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. 2025 Sep 3;16(36):9479–9487. doi: 10.1021/acs.jpclett.5c01314

The Lowest Excited State of Heptacenes Is Dark

Johannes Schöntag †, Philipp Frech ‡, Kathrin Zwettler †, Navid Fardan †, Ankit Somani †, Wolfgang Leis §, Markus Ströbele §, Michael Seitz §, Marcus Scheele ‡,*, Holger F Bettinger †,*
PMCID: PMC12434725  PMID: 40899811

Abstract

Understanding the electronic structure of polycyclic aromatic compounds is of fundamental importance for their potential applications. The optoelectronic properties of shorter acenes such as tetracene and pentacene have been extensively studied with regard to excitation, emission, and nonlinear effects such as singlet fission. The longer homologues present a unique challenge due to their low stability both in the solid state and in solution. In this work, we synthesized persistent 6,8,15,17-tetrakis­((triisopropylsilyl)­ethynyl)­heptacene and investigated its photophysical properties as well as those of the parent heptacene. Our steady-state electronic absorption and emission experiments combined with transient absorption spectroscopy show that the Franck–Condon electric-dipole-forbidden (“dark”) transition to the 21Ag singlet state is the lowest-energy excited state of heptacene. This contrasts with the optical properties of the well-known shorter acenes. Transient absorption data further suggest singlet fission or intersystem crossing as potential pathways to rapid population of the triplet state facilitated by the dark singlet state.

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The electronic structure of polycyclic aromatic compounds is the key to their application as organic optoelectronic materials. A particularly important class of condensed polyaromatics are oligoacenes, as their study has driven development in singlet fission research over the past decade. − The energies of the lowest-energy singlet and triplet states, S1 and T1, of the most important representatives, tetracene and pentacene, fulfill the energy requirement E(S1) ≈ 2E(T1) for singlet fission. ,− Electronic excitation from the ground state to the S1 state (11Ag → 11B2u transition) gives rise to a prominent band (p band according to Clar, 1La according to Platt) in the absorption spectra of tetracene and pentacene.

Increasing the conjugation length results in a bathochromic shift of the 1La band due to a decrease of the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). , A decreasing HOMO–LUMO gap can result in a change of the character of the S1 state. For example, from octatetraene onward, polyenes have a lowest excited state that has the same symmetry as the ground state (1Ag). − According to computations using the particle–particle random phase approximation in combination with density functional theory (pp-RPA-B3LYP) and the multiconfiguration coupled-electron pair approximation (MC-CEPA), heptacene is the first acene in which, within the Franck–Condon approximation, the electric-dipole-forbidden 11Ag → 21Ag (“dark”) transition is lower in energy than the electric-dipole-allowed (“bright”) 1Ag → 1B2u transition. , The electronic absorption spectra of heptacene in solution, in a cryogenic argon matrix, and in organic glasses and polymers show a weak feature at lower energy than the 1La band that is absent in the shorter homologues. ,− This feature, a shoulder in most cases and only in argon an individual new band, ,, was tentatively associated with a forbidden transition in a previous study based on comparison with the computational data.

We reasoned that the high kinetic stability of a heptacene derivative would be beneficial for the detailed optical spectroscopic investigations and opted for the synthesis of the persistent derivative 6,8,15,17-tetrakis­((triisopropylsilyl)­ethynyl)­heptacene (TIPS4Hep) (Scheme ). During the course of our work, this compound was obtained by Bunz et al. using a different synthetic route. , Here, by combining steady-state and transient absorption spectroscopy of TIPS4Hep and the parent heptacene (Hep), we show that the dark state 21Ag becomes the lowest singlet excited state in the acene series. Unlike the conventional S1 state (1La, 11B2u), this dark state has a different leading electronic configuration (H0L2; H = HOMO, L = LUMO) compared to the T1 state (H1L1). This change in state order facilitates energy transfer from the dark state to an excited triplet state via intersystem crossing or singlet fission because of El-Sayed’s rule.

