nm in DMF, and their corresponding Stokes shifts are also high (Table ). The observed decreasing trends of fluorescence intensity (Figure a,c), as well as the quantum yields of 1 (0.44% in DCM and 0.13% in DMF) and 3 (2.90% in DCM and 1.81% in DMF) with increasing solvent polarity are consistent with the occurrence of the ICT process …”