Photooxygenations and Self-Sensitizations of Naphthylamines : Efficient Access to Iminoquinones

A series of spiro [fluorene-9,7′-dibenzo[c,h]acridine]-5′-one (SFDBAO) derivatives have been concisely and cleanly obtained by exposure to sunlight without external photosensitizers in the higher yields than that when using a UV lamp. An interesting autocatalyst and self-sensitive procedure have been proposed to explain the effective photooxygenation. SFDBAO derivatives exhibit the continuous π-stacks in single-crystal and electron-withdraw properties with red light-emitting and photovoltaic properties.

Herein, we report a photochemical strategy for generating excited state species from naphthylamines, which does not rely upon any metal-or organic-based photoredox catalysts (Scheme 1).e reaction, which occurs at ambient temperature and requires irradiation by sunlight in order to proceed, is sufficient for achieving iminoquinones with synthetically useful results.In general, most photooxygenations require the effective photocatalysis or photosensitizers such as, Ir or Ru complexes [11], rose bengal [12][13][14], porphyrins [15], 9-mesityl-10-methylacridinium [16,17], inorganic TiO 2 [18,19], or organic molecule [20].However, our visible light mediates photooxygenation of naphthylamines with the advantages of high chemoselectivity without any external photosensitizers.In this reaction, naphthylamines play a dual role of providing both photosensitizer and substrate, and O 2 functions as triplet state trapping agent and oxygen sources.

Reagents and Instruments.
All the reagents used were of analytical pure.e UV lamp irradiation is carried out indoors by a household 23 W compact uorescent light (CFL) bulb. 1 H and 13 C NMR spectra were performed on a Bruker 400 MHz spectrometer in CDCl 3 or DMSO-d 6 with tetramethylsilane (TMS) as the interval standard.Mass spectra were recorded on a Shimadzu GCMS-QP2010 plus equipped with DB-5 ms column or a Shimadzu AXIMA-CFR plus spectrometer.For the MALDI-TOF MS spectra, the spectra were recorded in re ective mode, and substrates were used.Molecular weights of the samples were measured by gel permeation chromatography (GPC) on a Shimadzu LC-20A HPLC system equipped with 7911GP-502 and GPNXC columns.Elemental analyses were carried out in an Elementar Analysensysteme GmbH-vario EL III element analyzer.Absorption spectra were measured with a Shimadzu UV-3600 spectrometer at 25 °C, and emission spectra were recorded on a Shimadzu RF-5301(PC)S luminescence spectrometer.Cyclic voltammetric (CV) studies were conducted using a CHI660C electrochemical workstation in a typical three-electrode cell with a platinum sheet working electrode, a platinum wire counter electrode, and a silver/silver nitrate (Ag/Ag + ) reference electrode.X-ray crystallographic data were collected at 293 K on a P4 Bruker di ractometer equipped with a ne-focus sealed tube and a rotating anode utilizing graphite-monochromated Mo K α radiation (λ 0.71073 Å).Data processing was carried out using the program SAINT, while the program SADABS was utilized for the scaling of di raction data, the application of a decay correction, and an empirical absorption correction based on redundant re ections.e structures were solved by direct methods and rened against F 2 with the full-matrix, least-squares methods using SHELXS-97 and SHELXL-97, respectively.Column chromatography puri cation was carried out using the silica gel (200-300 mesh).

Syntheses of the Intermediates and the Target Compounds
2.2.1.General Procedure for Preparation of 2 and 3.In a round bottom ask, compound 1 (2 mmol) was dissolved in MeCN (400 mL) and the mixture was stirred in the sunlight irradiation under air atmosphere for 6 hours to produce compound 2. e crude product was puri ed by ash column chromatography (silica gel, CH 2 Cl 2 /petroleum ether, 1: 3) to give puri ed products.
e organic portion was separated and washed with brine before dried over anhydrous MgSO 4 .e solvent was evaporated off, and the solid residues were purified by flash column chromatography using CH 2 Cl 2 : petroleum ether � 1 : 3 to afford TSFDBAO (0.437 g, 0.83 mmol) as red solids with the yield of 83%. 1