1. Synthesis of TIPS4Hep Starting from Heptacene-6,8,15,17-diquinone 1 .

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The synthesis of TIPS4Hep was performed using heptacene diquinone 1 (Scheme ). The latter was synthesized following the route of Baxter et al. In addition to the analytical data given previously (IR, UV/vis, and EA), we achieved characterization by HR-APCI-MS as well as 1H NMR in D2SO4 (see the Supporting Information (SI)). Treatment of the diquinone with lithium triisopropylsilylacetylide at 50 °C not only gives the fourfold addition product tetrol 2 (18%) but also the threefold addition product triol 3 (23%). When this reaction is performed at room temperature, only threefold addition occurs. Tetrol 2 was further reduced with SnCl2·2H2O to the fully aromatic TIPS4Hep (Scheme ). This reaction was performed in the absence of light in degassed solvents and gave the product in 44% isolated yield. TIPS4Hep is stable enough for chromatography to be performed over a short column in the dark, thus achieving the purity needed for detailed photophysical measurements.

TIPS4Hep was fully characterized by NMR, HRMS, and single-crystal X-ray crystallography. Bunz et al. recorded the 1H NMR spectrum in CDCl3 and justified the broad signals with the diradicaloid character of heptacene. We measured the 1H NMR spectrum in C6D6 and obtained sharp signals (see the SI). The broadening in CDCl3 likely stems from aggregation at the concentration required for NMR spectroscopy. We were able to measure a 13C NMR spectrum in C6D6 (see SI), while Bunz et al. stated that this was impossible due to the instability of TIPS4Hep. Unlike Bunz et al., we could not detect an EPR signal in DCM, C6F6, or n-hexane. We could grow single crystals of TIPS4Hep suitable for single-crystal X-ray analysis (Figure S1, CCDC 2272243, R = 1.9%) by slow evaporation of a solution in DCM/n-hexane. The compound crystallized in space group P21/c. Bunz et al. reported two crystal structures of this compound, one in space group P1̅ (CCDC 2254466, C6H6, R = 5.7%) and the other one also in P21/c (CCDC 2254465, CHCl3/MeOH, R = 5.5%).

The absorption spectrum of TIPS4Hep shows unusual features in the low-energy range compared to shorter acenes besides the known p band at 851 nm (A1), namely the shoulder at 887 nm (A2) and the weak band at 1010 nm (A3) (Figure ). C6F6 proved to be suitable for our measurements, as it does not possess any absorption bands below 2000 nm, unlike other solvents with C–H bonds (Figures S8 and S9). Similar features were observed for TIPS4Hep by Bunz et al. as well as for other substituted heptacenes by Anthony et al. and Wudl et al. (Figures S4 and S5). Their origin was, however, not discussed in previous studies. TD-DFT calculations of the electronic absorption spectrum and the vibrational coupling for the parent heptacene cannot explain these peaks, as previously discussed (Figure S7).

1.

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Absorption spectrum (1 × 10–5 M, rt) of TIPS4Hep in C6F6.

We performed a series of absorption spectroscopy experiments to confirm that the shoulder (887 nm, A2) as well as the 1010 nm band (A3) are due to TIPS4Hep. Measurements in solvents of different polarities revealed that A2 and A3 become more prominent with increasing polarity (Figure a). In n-hexane A3 was barely visible, whereas in 1:3 DCM/CH2I2 shoulder A2 and A3 are most prominent. The concentration-dependent measurements (Figure b) in the range 5 × 10–6 to 2 × 10–4 mol/L verified that neither transition is due to aggregation phenomena. Temperature-dependent measurements (Figure c) in toluene from +80 °C to −80 °C showed that with decreasing temperature both A3 and A2 become more prominent, ruling out a possible hot band. At lower temperature another shoulder of the second vibrational progression at 800 nm appears. Oxidation of TIPS4Hep with Ag­[Al­(OC­(CF3)3)4] (Figure d) resulted in a completely different spectrum, which fits into the series of monocationic acenes reported by Krossing et al. and is quite similar to that of the heptacene radical cation reported earlier under matrix isolation and gas-phase conditions. , Hence, the latter experiment proves that the additional peaks are not due to traces of the radical cation. An EPR spectrum of this radical cation could be recorded (Figure S15) and is comparable to the EPR signals of a radical cation of sixfold-substituted heptacene reported by Bunz et al.

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Absorption spectroscopy experiments: (a) solvent-dependent (1 × 10–5 M), (b) concentration-dependent (5 × 10–6 to 2 × 10–4 M, C6F6, rt), (c) temperature-dependent (toluene, 1 × 10–5 M, −80 to 80 °C in 10 °C steps), and (d) oxidation of TIPS4Hep with Ag­[Al­(OC­(CF3)3)4] (C6F6, 1 × 10–5 M, rt).

