Modulation of the Nonlinear Optical Properties of Dibenzo[ hi , st ]ovalene by Peripheral Substituents

Dibenzo[ hi , st ]ovalene (DBOV) is a nanographene molecule with quasi-zero dimensional electronic confinement, which displays relatively high oscillator strength, remarkable photostability and optical gain property. For these reasons, DBOV has been proposed as gain medium and active material for achieving strong exciton-photon coupling in microcavity. Here, we study the stimulated emission properties of three DBOV derivatives with different substitution patterns. We found that these molecules likely undergo ultrafast intermolecular charge transfer processes occurring within their π-aggregates, which ultimately leads to quenching of stimulated emission and increase of the amplified spontaneous emission threshold. These effects can be minimized by installing bulky substituents on the peripheries that prevent π-π stacking. We can thus selectively favor either the luminescence/gain properties or the charge transport features by engineering the side groups.

The geometric confinement of graphene into few nanometers has allowed to open a finite band-gap in its electronic structure, which rendered the resulting nanographenes highly promising for a range of optoelectronic applications. [1][2][3] Top-down approaches have been successfully used to fabricate nanographenes that are represented by quasi-1D graphene nanoribbons (GNRs), 4

and quasi-0D
graphene quantum dots (GQDs), 5 although with certain degree of defects. On the other hand, bottomup synthesis has provided atomically precise GNRs 6-7 and molecular GQDs. 6,[8][9] The latter approach has offered efficient control over energy gap and optical absorption/emission by tailoring the size and edge structures. 6,[10][11][12][13] Such bottom-up synthesized nanographenes are thus highly promising for applications in the field of nanotechnology, optics, and optoelectronics. [14][15] Toward photonic applications, nanographene molecules have attracted a growing research interest very recently, owing to their optical absorption and luminescence properties that depend strongly on the degree of nanoconfinement and edge configuration. [16][17][18][19][20][21][22] A large variety of GNRs and molecular GQDs with armchair edges have been synthesized and characterized by various methods, 6,13 but there are still limited number of reports on nanographenes with zigzag edge although they generally exhibit intriguing properties, such as low energy gaps, biradical ground states character, and localized edge states. 12,[16][17][22][23][24][25] This is mainly due to the synthetic challenge in the incorporation of zigzag edges and their high environmental instability, which hampers their characterizations and utilization as functional materials. 26 To this end, we have recently proposed dibenzo[hi,st]ovalene (DBOV) as a new molecular GQD with both armchair and zigzag edges, which displays remarkable stability, low energy gap, high oscillator strength, and a photoluminescence quantum yield (PLQY) as high as 79%. 27 Moreover, DBOV exhibits optical gain properties, amplified spontaneous emission (ASE) with a relatively low excitation threshold for organic emitters (60 µJ cm -2 ), 27 and, interestingly, strong polaritonic emission at room temperature in microcavity. 28 In our previous spectroscopic investigation on DBOV, 27 we have noticed a relationship between the stability of the stimulated emission (SE) signal and intermolecular distance. Indeed, we have observed a dramatic and ultrafast (within 200 fs) quenching of SE when passing from solution (lifetime ≈ 300 ps) to solid films, whereas we were able to partially restore SE in diluted polymer matrix. We have tentatively attributed this effect to the formation of intermolecular polaronic-like species in solid film, whose absorption overlaps effectively with the SE band, in analogy with the photodynamic landscape depicted for conjugated polymers. [29][30][31] Although the aggregation behavior as well as the charge transport properties in supramolecular adducts of graphene molecules, in particular hexa-peri-hexabenzocoronene derivatives, has been studied extensively, 32 to date there are no detailed reports on the possible impact that charge generation in nanographenes can have on their optical properties. Such information can be highly relevant for the applications of nanographenes as emitters in light-emitting diodes and lasers, and as charge transport materials in solar cells and photodetectors.
Here, we report spectroscopic studies on DBOV derivatives with different substituents, providing evidence for strong competition between stimulated emission and charge generation in nanographenes. By systematically engineering the peripheral substituents, we found that the more substituted DBOV-Mes-C12 exhibits a sensibly slower SE deactivation dynamic (> 1 ns) 33 and lower ASE threshold in polystyrene (PS) matrix (60 µJ cm -2 ) than the less functionalized DBOV-Mes (180 µJ cm -2 ) and DBOV-Ph (no ASE observed) (see Figure 1a for the structures). We attribute this effect to the higher tendency of the less functionalized DBOVs to form π-stacking aggregates with intermolecular charge transfer (CT) character, in which charges quench stimulated emission.
