Necessary and Sufficient Condition for Organic Room‐Temperature Phosphorescence from Host–Guest Doped Crystalline Systems

Controlling and predicting the long‐lived room‐temperature phosphorescence (RTP) from organic materials are the next challenges to address for the realization of new efficient organic RTP systems. Here, a new approach is developed to reach these objectives by considering host–guest doped crystals, as well‐suited model systems in that they allow the comprehensive understanding of synergetic structural interactions between crystalline host matrices and emitting guest molecules, one of the key parameters to understand the correlation between the solid‐state organization and crystal RTP performances. Two series of σ‐conjugated donor/acceptor (D‐σ‐A) carbazole‐based matrices and isomeric 1H‐benzo[f]indole‐based dopants are designed, capable of exploring a wide variety of conformations thanks to large rotational degrees of freedom provided by the σ‐conjugation. By correlating the results of single‐crystal X‐ray diffraction analysis and photoluminescence properties, a necessary and sufficient condition for RTP is established that paves the way for the development of new long‐lived RTP host–guest doped systems.


Introduction
Metal-free organic materials involving triplet excited states in their luminescence process at ambient conditions have attracted widespread interest during the last decade, since they electron-acceptor (A) moieties connected via a π-conjugated spacer (e.g., D-π-A type molecules). Interestingly, one of the most used D units for building relevant TADF, [27] RTP, [19] and LPL luminophores [28] is the carbazole (Cz) motif, also commonly used as a well-suited hole-transporting unit for organic electronics. [27,29] Surprisingly, despite a plethora of works on Cz-based systems, it was pointed out not long ago that their efficient RTP emission could originate from impurities traces. [30] However, it is only very recently [31] that Cz from commercial sources was evidenced to contain a low-concentration isomeric impurity, namely 1H-benzo[f]indole (Bd), that can be also derivatized during the synthesis of Cz-based emitters, leading to the incorporation of the isomeric Bd-based material in very low content. Thus, while isomer-free materials obtained from pure Cz did not exhibit any afterglow, in contrast, crystals of Cz-based emitters synthesized from commercial Cz, as well as isomer-free crystalline systems doped with controlled amounts of their corresponding isomeric impurities exhibited obvious RTP. [30,[32][33][34] Therefore, this new context brings RTP from single-component Cz-based systems into question, and leads to discussing them as multi-component RTP systems (host-guest doped systems). More generally, it is obvious that establishing new emitters rational design guidelines, as well as structure-properties relationships still remains highly challenging, most notably in the case of purely organic host-guest-doped RTP systems.
The reason lies in that beyond the emitter intrinsic molecular structure, several other extrinsic factors have to be taken into account to achieve efficient RTP materials. [15,[35][36][37] To minimize the non-radiative deactivation pathways from the triplet excited state (T 1 state) to the singlet ground state (S 0 state), RTP hostguest materials have attracted special attention in these years. [38][39][40][41] In this multi-component approach, the organic phosphor is embedded as a doping guest into an amorphous or crystalline host matrix that suppresses by diluting effect the aggregationinduced self-quenching, restricts guest molecules motions, and prevents quenching with oxygen and moisture. [19 , 42-44] Additionally, it was reported that this strategy can also enable RTP by synergetic effects between the crystalline host matrix and guest molecules through the control of excited states energy transfers. [45][46][47][48][49] Very recently, another cooperative effect based on photo-induced charge-separated (CS) states between the host and guest entities was proposed in the case of Bd-doped Cz systems. [30] The mechanism relies on a charge trap/detrap model, as for LPL host-guest materials. [4] However, the ability of the studied system to generate long-lived intermediate CS states remains unclear. Moreover, the charge recombination should lead to an exciplex emission characterized by a broad structureless band whereas Bd-doped systems, intentionally or otherwise exhibit a highly distinctive emission spectrum with vibrational peaks.
