Daniel
Blasco
*a,
Rinat T.
Nasibullin
b,
Rashid R.
Valiev
b,
Miguel
Monge
a,
José M.
López-de-Luzuriaga
a and
Dage
Sundholm
*b
aDepartamento de Química, Instituto de Investigación en Química (IQUR), Universidad de La Rioja, Madre de Dios 53, 26006, Logroño, Spain. E-mail: daniel.blascos@unirioja.es
bDepartment of Chemistry, Faculty of Science, University of Helsinki, P. O. Box 55 (A. I. Virtasen aukio 1), FIN-00014, Helsinki, Finland. E-mail: dage.sundholm@helsinki.fi
First published on 31st January 2024
Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated ab initio levels of theory also show that 1 has a large inverted singlet–triplet (IST) gap, a non-vanishing spin–orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.
Fig. 1 The molecular structure of heptazine with the canonical numbering of the atomic positions and the HOMO and LUMO orbitals. |
The lowest-energy excited singlet (S1) and triplet (T1) states of heptazine have the same orbital composition consisting of a localized set of atom-range multiple resonances (MR) from the HOMO, located on the six electron-rich peripheral nitrogen atoms, to the LUMO, on the carbon and central nitrogen atoms (see Fig. 1, right).7 The character of these excitations is almost insensitive to corner functionalization (atomic positions 2, 5, and 8), even when the substituent is a heavy metal such as gold.8 The small HOMO–LUMO overlap minimizes the exchange integral leading to a small energy difference between S1 and T1 [ΔEST, calculated as ΔEST = E(S1 ← S0) − E(T1 ← S0)].9
By performing calculations at different levels of theory, several research groups have recently demonstrated that the energy order of S1 and T1 in heptazine is unexpectedly inverted (IST, see Fig. 2).7,8,10–22 The violation of Hund's maximum multiplicity rule23 by non-alternating polycyclic polyenes is not a new phenomenon. A more stable S1 with respect to T1 was observed for the symmetric forms of propalene, pentalene, heptalene,24 and, more recently, in isopyrene.25–27 However, far from just being a scientific curiosity, IST is expected to give rise to a new kind of light-emitting molecules, the so-called fifth generation of emitters with applications in OLED devices.28,29 The singlet–triplet inversion is expected to eliminate the need of thermal activation for achieving maximum external quantum efficiency. It also supresses the triplet decay channel, overcoming potential bimolecular annihilation processes that ordinary TADF emitters suffer from due to the long lifetime of the excited states, which are detrimental to the performance and stability of OLED devices.
Fig. 2 Energy diagram of the first excited states of a molecule (a) obeying Hund's multiplicity rule, and (b) with an inverted singlet–triplet gap. |
The origin of IST is attributed to dynamic spin polarization effects of the first excited state,24,30–32 which are accounted for at configuration interaction (CI) levels including double excitations. For a given active orbital space, Pauli's exclusion principle restricts the number of possible doubly-excited triplet configurations with respect to the number of doubly-excited singlet ones. When this extra dynamic spin polarization stabilization of S1 exceeds the exchange energy, ΔEST becomes negative.7,24,30 Thus, a proper description of the IST needs either single or multireference correlated wavefunction-based (ab initio) computational levels that accurately capture contributions from double excitations. These methods, in virtue of an improved description of the electronic structure, are computationally expensive and resource penalties preclude a widespread usage, justifying the use of cheaper but reliable approximate computational levels such as the approximate second-order coupled-cluster (CC2) and algebraic diagrammatic construction of second order [ADC(2)] methods.33–37
Despite the increasing plethora of potential IST molecules identified by high-throughput virtual screenings,10,17,26,38 there are still very few experimental studies of their photophysical properties.7,10,13 In their seminal paper from 2022, Aizawa et al. proposed a decreasing trend of the delayed fluorescence lifetime component with decreasing temperature as a conclusive experimental proof of the so-called heptazine-type delayed fluorescence arising from IST.10 However, we believe that the paucity and non-homogeneity of the available experimental data precludes a generalization of the photophysical behaviour of IST molecules. Thus, it is mandatory to continue gathering experimental data from molecules for which IST gaps are computationally detected, to gain a more complete picture of the actual behaviour and performance of these promising molecules.