Results and Discussion
Initially, our explorations toward an photooxygenation protocol under mild reaction conditions focused on the CISFDBAO, 2b reaction of 14H-spiro-[dibenzo[c,h]acridine-7,9′-uorene] (SFDBA, 1a) (Table 1).First, the reaction of 1a was purposively carried out in CCl 4 by exposure to UV irradiation of 365 nm for 24 h, giving the chloro-substituted oxidative product (ClSFDBAO) 2b in 40% yield, whereas the SFDBAO product 2a was not observed (entry 1).e structure of 2b was con rmed unambiguously by X-ray single-crystal diffraction (Figure 2).It is exciting that the conversion yield of 2b was improved up to 82% by exposure to sunlight at room temperature in CCl 4 (entry 2).A possible mechanism for the formation of ClSFDBAO (2b) is depicted in Figure S6 (see Supplementary Materials).When acetone was used as the solvent and conducted at room temperature, the desired product (SFDBAO) 2a was obtained in 76% yield, and 9% of 5H-spiro[dibenzo[c,h] acridine-7,9′-uorene]-5,6(14H)dione (SFDBAOO) 3 as a by-product was detected (entry 3).e structure of 3 was con rmed unambiguously by X-ray single-crystal di raction (Figure 2).Solvent screening indicates that MeCN was the most e cient.Other solvents such as, MeOH, THF, DMF, and toluene gave relatively low   yields of 2a (entries 4-8).No target compound 2a was obtained in the dark when heating up more than 80 °C (entry 9).Additionally, no product 2a was obtained in N 2 atmosphere (entry 11), indicating oxygen in the atmosphere participated in the visible-light-mediated photooxygenation of 1a. is result was further supported by the control experiments using the puri ed O 2 source to give 2a in 83% yield (entry 10).Moreover, a write-once, read-many (WORM) type memory device based on SFDBAO derivative nanosheets as electroactive layers shows an ON/OFF ratio of 6.0 × 10 4 , accompanied by an interesting photoswitching behavior.e results presented have shown that SFDBAO derivatives are promising electroactive materials for the memory device application [22,23].Compared with our previous reports, further study of the conditions and the scope of this reaction were conducted in detail.Under the optimized conditions (Table 1, entry 8), a range of reactions were performed with various substrates 1 (Scheme 2).e reactions of spiro uoreneacridines proceeded smoothly to a ord the corresponding photooxygenation products 2(a-d) in good to excellent yields (70-82%).en, di (naphthalen-1-yl) amine was compatible with this reaction, a ording the desired product 2e in 82% yields.To our disappointment, no desired products were obtained when naphthylamine and anilines were employed in the present reaction system (see Supplementary Materials Figure S6).
Chemical structures of the SFDBAO derivatives were unambiguously con rmed by GC-MS, 1 H and 13 C NMR, * *   S1.ClSFDBAO and SFDBAOO from a mixture of dichloromethane/alcohol and a mixture idines in ClSFDBAO are incompletely perpendicular with uorene plane according to X-ray crystallography, obviously di erent from spirobiuorenes.ClSFDBAO exhibit well-de ned consecutive intermolecular π-stacked motifs among adjacent dibenzoacridines with a close distance of 3.346 Å in the molecular packing graph (Figure 2), which is in the range of the distance of the typical π-π stacking interactions relative to range over 3.4-3.8Å. e closely interplanar distances are favorable for the electron delocalization and the improvement of mobility in organic lms and devices.
To clarify the SFDBA and SFDBAO together as a photosensitive catalyst involved in the reaction leading to SFDBAO, control experiments were performed (Figure 3).e optical absorption properties of the SFDBA and SFDBAO were investigated by UV-vis absorption spectra as shown in Figure 3. e absorption spectra in dilute CHCl 3 of SFDBA and SFDBAO show absorption at 370 nm and 470 nm, respectively.
erefore, we tested the absorption spectra of SFDBA 1a at di erent reaction times under the sunlight, the 365 nm UV lamp, and the 490 nm UV lamp, respectively.As a result, we found that the yield of SFDBAO increased gradually with the increase of reaction time under the sunlight and 490 nm UV lamp.However, the yield of SFDBAO increased very slowly with the increase of reaction time under the 365 nm UV lamp.e experimental results indicate that SFDBA acts as a photosensitive catalyst to induce the formation of SFDBAO in the initial reaction period, and then, SFDBAO also acts as another photosensitive catalyst accelerating the conversion together with SFDBA.
On the basis of all the results described above, a plausible and preliminary mechanism has been proposed, as depicted in Scheme 3. In the self-sensitizations cycle, naphthylamines is converted to the excited singlet state under the sunlight irradiation.Subsequently, the excited triplet singlet state is formed through intramolecularly intersystem crossing (ISC) of the excited singlet state.e triplet energy of the naphthylamines can be transferred to the ground state oxygen molecule to produce singlet oxygen to close this catalytic cycle.After that, an electron transfer of singlet oxygen from 1 occurs, leading to the formation of radical cation species I/II.en, a [4 + 2] cycloaddition is given to intermediate II, and the O-O bond cleavage of intermediate III led to intermediate IV, which was readily oxidized by dioxygen to produce the desired iminoquinones 2 [24][25][26].
iophene-substituted SFDBAO exhibit the new Journal of Chemistry electronic absorption bands at 322 nm for TSFDBAO and 353 nm for DTSFDBAO without the obvious change of PL peaks, expect for the reduced emission intensity probably owing to heavy atom effects of sulfur.e first turn-on oxidation potentials of SFDBAO occur at 0.98 V (vs Fe + /Fe) for SFDBAO, 0.96 V (vs Fe + /Fe) for TSFDBAO, and 0.71 V for DTSFDBAO, respectively.
ese results indicate that DTSFDBAO has better electron-donating properties owing to the introduction of the double thiophene groups.Furthermore, all the SFDBAO derivatives exhibit the reversible reduction processes with the similar onset first potential of around −1.00 V (vs Fe + /Fe).eir LUMO energy levels are estimated and determined from the onset of the reduction to be −3.73 eV for SFDBAO, −3.78 eV for TSFDBAO, and −3.77 eV for DTSFDBAO with regard to the energy level of ferrocene (4.8 eV below vacuum), respectively.e lower LUMO is probably assigned to the spiro-dibenzoacridine moieties with the electron-withdrawing properties, which are in the range of the LUMO energy level of acceptor material in solar cells [27,28].eir bandages are 2.02 eV for SFDBAO, 1.95 V for TSFDBAO, and 1.71 V for DTSFDBAO, respectively.ese data suggest that the SFDBAOs are potential n-type green organic semiconductors.