Although there are several reports explaining the RTP enhancement of crystalline host-guest architectures by synergetic electronic effects, a comprehensive understanding of synergetic structural interactions between crystalline host matrix and guest molecules is still lacking. Thus, the main objective of the present work is to uncover this synergy at the structural level by studying a wide variety of model systems with different crystalline structures but where the matrices, just as the dopants, both keep similar electronic properties. With this aim, we designed and synthesized from pure carbazole and 1H-benzo[f]indole two series (Cz and Bd) of isomeric σ-conjugated (D-σ-A) derivatives selected as guests and hosts (Figure 1a), respectively, to yield a wide host-guest systems library. σ-Conjugation is introduced to i) provide large rotational degrees of freedom to the rigid D and A planes, like molecular rotors, thus allowing the molecules to adapt their conformations into the crystals, and ii) slightly electronically decouple the D and A units allowing them to act independently to some extent. [50] Each series comprises of two subsets of molecules where the sp 3 methylene spacer attached to the D unit (Bd or Cz cycle) and the A unit (bromo-pyridine motif) are linked in one subset by a triple bond and in the other one by a double bond. The molecular structural variations in each series are expected to vary both their conformation and organization into the host-guest doped crystals, with only minor changes of the molecular electronic properties, in order to finally understand the structure-properties relationships.
Here, we reveal and establish for the first time a necessary and sufficient condition for RTP in host-guest doped materials by highlighting the structural synergistic action between the matrix and the dopant. First, while the isolated Cz-and Bdbased molecules in the rigid environment of frozen solution exhibit blue and orange long-lived luminescence, respectively, none of the single-component Cz-based host crystals displays RTP. Second, doping a host matrix with a Bd-based guest molecule does not lead systematically to room temperature (RT) afterglow, which, when observed, has the same features as the individual Bd-based molecules isolated in a rigid environment. Therefore, doping, although necessary, is not sufficient to generate RT long-lived luminescence. This condition is revealed by a doped system single-crystal X-ray diffraction analysis highlighting that the Cz units inside all host matrices of systems capable of RTP adopt the same specific organization. Moreover, a plausible mechanism of the guest molecule incorporation is developed, assuming that its conformation mimics the one displayed by the host at the solid state, thanks to the degrees of freedom provided by the σ-conjugated bridge. All these results provide a key for revisiting the RTP of Cz-based systems, and guidelines for constructing new efficient long-lived RTP crystalline host-guest doped materials.

Synthesis
To ensure the purity of the target D-σ-A compounds, we first started with the synthesis of the pure Cz and Bd building blocks. Pure Cz was synthesized from 2-aminobiphenyl according to the literature, through intramolecular CH amination of sulfimine intermediate (Scheme S1, Supporting Information). [51] As for pure Bd, since the only few reported synthesis routes may include up to seven low-yield steps, [34] we developed a new efficient four steps synthesis strategy (Scheme S2, Supporting Information). Starting from 2-aminonaphtoic acid, three high-yield (82-87%) steps provide ortho-acetylenic amine whose cyclization under copper catalysis finally leads to the Bd core (67%). Then, the starting key building blocks Bd and Cz are derivatized, to prepare a library of acetylenic and vinylic Bd-and Cz-based targets ( Figure 1a). The D-σ-A compounds are synthesized through a two steps procedure consisting of i) the preparation of an acetylenic or allylic Bd (Cz) precursor, and ii) its subsequent cross-coupling with a bromo-iodo-pyridine via either a Sonogashira coupling or a Heck reaction (Scheme S3, Supporting Information). The detailed synthetic routes are depicted in Supporting Information (Scheme S4, Supporting Information), and both of them afford the targeted compounds in good yield (76-90%) after the last coupling step. All the compounds were fully characterized by 1 H-and 13 C-NMR spectroscopy and mass analysis (see Supporting Information). Single crystals were grown by slow evaporation from a mixture of n-hexane and methylene chloride at RT and analyzed by singlecrystal X-ray diffraction (SC-XRD) measurements.

