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Large Polycyclic Aromatic Hydrocarbons as Graphene Quantum Dots: from Synthesis to Spectroscopy and Photonics

Synergistic research efforts in multistep chemical synthesis and state‐of‐the‐art optical spectroscopy have permitted the realization of graphene quantum dots (GQDs) with defined structures and properties, paving the way towards their possible applications in photonics and optoelectronics. One such fruitful feedback loop between synthesis and spectroscopy is represented by the recent development of stable GQDs with zigzag edges, displaying excellent spectral features such as optical gain and strong absorption/emission in the visible/near‐infrared regions, which are appealing for applications in photonics. In this progress report, the authors provide an overview of the latest developments in the chemical synthesis and advanced spectroscopic investigations of atomically precise GQDs, with particular emphasis on those with zigzag‐edged structures. The latest developments in their applications are also introduced, for example, in lasing and optical imaging, and new opportunities in the field of GQDs are discussed, including chiral GQDs with chiroptical properties and possible applications of GQDs for long‐lived luminescence. The authors highlight the role and power of chemical synthesis and spectroscopy in providing new GQDs with promising optical properties that pave the way for new and exciting applications in photonics.


Introduction
The development of high-performance, stable organic semiconductors (OSCs) for applications in optoelectronics and photonics has been a topic of intense multidisciplinary research in the last three decades, benefitting significantly from progress typically fabricated through solvothermal treatment of small molecular precursors (bottom-up) or "shearing" of graphene sheets (top-down), and their versatile applications have been demonstrated, for example, in the (opto)electronics and biomedicine. [33][34][35][36][37] Nevertheless, GQDs prepared by such methods mostly do not have defined chemical structures, and it is in some cases even difficult to clearly distinguish them from the so-called carbon dots, namely fluorescent carbon nanoparticles. [38,39] GQDs prepared by different methods can have largely different chemical structures, leading to certain controversies in the nomenclature and the discussion of structure-property relationship, particularly the PL mechanism. [34,40,41] In contrast, organic chemistry methods allow the synthesis of large PAHs that can be unambiguously identified, which can also be regarded as monodispersed GQDs with uniform sizes and welldefined chemical structures. [16,25,26,42] Large PAHs are thus at the interface of OSCs and GQDs, providing a unique opportunity for experimental study of the structure-property relationships of GQDs, which are largely studied by theoretical methods, [43,44] and to explore their applications. The synthesis of large PAHs can be typically carried out through the oxidative cyclodehydrogenation of tailor-made dendritic polyphenylene precursors, which are "planarized" and "graphitized" into disk-shaped molecules consisting of sp 2 carbons. [45] Various PAHs can be obtained by varying the precursor structures and utilizing other synthetic methods, providing structurally defined GQDs with different sizes and edge structures. [16][17][18][19][20][21] These studies have revealed that not only the size but also the edge structure of GQDs plays a critical role in tuning their optical, electronic, and magnetic properties. [26,46] Specifically, GQDs typically feature two types of edge configurations, namely, armchair and zigzag edges (Figure 1). GQDs with only armchair edges mostly have nonmagnetic properties with relatively large energy gaps, while the introduction of zigzag edges can lead to smaller energy gaps with intriguing optical features and even to an open-shell character, [47][48][49][50] as demonstrated, for example, by the syntheses of anthenes [51,52] and periacenes. [53][54][55][56][57] Therefore, GQDs with armchair edges hold promise for application in photovoltaics [58,59] and logics, while those with zigzag edges are potential materials for spintronics and photonics. [50,[60][61][62][63] Although zigzag-edged GQDs typically show low chemical stability, [51,64] the synthesis of an increasing number of examples of stable GQDs with zigzag edges has been demonstrated in recent years, displaying optical properties that are appealing for applications in photonics, such as intense long-wavelength absorption and very bright emission. [49,53,65,66] In this article, we describe the latest developments in the synthesis and spectroscopic characterization of large PAHs as structurally defined GQDs with an emphasis on zigzag-edged structures and optical gain properties. We also introduce new opportunities in chiroptical and long-lived luminescence applications and discuss the potential of GQDs in photonics as promising advanced optical materials.

