One‐Pot Synthesis and Excited‐State Dynamics of Null Exciton‐Coupled Diketopyrrolopyrroles Oligo‐Grids

Exciton coupling in molecular aggregates plays a vital role in impacting and fine‐tuning optoelectronic materials and their efficiencies in devices. A versatile platform to decipher aggregation‐property relationships is built around multichromophoric architectures. Here, a series of cyclic diketopyrrolopyrrole (DPP) oligomers featuring nanoscale gridarene structures and rigid bifluorenyl spacers are designed and synthesized via one‐pot Friedel–Crafts reaction. DPP dimer [2]Grid and trimer [3]Grid, which are cyclic rigid nanoarchitectures of rather different sizes, are further characterized via steady‐state and time‐resolved absorption and fluorescence spectroscopies. They exhibit monomer‐like spectroscopic signatures in the steady‐state measurements, from which null exciton couplings are derived. Moreover, in an apolar solvent, high fluorescence quantum yields and excited‐state dynamics that resembled DPP monomer are gathered. In a polar solvent, the localized singlet excited state on a single DPP dissociates into the adjacent null coupling DPP with charge transfer characteristics. This pathway facilitates the evolution of the symmetry‐broken charge‐separated state (SB‐CS). Notable is the fact that the SB‐CS of [2]Grid is, on one hand, in equilibrium with the singlet excited state and promotes, on the other hand, the formation of the triplet excited state with a yield of 32% via charge recombination.


Introduction
The properties of organic optoelectronic materials and devices are determined by the interactions or aggregations between chromophores.[3][4] Controlling these interactions and investigating their photophysical properties is one of the major challenges in the field.In solid-state studies, multiple interactions and disorder packings render it difficult to pinpoint the influence factors of optical properties.Hence, the studies of oligomers in dilute solutions have been motivated to provide crucial insights.However, in the absence of a statically defined environment as solid-state samples, a level of complexity can be produced due to the flexibility of molecular conformation in such cases.7][8] In the Kasha model based on Coulombic couplings, the aggregation of dimers is typically divided into two types.It is, on one hand, H-aggregates, in which the transition dipole moments are "side-by-side", leads to a blue-shift spectral behavior.11] By means of considering short-range couplings, Spano and coworkers proposed another type of aggregates called "nullaggregates".In null-aggregates, total destructive interference between Coulombic couplings and short-range couplings leads to identical absorption spectra when compared to the corresponding monomers. [12,13]Recently, several studies focused on nullaggregates in theoretical aspects and condensed states.Gopidas and Nayak concluded that bis-inclusion complexes are selfassemble into null-aggregated 1D fibers with stack rotations of 60°-72 o .Importantly, the photophysical properties of the resulting networks were identical to those of the monomers. [14]ariharan and coworkers demonstrated that a 90 o rotation angle between chromophores is helpful to minimize excitonic Scheme 1. Synthetic route of compounds [2]Grid and [3]Grid.
interactions.[17] To get deeper insights into the excited state dynamics in null-aggregated chromophores, novel dimers and trimers with the null-type steady-state spectroscopic behaviors are designed and studied in solution.Spano and coworkers synthesized perylene diimide (PDI) dimers with different aggregate types and observed that the dimers with nulllike HJ coupling displayed efficient SF through a charge-transferassisted mechanism. [18]Hariharan and coworkers reported spiroconjugated PDI dimers, which integrate null-exciton splitting and symmetry-broken charge separation. [19]Wasielewski and coworkers explored insights into electronic state mixing and symmetry-breaking charge separation by comparing covalent null-type, slip-stacked PDI dimers and trimers with a xanthene spacer. [20]The role of null-aggregates is significant to drive excited state properties and optoelectronic applications.In turn, investigations on the relationship between null-aggregates and photophysical properties remain in demand.
23][24][25] For DPPs, the energy of the singlet excited state is comparable to double the energy of triplet excited states.[38][39][40] Nanogrids represent a class of closed-loop molecular building blocks that are created through covalently linking diarylfluorenes.These building blocks are designed with an emphasis on the spatial arrangement, multi-resistance integration, and photoelectric properties of wide-bandgap semiconductors.Moreover, their molecular structure offers inherent advantages such as scalability, extensibility, functionalization, and modification.[40][41] Here, we introduced TDPP-Ref into nanogrid by using Bifluorenyls to act as covalent bridges, in which two 9-position carbons (sp 3 ) serve as nodes, to attach the thienyl units of TDPP-Refs via one-pot Friediel-Crafts gridization.Two ladder-type DPPs nanogrids are designed with different sizes, namely dimer [2]Grid and trimer [3]Grid (Scheme 1).This strategy enables to hinder molecular flexibility and improves the control of molecular spacing.Furthermore, the bifluorenyl spacers, have no absorption in the range of S 0 -S 1 transition of DPP and C (sp 3 ) nodes break the conjugation between spacers and chromophores while providing a platform for targeted studies on DPPs.
To study the effects of molecular spacing on the photophysical properties, we systematically study the properties of [2]Grid and [3]Grid via steady-state and time-resolved absorption and emission spectroscopy.Both [2]Grid and [3]Grid are demonstrated to generate unique null-aggregates, exhibiting monomer-like steady-state spectral signatures and high fluorescence quantum yields in non-polar toluene.Strikingly, these null-exciton coupled grid oligomers exhibit symmetry-broken charge separation in polar benzonitrile via transient absorption spectroscopy.In particular, [2]Grid with its smaller size displays faster and more efficient formation of symmetry-broken charge-separated state (SB-CS) in benzonitrile.Notably, the SB-CS in [2]Grid is found to form an equilibrium with a singlet excited state and gives rise to delayed fluorescence by time-resolved emission spectroscopy.Subsequently, triplet excited states were generated through charge recombination with overall quantum yields of 32%.The TTA-UC properties are also investigated by using Pd(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine (PdPc) as the photosensitizer.Both [2]Grid and [3]Grid act as annihilators and undergo TTA-UC.[2]Grid displays a more efficient TTA-UC than TDPP-Ref, while [3]Grid behaves even less efficiently.Our gridization strategy may provide a path to mimic the discrete chromophores in the solid state and systematically uncover the relationship between molecular properties and inter-chromophores interactions.

