1,6-Diazapyrene: A Novel, Well-Defined, Small-Size Prototype System for Nitrogen-Containing PAHs

Abstract

The quest for nitrogen-doped (N-doped) polycyclic aromatic hydrocarbons (PAHs) requires well-defined prototype systems to understand the relationship between the structure and the resulting photophysical and photochemical properties. To this end, a novel, simple, and small compound, 1,6-diazapyrene, was synthesized. In-depth analysis, employing optical spectroscopy and (time-dependent) density functional theory, (TD-)­DFT, elucidates the optical excitations on the basis of MO symmetry, energy, and topology considerations; the study further unveils the photophysical and photochemical deactivation kinetics after photoexcitation, revealing extreme changes against pyrene as well as against the well-known 2,7-diazapyrene isomer. The high sensitivity of the aza-substitution position to generate such changes is considered as highly relevant for the targeted design of N-doped PAHs in general.

Nitrogen-doped (N-doped) polycyclic aromatic hydrocarbons (PAHs) have found much interest in the past years to tune the electronic, optical and photophysical properties of PAHs, graphene, and carbon dots in (opto)­electronic, (photo)­catalytic and biochemical and biomedical applications. − Nevertheless, the modulation of the properties depends significantly on the type of PAH annulation (linear, angular, peri-annulated) and the number of annulated rings, as well as on the type of N-doping (pyridinic, pyrrolic, graphitic) and the number and positioning of N atoms. The complexity of possible manifestations of N-PAHs demands the investigation of prototype systems, for instance by systematic structure variation, − or computational screening of small model systems; in particular, for the latter, pyrene (Py) was used as a paradigmatic platform for N-doping. This is inter alia connected with the outstanding position of Py in materials and life science applications, , as well as serving as a prolific synthon to create functional dyes. − In any case, little attention was paid to the detailed understanding of N-doping of Py for photophysical and photochemical deactivation pathways, although this is of central importance for the functionality in the various applications. This is done in the present study, on a novel, small-size prototype system, without bearing any substitutes other than hydrogen atoms, that is 1,6-diazapyrene (DAP16), being a long-imagined, but until now not synthesized DAP isomer.

In fact, somewhat surprisingly, out of the 15 possible neutral DAP isomers ,− only five have been synthesized (i.e., 1,3-, 2,7-, 4,5-, 4,9-, 4,10-DAP). Among these, in particular derivatives of the 2,7-isomer (DAP27) were studied for the purpose of organic electronics, intercalation of nucleic acids, − sensors, building blocks for metal–organic frameworks and molecular machines, , as well as model systems for N-PAHs and carbon dots. In the case of solvothermally synthesized carbon dots it has actually been shown that N-PAHs are responsible for their photoluminescence, even though the presence of specific DAPs has not been detected. Although it is known from computational studies that the UV–vis spectra may significantly change upon structural variation in DAP isomers, the changes in photophysics and -chemistry upon structure variation have not been studied so far. This is, however, of crucial importance for the application of N-PAHs. In fact, in comparison with Py and DAP27, the new DAP16 isomer exhibits enormous changes of the optical and photophysical properties, providing violet fluorescence with a (moderately high) quantum yield of Φ_F = 34% with sub-ns decay time (τ_F = 0.73 ns) in comparison with up to 400 ns in Py , and 10 ns in DAP27. Furthermore, upon intense UV laser irradiation, complete, reversible photooxidation of DAP16 is observed; in all, this clearly evidences the need for a detailed understanding of the excited state properties and deactivation properties.

Such in-depth investigation is performed in the current work on the novel compound DAP16, combining detailed steady-state and time-resolved photoluminescence spectroscopy with (time-dependent) density functional theory, (TD) DFT, to fully rationalize the optical excitations on the basis of MO symmetry, energy and topology considerations. TD-DFT further tracks the observed photophysical and -chemical deactivation kinetics after photoexcitation, revealing the origin of the differences to DAP27 and Py. Overall, DAP16 is identified as the smallest possible structural change in the Py core, which leads to these dramatic alterations of the photophysics and photochemistry. The extreme sensitivity of the aza-substitution position to generate such changes is thought to be of utmost relevance for the targeted design of N-PAHs in general.