Hexa-Peri-Hexabenzocoronene (HBC) Derivatives
Hexa-peri-hexabenzocoronene (HBC), as a "superbenzene", has served as the prototypical example of a large PAH and can be considered a representative GQD with only armchair edges. [61] Since almost two decades ago, synthetic approaches have been made to introduce extra K-regions into HBC, namely, additional C2 units, which will form zigzag edges and alter its electronic and optical properties, for example, leading to modulated orbital energy levels and smaller optical gaps. [67][68][69][70][71] While two isomers of HBCs with two K-regions in the "meta" and "para" configurations and HBC with three K-regions were synthesized, the remaining isomer of HBC with two K-regions in the "ortho" configuration and the addition of more than three K-regions remained elusive. In 2016, the synthesis of HBC 6 with four K-regions was achieved with the oxidative cyclodehydrogenation of precursor 5 as the key step, as presented in Figure 2. [67] Tetrabromo-p-terphenyl 1 was subjected to Suzuki coupling with boronic ester 2 to give triisopropylsilyl-protected precursor 3. After the deprotection of 3 with tetrabutylammonium fluoride, fourfold Sonogashira-Hagihara coupling with 1-tert-butyl-4-iodobenzene yielded precursor 4. Bis(pyridine)iodonium tetrafluoroborate promoted the fourfold cycloaromatization of 4 to afford diiodinated benzo[m]tetraphene dimer 5. Finally, the oxidative cyclodehydrogenation of 5 using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the oxidant together with triflic acid (TfOH) provided HBC 6 with four K-regions. [67] The aromatic core structure of 6 with zigzag edges featured a HOMO-LUMO gap of 2.51 eV, as calculated by density functional theory, which was significantly smaller than the 3.63-eV gap of the parent HBC and demonstrated extended optical absorption up to ≈700 nm, while the latter absorbed only up to ≈400 nm. However, HBC with four K-regions could not be obtained without iodo groups, which quenched fluorescence. Attempts to remove the iodo groups or to use them for coupling reactions also failed, precluding further investigation of the fluorescence properties of HBC with four K-regions.
On the other hand, a stable HBC 10 with two extra K-regions in the "ortho" configuration, forming a zigzag edge, could be synthesized, as shown in Figure 3. [68] The nucleophilic oxygen/ carbon exchange of pyrylium salt 7 led to U-shaped precursor

DBOV Derivatives
In 2017, our group demonstrated for the first time the occurrence of optical gain in GQDs. Specifically, we reported dibenzo[hi,st]ovalene (DBOV) as a new GQD with a combination of armchair and zigzag edges, which revealed strong red emission (637 nm) with a high quantum yield approaching 80%. [72] In the first report, DBOV was synthesized via 12 steps in a linear sequence from commercially available starting materials. Precursor 13b with two formyl groups was the key intermediate, and its cove edges were converted to zigzag edges through treatment with the mesityl Grignard reagent and subsequent Friedel-Crafts cyclization, followed by oxidation, to yield DBOV 15b (Figure 4). More recently, we have developed an optimized synthetic route to enable gram-scale synthesis of DBOV derivatives ( Figure 4). [73] In this new route, the key intermediate  13a was synthesized by the iodine monochloride (ICl)-promoted cyclization of diaryldiyne 11 to afford diiodinated bichrysene 12, followed by photochemical cyclodehydroiodination. This facile and efficient method provided access to various DBOV derivatives with different substituents, including alkyl, aryl, and ethynyl groups, with a total yield of 23-41% over 7 steps.