Conclusions
We have developed a green efficient access to iminoquinones from naphthylamines based on the photooxygenation and self-sensitizations, in which naphthylamines play a dual role of providing both photosensitizer and substrate, and O 2 functions as triplet state trapping agent and oxygen source.Air atmosphere and sunlight irradiations are crucial conditions of the photooxygenation reaction to obtain the highyield products.is sunlight-induced C-H activation gives a kind of new n-type spiro-based organic semiconductors.Further work on the application of SFDBAO-based materials is ongoing in our laboratory.

Figure 2 :Scheme 2 :
Figure 2: Single-crystal X-ray structures of 2b and 3 and the packing structure of 2b.

Figure 3 :
Figure 3: (a) e absorption spectra of SFDBA and SFDBAO.Absorption spectra of 1a at di erent reaction times: (b) under the sunlight, (c) under the 365 nm UV lamp, and (d) under 490 nm UV lamp.

Figure 4 :
Figure 4: e photophysical and electrochemical properties of SFDBAO.(a) UV-vis electronic absorption spectra and photoluminescence spectra.(b) Reductive and oxidative cyclic voltammograms, 0.1 M n-Bu 4 NPF 6 in THF (reduction) and 0.1 M n-Bu 4 NPF 6 in CH 2 Cl 2 (oxidation), were used as supporting electrolytes.A platinum sheet electrode was used as the working electrode; the scanning rate was 100 mV/s, where E + (Fc/Fc) is about 0.03 V.