Solvent dielectric delimited nitro–nitrito photorearrangement in a perylenediimide derivative

The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO2-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (SCR1) of NO2-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the SCR1 state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO2-PDI.


Section 1: Materials and Methods
All chemicals were obtained from commercial suppliers and used as received without further purification.All reactions were carried out in oven-dried glassware prior to use.Solvents were dried and distilled by standard laboratory purification techniques.TLC analyses were performed on recoated aluminum plates of silica gel 60 F254 plates (0.25 mm, Merck), and developed TLC plates were visualized under short and longwavelength UV lamps.Flash column chromatography was performed using silica gel of 200-400 mesh employing a solvent polarity correlated with the TLC mobility observed for the substance of interest.Yields refer to chromatographically and spectroscopically homogenous substances.Melting points were obtained using a capillary melting point apparatus. 1H and 13 C NMR spectra were measured on a 500 MHz Bruker advance DPX spectrometer.The internal standard used for 1 H and 13 C NMR is tetramethylsilane (TMS).High-resolution mass spectra (HRMS) were recorded on Thermo scientific Q exactive mass spectrometer using the Atmospheric pressure chemical ionization (APCI, positive mode) technique.Photophysical measurements of the derivatives were carried out in a cuvette of 3 mm path length.Absorption and emission spectra were recorded on Shimadzu UV-3600 UV-VIS-NIR and Horiba Jobin Yvon Fluorolog spectrometers, respectively.

Computational Analysis
All the calculations are carried out in Gaussian 16 employing the CAM-B3LYP functional and 6-311++G(d,p) basis set at the DFT level of theory in vacuum unless stated otherwise.Vertical excitation energies and oscillator strengths were calculated employing time-dependent DFT (TD-DFT) at the CAM-B3LYP functional and 6-311++G(d,p) level of theory.The frontier molecular orbitals (FMO) of NO2-PDI was obtained from the generated cube files of energy calculations, high quality FMO isosurfaces were created using GaussView 5.0.8 software.Geometry optimizations and single point energy (SPE) calculations were carried out using CAM-B3LYP functional and 6-311++G(d,p) basis set at the DFT level of theory in Gaussian 16 1 .Transition state (TS) analysis was done using quadratic synchronous transit-3 (QST3), optimized using transition state (TS-Berny) and verified using intrinsic reaction coordinate (IRC) methods.Homolytic Bond Fragmentation (HBF) and Natural Bond Orbital (NBO) analyses were performed on the optimized geometry of NO2-PDI.Electron localization function (ELF) isosurfaces along with the molecular electrostatic surface potential (MESP) maps were calculated for the optimized TS geometry in Multiwfn 3.8 2 and GaussView 5.0.8 3 softwares respectively.SOC values were calculated using PySOC 4 package implemented in Gaussian.

Femtosecond Transient Absorption (fsTA) Measurement
A Spectra-Physics Mai Tai SP mode-locked laser (86 MHz, 800 nm) was used as a seed for a Spectra-Physics Spitfire ace regenerative amplifier (1 kHz, 5.5 mJ).A fraction of the amplified output was used to produce a 440 nm pump pulse by TOPAS.A residual pulse of 800 nm was sent through an optical delay line inside an ExciPro pump-probe spectrometer to produce a white light continuum by employing a sapphire crystal.The white light continuum was split into two, and the streams were used as a probe and reference pulses.The femtosecond transient absorption spectra of the sample were recorded using a dual diode array detector, having a 200 nm detection window and 3.5 ns optical delay.Sample solutions were prepared in a rotating cuvette with a 1.2 mm path length.Determination of an appropriate instrument response function (IRF) is needed for accurate deconvolution of recorded transient absorption data.The IRF was determined by a solvent (10% benzene in methanol) two-photon absorption and was found to be ∼110 fs at about 530 nm.A neutral density filter (80%) was used for controlling the incident flux on the sample.fsTA measurement of NO2-PDI in toluene solvent was recorded by photoexciting the sample with 470 nm, 200 nJ, and 100 fs pulses to moderate singlet-singlet annihilation that often arises in multi-chromophoric assemblies 5 .The observed kinetic components were laser intensity-independent, ruling out the chance of singlet-singlet annihilation.fsTA measurements of NO2-PDI in acetonitrile were recorded in low laser fluence and shorter acquisition time to avoid any undesired photoisomerization/degradation of the NO2-PDI sample during measurement.

Nanosecond Transient Absorption (nsTA) Measurement
Nanosecond laser flash photolysis experiments of the nitrogen-purged solution of NO2-PDI in toluene and acetonitrile was done in an Applied Photophysics Model LKS-60 laser kinetic spectrometer using the third harmonic (532 nm, pulse duration ≈10 ns) of a Quanta Ray INDI-40-10 series pulsed Nd:YAG laser as the excitation source.In acetonitrile, the measurement was completed under low fluence (laser power 5) to avoid unwanted rapid photodegradation/photodissociation.Exponential fitting of the decay traces obtained from the single wavelength (420 nm) nsTA decay kinetics experiment was performed with the help of OriginPro software 6 .