The TA spectrum of DBOV 15b measured in a diluted toluene solution revealed an SE signal at 695 nm, while this signal was overwhelmed by a strong absorption from the excited states in a neat film of 15b (Figure 5a). [72] We ascribed these phenomena to ultrafast and nonradiative intermolecular charge-transfer processes in the solid-state that are promoted by strong π-π stacking. [74] To exploit the optical gain properties of DBOV 15b, we dispersed it in a polystyrene (PS) matrix to minimize intermolecular interactions and aggregation. In a thin film of PS with 1% DBOV 15b, we could thus observe the recovery of the SE signal and, interestingly, the occurrence of amplified spontaneous emission (ASE) with high operational stability (Figure 5b). [72] Since the first report in 2017, DBOV derivatives have already been employed in a range of photonic studies, spanning from optical switching [75] to exciton-photon coupling [76] and imaging. [77] For instance, our group has achieved ultrafast all-optical switching with DBOV 15b by using a three-beam pump-push-pull experiment (Figure 6a). [75] Specifically, we exploited the spectral overlap between SE and absorption from dark states with charge separation, which is usually detrimental for the ASE and lasing action in solid-state devices. When the DBOV molecule is adequately isolated, intramolecular geminate recombination of charges allows ultrafast (sub-picosecond) and effective on/off resonant switching. Although the resonant process requires efficient energy dissipation, thus posing critical stability issues, the relatively high photostability displayed by DBOV derivatives can be a key asset for their future applications in plastic optical fibers. The narrow and well-separated optical transitions characteristic of DBOV lend themselves to strong exciton-photon coupling. Indeed, Lidzey and collaborators demonstrated the formation of polariton states upon placing DBOV 15a into an optical microcavity (at RT), which resulted from the hybridization of the cavity mode with the 0 → 0′ optical transition of 15a (Figure 6b). [76] These data suggested that the remarkable photostability and high emission efficiency make DBOV an ideal material for generating polariton condensation. Moreover, a recent detailed single-molecule characterization of DBOV permitted access to important details about its photophysics and its potential use as a single quantum emitter. [73] In particular, the fluorescence autocorrelation function displayed high-contrast photon antibunching and bunching ( Figure 6c). From these data, the fluorescence decay rate and the triplet population and   [68] Copyright 2020, the Royal Society of Chemistry. depopulation rates could be retrieved. Interestingly, the intersystem crossing rate into the triplet state decreased by more than an order of magnitude at low temperature, demonstrating that single-photon emission in DBOV can be achieved via temperature control. Applications of DBOV in imaging have also been reported ( Figure 6d). The outstanding photophysical properties of DBOV, such as intrinsic blinking even in air, excellent fluorescence recovery, and stability over several months, have rendered it a promising fluorophore for superresolution microscopy. [77] Furthermore, the application of DBOV 15a, as well as HBC 6, as a light-absorbing material in all-carbon photodetectors has been demonstrated, where these GQDs were deposited on graphene as the conducting layer ( Figure 6e). [78] Furthermore, postsynthetic functionalization of DBOV 15a was achieved, starting with its selective bromination at the bay regions. [79] 3,11-Dibromo-DBOV 17 was thus obtained and successfully used for transition-metal-catalyzed coupling reactions to introduce aryl or ethynyl groups (Figure 7). Circumarenes are a type of zigzag-edged PAH containing a small aromatic core surrounded by a single layer of benzene rings. [46] Circumarenes with central aromatic core sizes up to three fused benzene rings have been reported, with circumanthracenes as the largest examples to date, reported by Diederich et al. in 1991 [57] and then Feng et al. in 2018. [53] In 2020, our group reported the successful synthesis of a new member of the circumarene family, namely, a circumpyrene, by the π-extension of DBOV. [80] Circumpyrene 20 could be obtained through the twofold palladium (Pd)-catalyzed benzannulation of 3,11-dibromo-DBOV 17 with diphenylacetylene, together with monoadduct 19 ( Figure 7). The π-extension from DBOV to 19 and then 20 increased the optical and electrochemical energy gaps, accompanied by blueshifts in the optical absorption and emission ( Figure 7). These results highlighted the significance of the postsynthetic π-extension of GQDs to selectively obtain novel GQD structures, thereby modulating the optoelectronic properties. The possible optical gain properties and other photophysical characteristics of 19 and 20, as well as aryl-substituted DBOV 18 with different functional groups, are currently under investigation in our laboratories.

