Making Nitronaphthalene Fluoresce

Abstract

Nitroaromatic compounds are inherently nonfluorescent, and the subpicosecond lifetimes of the singlet excited states of many small nitrated polycyclic aromatic hydrocarbons, such as nitronaphthalenes, render them unfeasible for photosensitizers and photo-oxidants, despite their immensely beneficial reduction potentials. This article reports up to a 7000-fold increase in the singlet-excited-state lifetime of 1-nitronaphthalene upon attaching an amine or an N-amide to the ring lacking the nitro group. Varying the charge-transfer (CT) character of the excited states and the medium polarity balances the decay rates along the radiative and the two nonradiative pathways and can make these nitronaphthalene derivatives fluoresce. The strong electron-donating amine suppresses intersystem crossing (ISC) but accommodates CT pathways of nonradiate deactivation. Conversely, the N-amide does not induce a pronounced CT character but slows down ISC enough to achieve relatively long lifetimes of the singlet excited state. These paradigms are key for the pursuit of electron-deficient (n-type) organic conjugates with promising optical characteristics.

Revisiting the non-fluorescence of nitroaromatics: presumption versus reality

“…, . With S usually smaller than 1, 8,20 eqn (2b) also predicts a behavior consistent with the energy-gap law. On the other hand, a large displacement between the minima of the potential surfaces of the excited singlet and triplet states leads to a substantial reorganization energy and can result in the emergence of crossing points or conical intersections (Fig.…”

“…With Plotnikov's approximation for calculating k ISC , the nuclear contributions to the rate can assume the following form (eqn (4)): 8,19,20 where the Huang–Rhys factor, S , accounts for the shift of the equilibrium position of the normal modes, and n ν is an integer approximation of the ratio between the singlet–triplet energy difference and the energy of the receiving vibrational mode, i.e. , .…”

“…22 Preventing such distortions of the nitro group can suppress ISC and decreases k ISC by orders of magnitude. 8 There is still no consensus as to whether it is the pyramidalization, the torsional motion, or the bending and stretching modes of the nitro groups that are responsible for the highly efficient ISC. 10 Nevertheless, constraining the nitroaromatic to be as planar as possible can suppress triplet formation and increase the fluorescence quantum yields.…”

“…Thus, in acetonitrile the dipoles of S 0 and the FC S 1 states are 6.6 D and 10 D, respectively. 8 Upon relaxation, the S 1 state loses its CT character while the nitro group twists out of plane and assumes asymmetric pyramidal structure, resulting in k ISC > 10 12 s −1 . 8…”

“…Thus, for –CN, σ p = 0.66, σ m = 0.56, = 0.51 and = 0.15; for –CF 3 σ p = 0.54, σ m = 0.43, = 0.38 and = 0.16; and for –CO 2 CH 3 σ p = 0.45, σ m = 0.37, = 0.34 and = 0.11. Nitro groups induce positive shifts in the reduction potentials of polycyclic aromatic hydrocarbons, such as pyrene and naphthalene, that can exceed 1 V. 7,8 In comparison, the shifts induced by nitrile and carbonyl substituents range between 0.2 and 0.7 V. 9 The attachment of one or more nitro groups to the rings of an aromatic compound induces bathochromic spectral shifts, which is potentially an attractive means of shifting the optical absorption from the UV to the visible region of the spectrum or even from the blue to the red or near-infrared regions. Although there are substituents such as –N 2 + ( σ p = 1.91, σ m = 1.76, = 1.58 and = 0.33), –IF 4 ( σ p = 1.15, σ m = 1.07, = 0.98 and = 0.17), and –SF 3 ( σ p = 0.80, σ m = 0.70, = 0.63 and = 0.17) that are more strongly electron-withdrawing than –NO 2 , the resulting compounds are either chemically unstable and/or synthetically challenging.…”