Global Analysis
Global analyses of the fsTA and nsTA spectra were performed using the Glotaran software 7 .The procedure evaluates the instrument time response function and group velocity dispersion of the white continuum and allows one to compute decay time constants and dispersion-compensated spectra.In global analysis, all the wavelengths were analyzed concurrently, employing a sequential model to give species associated difference spectra (SAS).The SAS indicate that the evolution of the spectra in time and do not necessarily denote a real physical/chemical species.SAS designate the spectral changes that occur with their associated time constants.

Fourier-Transform Infrared Spectroscopic (FTIR) Analysis
Fourier transform infrared spectroscopy (FTIR) measurements of NO2-PDI and ONO-PDI were recorded on a Shimadzu IR Prestige-21 FTIR spectrometer as KBr pellets.

Raman Spectroscopic Analysis
Raman spectroscopic analysis of NO2-PDI and ONO-PDI in the solid powder state were recorded using a HR800 labRAM confocal Raman spectrometer, operating at 20 mW laser power using a Peltier cooled (-74 C) CCD detector.Raman spectra were recorded using a He-Ne laser source having an excitation wavelength of 632.8 nm with an acquisition time of 5 s using a 50x objective.

X-ray Photoelectron Spectroscopic (XPS) Analysis
X-ray photoelectron spectroscopic (XPS) measurements of NO2-PDI and ONO-PDI solid samples were performed using an ESCA Plus spectrometer (Omicron Nanotechnology Ltd, Germany).Mg Kα (1253.6 eV) was used as the X-ray source operating at 100 watts.General scans and core-level spectra were acquired with 1 eV and 50 eV pass energy respectively.Spectral Background (Shirely) de-convolution was done by using CasaXPS software 8 .

Photoirradiation Experiments
The photoirradiation experiments were performed on nitrogen-purged acetonitrile and toluene solutions of NO2-PDI using 532 nm-10 mW diode laser.The photoirradiation experiment with 532 nm laser was continuously monitored using UV-vis absorption and fluorescence emission measurements.

Electron Paramagnetic Resonance (EPR) Experiments
Continuous-wave EPR (CW-EPR) measurements with X-band (8.75-9.65 GHz) were carried using JEOL JES-FA200 ESR spectrometer at room temperature and liquid nitrogen (77 K) temperatures.Samples (5 mM conc.) were prepared by loading the solutions of NO2-PDI in 5 mm o.d. ( 4 mm i.d.) quartz tubes, subjecting them to nitrogen purging cycles and was sealed later using a rubber septum.Samples were photoexcited inside the EPR cavity with a USHIO Optical Modulex-XENON lamp-ES-UXL 500 (input current = 20 amperes).The acquisition of the EPR samples were carried out with a modulation frequency (100 KHz, width = 0.1 mT), phase (0 degree), sweep time (30.0 s) and time constants of 0.1 and 0.03 s.

Synthesis of PDI
The compound synthesized according to the literature procedure 9 .5 g PTCDA was accurately weighed out in a 250 ml round bottom flask.Then about 125 g of Imidazole was added and mixed well.The temperature was raised from room temperature to 140 ο C. When imidazole melts entirely and forms a homogeneous solution with PTCDA, 2.5 equivalents (3.7 g) of 3-amino pentane were added slowly under the N2 atmosphere.This reflux was continued for about 6 hr.After the completion of the reaction, washed with a mixture of 1N HCl and Ethanol (40:60).The precipitated product was filtered and purified using column chromatography using DCM: PET ether to afford dark red coloured solid (5.75 g, 85% yield).M.p. > 300 °C.

Synthesis of NO2-PDI
The compound was synthesized according to the literature procedure 10 .To a solution of PDI (900 mg, 1.6 mmol) in CH2Cl2 (100 mL), conc.HNO3 (0.1 M, 3.0 mL) and cerium (IV) ammonium nitrate (CAN) (1.2 g, 2.2 mmol) was added and the mixture was stirred at room temperature for 2 hours under N2.The mixture was neutralized with 10% KOH and was extracted using CH2Cl2.The crude product was purified by a silica gel column chromatography, using CH2Cl2 as eluent to afford shiny red coloured solid (829 mg, 95% yield).M.p. > 300 °C.