GQDs with Four Zigzag Edges (FZ)
Recently, Wu and his colleagues demonstrated a systematic synthesis of a series of GQDs having four zigzag edges (FZs), including FZ1, FZ2, and FZ3 (Figure 8). [65,66] For the synthesis of FZ1, dialdehyde 24 was prepared from diol 23 through Swern oxidation. Dialdehyde 24 was then treated with mesitylmagnesium bromide and quenched with water to give a   (Figure 8a). FZ2 and FZ3 were also synthesized in a similar manner, through tetraaldehydes 27 and 30, respectively (Figure 8b,c). The zigzag edges of FZ1, FZ2, and FZ3 are kinetically protected by bulky aryl groups, and these functional groups ensure their adequate stability and solubility. [66] FZ1, FZ2, and FZ3 have shown intriguing photophysical properties that can be appealing for photonic applications, similar to the case of DBOV derivatives. Moreover, analysis of their ASE spectra revealed a change in the emission mechanism as a function of the molecular size, with the larger FZ3 displaying ASE from the first vibronic replica (0′ → 0 transition) and not from the second (0′ → 1 transition), as in the cases of FZ1 and FZ2 as well as most other organic dyes (Figure 9b). [72] The authors attributed this phenomenon to changes in the electron-vibration mechanism along the relevant electronic excitation, related to the more extended 2D π-electron delocalization in FZ3 than in FZ1 and FZ2. [62] Furthermore, these FZ-GQDs were blended in a PS matrix to attain lasing action. Specifically, the authors employed a fully solution-processed distributed feedback laser (DFB) based on a top-layer resonator (Figure 9c). Notably, efficient lasing was observed from the blue to the near-infrared (NIR) region. These GQDs also showed excellent environmental stability (i.e., against air and moisture) and photostability, exhibiting an outstanding operational lifetime. Very recently, FZ3 was demonstrated to sustain dual ASE and lasing at both 685 and 749 nm, even though the excitation threshold to achieve ASE was 10 times larger than those observed for FZ2 and FZ3. [81] This high threshold could be attributed to the spectral overlap between gain and absorption from charge-separated states in the NIR.

Perylene-Fused GQDs with Partial Zigzag Peripheries
In 2020, the groups of Díaz-García and Wu showed that lasing action in the visible could be attained by using aggregationfree perylene-fused GQDs ZY-01 and ZY-02 with partial zigzag peripheries (Figure 10). [82] The introduction of a C(sp 3 )-bridged five-membered ring at the bay region of the perylene moiety, as reported previously by Wu and his colleagues, [83] suppressed the aggregation of these GQDs, thus minimizing the aggregation-induced PL quenching. GQDs ZY-01, ZY-02, and ZY-03 were synthesized as shown in Figure 10a. Ethynyl-substituted perylene derivative 32 was initially prepared as the key intermediate, and subjected to the Diels-Alder cycloaddition with tetraphenylcyclopentadienone 33 to provide precursors 34 in 78% yield. Interestingly, oxidative cyclodehydrogenation of 34 by using DDQ and methanesulfonic acid (MeSO 3 H) in dichloromethane (CH 2 Cl 2 ) at room temperature (RT) selectively resulted in the partially fused product ZY-01 in 85% yield. Prolonging the reaction time did not yield the fully fused product ZY-02, but the use of iron(III)chloride in nitromethane/CH 2 Cl 2 gave ZY-02 in 36% yield from precursor 34. In addition, ZY-02 could also be obtained through treatment of ZY-01 with iron(III)chloride in 50% yield. ZY-03 was synthesized through the Diels-Alder reaction of 32 with biscyclopentadienone 35 to afford precursor 36 in 38% yield, followed by the oxidative cyclodehydrogenation with DDQ and MeSO 3 H in CH 2 Cl 2 at RT to give ZY-03 in 78% yield. Similar to the above-described case of FZs, the authors exploited a solution-processed DFB device to achieve lasing action in the visible (515-523 nm) from ZY-01 and ZY-02, which displayed the optical gain (Figure 10b) while ZY-03 did not show net gain due to the overlap between Adv. Optical Mater. 