Photophysical Properties of Single Components in Frozen Solutions at 77 K
To evaluate the D-σ-A molecules' photoluminescence (PL) properties, the effect induced by the donor difference character between Bd and Cz moieties on the compound's optical features, and to estimate the energies of the lowest energy singlet (S 1 ) and triplet (T 1 ) excited states, we recorded the fluorescence and phosphorescence spectra in deaerated diluted ethanol/ methanol (EtOH/MeOH) (80:20) solution (10 −5 m) at 77 K. After stopping the UV-vis irradiation at 365 nm, all the Cz-based compound solutions exhibit an obvious long blue afterglow ( Figure 1b). The typical steady-state PL spectrum shows a band with well-defined peaks at 340 and 357 nm and a shoulder at 372 nm followed at lower energy by a less intense transition with the main peak and shoulder peaks located at 429, 410, and 450 nm, respectively ( Figure 1c). Comparison with timegated spectra allows attributing the short-and long-wavelength transitions to fluorescence and phosphorescence, respectively. Interestingly, the Bd-based molecule's frozen solutions exhibit markedly distinct features. Whereas an intense blue emission is observed under irradiation at 365 nm, the emission color changes after removing the UV excitation source and shows an obvious orange afterglow ( Figure 1b). This behavior is confirmed by the photoluminescence spectra that show dual structured emission bands with one comprising peaks at 402 and 424 nm and a shoulder at 448 nm and the other one in the 550-700 nm range ( Figure 1c). Time-gating of the PL eliminates the high energy transitions thus ascribed to fluorescence and allows to highlight the phosphorescence with vibrational emission peaks at 565, 622, and 678 nm separated by ≈0.16-0.18 eV (≈1500 cm −1 ), indicative of emission from a localized excited state. Finally, the energy level of the S 1 and T 1 states of the Cz-and Bd-based derivatives were estimated from the onset of the phosphorescence spectra ( Figure 1d). All these results show that within each series, the optical properties are similar, as expected from the D-σ-A molecular design. In contrast, compared to the series Cz, the series Bd displays redshifts of fluorescence (ΔE = ≈0.55 eV) and phosphorescence (ΔE = ≈0.9 eV), due to the stronger donor character of the Bd unit compared to the Cz one.

Photophysical Properties in the Crystalline State
We then investigated the PL properties of the single-component molecular host crystals. At RT, whatever the matrix, only an extremely faint blue-violet emission is observed under UV-vis irradiation and no afterglow is observed by the naked eyes after ceasing the irradiation ( Figure S1, Supporting Information). As shown in Figure S2, Supporting Information, for CzT24, the PL spectrum exhibits only a peak at ≈363 nm, which is consistent with the above observations. At low temperatures (5 K), a very weak green persistent emission is observed. This phosphorescence is detected on the time-gated PL spectrum as a broad band centered at ≈550 nm with a lifetime of 170 ms ( Figure S2, Supporting Information). The same behavior is observed for the other pure matrices, with a very weak green phosphorescence detected only on time-gated spectra at low temperatures (77 K) as broadband for the triple bond containing compounds, and as a structured band at ≈550 and ≈600 nm for the double bond containing compounds ( Figure S3, Supporting Information). It is noteworthy that the T 1 excited state energy of the Cz-based matrices (Figure 1d), estimated from the onset of the low-temperature phosphorescence spectrum, is red-shifted compared to one of the individual molecules (ΔE = ≈0.35 eV). On the other hand, the structuration of the spectrum of the Bd-based derivatives in frozen dilute solution reveals that the triplet excited states are π−π* transition in nature (i.e., local excited state), which are not solvatochromic as evidenced in Figure S4, Supporting Information. Thus, the triplet energy gap between host and guest (ΔE T1-T1' ) can be evaluated from the T 1 state energies of the Cz-based matrix and the Bd-based dopant in a frozen dilute solution. Since this value (0.53eV, Figure 1d) is larger than the one required to confine guest long-lived triplet excitons, [47] reverse energy transfer to the matrix should be avoided.