Experimental and Computational Details

DAP16 was synthesized by acceptorless dehydrogenative condensation , from naphthalene-1,5-diamine (1 mmol) and ethylene glycol (5 mL) employing both Pt/Al₂O₃ (1.7 mol %) and ZnO (4.5 mol %) as catalyst at 175 °C. As shown in Scheme , this reaction provided an almost equimolecular mixture of DAP16 and 3,8-dihydroindolo­[7,6-g]­indole BI­[7,6- g ], and fully characterized by ¹H-, ¹³C NMR and HRMS (high resolution mass spectrometry). This glycol/aniline methodology has been employed in the past for the preparation of many different indole derivatives. , Aromatic amines are prone to react at the ortho position creating five membered rings under these conditions. However, the particular structure of naphthalene-1,5-diamine allows a different, and unreported, reaction path at the peri position which allows the formation of the six membered ring. DAP16 could be identified due to the J ^H2H3 ∼ 5 Hz coupling constant, this value is typically observed between H2 and H3 in pyridines. In contrast, BI­[7,6- g ] presented well reported signals from indole core including broad NH peak around 12 ppm. BI­[7,6- g ] can be removed from the reaction mixture by trituration in chloroform due to its limited solubility. Pure samples of DAP16 can be obtained by chromatography or by sublimation.

1. Synthesis of DAP16 .

Analysis of DAP16: pale yellow solid, (77 mg, 38%). Mp 223–224 °C, ¹H NMR (300 MHz, CDCl₃) δ 9.34 (d, J = 5.2 Hz, 2H), 8.51 (d, J = 9.2 Hz, 2H), 8.31 (d, J = 9.2 Hz, 2H), 8.13 (d, J = 5.1 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃) δ 148.7 (2xC), 147.8 (2xCH), 135.0 (2xC), 133.5 (2xCH), 129.6 (2xCH), 119.6 (2xCH), 118.9 (2xC). HRMS (ESI-TOF) m/z: [M⁺] calculated for C₁₄H₈N₂ 205.0760; found: 205.0758. Analysis of BI­[7,6- g ]: black solid (74 mg, 36%). Mp 207–208 °C ¹H NMR (300 MHz, DMSO-d₆) δ 11.80 (s, 2H), 7.96 (d, J = 8.5 Hz, 2H), 7.73–7.66 (m, 2H), 7.38–7.32 (m, 2H), 6.58 (dd, J = 3.0, 1.9 Hz, 2H). ¹³C NMR (75 MHz, DMSO-d₆) δ 132.3­(2xC), 123.0 (2xCH), 122.9 (2xC), 119.9 (2xCH), 118.0 (2xC), 113.3 (2xCH), 103.1 (2xCH). HRMS (ESI-TOF) m/z: [M – H⁺] calculated for C₁₄H₁₀N₂ 205.0760; found: 205.0759. For further details, see text and Figures S1–S12.