2021, 9, 2100508 Figure 5. a) TA spectra of DBOV in diluted toluene solution, in a solid neat film, and in a blend with PS. Although the SE signal is overwhelmed by excited-state absorption in the neat film, this feature can be recovered by suppressing the intermolecular interactions in the PS matrix (1% blend). b) ASE action in the DBOV 15b:PS blend. Notably, ASE with high operational stability is observed (only 30% decay after 30 min of intense irradiation (2 × 10 6 pulses, at 320 µJ cm −2 )). Reproduced with permission. [72] Copyright 2017, Wiley-VCH. SE and absorption from excited states that were attributed to triplets.

π-Truncation of GQDs
Novel synthetic approaches towards the development of advanced functional GQDs are currently being deployed. For instance, we have recently reported the synthesis of peralkylated GQD 38 via the regioselective hydrogenation of GQD 37 with 60 contiguous sp 2 carbons in its aromatic core (Figure 11a). [84] The hydrogenation of 37 was carried out with Pd/C in dry tetrahydrofuran (THF) at 120 °C, which could be completed either in two weeks at 120 bar and 120 °C or in one week at 150 bar (Figure 11a). While most of the GQDs were synthesized through the "π-expansion" of smaller aromatic molecules, this synthesis of GQD 38 with 38 sp 2 carbons from GQD 37 with the larger aromatic core can be considered a rare example of "π-truncation", which can potentially extend the scope of GQD synthesis. In addition to the novelty of the synthetic method, we showed that peralkylated GQD 38 holds potential for future applications in photonics. Specifically, TA measurements indicated that GQD 38 displays an SE signal at 500 nm, whereas absorption from excited states overwhelms the SE in the case of Adv. Optical Mater. 2021, 9, 2100508 Figure 6. a) All-optical switching in DBOV 15b solution and DBOV:PS film by using the three-beam pump-push-probe technique. Adapted with permission. [75] Copyright 2019, Wiley-VCH. b) Exciton-photon coupling is achieved by filling an optical microcavity with DBOV 15a. c) Single-molecule spectroscopy on a DBOV derivative with 2,6-dimethylphenyl groups reveals the potential of this molecule as a single quantum emitter. Reproduced under the terms of the Creative Common CC BY license. [73] Copyright 2019, American Chemical Society. d) Application of DBOV 15a in superresolution microscopy. Reproduced under the terms of the Creative Common CC BY license. [77] Copyright 2019, The Authors, published by Wiley VCH. e) Color-sensitive photodetector fabricated by using DBOV 15a as a light-absorbing material. Reproduced with permission. [78] Copyright 2019, American Chemical Society. GQD 37 (Figure 11c). The reason GQD 38 displays SE likely lies in its improved solubility brought about by the peripheral alkyl groups as well as the smaller aromatic core, based on our previous observation in DBOV derivatives with different solubilities that intermolecular interactions can lead to the formation of aggregates with charge-transfer character, where the charge generation quenches the SE and gain. [74]

π-Extended Helicenes as Chiral GQDs
Nonplanar GQDs represent another important class of molecules that display unique features, such as optoelectronic properties, conformational dynamics, and intriguing supramolecular characteristics, which are distinct from those of planar GQDs. [56] Chirality can also arise in nonplanar π-conjugated structures with chiral axes, which is among the most appealing features of such systems. For example, field-effect transistors based on chiral π-conjugated molecules can be utilized to detect circularly polarized light, which is intrinsically difficult with conventional photodetectors. [85] In this case, the nonplanar core has to ensure both efficient π-stacking, to guarantee efficient intermolecular charge/electron transfer, and extended π-conjugation. Moreover, chiral GQDs could also find potential application for in vivo bioimaging and NIR light-emitting devices, where NIR-emitting systems could be especially interesting.