The structures of DBOV-Mes-C12, DBOV-Mes, and DBOV-Ph are displayed in Figure 1a and the synthesis was carried out through the method which we reported previously (see the SI). [27][28] The peripheral substituents were varied to modify the propensity for aggregation: DBOV-Mes-C12 carries two bulky mesityl groups and two dodecyl chains and DBOV-Mes possesses only two mesityl groups while the DBOV-Ph has two smaller phenyl groups. The absorption spectra of these three derivatives in toluene (Figure 1b) show similar spectral features: i. a relatively strong peak at 626 nm, 609 nm and 608 nm for DBOV-Mes-C12, DBOV-Mes, and DBOV-Ph, respectively, corresponding to the 00' electronic transition; ii. two weaker vibronic replica at 563-576 nm (01') and 513-520 nm (02'); iii. a broad absorption band peaked at 340-370 nm that can be attributed to a convolution of more energetic transitions (for the density functional theory calculations see S.I.). [27][28] The lower I0-0'/I0-1' ratio of DBOV-Ph (2) than that of DBOV-Mes-C12 (2.8) and DBOV-Mes (2.7), and the appearance of a broad and red-shifted band for the less substituted molecules (DBOV-Mes and DBOV-Ph) (Figure 1b, inset), suggest the formation of stable π-stacked aggregates especially for the molecules lacking alkyl substitution. 33-34 35 In addition, we embedded the three derivatives into a polystyrene (PS) matrix at 1 wt%, a weight ratio that was already optimized for observing PL and ASE action in DBOV-Mes-C12. 27 The optical absorption of the films (Figure 1c) shows regular fringes that can be attributed to interference phenomena (see Figure S4 for the reflection spectra), which in-fact hinder a detailed UV-VIS absorption characterization of the diluted blends in polymer matrix. On the other hand, the PLE spectra (Figure 1d   In Figure 2 we present the differential transmission spectra and dynamics in toluene solution (0.1 mg/mL) and PS matrix. The transient spectra of the DBOV molecules (Figure 2 a,b,d,e,g,h,) display three main peaks, namely: i. a negative feature at around 500 nm that can be linked to photoinduced absorption (PA) from the first excited state (S1) to higher lying states (Sn); ii. the main peak centered at 625 nm for DBO1 and 610 nm for DBOV-Mes and DBOV-Ph, can be attributed to the depletion of the ground state due to the 00' transition (photobleaching PB); iii. a side positive peak at 560-570 nm corresponding to its vibronic replica (01'); iv. a positive peak at 670-680 nm that can be associated to the stimulated emission signal (SE). 27 The spectra of the three molecules exhibit an intriguing behavior as a function of pump-probe delay. In particular, we observe the complete suppression of the SE and its incorporation into a large negative PA feature (650-720 nm), as well as the appearance of a well-defined derivative-shaped peak resembling an electroabsorption signal. Such an effect seems to be clearer in the less substituted molecules, and especially for the least substituted DBOV-Ph for which we see a progressive build-up of the electroabsorption-like signal.
Conversely, if we either diluted the solutions down to 0.01 mg/mL ( Figure S5) or embedded the molecules in a diluted PS matrix (Figures 2b,e,h), both the PA signal and the delayed electroabsorption are suppressed. Such behavior is thus ultimately related to the intermolecular distance experienced by the nanographenes. Therefore, we preliminary assign the derivative shape signal to a Stark shift due to charge formation, whereas the PA signal can be connected to charge absorption. 29 Note that we could not observe such effect in the previous study on DBOV-Mes-C12, 27 as we used a lower concentration (0.05 mg/mL) than in the work reported here. The delayed appearance of these two peaks can be linked to depopulation of the excited states (decay of the PB and SE signals) followed by the rise of the charge state. Furthermore, we reckon that one single nanographene molecule cannot sustain the formation of a stable charge population, therefore these effects occur most likely in intermolecular adducts (i.e. dimers, trimers), rather than in one single nanographene unit only. These hypotheses are corroborated by the fact that the less substituted DBOV-Ph, which possesses the highest tendency to form aggregates as indicated by the steady-state absorption and PLE data, exhibits the best-defined electroabsorption signal among the three molecules.