For the spectroscopic investigations at room temperature (RT; 22 °C), DAP16 was dissolved in dichloromethane (DCM; c = 5 × 10^–6 M) of spectroscopic quality (Uvasol, Sigma-Aldrich). Absorption spectra were recorded on an Agilent Technologies Cary 60 UV/vis absorption spectrometer. Fluorescence spectra were obtained with a Horiba FluoroMax-4 spectrofluorometer; spectra were corrected for the wavelength dependence of the detection unit. The fluorescence quantum yield of DAP16 was determined against quinine sulfate dihydrate as a standard (Φ_F,st = 0.59). Fluorescence lifetimes were measured by time-correlated single photon counting (TCSPC) using a 320 nm pulsed LED (pulse width 933.6 ps) at 10 MHz repetition rate (Edinburgh Instruments) as excitation source, and a HydraHarp 400 (Picoquant) multichannel time correlator. Photons emitted at a particular wavelength were thereby detected with a thermoelectrically cooled Hamamatsu photomultiplier coupled to a f = 0.5 m spectrometer (Acton SP2500, Princeton Instruments) equipped with a 600 lines per mm grating blazed at 500 nm. The fluorescence decay curves were fitted via reconvolution with the instrumental response function (IRF) using the EasyTau software (PicoQuant). For phosphorescence measurements, samples of DAP16 doped in poly­(methyl methacrylate), PMMA (0.1 wt %), were mounted in a Magnex Scientific needle valve optical cryostat operated with liquid nitrogen. The nitrogen was pumped down to 65 K and allowed to warm up slowly to remove bubbles; for further details on the setup, see Supporting Information. For photooxidation experiments, all the samples were dissolved in DCM (c = 5 × 10^–6 M), irradiated with a 355 nm Nd:YAG laser (pulse energy 20 μJ, repetition rate 1 kHz, beam diameter 3 mm at the cuvette) under continuous stirring for several minutes and absorption and emission spectra were periodically recorded using an Agilent Technologies Cary 60 UV/vis absorption spectrometer and a Horiba FluoroMax-4 spectrofluorometer, respectively. For the comparison with Py and DAP27, we relied on available literature data; , nevertheless, we remeasured the spectra of Py in DCM (purchased from Acros organics; purity 98%). For comparative photooxidation studies with DAP16, DAP27 was resynthesized following the protocol of ref. . The absorption spectrum of the product, however, turned out to be of inferior quality compared to the former report, so that to compare the spectral properties, we relied on the latter; although this was measured in H₂O, the moderately small solvent shifts observed were irrelevant for the qualitative comparison of the three compounds below.

DFT geometry optimizations were done in the highest possible point groups (PGs), that is D_2h for Py and DAP27, and C_2h for DAP16. In the PG D_2h, the irreducible characters of the Gaussian16 output files correspond to the standard orientation in group theory, as recommended by the International Union of Pure and Applied Chemistry (IUPAC); accordingly, the x-axis is oriented perpendicular to the molecular plane and the z-axis is passing through the 2,7 positions of the pyrene core. Singlet (S_n) and triplet energies (T_n) were obtained as single point TD-DFT calculations in vacuum (6-311G­(d,p) basis set) within the linear response formalism, as implemented in the Gaussian16 program package. While the TD-DFT treatment of DAP16 (and DAP27) is rather straightforward, the comparison with Py is a delicate issue. In fact, a precise theoretical treatment of Py requires multireference methods, , which is beyond the current scope, aiming at a qualitative comparison. At the TD-DFT level, the choice of the functional (and possible implicit inclusion of solvent, through the polarizable continuum model; PCM) is an intricate matter for Py; in fact, it is known that standard DFT functionals like B3LYP suffer to reproduce the correct state ordering of Py, , while e.g., CAM-B3LYP, ωB97XD or M06–2X in vacuum give the correct ordering. However, the stabilization of S₁ is substantially underestimated, giving ΔE ₁₂ < 0.1 eV; , inclusion of implicit solvents may again reverse the state ordering. Thus, we consistently used M06-2X in vacuum in the current work. We note in this context that the absolute transition energies are considerably overestimated by the M06-2X functional; however, for the intended qualitative comparison of state ordering and composition between the three compounds, this functional represents the best choice. Comparison of different functionals (M06-2X, ωB97XD, PBE0, D3-B3LYP, CAM-B3LYP) in vacuum and DCM for all three compounds is found in Table S3. Furthermore, M06-2X calculated adiabatic energies for relevant excited of all compounds in vacuum and DCM are given in Table S4. The spin–orbit coupling (SOC) matrix elements were calculated between the S_1,2 and the accepting triplet states T_n based on the S₀ geometry, in vacuum and DCM, via quasi-degenerate perturbation theory, as implemented in the ORCA package, − applying a full TD-DFT scheme (M06-2X/6-311G­(d,p)). Intersystem crossing (ISC) rates were then estimated via a Marcus-type expression; for details, see eqs S1 and S2; all results are given in Table S5. In order to investigate the possible deactivation paths, the minimal energy crossing points (MECPs) between different triplet and singlet states were located; for this, the geometries of all respective excited states were optimized, see Figure S15 and Tables S6–S8 for details. For the identification of the photoproducts, M06-2X in DCM was used (Tables S9–S12); for the most probable (peroxide) product, D3-BLYP calculations in DCM were done for comparison (Table S13). Furthermore, we explored the reaction path along the potential energy surface, identifying energies and equilibrium geometry of the intermediates and locating the transition state (TS); see Figure S26.