Here, we report a thorough experimental study of the photophysical properties of 2,5,8-tris(phenylthiolato)heptazine (molecule 1, see Fig. 3), which is identified to have a large inverted singlet–triplet gap at several ab initio correlated levels of theory. We show that the completely symmetrical derivatives of heptazine, which are excluded at the first stage of virtual screenings due to their vanishing S1 ← S0 oscillator strengths, can also be efficient IST emitters thanks to vibronic coupling. Besides, some of the measurements are performed on pure powder samples, a state that had not been considered in previous studies. We have focused on ether-like derivatives of heptazine due to their relative ease of synthesis and introduction of structural modifications in the pending moieties. Posern et al. synthesized a family of symmetric heptazine arylthio- (aryl = phenyl, p-tolyl, p-chlorophenyl, p-methoxyphenyl, 2-naphthyl, 1-methyltetrazyl) and phenylselenoethers starting from 2,5,8-trichloroheptazine, and studied their thermal stability. X-ray single-crystal structures were determined for the phenylthiolato and phenylselenolato derivatives as mesitylene (1,3,5-trimethylbenzene) solvates, revealing a three-fold symmetry axis and a staggered conformation of the phenyl planes with respect to the heptazine one. Besides, the title molecule 1 was identified as photoemissive in chloroform solution (λem at 552 nm with a λex of 288 nm). Galmiche et al. proposed a new route to heptazine arylthioethers starting from the more soluble 2,5,8-tris(3,5-diethylpyrazolyl)heptazine and presented a more complete study of the photophysical properties of molecule 1. Interestingly, the authors noted that there is a weak absorption band at low energies in all heptazine derivatives they studied. The band was assigned with time-dependent density functional theory (TD-DFT) calculations to a symmetry-forbidden S1 ← S0 transition in the heptazine core.
The synthesis of 1 is described in Section 2.1 and in more detail in the Experimental Section 4. The experimental studies of the photophysical properties are discussed in Section 2.2, which is followed by computational characterization in Section 2.3 that comprises calculations of photophysical properties, the magnetically induced current density of 1, and calculation of optical properties of related compounds where the S atoms are replaced by O (2), Se (3) or Te (4). The main results are summarized in the Conclusions Section 3.
The emission spectrum and average lifetimes were also measured at low temperature (see Fig. 4, right and the ESI†). The emission intensity nearly triples upon cooling to 78 K as the restriction of vibrational motion disfavors non-radiative deactivations. The vibronic structure of the ground state is also revealed in the emission profile (see Fig. S1, ESI†). The energy difference between consecutive vibronic peaks is roughly 300 cm−1, which is of the same size as the calculated energy of the out-of-plane vibration of the nitrogen atoms of the heptazine core (see Fig. S2, ESI†). The average emission lifetime at 78 K of 445 ns is about a factor of 2.5 longer than the one of 171 ns measured at 300 K indicating the influence of excited vibrational motion on the relaxation of S1. However, it is still fast as compared to other relaxation channels. There is no evidence of participation of the triplet state at 78 K, precluding phosphorescent or TADF behavior.
The photophysical properties of molecule 1 have also been measured in toluene solution (see Fig. 5), which was not considered in the study of Galmiche et al.3 The UV-vis spectrum (see Fig. 5a) shows a structured absorption at 303 nm similar to the one observed by DR-UV-vis. A structured spectral trace corresponding to the S1 ← S0 transition is again observed around 410 nm, in line with the findings of Galmiche et al. for other solvents3 (see Introduction Section 1). The spectrum has been measured at different temperatures in the 20 °C to 70 °C range. The spectral profile is slightly modified upon heating, whereas the absorption energies do apparently not depend on the temperature since the position of the maximum is not shifted. Notably, the absorbance of the spectral trace increases. Since the S1 ← S0 transition is symmetry-forbidden, a thermally-prompted increase of the promoting vibrational motion breaks the molecular symmetry and relaxes the prohibition rule, thus increasing the absorbance. The photoemissive properties have been measured in the absence and presence of molecular oxygen, which is a well-known quencher of luminescence via collisional energy transfer and formation of singlet oxygen. Besides that long-lived excited states are sensitive to the presence of molecular oxygen, it can also form an exciplex with the excited emitter molecule and thereby contribute to the luminescence quenching.40
Molecule 1 is extremely luminescent in solution, exhibiting intense blue emission with a PLQY of 0.71 even at μM concentrations (see Fig. 5b). The emission profile resembles the one in the solid state, with a maximum at 459 nm and a tail up to 700 nm, with a long emission lifetime of 462 ns. As expected, the presence of oxygen in solution leads to a drastic drop in the emission intensity, which is reflected in a shortening of the lifetime to 316 ns and a decrease in the PLQY to 0.38. However, it is noteworthy that the emission in not completely quenched, what would be expected if it were of phosphorescent nature. Moreover, the spectral profiles are virtually indistinguishable without loss of vibronic fine structure (see Fig. 5c). This demonstrates that the triplet state is not populated at room temperature, since no part of the spectrum originates from triplet decay. The transient photoluminescence decay data shows monoexponential behaviour over the whole considered time range (see Fig. 5d and Fig. S5, ESI†). The longer lifetime and higher PLQY of 1 in solution than in the solid state, irrespective of the absence or presence of oxygen, suggest the formation of π-stacked exciplexes with toluene molecules, what is supported by the observation of π-stacks with mesitylene molecules in the crystal state.39 The photophysical properties of molecule 1 are summarized in Table 1.