The shoulder A2 in the absorption spectrum of TIPS4Hep resembles features observed previously for Hep under cryogenic (Ar, 10 K) and room-temperature (PMMA) matrix isolation conditions and in solution (1-methylnaphthalene, 230 °C). ,, In the solid argon spectrum of Hep, a distinct band with a maximum shifted by 730 cm–1 from the 1La maximum is observed, while in solution there is a shoulder shifted by about 650 cm–1 from the maximum to lower energies (see Figure S6). The shoulder in TIPS4Hep is similarly shifted in C6F6 by about 590 cm–1 from the maximum. This suggests that the carrier of A2 is related in Hep and TIPS4Hep. The weak maximum of TIPS4Hep at 1010 nm (A3, bathochromic shift of ∼1850 cm–1 with respect to the maximum), on the other hand, is not related to any known transition of Hep.

To investigate the possible origin of the A3 band of TIPS4Hep further, we employed fluorescence spectroscopy in different solvents. Characteristic for the fluorescence spectra of acenes are the small Stokes shift and the vibrational fine structure of the emission band that mirrors the absorption spectrum. The longest acene for which fluorescence spectra are known is hexacene. Fluorescence spectra of larger acenes are not available, to the best of our knowledge. We could obtain fluorescence spectra of TIPS4Hep in C6F6, toluene, and n-hexane (Figure a). CCl4 and C2Cl4 turned out to be unsuitable, as it appears that photoexcited heptacene can react with these solvents. This was shown by a red coloration of the initially almost colorless solution (Figure S10) and no detectable fluorescence. In all cases the strongest maxima of the fluorescence spectra were at very long wavelengths (C6F6, 1040 nm; toluene, 1064 nm; n-hexane, 1031 nm). The measured spectra looked very similar, showing consistency across the measurements in different solvents. In n-hexane (red), more features of the spectrum are resolved, as the shoulder at 1162 nm became more prominent compared to the other solvents and another shoulder at 1220 nm could be detected. Measurements in frozen solutions at 77 K (Figure b) revealed more features in toluene (1028, 1176 nm) and particularly in n-hexane (1019, 1064, 1158, and 1239 nm), as this solvent can form a Shpolskii matrix, − which surrounds the molecule with a well-defined frozen solvent structure. The features of the corresponding excitation spectra largely follow the absorption spectra (Figure c,d). At higher energies, all main features are visible, but at longer wavelengths, the agreement is not as strong. Especially the band at 850 nm does not contribute to the emission as much as the less intense absorbing peak at approximately 760 nm. This possibly stems from effects of the gratings, which do not perform very well in these wavelength regions.

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(a, b) Fluorescence spectra of heptacene in different solvents (1 × 10–5 M) with λexc = 350 nm at (a) room temperature and (b) 77 K. (c, d) Excitation spectra vs absorption spectra of TIPS4Hep in (c) n-hexane (λem = 1028 nm) and (d) in C6F6 (λem = 1040 nm).

To further support these measurements, we decided to take a closer look at parent Hep. Studying the photophysics of this molecule is quite challenging, as it can dimerize or polymerize in solution, although measurement in 1-methylnaphthalene at 230 °C allowed an absorption spectrum in solution to be recorded. A convenient alternative is to use a photoprecursor of Hep and a solid matrix, as has been shown by Neckers, Bettinger, and co-workers. ,, In the present work, we used the monocarbonyl precursor COHep (Figure ), which is known to give Hep upon thermolysis in the solid state, as shown by Chow et al. The absorption spectrum of Hep in an argon matrix generated from COHep (Figure S14) is very similar to that obtained from the α-diketone precursor reported previously. , Irradiation of the precursor COHep in a 2-methyltetrahydrofuran (MeTHF) glass at 77 K using a medium-pressure mercury vapor lamp and a dichroic mirror (350–450 nm) resulted in Hep (Figure a,b), as confirmed by characteristic signals of the heptacene π system in the 700–800 nm range. ,,,

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(a) Formation of Hep via monocarbonyl photoprecursor COHep in 2-MeTHF at 77 K and (b) the resulting absorption spectra before and after irradiation. (c) Emission spectrum of Hep in MeTHF at 77 K with λexc = 350, 760 nm. (d) Excitation spectrum (λem = 1030 nm) of Hep in MeTHF inside a J. Young tube at 77 K compared to the absorption of Hep inside a cuvette.