Results and Discussion

The absorption spectrum of DAP16 in DCM solution in the near-UV range is dominated by a vibronically well-structured absorption band with an intense apparent 0–0 band at 3.32 eV (374 nm), see Figure . Emission occurs without notable Stokes shift and with approximate mirror symmetry vs absorption, all reflecting the rigid nature of the molecular backbone, so that the absorption features are assigned to the S₀ → S₁ transition. Small deviations from mirror symmetry are ascribed to the presence of a higher electronic state (S₂) in absorption, located about 0.14 eV above S₁ according to the TD-DFT calculations, vide infra. The fluorescence quantum yield is determined as Φ_F = 0.34, and the lifetime is τ_F = 0.73 ns, see Tables and S16. While fluorescence color and quantum efficiency are not very different from Py, the extreme shortening of the lifetime by almost 3 orders of magnitude, and the vanishing apparent Stokes shift suggests a very substantial change of the electronic nature in comparison with Py.

1.

Absorption (blue) and fluorescence (red) spectra of the compounds under study. Top: DAP16 (in DCM solution, at RT). Center: DAP27 (in H₂O). Bottom: Py (in DCM). State assignments correspond to the TD-DFT results; the low-intensity (nσ)­π* transitions (1,2¹A_u; in gray) for DAP16 are estimations. The thin black line for Py is a 20× magnification of the low-energy absorption region.

1. Fluorescence Quantum Yield and Lifetime (Φ_F, τ_F) of DAP16 in DCM Solution (RT) in Comparison with DAP27 and Py .

	ΦF	τF/ns	kr/s–1	knr/s–1
DAP16	0.34	0.73	4.7 × 108	9.0 × 108
DAP27	0.50	10	5.0 × 107	5.0 × 107
Py	0.65	382	1.7 × 106	9.2 × 105

^aRadiative and nonradiative rates k _r, k _nr are calculated via k _r = Φ_F/τ_F and k _nr = (1 – Φ_F)/τ_F.

^bIn H₂O; from ref. .

^cIn cyclohexane; from ref. .