If we pass to the transient dynamics of the stimulated emission signal (Figure 2e, f,) that is of great interest for applications of these molecules in photonic devices, we can detect a clear connection between the degree of functionalization and the photodynamic. Starting from DBOV-Mes-C12, the SE decay rate follows nicely the dilution series, as the signal lifetime is 400 ps for the 0.1 mg/mL solution and 1 ns for the PS film, whereas it does not decay at all for the 0.01 mg/mL solution (see table 1 for the SE lifetimes). This can be explained in terms of the relatively high solubility of this derivative containing two dodecyl chains, which allows one to tune easily the degree of aggregation.
DBOV-Mes displays a general decrease of the lifetime if compared to DBOV-Mes-C12, as well as a virtually identical photodynamic between the 0.1 and 0.01 mg/mL solutions. It is worth to note a hint of an initial ultrafast decay for this molecule that in principle can be a signature of ultrafast charge transfer processes (< 100 fs), which unfortunately we could not resolve with the temporal resolution of our experiment (150 fs). Finally, DBOV-Ph solutions exhibit further lower lifetimes than the other derivatives and, despite the noise signal, in this case we can clearly discriminate an initial decay for the DBOV-Ph film, which loses ≈ 60% of the population in 200 fs. This fast decay can be due to ultrafast charge transfer processes in the DBOV-Ph aggregates still present in PS dilute matrix, whereas the long decay time (1 ns) can be related to hole bleaching. These effects most probably arise from the poorer solubility of DBOV-Ph than that of the other molecules, both in solution and polymer matrix. We can thus conclude that the SE deactivation rate increases with decreasing bulkiness of the substituents, an effect that can be ascribed to an augmented tendency for the least substituted molecules to form aggregates with charge transfer character, in which charge generation prevails over SE.   27 In particular, we have seen that such a concentration allows to minimize the occurrence of intermolecular charge transfer processes that would suppress gain, while maintaining a sufficient amount of material necessary for waveguiding the excitation and achieving ASE action. In agreement with the abovementioned findings, we observe a three-fold increase of the ASE threshold for the DBOV-Mes (180 µJ/cm 2 ) than for DBOV-Mes-C12 (60 µJ/cm 2 ), whereas we could not observe any ASE action for DBOV-Ph. This indicates that at the same concentration the three molecules experience three different intermolecular environments which strongly depend on the presence of substituents acting as intermolecular spacers. For instance, the lack of substitution in DBOV-Ph leads to a stronger tendency to aggregation not only in solution, but also in the diluted polymer matrix. In these aggregates, gain and photoluminescence are quenched due to the strong competition with ultrafast charge transfer phenomena, as it has been shown in the TA section. On the other hand, although we can still observe ASE action for DBVO-Mes, the fluency threshold to achieve the ASE regime is higher than for DBOV-Mes-C12, as a result of a closer intermolecular distance in this molecule as compared with the dodecyl substituted case.

Supporting Information for Modulation of the Nonlinear Optical Properties of Dibenzo[hi,st]ovalene by Engineering of Peripheral Substituents Experimental details
General: All reactions dealing with air-or moisture-sensitive compounds were carried out in a dry reaction vessel under argon. Analytical thin layer chromatography (TLC) was performed on silica gel coated substrates "60 F254" from Merck. Preparative column chromatography was performed on silica gel from Merck with a grain size of 0.063-0.200 mm (silica gel) or 0.040-0.063 mm (flash silica gel). High-resolution mass spectra (HRMS) were recorded by matrix-assisted laser decomposition/ionization (MALDI) using 7,7,8,8-tetracyanoquinodimethane (TCNQ) as matrix on a Bruker Reflex II-TOF spectrometer. Raman spectra were recorded with a Bruker RFS 100/S Raman spectrometer excited by a 488 nm laser. FT-IR spectra were obtained using a FT-IR spectrophotometer TENSOE II Brucker equipped with platinum attenuated total reflection (ATR) in the range between 400 and 4000 cm -1 with a resolution of 4 cm -1 . An average of 64 scans has been taken for each sample.