Helicenes, consisting of consecutive ortho-condensed aromatic rings, are among the most studied nonplanar chiral π-conjugated molecules. [86] Helicene derivatives with extended aromatic structures have recently attracted growing attention, including both π-extended single helicenes and molecules containing multiple helicene moieties, which display enhanced chiroptical responses and/or unique self-assembling characteristics distinct from those of single helicenes. [87][88][89][90][91][92][93][94][95][96] FZ1, FZ2, and FZ3. a) Absorption, PL, and ASE spectra of the three FZ-GQDs. b) Sketch of the DFB resonator, consisting of a top-layer polymeric resonator with an engraved relief grating (Λ, grating period; d, grating depth), located over an active film (h, film thickness) of GQDs dispersed in PS, deposited on a fused silica substrate. c) Spectra of the DFB lasers based on FZ1, FZ2, and FZ3 (indicated as 1, 2, and 3 in the Figure). The laser line consists of either one or two peaks (modes), each associated with a given waveguide mode of the film, TE 0 or TM 0 , whose light is polarized parallel or perpendicular to the DFB grating lines, respectively. The top red arrows indicate the ASE wavelength for each case. Reproduced with permission. [62] Copyright 2019, Springer Nature.
More recently, we reported the synthesis and characterization of a π-extended double helicene 43 with fused pyrene substructures in a similar manner to the preparation of 41, employing 2-pyrenylbornic acid pinacol ester as the coupling partner of 39 (Figure 12a). [100] π-Extended double helicene 43 featured a broad and highly redshifted emission in the NIR (from 600 to 900 nm) compared to that of 41 at 565 nm. This unique fluorescence of 43 was tentatively ascribed to the formation of an intramolecular excimer based on TA spectroscopy analyses. In addition, this system displayed a relatively high chiroptical stability, as indicated by the high isomerization barrier of 46.3 kcal mol −1 (Figure 12c). This allowed separation of the enantiomers with opposite helicity, as confirmed by mirror-symmetric circular dichroism spectra. The chiroptical properties and the enhanced intra-and intermolecular π−π interactions can enable the application of [7]carbohelicene as a chiral semiconducting and lightemitting material.

GQDs for Long-Lived Luminescence
One of the research areas in which OSCs excel is room-temperature phosphorescence (RTP). [101][102][103][104] Such systems hold great potential for applications in organic photonics, with possible use, for example, in bioimaging, lighting, nonlinear optics, and lasing. [105,106] The recent past witnessed a very large jump in the new molecular design and exploration of organic phosphors, [105,[107][108][109][110][111][112] demonstrating RTP that persists longer than 100 ms with an afterglow and even exceeds the lifetime of 1 s after cessation of excitation. [58,60,113] This ultralong RTP property is enabled by the specific molecular organization of Figure 10. a) Synthetic routes towards ZY-01, ZY-02, and ZY-03. b) Spectra of the DFB lasers in which the active material is ZY-01 (blue lines) and ZY-02 (green lines). The number on the device label (1 or 2) refers to the compound used (ZY-01 or ZY-02, respectively), while the letters on the labels refer to devices with different geometrical parameters, as reported in Ref. [82]. c) Picture of the green light emitted by laser 1 A , projected on a screen in the left top corner. The bright spot in the center of the sample is a mixture of the pump and emitted laser light. Reproduced with permission. [82] Copyright 2020, Wiley-VCH.