It is well-known that Py exhibits a peculiar electronic structure, − as reflected in the subsequent state ordering, ,,− vibronics, − photokinetics , and the ability for dynamic excimer formation. − As seen in Figure , the absorption spectrum of Py in the near UV is dominated by the S₀ → S₂ transition (1¹B_1u; or ¹L_a in the Platt notation), with an apparent 0–0 band at 3.68 eV, i.e., λ = 337 nm, and an oscillator strength f ≈ 0.5; see Table S3. This transition is oriented along the long molecular axis (z), and is described almost exclusively by a monoelectronic excitation (Φ₁) between the highest occupied and lowest unoccupied MOs (HOMO, LUMO), here abbreviated as H → L. The subsequent excited symmetry-allowed one-electron configurations H → L + 1 and H – 1 → L (Φ_2,3) exhibit B_2u symmetry and are oriented along the short axis (y) of Py. Equal energy spacing in the sets of occupied and unoccupied MOs is observed, as in fact expected for an alternant hydrocarbon; in particular ΔE _H,H–1 ≈ ΔE _L,L+1. This gives rise to strong first order configuration interaction (CI) between Φ_2,3 with almost equal CI coefficients, and generates substantial splitting of the resulting (symmetry allowed) B_2u states. Subsequently, the lower state (1¹B_2u; ¹L_b in the Platt notation) carries only very little oscillator strength (f ≈ 6 × 10^–4), ,, due to this alternant pairing effect. Experimentally, 1¹B_2u is found as the S₁ state (with an onset at about 3.3 eV in DCM, see Figure ) below 1¹B_1u (S₂; onset at about 3.6 eV); the actual energy difference ΔE ₁₂ (in DCM 0.3 eV) is very sensitive to the environment as solvent shifts depend significantly on f. − Within the TD-DFT framework, standard DFT functionals like B3LYP reproduce quite well the energies of bright states (i.e., with large f; like 1¹B_1u), but have difficulties to correctly govern alternant pairing (like for 1¹B_2u); in particular, they give the wrong state ordering for Py (see Table S3). On the other hand, M06-2X overestimates the absolute transition energies (by about 0.7 eV), and underestimates ΔE ₁₂, giving only 0.05 eV (in vacuum); however, it gives a good estimation of the oscillator strength (f ₀₁ = 5.7 × 10^–4), and, importantly in the current context, the correct state ordering. , The good performance of multiconfigurational methods for 1¹B_2u of Py , points to significant second order CI contributions, which are thought to be promoted by the large exchange integral, reflected in the singlet–triplet gap of 1.5 eV between 1¹B_1u and 1³B_1u; see Tables S1, S2 and Figure S13 with the detailed discussion there. While S₁ (1¹B_2u) is essentially dark in absorption (Figure ), it is not dark in emission. This is due to the fact, that the very small radiative rate k _r, which result from the small f ₀₁, competes well with the very small rate of nonradiative decay k _nr (Table ); the latter proceeds both through (thermally activated) internal conversion (IC, and subsequent vibrational relaxation; VR) to S₀, and intersystem crossing (ISC) to the triplet manifold. , The low k _IC and k _ISC rates are due to the rigid nature of Py, and, at the same time, sole π­(π*) character of the relevant frontier MOs (Figure ), so that ISC is El-Sayed forbidden, resulting in small spin–orbit coupling (SOC) for ISC. In all, the small k _IC and k _ISC gives rise to a high Φ_F, and, at the same time, an extraordinarily long τ_F for Py; e.g., in degassed cyclohexane, Φ_F = 0.65, and τ_F = 382 ns, , see Table .

2.

Frontier MO diagrams (energies, topologies and symmetries; H = HOMO, L = LUMO) for Py, DAP27 and DAP16, as calculated by DFT (M06-2X, 6-311G*); below: TD-DFT calculated low-lying symmetry-allowed vertical excited state energies, oscillator strengths and MO descriptions (contributions >5%). Insets: convention for the symmetry axes in the PG D_2h, according to IUPAC; excited state notation according to group theory, Platt and Kasha.