Molecule | λ abs | λ em (λex) | τ | ϕ (λex) |
---|---|---|---|---|
a Presently we do not have instrumentation to measure these values. | ||||
Solid state, 300 K | 295, 321, 397 | 456 (354) | 171 ± 4 | 0.33 (374) |
Solid state, 78 K | —a | 472 (354) | 445 ± 17 | —a |
Toluene, O2 free | 303, 398, 418 | 459 (306) | 462 ± 2 | 0.71 (290) |
Toluene, O2 saturated | 303, 398, 418 | 459 (306) | 316 ± 2 | 0.38 (290) |
Transition | Irrepa | Energy | f | Contributions (%) | |
---|---|---|---|---|---|
a Irreducible representation. | |||||
TD-DFT/PBE0 level of theory | |||||
S1 ← S0 | a′ | 361 [3.437] | 0.00 | 37a′′ → 38a′′ | (97.0) |
S6 ← S0 | a′ | 289 [4.290] | 0.44 | 36a′′ → 38a′′ | (98.4) |
S7 ← S0 | a′ | 289 [4.290] | 0.44 | 35a′′ → 38a′′ | (98.4) |
S26 ← S0 | a′ | 241 [5.139] | 0.42 | 34a′′ → 39a′′ | (67.7) |
31a′′ → 39a′′ | (12.0) | ||||
S27 ← S0 | a′ | 241 [5.146] | 0.42 | 34a′′ → 40a′′ | (67.7) |
31a′′ → 40a′′ | (12.0) | ||||
T1 ← S0 | a′ | 381 [3.251] | — | 37a′′ → 38a′′ | (97.2) |
ADC(2) level of theory | |||||
S1 ← S0 | a′ | 414 [2.993] | 0.00 | 37a′′ → 38a′′ | (50.1) |
34a′′ → 38a′′ | (44.5) | ||||
T1 ← S0 | a′ | 382 [3.246] | — | 37a′′ → 38a′′ | (51.8) |
34a′′ → 38a′′ | (45.7) |
There are two pairs of doubly-degenerate transitions of high intensity at 289 and 241 nm. They are considerably blue-shifted with respect to the absorption maxima. The energy mismatch may be due to stabilizing packing effects in the solid state, that are not taken into account in the calculation. The S6 ← S0 and S7 ← S0 transitions correspond to charge transfers from the HOMO−1 and HOMO−2, mainly located on the sulfur atoms of the pending phenylthiolate substituents with smaller contributions on the phenyl rings and the peripheral nitrogen atoms of the heptazine core, to the LUMO, which, apart from the expected contribution on the peripheral carbon and central nitrogen atoms of the heptazine core (see Fig. 1, right), shows a non-negligible contribution from the sulfur atoms. The S26 and S27 states, on the other hand, involve transitions from HOMO−6 and HOMO−9, which have a bigger contribution from the π-bonding electron density of the phenyl rings, to LUMO+1 and LUMO+2, which are not as clearly defined on the heptazine ring as the LUMO.