OSCs using a proper host material, which stabilizes the excited triplet state, prolonging both their radiative and nonradiative lifetimes. [114][115][116] The RTP of PAHs has been studied over decades, [117][118][119] but the application of coronene (44, Figure 13a) has recently been revisited to achieve long-lived RTP using new host materials, [120,121] indicating the potential of such PAHs in this field of photonics. On the other hand, the RTP features of conventional GQDs and carbon dots have also attracted attention, as summarized by a recent review article by Yang, He, Ding, and their colleagues, including the discussion of the mechanism and potential applications. [34] In 2016, the Adachi group reported 3-(N-carbazolyl)-androst-2-ene (CzSte) as a new host material while using coronene-d 12 (45) as the RTP emitter, demonstrating an extended phosphorescence lifetime (τ p ) of 4.7 s and a phosphorescence quantum yield (φ p ) of 5.3% (Figure 13a,b). [120] The planar structure of 45 was found to be suitable for efficient intermolecular CH-π interactions with the steroid moiety of the CzSte-based host, which presumably suppressed the molecular vibration and thus the nonradiative decay of 45. Furthermore, LED devices could be fabricated using the prepared host-guest 45@CzSte samples, displaying electroluminescence from 45 during electrical excitation, followed by RTP with an afterglow lifetime of 4.3 s after turning off the voltage (Figure 13c). On the other hand, Hirata and Vacha reported a τ p of 17 s for coronene-d 12 (45) embedded in β-estradiol as the host as well as efficient accumulation of triplet excitons, which led to reverse saturable absorption, causing a decrease in the optical transmittance even by weak excitation with similar power to that of sunlight. [122,123] Metal-organic frameworks (MOFs) are promising porous materials that can serve as hosts of various guest molecules for multiple applications. [124][125][126] Kabe, Adachi, and coworkers proposed the use of an MOF, in particular a zeolitic imidazolate framework (ZIF-8), as a host for RTP emitters and demonstrated a long-lived RTP from coronene (44) with a τ p of 7.4 s. [121] Moreover, an extended lifetime of 22.4 s was achieved for coronene-d 12 (45) in ZIF-8 under the same experimental conditions. The enhanced lifetime of 45@ZIF-8 was ascribed to the lower vibrational energy of the C-D stretching mode compared to the C-H stretching mode. [127] Additionally, temperature-dependent lifetime measurements confirmed the suppression of nonradiative deactivation of coronene (44) by encapsulation in ZIF-8.
Moreover, in 2020, Kuila and George proposed another approach to achieve long-lived luminescence through tripletto-singlet Förster resonance energy transfer (FRET), where coronene tetracarboxylate salt (CS) 48 was employed as the triplet energy donor. [128] For the synthesis of CS 48, perylene (46) was subjected to twofold Diels-Alder cycloaddition with N-ethylmaleimide followed by oxidation, to give coronene diimide 47, which was further treated with KOH in methanol to yield 48 (Figure 14a). After embedding in a poly(vinyl alcohol) (PVA) matrix, CS 48 showed RTP in the range of 500-700 nm with  The smaller average phosphorescence lifetime of 52 in PVA compared to that of 51 was probably due to the heavy atom effect of the iodide counter ions, leading to enhanced spin-orbit coupling and faster intersystem crossing. [129] Notably, a combination of CS 48 and triazatruxene 51 in the PVA matrix at the appropriate ratio (10 wt.% 48 with respect to 51) allowed white phosphorescence with a φ p of 15.8% upon excitation at 346 nm, followed by a white afterglow that persisted over 7 s. Whereas GQDs larger than coronene (44) have rarely been investigated for long-lived luminescence recently, Clar Adv. Optical Mater. 