The fluorescence spectrum was measured under argon inside a J. Young tube in MeTHF at 77 K (for the absorption spectrum in a J. Young tube, see Figure S2), and a signal at approximately 1030 nm was the only signal that we could detect (Figure c). Comparison of the fluorescence excitation and absorption spectra (Figure d) shows some resemblance at short wavelengths, while at longer wavelengths discrepancies are apparent. This is consistent with the fluorescence excitation and absorption spectra of TIPS4Hep (Figure ) discussed above. Emission at around 1030 nm would imply a very large Stokes shift if the longest-wavelength absorption at around 800 nm were considered as the emitting state, which seems highly unlikely in view of the typically very small Stokes shifts exhibited by shorter acenes.

A plausible interpretation of the absorption and emission spectra of Hep and TIPS4Hep involves the 21Ag state as the lowest-energy excited singlet state S1 of the heptacene framework. The lowest-energy excitation, 11Ag → 21Ag (S0 → S1), is electric-dipole-forbidden within the Franck–Condon approximation in this case and not detectable for Hep but appears as a low-intensity band (A3) at 1010 nm for the lower-symmetry TIPS4Hep. The energy difference between the p band maximum A1 (11Ag → 11B2u) at 852 nm and A3 in n-hexane is 0.23 eV (0.23 eV in C6F6, 0.22 eV in toluene, and 0.22 eV in 1:3 DCM/CH2I2), which is in reasonable agreement with the energy gap computed previously for these states (0.17 eV). Assuming a similar energy gap between 11B2u and 21Ag for Hep as for TIPS4Hep, we would expect the 11Ag → 21Ag absorption to be located at around 932 nm for Hep. Emission spectroscopy can reveal the 21Ag → 11Ag transition due to its sensitivity for both Hep and TIPS4Hep. The Stokes shift of 0.02 eV (181 cm–1) for TIPS4Hep in n-hexane is reasonably small, as typically observed for acenes. The predicted 11Ag → 21Ag absorption of Hep at 932 nm could correspond to a Stokes shift of 0.13 eV (1021 cm–1), which appears to be too large. Adding the Stokes shift of TIPS4Hep to the fluorescence signal of Hep would lead to a predicted 11Ag → 21Ag absorption at 1016 nm. Thus, we would expect the missing transition of Hep to be between 932 and 1016 nm. The vibrational fine structure of the fluorescence spectrum of TIPS4Hep indicates that the 21Ag → 11Ag transition goes along with vibrational excitation, and the same must be expected for the 11Ag → 21Ag absorption. The first vibrational band in the fluorescence spectrum of TIPS4Hep in n-hexane at room temperature at 1162 nm is shifted by 1094 cm–1 (C6F6, 995 cm–1; n-hexane (77 K), 1178 cm–1; toluene, 1022 cm–1; toluene (77 K), 1225 cm–1) from the fluorescence origin (Figure a), and assuming a similar shift in the 11Ag → 21Ag absorption, the shoulder (A2) at 890 nm (C6F6, 887 nm; toluene, 898 nm) is due to the vibrational feature of this transition (expected at 911 nm with these numbers; C6F6, 919 nm; toluene, 930 nm; Figure S3). This gains intensity from the nearby 11Ag → 11B2u transition. The low-temperature absorption spectrum in toluene in Figure c shows another shoulder at the first p vibrational band. Between this shoulder, the main shoulder A2 and the 1010 nm band A3, an energy gap of 0.18 eV (1450 cm–1) can be determined, which is a further indication of a vibrational progression. Likewise, the weak band at 768 nm in the matrix spectra of Hep is due to a vibrational progression of the 11Ag → 21A1g transition. ,

We performed transient absorption spectroscopy (TAS) of a 50 μM TIPS4Hep solution in C6F6 under argon to further corroborate our hypothesis of a 21Ag singlet excited state. TAS probes the temporal evolution of the excited state by determining the differential absorbance ΔA, which is the difference in absorbance between the excited and ground states. We pumped TIPS4Hep with 90 fs laser pulses at 900 nm to selectively target the A3 transition and monitor the excited-state absorption within a time window of 200 fs to 20 μs. Experiments at short time scales (<2 ns) were carried out with 0.9 μJ/pulse. To enhance the visibility of the weaker signals at long delay times, we display here the spectra obtained with 8 μJ/pulse but note that similar results (with less favorable signal-to-noise ratio) were observed with 0.9 μJ/pulse (see the SI for more details).