To a solution of benzo[a]dinaphtho[2,1,8-cde:1',2',3',4'-ghi]perylene-5,14-dicarbaldehyde (1) 1 (5.0 mg, 9.9 mmol) dissolved in anhydrous tetrahydrofuran (5 mL) was added PhMgBr (0.080 mL, 0.16 mmol, 2.0 M in tetrahydrofuran). After stirring at room temperature for 2 h, the reaction was quenched by addition of saturated aqueous solution of NH4Cl (10 mL) and extracted with ethyl acetate (10 mL) for three times. The combined organic phases were washed with brine, dried over Na2SO4, and evaporated to give an orange solid, which was used for the next step without further purification. The obtained solid was dried under vacuum for 2 h and dissolved in anhydrous dichloromethane (5 mL). After addition of BF3•OEt2 (0.5 mL), the solution was stirred at room temperature for 2 h under air. Methanol (20 mL) was then added and the suspension was filtered, followed by washing with dichloromethane (50 mL) to give the title compound (5 mg, 80% yield) as blue solid. Measurement of 1 H NMR was not possible even at 140 °C in o-dichlorobenzene (d4) most probably because of the low solubility and strong aggregation in organic solvents. HRMS (MALDI-TOF, positive) m/z: calcd. for C50H24 + (M) + : 624.1873; found: 624.1859 (error = 2 ppm). Additional structural evidences could be obtained by comparison of FTIR, Raman, UV-VIS absorption, and PLE spectra with those of DBOV-Mes, which could be fully characterized by NMR in our previous report (see Figure 1 and S6). 1

Steady-state absorption and photoluminescence excitation.
For the UV-VIS absorption measurements, we used a Perkin Elmer Lambda 1050 spectrophotometer, equipped with deuterium (180-320 nm) and tungsten (320-3300 nm) lamps and a photomultiplier detector (180-860 nm. All the absorption spectra were corrected for the reference spectra taken at 100% transmission (without the sample) at 0% transmission (with an internal attenuator), and for the background spectrum (toluene only or glass). The PLE spectra were taken with a Horiba Nanolog Fluorimeter, equipped with a xenon lamp, two monochromators and two detectors (photomultiplier and InGaAs).
Ultrafast pump-probe spectroscopy. For the non-degenerate pump and probe measurements, the molecules were dissolved in toluene with a concentration of 0.1 and 0.01 mg mL -1 . For DBOV: polystyrene solid blends (1% weight ratio), we dissolved the proper amount of material in a 40 mg/mL polystyrene solution in toluene (PS, Aldrich, Mw = 200,000). Then, the blends were spin-cast onto a glass substrate with a spin speed of 1000 rotations per minute yielding a thickness of ≈ 400 nm, as measured by profilometer. We employed an amplified Ti:sapphire laser with 2 mJ output energy, 1 kHz repetition rate and a central energy of 1.59 eV (800 nm). We used a pump wavelength of 610 nm, which is resonant with the main π  π* transition. Such pump pulses were generated by using a visible optical parameter amplifier (OPA). Pump pulses were focused on a 200 µm spot (diameter), keeping pump fluences at ~ 50 µJ cm-2. As a probe pulse, we used a broadband white light supercontinuum generated in a sapphire plate from 450 nm to 780 nm.
Amplified spontaneous emission measurements. For ASE measurements, we employed the same films produced for pump-probe measurements. ASE characterizations were performed by using an amplified Ti:sapphire laser with 2 mJ output energy and 2 kHz repetition rate at 625 nm for DBOV-Mes-C12 and 610 nm for DBOV-Mes and DBOV-Ph. We used a 7.5 cm cylindrical focal lens to focus the pump beam into a 2 mm × 0.1 mm strip and collected the emission from the edge of the film with a fibre spectrometer (resolution 0.5 nm).

Density functional theory calculations
Optimized geometries and simulated spectra. The DBOV molecules have been sketched with the Avogadro package. 3 The optimization of the ground state geometry and the calculation of the electronic transitions have been performed with the package ORCA 3.0.3, 4 using the B3LYP functional 5 in the framework of the density functional theory. The Ahlrichs split valence basis set 6 and the all-electron nonrelativistic basis set SVPalls1 7, 8 have been employed. Moreover, the calculation utilizes the Libint library. 9 Optimized geometries Figure S1. Optimized geometries of DBOV-Mes-C12.     Reflectance spectra for the DBOV:PS 1 wt.% films Figure