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.

Chart 1.. Nitronaphthalenes with Electron-Donating Groups, along with Examples of Broadly Used Push–Pull Naphthalene Dyes^a

^aThe reduction potentials of 1, 2, and 3, $E_{\text{X|\hspace{0.33em}}{\text{X}}^{•-}}^{\left(1/2\right)}/V$ vs SCE, measured for acetonitrile (Figure 1), are listed in the parentheses. For naphthalene and 1-methylnaphthalene, $E_{\text{X|\hspace{0.33em}}{\text{X}}^{•-}}^{\left(1/2\right)}\approx -2.5\text{\hspace{1em}V}$ vs SCE;^, for dansylamide, $E_{\text{X|\hspace{0.33em}}{\text{X}}^{•-}}^{\left(1/2\right)}\approx -2.05\text{\hspace{1em}V}$ vs SCE; and for prodan, $E_{\text{X|\hspace{0.33em}}{\text{X}}^{•-}}^{\left(1/2\right)}\approx -2.09\text{\hspace{1em}V}$ vs SCE.

Figure 1.

Dependence of the half-wave potentials, E^(1/2), of 1, 2, and 3 (extracted from their cyclic voltammograms) on the electrolyte concentration, C_el, in CH₃CN. The supporting electrolyte is N(n-C₄H₉)₄PF₆. The dotted lines represent extrapolations to C_el = 0 yield estimates of E^(1/2) of 1, 2, and 3 for neat acetonitrile.^,

Figure 2.

Optical absorption and fluorescence of 2 and 3. (a,d) Absorption and emission spectra for different solvents. The fluorescence is divided by $\left(1-{10}^{-A\left({\lambda }_{ex}\right)}\right)$; thus, the spectra have areas under them proportional to ϕ_f, and they are plotted in logarithmic scales for improved visualization. The concentrations of 2, C(2), are 18 μM for toluene, 140 μM for CHCl₃ and CH₂Cl₂, and 150 μM for CH₃CN, and the concentrations of 3, C(3), are 130 μM for toluene and CHCl₃, 510 μM for CH₂Cl₂, and 180 μM for CH₃CN. (b,c,e,f) Concentration dependence of the fluorescence spectra (with normalized areas), along with the excitation spectra recorded at the short-wavelength and long-wavelength bands ascribed to emission from monomers and aggregates (for 2, λ_ex = 410 nm; and for 3, λ_ex = 350 nm).

Figure 3.

Dependence of excited-state properties of 2 and 3 on solvent polarity as represented by the Onsager function: f_O(x) = 2(x – 1)/(2x + 1), f_O(ε, n²) = f_O(ε) – f_O(n²). (a) Energies of the absorption maxima, ε_abs, obtained from absorption and excitation spectra, and fluorescence maxima, ε_f, from emission spectra. (b) Stokes’ shifts, Δε = ε_abs – ε_f. (c) Fluorescence quantum yields, ϕ_f, and S₁ lifetimes, τ, for the aggregated species that dominate the photophysics. (d) Rate constants of the radiative, k_f, and nonradiative, k_nd, decays.

Figure 4.

TA dynamics of 2 and 3 in solvents with different polarity (λ_ex = 400 nm, 4 μJ per pulse). TA spectra of (a) 2 (700 μM) and (b) 3 (2 mM). TA decays of the S₁ states (with ordinates in logarithmic scales) of (c) 2, recorded at 454 nm for toluene, 450 nm for CHCl₃ and CH₂Cl₂, and 630 nm for CH₃CN, and (d) 3, recorded at 450 nm for toluene and CHCl₃ and 460 nm for CH₂Cl₂ and CH₃CN.

Figure 5.

Jablonski diagrams depicting the excited-state dynamics of (a) 1, 2, and 3 monomers and (b) their dimers, as revealed by (TDA)-TDDFT analysis at the wB97X/TZP level of theory with toluene solvent implemented within the COSMO model. Under the labels of the singlet states, their permanent electric dipoles are listed in parentheses. The oscillators strengths of the absorption, hν_abs, and fluorescence, hν_fl, transitions are listed in parentheses along the corresponding arrows. For each S₁ → T_j ISC transition, the value the SOC matrix element (in cm⁻¹) is listed in parentheses.