Due to the peculiar electronic situation in Py, essentially all chemical modifications of the pyrene backbone break the symmetry between the sets of occupied and unoccupied MOs and modulate electronic transition energies and strengths. However, the resulting effect depends subtly on the nature, number and position of the substituents. In particular, significant differences are expected for 2,7- vs 1,6-substitution, as the H, L orbitals of Py exhibit a nodal plane passing through the 2,7 positions. , Therefore, substitution at the 2,7-positions affects H – 1 and L + 1 energies much more compared to H, L. Depending on the nature of the substituents, inter alia substantial stabilization or destabilization of the MOs is observed, with, in most cases, enhanced f ₀₁. ,− This is exactly what is observed for DAP27, where the higher electronegativity of nitrogen compared to carbon stabilizes H – 1 and L + 1 much stronger compared to, H, L, respectively, so that ΔE _H,H–1 is substantially larger than ΔE _L,L+1 (Figure ). As the PG of DAP27 is the same as Py (i.e., D_2h), S₁ is equally formed by linear combination of Φ_2,3. However, as ΔE _H,H–1 ≫ ΔE_L,L+1, the CI coefficients are now substantially different, which strongly enhances f ₀₁ in DAP27 in comparison with Py. According to our TD-DFT (M06-2X) calculations, the enhancement amounts to almost 2 orders of magnitude (with f ₀₁ = 7.0 × 10^–2, see Figure ). This is in good agreement with the observed spectral features of DAP27 (Figure ); the enhanced f ₀₁ (and, therefore, equally enlarged k _r) is as well responsible for the substantial reduction of τ_F compared with Py to about 10 ns (in H₂O), while Φ_F stays high (50%); see Table . In any case, it is noted that the appearance of the absorption spectrum of DAP27 is still not fundamentally different from Py, with a dominating S₂ band in the near UV, and a relatively weak S₁ band, see Figure .

While the modulation of pyrene photophysics in DAP27 is still moderate, DAP16 exhibits much more drastic changes, despite the fact that the alterations in MO energies are less pronounced compared to 2,7-substitution. In fact, in DAP16, ΔE_H,H–1 is now smaller than ΔE _L,L+1 (see Figure ), because the LCAO coefficients at the nitrogen atoms are smaller for H – 1 and L + 1 compared to H, L. The overall asymmetry in the energies between the occupied and unoccupied MOs is however much less pronounced compared to DAP27, because of the mentioned symmetry of the nodal plane in H, L. Therefore, the reason for the much more manifest spectral changes for DAP16 is attributed to the reduction in the molecular symmetry when going from Py or DAP27 (PG D_2h) to DAP16 with PG C_2h. For this reason, H, and H – 1 in DAP16 belong to the same irreducible representation (b_g), whereas L, L + 1 belong to a_u, see Figure . This mixes all three one-electron configurations Φ_1,2,3, i.e., H → L, H – 1 → L, and H → L + 1, resulting in three optically allowed ¹B_u states of similar intensities; in particular, the S₁ state of DAP16 carries substantial oscillator strength. The TD-DFT calculations in fact give f(1¹B_u) = 0.17, and f(2¹B_u) = 0.13, which reproduce well the observed experimental absorption features in Figure . The enhancement of f ₀₁ vs Py amounts to a factor of about 3 × 10² (Figure ); this correlates well with the enhancement of the radiative rate, see Table .

In contrast to Py, the presence of the free electron pair of nitrogen in DAP16 generates an energetically high-lying nσ-type MO (being H–2; see Figure ), while the corresponding σ-type MO in Py is very low in energy, i.e., H–6. In DAP16 with PG C_2h, this gives rise to a symmetry-allowed (nσ)­π*-type transition, which is found as S₂ state (1¹A_u) of low oscillator strength (f ₂ = 2.4 × 10^–3) between the two first ππ*-type ¹B_u states, see Figure and Table S1. This should somewhat broaden the absorption features in the region of the S₀ → S₁ transition, as indeed experimentally observed (vide supra). The correlated (nσ)­π*-type triplet state (T₃; 1³A_u) is calculated to be only 0.26 eV below S₁ (see Tables S1 and S2) due to the small exchange integral for 1A_u. This opens a new, El-Sayed allowed ISC channel in DAP16, which is expected to significantly enhance k _ISC. This agrees with the largely increased k _nr compared to Py; i.e., by 3 orders of magnitude, see Table ; TD-DFT calculations of k _ISC confirm the enhancement in a good qualitative manner when comparing the compounds; however, the calculations fail to quantitatively reproduce the effect (see Table S5a). In DAP27, the (nσ)­π*-type singlet and triplet states (1¹B_3u, 1³B_3u) are significantly higher in energy compared to DAP16 (Figure ), so that 1³B_3u is found 0.17 eV above S₁ (see Tables S1 and S2); therefore, k _nr (mainly via k _ISC) in DAP27 takes a position in between Py and DAP16, see Table , which is again qualitatively confirmed by the TD-DFT calculations, see Table S5b. The competition of radiative decay and ISC in DAP16 results in the moderately high Φ_F of 34% and the short lifetime of τ_F = 0.73 ns; thus, τ_F is not only drastically shorter than in Py, but also considerable shorter compared to DAP27, see Table .