We then explored PL behaviors at RT of host-guest molecular crystals where a crystalline Cz-based matrix (vertical entry in Figure 2a and Figure S1, Supporting Information) is used to confine Bd-based chromophores (horizontal entry). Thus, Figure 2a and Figure S1, Supporting Information, point out whether the investigated host-guest systems exhibit afterglow or not, after turning off the UV excitation. First, the results show obviously that incorporation of the Bd-based emitters does not systematically lead to the observation of a long-lived luminescence of the host-guest crystals. More precisely, let us first consider crystals where the Bd-based guest chromophore is inserted in its isomeric Cz-based host matrix (on-diagonal terms). With the naked eye, an orange afterglow can be seen only for the systems based on CzT24, CzT13, and CzD24 host matrices whereas there is no persistent emission for CzT34 and CzD23 matrix-based systems ( Figure S1, Supporting Information). Steady-state photoluminescence spectra of the hostguest systems CzT24/BdT24, CzT13/BdT13, and CzD24/BdD24 (Figure 3a,b,d, respectively) show dual emission with a band in the 350-450 nm range and the other in the 500-700 nm range. The short-wavelength band consists of peaks at 363, 405, and 427 nm with luminescence lifetimes ranging from 0.5 to 2 ns, indicating the fluorescence nature of the emission ( Figure S5, Supporting Information). The long-wavelength emission peaks are located at 568, 620, and 678 nm with a vibronic progression (0.17-0.18 eV) characteristic of the emission from a localized excited state, showing long luminescence lifetimes up to ≈316 ms usually associated with phosphorescence, that is, long-lived RTP ( Figure S6, Supporting Information). The phosphorescence quantum yields were evaluated to 20%, 9%, and 2.5% for the CzT13/BdT13, CzT24/BdT24, and CzD24/BdD24 systems, respectively. By contrast, CzD23/BdD23 spectrum only displays the structured emission band of fluorescence between 350 and 450 nm (Figure 3f), while CzT34/BdT34 is barely emissive (Figure 3c). Interestingly, with respect to cross-systems where the Cz-based host and Bd-based guest differ by the nature of the bridge and/or terminal pyridinic ring (off-diagonal terms), three different behavior are observed (Figure 2a and Figure S1, Supporting Information). While after ceasing the photoexcitation systems based on CzT24 and CzT13 matrices display the same orange luminescence for all the investigated guests, crystals based on the CzD24 host matrix show afterglow depending upon the guest chromophore structure, whereas those based on CzT34 and CzD23 host matrices show no persistent emission at RT whatever the guest. PL spectra of the cross-systems are consistent with these features observed by the naked eye. For example, for systems from the CzT24 and CzT13 matrices, the PL spectra display the long-wavelength emission peaks located at 568, 620, and 678 nm with a vibronic progression ( Figure S7, Supporting Information). For most of the systems, the phosphorescence quantum yields are lower than the fluorescence ones and also lower than those of these matrices doped with their own Bd-based isomer. Also in line with the visual observations, the PL spectrum of CzD24/BdT24 displays the long-wavelength emission peaks, whereas PL spectra of the other crosssystems from the same matrix confirm that these host-guest doped materials do not display any RTP ( Figure S7, Supporting Information). As for cross-systems from the CzD23 matrix, PL spectra, as well as lifetimes (µs range), confirm that these materials do not exhibit any long-lived RTP ( Figure S7, Supporting Information). As for CzT34, the spectra are structureless in the long-wavelength region as compared to other matrices ( Figure S7, Supporting Information). This change suggests that this emission arises from charge transfer excited states that are either due to intramolecular or intermolecular. Clearly, in this matrix, the structured spectra due to π-π* transitions observed in other matrices are absent most likely due to quenching by this lower excited triplet CT state.
To verify the origin of the phosphorescence structured emission, we further investigated the PL properties at 77 K of the on-diagonal systems. Spectra of systems capable of RTP exhibit also the typical vibrational emission peaks between 500 and 700 nm, as shown in Figures S8a, S8b, and S8e, Supporting Information, for CzT24/BdT24, CzT13/BdT13, and CzD24/BdD24, respectively. Since the energy values are similar to those of the molecular dopant, this indicates that the phosphorescence of these doped materials originates from the guest emitter. Interestingly, the CzD23/BdD23 spectrum exhibits also the same features, while it shows no RTP ( Figure S8d, Supporting Information). This suggests that an interaction between the matrix and the dopant could inhibit the RTP, as discussed later. As for CzT34/BdT34, the spectrum, as observed at RT, does not display the structured emission in the long-wavelength region ( Figure S8c, Supporting Information).

Single-Crystal X-ray Diffraction Analysis -Influence of the Packing on the RTP
To gain a deeper insight into the correlation between the longlived emission of the host-guest systems and the host matrix crystalline structure, we have performed SC-XRD analyses (Tables S1-S7, Supporting Information). As already evidenced in the literature [24,31,38,52] -and further verified in the series herein described-it is noteworthy that SC-XRD results are identical for the crystalline host pure matrix and the associated host-guest crystals, because of the low concentration (0.5 mol%) of the guest molecules combined with their random distribution into the matrix. Thus, we investigated the five pure matrices (CzT24, CzT13, CzT34, CzD24, and CzD23). We mainly focused on the organization of the Cz units, and the results are summarized in Figures 2b and 4, and Figure S9, Supporting Information.