The S1 and T1 states calculated at the ADC(2) level consist of the two transitions of similar weight from the HOMO and HOMO-6 to the LUMO that are depicted in Fig. 7. The spatial distribution of the HOMO and HOMO−6 is identical, with only a variation of the sign of the orbital lobes located on the peripheral nitrogen atoms of heptazine. The usual MR pattern of heptazine is accompanied by a charge transfer from a bonding π-orbital of the pending phenyl groups to an empty atomic orbital of the bridging sulfur atoms. The calculated energies at this level of theory are close to the local absorption maximum of the band edge (see Fig. 6, bottom). More importantly, the S1 ← S0 excitation energy is lower (longer wavelength) than the T1 ← S0 one, meaning that the singlet–triplet gap is inverted. The IST is consistent with the strong fluorescent property of molecule 1.
Fig. 7 Molecular orbitals and their contributions to the S1 (T1 in red) transition calculated at the ADC(2)/def2-TZVP level of theory for molecule 1. |
The excitation energies to the S1 and T1 states have also been calculated at the complete active space self-consistent field (CASSCF) and the extended multiconfigurational quasidegenerate perturbation theory of second order (XMC-QDPT2) levels, with an active space of 6 electrons in 6 orbitals as well as with 12 electrons in 12 orbitals. The excitation energies and the singlet–triplet gaps calculated at the different levels of theory, together with the spin–orbit coupling matrix elements between S1 and T1 (〈S1|SO|T1〉) and the S1 ← T1 intersystem crossing rate constants (kISC), are collected in Table 3.
As expected, TD-DFT gives a positive IST gap. The correlated ab initio methods predict that the excitation energy of the S1 ← S0 transition is smaller than the T1 ← S0 transition leading to a negative singlet–triplet gap. In fact, the ΔEST of −3305 cm−1 obtained at the XMC-QDPT2(12,12) level is large despite the excited singlet and triplet states have the same orbital composition, suggesting a noticeable dynamic spin polarization effect. However, the excitation energies calculated at the XMC-QDPT2 level are for some unclear reason much smaller than the ones calculated at other levels of theory and too small as compared to the energy of the emitted light of 21790 cm−1 corresponding to a wavelength of 459 nm. The excitation energies calculated the CASSCF level are rather independent of the size of the active space and in fair agreement with those calculated at other levels of theory and with the experimental value. The SOC matrix element between T1 and S1 is 0.15 cm−1 at the CASSCF(12,12) level yielding a kISC of the order of 106 s−1, which is fast enough to direct the excitons to the fluorescing S1 state. The calculated phosphorescence rate constant from T1 to S0 is 400 s−1 at the CC2 level. The calculated rate constant for the reverse ISC (kRISC) from S1 to T1 is 0.07 s−1 at the XCM-DQPT2(12,12) level. These rate constants are several orders of magnitude smaller than the kISC rate constant from T1 to S1 of 5.5 × 105 s−1, which was calculated at the XMC-QDPT2(12,12) level, the internal conversion rate constant (kIC) from S1 to S0 is 4.0 × 104 s−1, and the experimental fluorescence rate constant (kfluor) from S1 to S0 is 3.1 × 105 s−1 implying that practically all excitons reach S1 where they decay to S0 either by fluorescence or non-radiatively through internal conversion.
Previous spectroscopic studies suggested that pristine heptazine is aromatic because it has the δ (1H) signal at 8.3 ppm44 and that cyclazine (9b-azaphenalene) is antiaromatic, with δ (1H) signals at 2.1–3.7 ppm.45–47 In the latter, the central nitrogen atom does not participate in the π-conjugation, leading to a rim consisting of a 12π-electron annulene.
We have performed MIC calculations with the GIMIC method and integrated the strength of the MIC passing through selected cut planes as shown in Fig. 8. The profiles of the MIC passing through the cut planes are depicted in Fig. S11 (ESI†). The integrated MIC strengths of molecule 1 are collected in Table 4. The GIMIC calculations show that molecule 1 is non-aromatic, with a very weak paratropic current of ca. −0.4 nA T−1 circulating around the edge of the molecule. Visualization of the streamlines of the current-density pathways reveals that the MIC consists of small local independent vortices at the nitrogen and sulfur atoms (Fig. S12, ESI†), with hardly any current density circulating around the carbon atoms. The global non-aromaticity of the heptazine core in 1 is thus a consequence of the alternation of carbon and nitrogen atoms along the external rim that precludes free electron circulation.