In the next step, we explored the deactivation paths of the triplet state. At room temperature (RT) under O₂-free conditions, Py exhibits a phosphorescence lifetime of τ_P = 9 ms, while at low temperatures (LT) the lifetime increases to 0.5 s. In contrast, no RT phosphorescence was seen for DAP16 and DAP27 in Ar-purged DCM solution, pointing to very efficient nonradiative decay via triplet-singlet crossing. In fact, nonradiative pathways are known to be effectively activated when going from PAHs to their aza-counterparts; see for instance naphthalene vs quinoline. We thus calculated the triplet-singlet minimum energy crossing point (MECP) which occurs from T₂ to the ground state S₀ for DAP16, which indeed shows a local out-of-plane distortion of one of the N-containing rings, see Figure a and Table S7. After initial population of T₃ (1³A_u) via ISC from S₁, the T₂ state (2³B_u) is rapidly populated by IC. From here, the MECP can be efficiently reached because of the pronounced energy gap of about 1 eV between T₁ (1³B_u) and T₂ (see Table S2); in the framework of Fermi’s golden rule, such large gap considerably slows down IC, as it was in fact shown for azulene derivatives. This is expected to hold in the current case for IC from T₂ to T₁, but permits access to the MECP; according to the DFT results, the latter is in fact located 0.02 eV below the T₂ minimum. Under LT conditions (65 K), for DAP27 still no phosphorescence was detected, while DAP16 does exhibit phosphorescence at 564 nm (Figure b), being 1.1 eV below S₁ (reasonably reproduced by TD-DFT, see Table S3). The corresponding lifetime amounts τ_P = 0.29 s (Figure S17); similar to Py, the long τ_P of DAP16 reflects the small SOC of T₁ deactivation due to the ππ* nature of the S₀ → T₁ transition (Table S2). T₁ is almost exclusively described by a H → L excitation (see Table S2), different from S₁ (Figure and Table S1), so that the natural transition orbital (NTO) topologies differ slightly in the central carbon atoms (Figure S14); this is subsequently reflected in the somewhat different vibronic patterns of the LT fluorescence and phosphorescence spectra; see Figure b.

3.

Excited state deactivation in DAP16. (a) Schematic path at RT (based on the experimental and computational results). Main channels from S₁ (1¹B_u) are k _F and k _nr via El-Sayed allowed ISC to T₃ (1³A_u), followed by IC to T₂ (2³B_u) and reverse (R)­ISC to S₀ via the MECP; each followed by VR. IC from T₂ to T₁ is a minor path (large ΔE); IC becomes active at LT, so that k _P opens. (TD)­DFT optimized geometries are given for S₀ and MECP. (b) ungated (black) and gated (blue) emission spectra of DAP16 in PMMA at LT (65 K).