In the crystals, Cz-based molecules take advantage of the degrees of freedom introduced by the sp 3 hybridized carbon atom to adopt various conformations in which the planes of the two nitrogen-containing heterocycles (pyridine and carbazole) are nearly perpendicular ( Figure S9, Supporting Information). On closer inspection, the comparison between the structures of CzD24 and its analogous CzT24 indicates that the replacement of the double bond with a triple bond has only a slight effect on the conformation in the solid state. In contrast, a comparison between isomers CzT24 and CzT34 (CzD24 and CzD23) evidences that the position of the N atom and/or Br atom substituent on the pyridinic ring strongly influences the conformations by rotating the pyridine plane.
As shown in Figure S9, Supporting Information, the crystal structures reveal that the carbazole units of CzT24, CzT13, CzD24, and CzD23 form a π-stacked tilted motif (θ = 27.3°, 27.3°, 52.7°, and 36.8°, respectively) along the crystallographic b-axis (a-axis for CzD23), with an interplanar distance d inter of 2.67, 2.65, 3.42, and 3.15 Å, respectively. The main feature of this stacked organization is that the Cz permanent dipole moment projection along the b-axis points in the same direction. More precisely, in CzT13 and CzT24, the stacks assemble by pair with a slip of the stacks by half the length of the stacking distance to produce a 1D columnar herringbone structure along the b-axis characterized by an "edge-to-face" arrangement. Therefore, the carbazole molecular columns self-assemble to give a 2D rectangular columnar network, with the Cz permanent dipole moment projection along the b-axis pointing in the same direction into each column. As for CzD24, the vinylic analogous of CzT24, another type of herringbone structure is shown, in which two adjacent carbazole molecules stacks are slipped simultaneously along the b and c crystallographic axis while maintaining the "edge-to-face" arrangement. The vertical and horizontal slippages correspond to half the stacking distance and carbazole length, respectively. At a larger scale, the Cz units form in the crystal parallel herringbone arrays where the Cz permanent dipole moment projection along the b axis is pointed in the same direction. As for CzD23, the carbazole units stacks adopt a quite different arrangement in that two adjacent stacks are not parallel but form a nearly 80° angle leading to a 2D network of parallel "zigzag" Cz chains perpendicularly to the stacking direction with an "edge-to-edge" arrangement. In contrast to CzT24, CzT13, CzD24, and CzD23, carbazole units in CzT34 do not pack into extended stacks but form antiparallel face-to-face pairs of molecules that interact in an "edge-to-face" fashion with four other pairs of molecules to form a kind of sandwich arrangement.
From the comparison between the photoluminescence properties of the host-guest systems (Figure 2a) and the Cz units arrangements into the crystalline matrices sketched in Figure 2b, it is obvious that there is a correlation between the columnar herringbone packing of the matrix Cz units and the RTP of the Bd-doped materials. Systems based on CzT24 and CzT13 matrices exhibit afterglow whatever the Bd-based guest whereas for CzD24 matrix long-lived luminescence is observed only when hosting either its own Bd-based isomer or its triple bond analogous. In contrast, none of the crystals based on CzT34 and CzD23 matrices shows any afterglow. At this stage, these behaviors obviously show that the PL properties of the doped materials do not result only from those of the dopant and that somehow, the herringbone packing provides the structural requirement that is necessary to favor the persistent emission.

