Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core

Abstract

Dipyrrolonaphthyridinedione appended with para- or meta-nitrophenyl substituents exhibits strong fluorescence from a 1ππ* S1 state.

Revisiting the non-fluorescence of nitroaromatics: presumption versus reality

“…Although an S 1 and T 1 states of π, π* character should result in rather small SOC, solvation and/or relaxation of the nuclear geometry can alter this order of energy levels and adiabatically change the electronic configuration of S 1 from π, π* to n, π*, promoting strong n, π*/π, π* SOC and thus fast ISC. 11 Moreover, when the molecule has upper triplet states intermediate in energy between S 1 and T 1 , the energy gap law points to a preference for intersystem crossing to occur via the initial population of these upper triplet states, followed by internal conversion to T 1 . Indeed, many nitroaromatics undergo picosecond and sub-picosecond S 1 → T 1+ j ISC, which is faster than the IC within the triplet manifold.…”

“…In a report of the synthesis of a highly symmetrical 3.7-diamino-1,5-dinitronaphthalene, for example, “recrystallization from acetonitrile gave dark prisms with greenish fluorescence.” 31 Assembling nitrophenyl acceptors and an electron-rich aromatic donor to produce acceptor–donor–acceptor (ADA) configurations has been shown to result in fluorescent conjugates. 11,32,33 The quadrupole symmetry of the ground and FC excited states of such ADA dinitroaromatics precludes solvatochromism in the absorption spectrum, i.e. , S 0 → S (FC) 1 .…”

“…The downside of nitroaromatics, particularly for photonic and optoelectronic applications, is that their electronically excited singlet states usually exhibit very short lifetimes in the sub-ns and often sub-ps time range. 10–12 Such short lifetimes render the use of nitroaromatics impractical for efficient light-emitting applications and have reinforced the long-standing presumption that they are effectively non-fluorescent. Despite this stigma with regard to their fluorescence, reports in the literature demonstrate that many nitroaromatics can indeed fluoresce, albeit with fluorescence quantum yields ( ϕ f ) that are often smaller than about 10 −3 to 10 −4 .…”

“…Tuning the CT character of the excited states by decreasing the electron-donating strength of the chromophore core allows for nitroaromatics which fluoresce, even when in polar media. 11 Such optimization, which suppresses ISC and the formation of strongly polarized CT states, however, can lead to the emergence of alternative pathways of non-radiative deactivation involving the adiabatic transition to singlet nπ* dark states and aborted photochemistry. 11…”

“…11 Such optimization, which suppresses ISC and the formation of strongly polarized CT states, however, can lead to the emergence of alternative pathways of non-radiative deactivation involving the adiabatic transition to singlet nπ* dark states and aborted photochemistry. 11…”