Synthesis of ONO-PDI
To a 20 mL quartz test tube equipped with a magnetic stir bar was charged with NO2-PDI (50 mg, 0.087 mmol) in acetonitrile (10 mL) as the solvent.The test tube was backfilled with argon and sealed, then irradiated with a 532 nm 10mW-diode laser for 2 hours at room temperature.The crude product was purified by silica gel column chromatography using EtOAc as eluent to afford yellow coloured solid (  11 .CHN elemental microanalysis was employed to measure the percentage elemental composition of the isomers NO2-PDI and ONO-PDI.Interestingly, both the isomers exhibited almost similar elemental composition percentages which were correlated with the theoretical/calculated data available.NO2-PDI displayed an error of 0.36% from the calculated data, while ONO-PDI displayed a marginal error of 0.17% from the calculated data (hexane used for solvent correction, Table S2) 12 .Molecular mass analysis of the red coloured isomer NO2-PDI and the yellow coloured isomer ONO-PDI was accomplished using high resolution mass spectrometry-atmospheric pressure chemical ionization (HRMS-APCI).Both the compounds demonstrated the same molecular ion peak.NO2-PDI molecular ion peak was observed at m/z=576.2116 (M+H) + (Figure S7a) while ONO-PDI molecular ion peak was observed at m/z=576.2120 (M+H) + (Figure S7b).The indication of the same molecular ion mass peak in HRMS spectra verifies the rearrangement of nitro to nitrito functionality in the course of photoisomerization.
Section 3: Tables Table S1: Selected publications showing the evolution of nitrite linkage isomers reported over the years.

Molecule Exhibiting Nitro to Nitrito Linkage Isomerization Method/Technique of Characterization
Genth and co-worker      Section D: Figures                          a)

Figure S4: 1
Figure S4: 1 H-NMR spectrum of NO2-PDI in CDCl3.Inset a) shows the splitting of the core aromatic protons.

Figure S6 :
Figure S6: Superimposed 1 H-NMR spectra of NO2-PDI (red spectrum) and ONO-PDI (blue spectrum) in CDCl3.Inset a) shows the splitting of the core aromatic protons.

Figure S8 :Figure S9 :
Figure S8: a) Normalized absorption spectra of PDI (black line) and NO2-PDI (red line), b) Emission spectra of PDI (black line) and NO2-PDI (red line) in CHCl3 at room temperature.Zoomed inset c) Showing the normalized emission of PDI (black line) and NO2-PDI x 345 units (red line).

Figure S10 :Figure S11 :
Figure S10: Resonant Raman spectrum of NO2-PDI measured in the powder state upon excitation at 488 nm with an acquisition time of 5 s using a 50x objective.

Figure S17 :Figure S18 :
Figure S17: Temperature-dependent nsTA spectra of NO2-PDI in a) toluene and b) acetonitrile.N. B.-At lower temperatures the lifetime of the NO2-PDI transient feature increases and the decay of this transient species is not complete.The most probable reason for the increase in the lifetime of the transient species is the rigidification of the molecule at lower temperatures, thereby reducing the total degrees of freedom and suppression of the deactivation channels of NO2-PDI.

Figure S21 :
Figure S21: NO2-PDI sample after EPR measurements under continuous light irradiation conditions at room temperature in a) toluene and b) acetonitrile solvents.N. B.-Red colour of NO2-PDI sample was intact in toluene after EPR experiment, whereas red coloured NO2-PDI transformed to yellow coloured ONO-PDI after EPR experiment in acetonitrile.

Figure
Figure S23: a) Single wavelength (420 nm) nsTA decay kinetics of NO2-PDI in toluene at 0 min, 20 min and 40 min.b) UV-vis absorption spectra of NO2-PDI at 0min and 40 min in toluene.

Figure S26 :
Figure S26: Exponentially fitted decay lifetime of the excited-state absorption of NO2-PDI in acetonitrile at a) 0 min, b) 20 min and c) 40 min obtained through the single wavelength (nsTA) decay kinetics experiment.

Figure S31 :
Figure S31: a) Transformation of NO2-PDI to yellow coloured ONO-PDI under visible light irradiation for 2 hrs in deuterated acetonitrile (CD3CN).b) HRMS spectrum showing the presence of non-incorporated deuterium product.

Table S2 :
NO2-PDI and ONO-PDI CHN elemental analysis (corrected with hexane as solvent)….SI 9 H NMR spectra of both NO2-PDI and ONO-PDI showed the presence of 7 aromatic deshielded protons and 22 aliphatic shielded protons.The observance of the 1 H NMR singlet peak (single proton) shifting from 8.64 ppm in NO2-PDI to 8.36 ppm in ONO-PDI confirms the rearrangement of the nitro (-NO2) group to nitrito (-ONO) group thereby altering the associated electronic effects (FigureS4-S6, 13 C NMR spectrum of ONO-PDI could not be obtained due to the very less stability of the compound in solution state for long scan times) 1

Table S4 :
UV-vis and fluorescence spectroscopic data of NO2-PDI and ONO-PDI measured in toluene at room temperature.

Table S9 :
Theoretical Bond Order of selected bonds in the proposed 6-membered TS calculated from the Wiberg Bond Indices evaluated in Multiwfn 3.8.

Table S11 :
Calculated dipole moment values of the transition state in vacuum, polar acetonitrile and non-polar toluene solvent models at CAM-B3LYP/6-311++G(d,p)) level of theory.