Guest Insertion Mechanism
To rationalize the correlation between the host-guest solid-state organization and the optical performances, it is also important to have a better understanding of the guest conformation into the crystalline host matrix. Considering the CzT24/BdT24 host-guest material as a model system, we propose a plausible mechanism for guest insertion from photoluminescence results and from the comparison of CzT24 and BdT24 crystal structures. First, we hypothesized that the dopant is introduced as a single molecule, as deduced from the PL spectra at 77 K of a molecular dopant (BdT24) in a polymer matrix (PMMA) at different concentrations ( Figure S2, Supporting Information). The evolution of the spectra shows that the concentration increase induces a broadening of the phosphorescence spectra. Since the PL spectra of the host-guest crystals do not exhibit such broad bands in the low-energy region, we thus infer that the molecular dopant is incorporated as an isolated molecule. As for crystal structures, despite CzT24 and BdT24 crystallize in the P21/n space group with Z = 4 molecules per unit cell, Cz-and Bd-based molecules adopt quite different conformations in the crystals ( Figure S10, Supporting Information). While the pyridine rings of both derivatives have the same twist angle θ 1 with the CH 2 group (C-sp 3 atom), the twist angle θ 2 between the CH 2 group and Cz and Bd unit drastically differ so that the angle between the pyridine and tricyclic units planes are ≈90° and ≈30° in CzT24 and BdT24, respectively. In the BdT24 crystal, despite the nearly coplanarity of the two heterocycles (Bd and Py) instead of the orthogonality previously observed for the Cz and Py units in CzT24, adjacent Bd moieties are also packed in adjacent slipped stacks in an edge-to-face manner with roughly similar tilt angle 25.6° (vs 27.3°) and interplanar distance 2.66 Å (vs 2.67 Å) and adopt a herringbone structure forming sheets instead of columns ( Figure S10, Supporting Information). Unexpectedly, pyridine rings in both BdT24 and CzT24 crystals similarly self-assemble in supramolecular linear chains through hydrogen bonding (N···HC = 2.45 and 2.31 Å) along the Bd and Cz stacking direction, respectively (Figure S10, Supporting Information).
All these results show that despite CzT24 and BdT24 exhibit different crystalline structures, both crystals display similar organization patterns for each part on both sides of the sp 3 linker. Thus, it is possible to envision a plausible manner in which a guest molecule fits into the host matrix: the guest adopts within this structure a conformation that mimics the one of the host by taking advantage of the degrees of freedom provided by the CH 2  group. As sketched in Figure 5a, BdT24 could adopt two conformations to fit into the CzT24 matrix: one in which the BdT24 Py ring is perfectly inserted into the supramolecular chains of CzT24 Py units and the directly connected Bd rotates as a rudder to slot into a CzT24 Cz stack, but slightly slipped; the other where the BdT24 Bd cycle exactly fits into a CzT24 stack and the directly connected "arm" containing the Py ring rotates, but not perfectly aligned with the plane containing the CzT24 cycle. Moreover, in the case of the CzT24/BdT24 model system, the self-assembly of the pyridine rings through hydrogen bonding in supramolecular linear chains along the stacking direction could enhance the proposed insertion mechanism ( Figure 5b). Overall, the main feature of the resulting stacking pattern is the replacement, into the matrix, of a Cz motif with a Bd one in a stack of the columnar herringbone arrangement of the Cz units. Interestingly, a comparison of BdT13 and BdT24 crystal structures demonstrates the feasibility of the proposed insertion mechanism. Indeed, BdT13 adopts a quite different conformation ( Figure S11a, Supporting Information) compared to the BdT24 molecule: nearly orthogonality of the heterocycles (Bd and Py) instead of nearly coplanarity, and drastically different twist angles. Moreover, in the unit cell, the BdT13 molecules are organized head to tail in pairs where the Bd units adopt a herringbone-like structure with an "edge-to-face" arrangement ( Figure S11b, Supporting Information). Thus, these results confirm that thanks to the degrees of freedom provided by the CH2 group, the σ-conjugated Bd derivatives can adapt their conformation to fit into a crystalline structure, and adopt a conformation that enables to insert the Bd units into a columnar herringbone structure.

Discussion
By combining photoluminescence properties and single-crystal X-Ray analysis, we found that RTP only occurs in host-guest doped systems that display a columnar herringbone-like arrangement of the matrix Cz units ( Figure 2). Therefore, the two following questions arise: what is the role of this organization in the RTP process, and is this condition for RTP applicable to other doped systems?