2021, 9, 2100508 Figure 13. a) Chemical structures of coronene (44) and coronene-d 12 (45). b) Transient PL intensity of 45@CzSte at 560 nm. c) Electroluminescence spectra of 45@CzSte during and after electrical excitation (blue and green lines, respectively). Reproduced with permission. [120] Copyright 2016, Wiley-VCH. Reproduced with permission. [128] Copyright 2019, Wiley-VCH. and Ironside noted long-life orange-yellow phosphorescence of HBC at low temperature in their report in 1958. [131] After almost 60 years, Hirata and Vacha demonstrated that hexa(2-ethylhexyl)-HBC 53, when embedded in β-estradiol as the host material, displays blue-green thermally activated delayed fluorescence (TADF) simultaneously with red phosphorescence under ambient conditions, leading to white afterglow (Figure 15). [130] The PL spectrum of 53@β-estradiol showed well-resolved vibronic peaks (Figure 15b), similar to the spectra of similar HBC derivatives in solution, with a PL quantum yield of 9.3%, indicating that the molecules of 53 were effectively isolated in β-estradiol. After cessation of the excitation light, white afterglow emission was observed at RT with a quantum yield of 2.6% and a lifetime of 3.9 s (Figure 15c,d). [130] A relatively small energy difference of 0.48 eV between the lowest singlet excited state and the lowest triplet excited state (ΔE ST ) was determined for 53 based on the 0-0 transition peaks of its delayed fluorescence (469 nm) and phosphorescence (572 nm), which accounted for the occurrence of TADF. The reduction in ΔE ST was ascribed to the effective separation between the two molecular orbitals associated with fluorescence, resulting from delocalization over a 2D extended aromatic core.

Conclusions and Outlook
In this progress report, we have summarized the recent developments in the synthesis of large PAHs as atomically precise GQDs and relevant spectroscopic studies, with a focus on their current and/or possible applications in photonics. In particular, we have highlighted the chemical synthesis of GQDs with zigzag edges, which have revealed unique optical properties, such as strong absorption/PL in the visible/NIR region, high fluorescence quantum yields, and optical gain. Multiple examples of stable and solution-processable GQDs with zigzag edges have been achieved in recent years, demonstrating fine tailoring of their size and peripheral structures and thus of their spectral features. Stateof-the-art optical spectroscopic studies have permitted access to the rich photophysics of such GQDs, leading to insights into the mechanism underpinning their intriguing optical features. For instance, ultrafast spectroscopy has highlighted the role of zigzag edges in the optical gain properties of GQDs, possibly owing to the presence of localized and emissive edge states. On the other hand, the interplay between the gain band and absorption from dark and likely charged states plays an important role in determining both the spectral shape/position of ASE and the pumping threshold necessary to achieve lasing action.
Adv. Optical Mater. 2021, 9, 2100508 These unique properties have prompted the exploitation of GQDs in a range of applications, where high and tunable emission and photostability are highly desirable. These applications range from fundamental demonstration of exciton-photon strong coupling, ultrafast resonant optical switching, and single-photon emission to more practical uses as active material in lasers covering the visible-NIR region, in photodetectors, and as stable fluorophores for superresolution imaging. Based on these promising results, the next steps could be further exploration of GQDs with novel aromatic core structures as well as their peripheral functionalization to improve and tune their properties for specific applications, for example, to achieve lower-energy emission, lower ASE thresholds, and sensitivity to external stimuli relevant to sensing applications. On the other hand, recent advancements in PAH synthesis have paved new avenues towards chiral GQDs with nonplanar, helical substructures, which display intriguing chiroptical properties. Moreover, the long-lived phosphorescence properties of PAHs have recently been revisited, demonstrating the potential of GQDs for attaining multicolor, intense afterglow emission. These results thus highlight the significance of the chemical synthesis of such atomically precise GQDs as advanced optical materials with tunable structure-dependent properties and suggest their promise in photonic applications.