Figure a depicts a superimposed 2D hyperspectrum as a function of the pump–probe delay time and ΔA, which is color-coded on the right. The white patches indicate spectral regions that cannot be probed due to scattered pump or weak probe light. Detailed ΔA spectra at selected delay times below 400 ps are shown in Figures b and S11. We find bands with negative ΔA centered at 350, 383, 850, and 1020 nm, which are consistent with the steady-state absorbance shown in red above the 2D spectrum in Figures a and . Negative ΔA values indicate ground-state bleaching (GSB) due to a partial depopulation of the ground state as well as Pauli blocking (S0 →̷ S n , n ≠ 0) due to partial filling of an excited state. The fact that the β band at 350 nm and the p band at 850 nm (A1) both exhibit GSB under 900 nm excitation is a strong indication that the band at 1020 nm (A3) involves the 11Ag ground state. In addition, we observe bands with positive ΔA, indicating excited-state absorbance (ESA) (S n → S n+x , n, x ≠ 0), which is most prominent at 620 nm and some features around 500 nm. In the NIR, weak ESA around 1120, 1290, and 1500 nm emerges, showing a possible vibrational progression (compare Figure S11).

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Transient absorption spectra of TIPS4Hep in C6F6 (λexc = 900 nm). (a) Superimposed 2D TA spectra consisting of fs TAS up to 2 ns (0.9 μJ/pulse) and ns TAS (8 μJ/pulse) up to 20 μs. A steady-state absorbance spectrum is shown in red at the top. (b) ΔA spectra of (a) at selected delay times. In the inset, the transition from ps to μs TAS is shown. (c, d) Kinetic traces and their corresponding fits at selected wavelengths. The residuals of the fits are displayed below the graphs. (e) Schematics of all relevant states, transition wavelengths, and lifetimes extracted from the TAS experiments. Blue and red arrows indicate ground-state bleaches and excited-state absorption, respectively. All term symbols are according to Chakraborty and Shukla.

We fitted the kinetics of all prominent GSB and ESA features in Figure c and found that most of the transient signals decay monoexponentially within 130 to 160 ps. Across the full probe spectrum, we observe no significant kinetics within the first picosecond other than the ∼400 fs population rise imposed by the instrument response and very weak residual coherent artifacts. The lifetimes of the three prominent GSB signals are 145 ps at 350 nm (red, β-band), 113 ps for the p band (cyan, A1), and 80 ps for A3 (purple). These overall comparable magnitudes are supporting evidence that the A3 transition belongs to TIPS4Hep. The small but significant differences in the lifetimes of the three transitions can be rationalized in terms of a shared ground state but different excited states. This is consistent with our attribution of the A3 band to a 11Ag → 21Ag transition. With respect to the ESA kinetics, the most notable finding is that the band at 635 nm exhibits complex kinetics with a near-monoexponential decay to 3% of the peak ΔA value, where it remains as a residual, constant ESA for >8 ns (see Figure S12).

In Figure d, we display the further monoexponential decay of this residual ESA and determine a lifetime of 1.6 μs. We note additional ESA signals with such long lifetimes in Figure a at 425, 585, and 635 nm, which are plotted in more detail in the inset of Figure b. We argue that these features are evidence for a populated triplet state and assign the ESA at 635 and 425 nm to the 13B2u → 13B1g and 13B2u → 13Ag transitions, respectively. We rationalize this with two previous findings: heptacene prepared in situ by the photodecarbonylation of α-diketones revealed a triplet with a lifetime of 11 μs featuring a sharp ESA centered at 580 nm (2.14 eV) and a broad band between 400 and 500 nm. ESA from the 13B2u triplet in heptacene has been calculated with a prominent transition at 580 nm (13B2u → 13B1g ) and a weaker transition at 393 nm (13B2u → 13Ag ). These values are in reasonable agreement with our experimental results for TIPS4Hep, accepting a shift of ∼0.2 eV to lower energies for both transitions, which may be the effect of the TIPS-ethynyl substituents. In line with this, our computed absorption spectrum (Figure S13) of T1-TIPS4Hep obtained using TD-UDFT (UB3LYP/6-311+G**//M06-2X/def2-SVP) shows two main transitions with high oscillator strengths at 703 nm (f = 0.85) and 457 nm (f = 0.14), that is, at lower energies than the computational results for heptacene at the same level (see the SI for more details). The unassigned third transition at 585 nm can be regarded as a vibrational progression of the transition at 635 nm.