For our matrices displaying the 1D columnar herringbone structure composed of a Cz stacks pair (CzT24 and CzT13) the proposed guest molecule insertion mechanism consists in removing a Cz-based molecule from the host matrix and inserting a Bd-based guest molecule into the vacancy. Consequently, this leads into the Cz stack, according to the two suggested insertion scenarios, to the substitution of the removed host molecule Cz unit by the inserted guest molecule Bd unit (Figure 5b). At this point, since the doped systems exhibit RTP with the same features as the dopant in diluted solid solution, as the first hypothesis we thus could suggest that this packing quite simply suppresses the nonradiative deactivation ways of the dopant T 1 exciton by confining it (Figure 1d) and restricting molecular motions of the isolated embedded Bd phosphor. In this case, the carbazole moiety would be considered just as a neutral component. However, the herringbone arrangement could in addition favor aromatic "edge-to-face" intermolecular interactions between Cz and Bd units facing each other in two adjacent stacks (Figure 5b). Therefore, we can speculate that intermolecular mechanisms of RTP could also occur. This second hypothesis is supported by comparing the behavior of host-guest systems based on CzD23 and CzD24 matrices. In the latter matrix, long-lived RTP is only observed when the guest is its own isomeric dopant (BdD24). Since at low temperatures, the doped systems exhibit an afterglow with the same features as the isolated dopants ( Figure S8e, Supporting Information) the "replacement" of a Cz unit with a Bd one in the Cz units stack structure can as well occur. However, since the Cz units adopt a "slipped" herringbone packing motif, the "edge-to-face" intermolecular interactions between slipped stacks could be more or less efficient according to the degree of overlap between the aromatic Bd and Cz units, which depends on the dopant structure, as experimentally observed. This could explain the shortening of the phosphorescence lifetime for the "slipped" herringbone packing compared to the columnar one ( Figure S6, Supporting Information). As for the CzD23 matrix, the observation at low temperature of the phosphorescence of the isolated dopant molecules ( Figure S8d, Supporting Information) suggests that they are embedded into the matrix and that the proposed guest insertion mechanism could also occur, despite no RTP. However, Cz units form only one of the stacks of the stacked pair of the columnar herringbone structure (Figure 2b), thus precluding "edge-to-face" intermolecular interactions that exist within a complete columnar herringbone arrangement like in CzT24 and CzT13 matrices. Thus, the suppression of intermolecular interactions within the "halfcolumn" packing motif could be the cause of the absence of RTP. Finally, we can speculate that not only the matrix confines the guest long-lived triplet excitons probably thanks to the herringbone-like packing but also that such herringbone packing favors intermolecular interactions between Cz and Bd units. Furthermore, the tailoring process of the D-σ-A guest emitter conformation appears as a powerful tool to understand, predict, and build host-guest systems exhibiting long-lived RTP. So for the applicability of the highlighted condition to other systems, deeply reanalyzing crystallographic data from the literature demonstrates for the first time that the columnar herringbone arrangement seems to be one common packing motif adopted by Cz-based systems-in reality doped systems-in the crystalline state that yields to RTP (Figures S12 and S13, Supporting Information). Most importantly, this finding paves the way for Figure 5. Sketch of the insertion mechanism of the guest emitter into the host matrix by mimicking the host conformation thanks to rotational freedom degrees provided by σ-conjugation. a) Comprehensive insertion scheme, for example, of the BdT24 emitter (in red) into the CzT24 matrix (in grey) showing two possible favored conformations (1) and (2) of the dopant, leading to the embedding of the Bd unit inside the herringbone Cz-stack (view perpendicular to the stacking axis). b) Insertion of the Bd unit of the BdT24 emitter (in red) into a 1D columnar herringbone arrangement of matrix Cz unit, and highlighting of the pyridine rings self-assembly by hydrogen bonds (side view along the stacking direction).

Conclusion
In summary, by designing and synthesizing from pure carbazole and 1H-benzo[f]indole two series of σ-conjugated D-σ-A molecules used as matrix and dopant, respectively, we demonstrated for the first time a necessary and sufficient condition for organic RTP from our host-guest doped crystalline model systems. Provided that the triplet energy gap between the host and guest emitter allows confining dopant long-lived triplet excitons, the host-guest doped system exhibits RTP only if the carbazole units adopt a specific herringbone type packing into the crystal. This structural synergistic action between the matrix and the dopant is made possible by assuming that the guest conformation mimics the one displayed by the host at the solid state, thanks to the rotational degrees of freedom provided by the σ-conjugated bridge. Thus, a guest insertion mechanism is proposed, that leads to the "replacement" of a matrix Cz unit with a dopant Bd one into the herringbone structure without alteration of the crystal structure. The resulting stacking pattern, so far never reported, opens the way for understanding and predicting RTP in host-guest doped systems. Thus, our findings not only allow for revisiting previous literature on RTP from carbazole-based materials but also provide some guidelines in terms of crystal engineering to develop and predict long-lived RTP host-guest doped systems.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.