Photo-Induced Partially Aromatized Intramolecular Charge Transfer

Abstract

Diverse models of intramolecular charge transfer (ICT) have been proposed for interpreting the origin of the charge-transfer (CT) state in donor–acceptor (D–A) dyes. However, a large variety of fused-heterocyclic dyes containing a pseudo-aromatic ring in the rigid structure have shown to be incompatible with them. To approximate a solution within the ICT concept, we reported a novel ICT model called partially aromatized intramolecular charge transfer (PAICT). PAICT involves the generation of a CT state from an ICT that occurred within a pre-excited D–A fused-heterocyclic structure possessing a pseudo-aromatic or unstable aromatic ring as the acceptor moiety. The model was proposed from the multiple-emissive mesomeric D–A N 1 -aryl-2-(trifluoromethyl)­benzo­[b]­[1,8]­naphthyridin-4­(1H)-one, whose excited mesomeric states, which are defined by the aromatic and pseudo-aromatic forms of the pyrindin-4­(1H)-one ring, led to a common partial aromatized CT state upon excitation via PAICT. The latter was supported through theoretical calculations on the excited mesomeric states, one-dimensional (1D) and two-dimensional (2D) excitation–emission measurements in different solvents, and the detection of three excited states by lifetime measurements upon 370 nm excitation. The existence of mesomerism was supposed from: (i) two overlapping bands at 370–390 (or 400–420 nm) in UV–vis spectra, (ii) the direct interaction between the pyridinic nitrogen of one molecule and the carbonylic oxygen of the other found in the solid state and, (iii) the detection of three excited states by lifetime measurements. The PAICT opens new perspectives for interpreting the charge-transfer phenomenon in fused-heterocyclic dyes, in particular, those containing a pseudo-aromatic or unstable aromatic ring as an acceptor moiety.

High CT-Fluorophore Featuring a Basic Moiety into D–A Chain as a pKa Probe

“…In this case, the protonation of the basic moiety may disrupt the ICT from (D) to (A), leading to a selective quenching response dependent on the protonation level (Scheme 2A). As a fundamental platform, we employed three fluorescent dyes of N 46 Previously, we demonstrated that the balance between the CT and LE states can be modulated by solvent changes, finding a dominant and intense CT state in hydrogen-donor media, such as water, methanol, and ethanol, whereas the nonhydrogen donor or low polar media favored the occurrence of the LE state (Figure 1A−C). 46 The fluorescence profiles of the dyes 3k and 3j were almost totally determined by the CT state at 476 nm and that of excited state showed to be highly stable with a prevalence inclusive in nonpolar media, such as hydrocarbons (n-hexane and n-heptane).…”

“…As a fundamental platform, we employed three fluorescent dyes of N 46 Previously, we demonstrated that the balance between the CT and LE states can be modulated by solvent changes, finding a dominant and intense CT state in hydrogen-donor media, such as water, methanol, and ethanol, whereas the nonhydrogen donor or low polar media favored the occurrence of the LE state (Figure 1A−C). 46 The fluorescence profiles of the dyes 3k and 3j were almost totally determined by the CT state at 476 nm and that of excited state showed to be highly stable with a prevalence inclusive in nonpolar media, such as hydrocarbons (n-hexane and n-heptane). The relative abundance of these excited LE and CT states was confirmed by lifetime measurements, finding a dominant long-lived state (∼80%, 10−14 ns) (CT state) and a minority short-lived state (∼20%) (LE state) for the high CT-probes 3j and 3k in ethanol media.…”

“…Further details were discussed in our previous report. 46 Photophysical data for dyes can be found in Table S1. Regarding the role of basic nitrogen as a hydrogen acceptor, we previously found that the CT state of the probes 3k and 3j was discretely quenched under a strong donor-hydrogen environment, such as water and methanol, which was attributed to a hydrogen bonding between quinolinic nitrogen-10 and the hydrogen atom of water/methanol.…”

“…S1). All N 1 -aryl-7-methoxy-2-(trifluoromethyl) benzo [b] [1,8]naphthyridine-4(1H)-ones 3g, 3j, and 3k were previously prepared by our group following a synthetic strategy of successive steps 65,66 The fluorescent profile of the mentioned pK a probes were previously studied by our group, 46 and some key photophysical properties are listed in Table S1. All Brønsted acids were purchased from commercial sources (Sigma-Aldrich, Fluka, or Merck), and they were used without further purification.…”

“…42−45 With this principle in mind, we planned the use of a high CT fluorophore, such as N 1 -aryl-7-methoxy-2-(trifluoromethyl)benzo [b][1,8]naphthyridin-4(1H)-one, featuring a basic moiety into the D−A chain as a convenient pK a probe (Scheme 1). Previously, we found that this chemical system displayed a high CT state upon excitation, 46 which was dominant inclusively under a nonprotic environment (Figure 1). This feature is of great importance for our purpose because the level of protonation of the basic moiety can promote a selective disruption of the ICT process with a quenching consequence dependent on the acidity strength of the Brønsted acids (Scheme 2A,B).…”