We conclude by discussing the population mechanism of the T1 state in TIPS4Hep based on our TAS data. We argue that the most likely pathway is via singlet fission (SF) or intersystem crossing (ISC) of the 21Ag singlet. This would also explain why the GSB of the 11Ag → 21Ag transition has the shortest lifetime, as it would decay due to relaxation to S0 and T1 simultaneously. An involvement of the 11B2u state is unlikely for three reasons: (1) for this state to be populated with a 900 nm pump pulse, two-photon absorption would be required, which is unlikely with the 0.9 μJ/pulse utilized by us; (2) the ISC from this state is expected to be slow due to El-Sayed’s rule, which is incompatible with the competition with the rather fast decay of the 11B2u state (113 ps); (3) SF of the 11B2u state occurs on short enough time scales for TIPS-ethynyl-pentacene derivatives, where the ideal scenario of E(S1) = E(1(TT)) is met almost perfectly. , For higher acenes, however, SF is increasingly exothermic, which greatly decreases its efficiency and favors triplet pair recombination before the free T1 state is formed.

Assuming a similar energy for the 13B2u state in TIPS4Hep as the 0.5–0.6 eV calculated for Hep, , SF should be much more favorable from the 21Ag state (1.2 eV). We note that SF typically requires concentrations larger than 4 × 10–4 M, which is slightly more than the 5 × 10–5 M used by us. A possible explanation could be the temporal formation of small aggregates in solution.

Alternatively, the T1 state may be populated from the 21Ag state via fast ISC, since it is favored by El-Sayed’s rule. Moreover, organic π radicals have shown ISC lifetimes on the order of picoseconds due to radical enhanced intersystem crossing (REISC). − Heptacene is known for its biradical character, and this property could render ISC competitive to the S0 relaxation with τ = 80 ps.

We summarize our TAS results concerning the nature of all observed transitions, their lifetimes, the two coexisting spin systems, and the pathway of transition between them in Figure e.

In conclusion, we have developed an alternative synthesis for TIPS4Hep and conducted a comprehensive photophysical study using steady-state and transient absorption spectroscopy. Comparing these results with complementary spectroscopic data on the less stable parent Hep suggests that TIPS4Hep is a suitable proxy to understand the electronic structure and excited-state dynamics of Hep. Its absorption spectrum reveals a weak band at 1010 nm, assigned to a forbidden 11Ag → 21Ag transition that is partially allowed due to vibronic coupling and proximity to allowed transitions. Similar spectral features are observed for Hep, although its higher symmetry renders the 11Ag → 21Ag origin unobservable. In accordance with computational spectroscopy, we assign this dark excited state to the S1 state of heptacene. We show that the dark state relaxes within 80 ps to a long-lived triplet state, and we discuss singlet fission and intersystem crossing as the most likely mechanisms for this action.

Supplementary Material

jz5c01314_si_001.pdf (2.6MB, pdf)

Acknowledgments

We are thankful to the European Research Council (ERC) for financial support of this work through SyG2023 (101071420, TACY). We also acknowledge support of the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through Grant INST 40/575-1 FUGG (JUSTUS 2 cluster) for computation facilities, Grant INST 37/1160-1 FUGG for the TAS measurements, and SCHE1905/9-1. We acknowledge support by the Open Access Publishing Fund of the University of Tübingen.

The data underlying this study are openly available in RADAR4Chem at DOI: 10.22000/j80kn6cses15pn7x.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.5c01314.

  • Synthesis of all compounds and their 1H NMR, 13C NMR, and HRMS spectra; computational and additional experimental details for absorption, fluorescence, and transient absorption spectroscopy; Cartesian coordinates; absorption spectra from other publications (PDF)

J. Schöntag: synthesis, analytics, computations, optical measurements, writingoriginal draft and visualization. P. Frech: transient absorption spectroscopy measurements, interpretation and visualization. K. Zwettler: synthesis and analytics. N. Fardan: synthesis and analytics. A. Somani: matrix isolation measurements and visualization. W. Leis: fluorescence spectroscopy measurements and interpretation. M. Ströbele: X-ray crystallography. M. Seitz: funding acquisition and interpretation. M. Scheele: funding acquisition, writing resources, writingreview, editing, and supervision. H. F. Bettinger: funding acquisition, project administration, resources, supervision, writingreview, editing, and supervision.

The authors declare no competing financial interest.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

jz5c01314_si_001.pdf (2.6MB, pdf)

Data Availability Statement

The data underlying this study are openly available in RADAR4Chem at DOI: 10.22000/j80kn6cses15pn7x.

Articles from The Journal of Physical Chemistry Letters are provided here courtesy of American Chemical Society

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