The efficient population of the triplet manifold of DAP16 (vide supra) allows for an intriguing photoreaction with O₂ under unpurged conditions, while this is not observed for DAP27, nor for Py (see Figures S20 and S21). In fact, under laser irradiation at λ_ex = 355 nm of DAP16 in DCM solution, new absorption features appear. The new bands resemble those of DAP16, but appear clearly somewhat broader and with a distinct bathochromic shift of 0.12 eV (13 nm), see Figure . After 20 min under the given excitation conditions, the reaction is complete, as the original DAP16 features have entirely disappeared. The reaction does not occur under Ar-purged conditions (see Figure S22). Strikingly, the photoreaction is reversible; 7 days after the formation of photoproduct, and keeping the solution at RT in a sealed vial, the original DAP16 spectrum slowly recovers (Figure S23). At elevated temperature (50 °C) in chloroform solution, the process is slightly accelerated (Figure S25) as all the photophysical and photochemical properties of DAP16 are identical in DCM and chloroform (Figure S24); a posteriori purging with Ar does not have a notable effect.

4.

Photooxidation of DAP16. Absorption spectra of in air-saturated DCM solution during irradiation with a 355 nm laser (left); suggested peroxide formation (right).

We have made significant efforts to identify and isolate the photoproduct by ¹H NMR and HRMS; however, these attempts have been unsuccessful. The solubility of O₂ and the observed reversibility of the process may account for the lack of evidence while reproducing this phenomenon in the required 10^–2–10^–3 M range. Therefore, in order to gain further insight in the nature of the photoproduct, we used TD-DFT to identify the most probable candidates; in fact, TD-DFT is known to predict reliably even subtle electronic and geometrical substituent effects in conjugated compounds, for varying nature, − number , and position − of the functional unit. The calculated possible photoproducts of DAP16 included (i) noncovalent complexes in different configurations, (ii) various cycloadditions, (iii) oxidation in different positions, as well as (iv) peroxide formation after singlet oxygen production via the T₁ state of DAP16 during irradiation. As detailed in Tables S9–S13, the resulting UV/vis absorption spectra of the photoproducts turned out to be largely different for the various reactions, so that comparison with the experimental spectrum of the photoproduct allows to specifically identify the most probable species formed. Taking together the experimental findings (that is the small bathochromic shift of the absorption against DAP16 while maintaining the spectral features, as well as the very slow back-reaction of photo-oxidation), the most probable scenario for the photoproduct is peroxide formation via insertion in the C–H bond at the 2-position of DAP16, see Figure and Table S12. This is plausible, as the α-position to the nitrogen is expected to be activated, and the photoproduct is additionally stabilized by intramolecular H-bonding with the N atom. In fact, the N···H distance is calculated to be 1.84 Å, matching typical values for NH hydrogen bonding; for additional information on the calculated reaction path, see Figure S26. To further elucidate the photochemical process, derivatives of DAP16, including nitrogen-alkylated forms and N-oxides, are currently being synthesized to explore their reactivity and mechanistic pathways.

In conclusion, we synthesized the 1,6-diazapyrene isomer, DAP16, which has not been reported so far, and which is a well-defined small size prototype system for nitrogen-containing PAHs. The new compound dramatically changes the optical excitation, and subsequent photophysics and -chemistry compared to pyrene and the well-studied 2,7-diazapyrene; in particular, effective triplet population was observed for DAP16. All changes could be rationalized by the peculiarities of DAP16 in the electronic structure and symmetry, as fully revealed by (TD)­DFT calculations on excited state properties and deactivation. Because of its unique photophysics, DAP16 shows an extreme sensitivity against oxygen, leading to reversible peroxide formation. The extraordinary sensitivity of positional isomerism in diazapyenes for photophysics and -chemistry is considered of high relevance for a general understanding of N-PAHs and their targeted design.

All detailed experimental and characterization data associated with this work are available in the Supporting Information, raw data are available at Zenodo repository: 10.5281/zenodo.14277234.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.5c01474.

• Detailed synthesis and characterization of all compounds, NMR and HRMS copy, symmetry considerations for pyrene, additional calculations, spectroscopic results and photochemical experiments (PDF)

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I.B., L.W., and N.G.-S. have contributed equally to the work.

The authors declare no competing financial interest.