Plane | Net | Diatropic | Paratropic |
---|---|---|---|
a | 0.03 | 6.00 | −5.97 |
b | −0.35 | 7.09 | −7.45 |
c | −0.43 | 7.73 | −8.16 |
d | −0.41 | 7.72 | −8.13 |
e | −0.34 | 7.10 | −7.44 |
The structural optimization at the DFT/PBE0-D3(BJ)/def2-TZVP level of theory of molecules 2–4 preserves, in the four cases, the three-fold symmetry of the starting C3h point group of molecule 1 as extracted from its X-ray crystal structure (see Fig. 9). For molecule 4, the σh plane is lost due to the torsion of the bulkier phenyltellurol groups. A selection of bond distances and angles of the optimized molecular structures of 1–4 is compared in the ESI,† with the existing X-ray structural data.39,48 There is no experimental evidence of molecule 4, probably due to the instability of tellurols. The agreement between the available experimental and calculated distances and angles is excellent, validating the use of the computed models for further simulations.
Fig. 9 The molecular structures of molecules 1–4 optimized at the DFT/PBE0-D3(BJ)/def2-TZVP level of theory. |
The vertical excitation energies to the S1 and T1 states have been calculated at the ADC(2) level of theory. The results are summarized in Tables 5 and 6. The orbital contributions to the most intense transitions are plotted in the ESI.†
Molecule | Irrepa | E(S1 ← S0) | Contributions (%) | E(T1 ← S0) | Contributions (%) | ||
---|---|---|---|---|---|---|---|
a Irreducible representation. | |||||||
2 | a′ | 3.494 | 31a′′ → 35a′′ | (72.4) | 3.787 | 31a′′ → 35a′′ | (74.3) |
34a′′ → 35a′′ | (21.4) | 34a′′ → 35a′′ | (22.5) | ||||
3 | a′ | 2.920 | 41a′′ → 47a′′ | (49.7) | 3.166 | 41a′′ → 47a′′ | (52.1) |
46a′′ → 47a′′ | (40.3) | 46a′′ → 47a′′ | (41.6) | ||||
4 | a | 3.272 | 140a → 141a | (84.0) | 3.164 | 140a → 141a′′ | (81.1) |
The chemical identity of the bridging chalcogen atom has an important effect on the calculated ΔEST, which becomes smaller upon increasing the atomic number in the series 2 (XO) > 1 (XS) > 3 (XSe) > 4 (XTe). In the extreme case of molecule 4, ΔEST is even positive. The reason for this may be found in the values of the vertical excitation energies and the orbital contributions. For molecules 1–3, for which ΔEST is predicted to be negative, there are two main contributions to the vertical transitions, regardless of the multiplicity of the arriving state. These are analogous to the ones calculated at the same level of theory for molecule 1 (vide supra). The occupied MOs at Se and Te contribute to the exciton of the S1 and T1 states, whereas for the O and S containing molecules they do not. Since Te is larger than O, S and Se the exciton is significantly localized to Te, whereas for the three other molecules the exciton is mainly at the heptazine moiety.
This study demonstrates the potential of three-fold symmetric ether, thioether, and selenoether derivatives of heptazine to also be among the fifth generation of emitter molecules. Further efforts will be devoted to the synthesis of new analogs of molecule 1 and its implementation in working OLED devices.
(1) |
(2) |
Absolute photoluminescence quantum yields were determined with a Hamamatsu Quantaurus-QY C11347 instrument. HPLC grade toluene was degassed and dried with a MBRAUN MB-SPS800 system. The solutions for photophysical measurements were prepared inside a MBRAUN LABstar glovebox under Ar atmosphere. For measurements in the presence of oxygen, air was bubbled through the neat solvent for 10 min.
(3) |
The magnetically induced current (MIC) densities were calculated using the gauge-including magnetically induced currents (GIMIC) method.41–43 Ring-current strength susceptibilities, which are denoted ring-current strengths, were obtained by numerical integration of the current density passing through a cut-plane perpendicular to a selected bond. The current density was calculated at the BHandHLYP/def2-TZVP level of theory (LIBXC ID 436).69–71
Footnote |
† Electronic supplementary information (ESI) available: Emission spectrum of molecule 1 in the solid state at 78 K, transient photoluminescence spectra of molecule 1, molecular orbital contributions to the most intense transitions of molecules 1–4, streamlines of the MIC pathways of molecule 1, selected bond lengths and angles of molecules 1–4, Cartesian coordinates of molecules 1–4. See DOI: https://doi.org/10.1039/d3cp05242g |
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