Long‐lived Charge‐Transfer States in 9‐Aryl‐Acridinium Ions; A Critical Reinvestigation

Abstract

In the recent literature a simple 9-aryl-acridinium ion was claimed to undergo an intramolecular, photoinduced charge shift to produce an extremely long-lived and very high energy charge-transfer state. The possible consequences of this observation are discussed and the tenability of the claims made is investigated via time resolved spectroscopy of a closely related system with spectroscopic characteristics allowing more solid identification of the actual photophysical events taking place. From the results obtained it appears likely that the long-lived species observed earlier in solution cannot be charge transfer in nature but must instead be identified as the lowest triplet state of the acridinium chromophore.

Long‐Lived Charge‐Transfer States in Compact Donor–Acceptor Dyads

“…This indicates that the yield of the local triplet state must be less than or equal to 14 % in pure acetonitrile. [46] The observations described above for 9 concur with and further augment earlier reports, especially the formation of the local acridinium T 1 triplet as the only long-lived species. These findings, however, are in strong contrast with the claimed formation of a very long lived CT state in 8.…”

“…[26] But the same reaction was found to occur between anthracene and the local acridinium triplet derived from 9. [46] This is consistent with the estimated reduction potential for the triplet excited state (+ 1.5 V vs SCE) and the oxidation potential of anthracene (+ 1.29 V vs SCE). [46] We therefore feel that it is essential to reinvestigate the photophysical and photochemical properties of 8 with some urgency.…”

“…[46] This is consistent with the estimated reduction potential for the triplet excited state (+ 1.5 V vs SCE) and the oxidation potential of anthracene (+ 1.29 V vs SCE). [46] We therefore feel that it is essential to reinvestigate the photophysical and photochemical properties of 8 with some urgency. In fact, studies are underway at the time of writing and will be reported separately.…”

“…This behavior has been observed before and, without doubt, can be attributed to CT fluorescence, that is, emissive back electron transfer from a 1 CT state populated by charge transfer from the naphthyl donor to the acridinium ion. [48][49][50] From the decay of this CT fluorescence, the lifetime of the 1 CT state in 9 could be determined directly as 3 AE 0.1 ns, [46] which is also in excellent agreement with earlier reports. [49,50] To study the rise and decay pathways of the 1 CT state in 9 in more detail laser flash photolysis measurements were performed with subpicosecond and nanosecond time resolution, respectively.…”

“…[45] This might make it difficult to discriminate between these species. We therefore (re)investigated the properties of 9 (see Figure 9), [46] which is closely related to 8 but has the advantage that the transient absorption spectra for its CT and lowest triplet states are markedly disparate (see later). The only significant difference between 8 and 9 is that the CT state for the latter is at slightly lower energy (ca.…”