Synthesis and Molecular Structures
Ladder-type DPP nanogrids, [2]Grid and [3]Grid, were synthesized via one-pot Friedel-Crafts gridization reaction between TDPP-Ref and difluorenols (DOH-Ref) by using CF 3 SO 3 H as an acid catalyst at room temperature (Scheme 1). [38,39,42,43]The reaction conditions were optimized to generate target nanogrids efficiently.It should be noted that under the condition of 1 mm TDPP-Ref concentration with TDPP-Ref, DOH-Ref, and CF 3 SO 3 H in a 1:1:3 equivalent ratio, the highest yields of [2]Grid and [3]Grid with values of 33 and 50%, respectively, were obtained.The chemical structures of [2]Grid and [3]Grid were characterized by1 H/ 13 C nuclear magnetic resonance (NMR), matrixassisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS), and high-performance liquid chromatography (Figure 1; Figures S1-S6, Supporting Information).As Figure 1a shows, the experimental mass values of [2]Grid (2006.75m z −1 ) and [3]Grid (30 10.12 m z −1 ) are in agreement with the theoretical simulation results, which are 2006.34and 3009.95m z −1 , respectively, thus suggesting the synthesis of target nanogrids.The isotope mass distribution of the experimental results is consistent with the theoretical results as well.The

Steady-State Spectroscopy and Electrochemical Characterization
Steady-state absorption spectroscopy of the references, that is, DPP monomer TDPP-Ref and bifluorenols DOH-Ref together with gridized oligo-DPPs, that is, [2]Grid and [3]Grid, were per-formed in toluene and benzonitrile to investigate the ground state interactions between the DPPs in the oligomers (Figure 3a; Figure S9, Supporting Information; Table 1).At first glance, the vibrational fine structure seen in the absorption spectra of [2]Grid and [3]Grid closely matches that of TDPP-Ref with a set of well-defined maxima in the 400 to 650 nm range, corresponding to S 0 -S 1 transition of the DPPs.A closer look reveals, however, a 25 nm red shift in [2]Grid and [3]Grid due to  an effective -extension by means of the fluorenyl spacer.The 300 to 400 nm absorption stems from overlapping DPP S 0 -S 2 and bifluorenyl S 0 -S 1 transitions.The extinction coefficients of, for example, [2]Grid is with 46 642 M −1 cm −1 more than twice that of TDPP-Ref with 22 224 M −1 cm −1 , while that of [3]Grid of 37 028 M −1 cm −1 is far less than three times of TDPP-Ref in toluene.The variance in extinction coefficients compared to TDPP-ref is attributed to the following factors: diverse packing modes among chromophores, [19,44] enhanced electron-donating character on the thiophenes, [45] and presence of bent DPPs [46] in [2]Grid and [3]Grid.Looking at the A 0-0,abs : A 0-1,abs intensity ratio of 1.23 for TDPP-ref, 1.14 for [2]Grid, and 1.31 for [3]Grid, suggests negligible exciton couplings among the DPPs in the gridized oligomers. [19]Similar results were concluded in polar benzonitrile.
Insights into excited state activity came from fluorescence spectroscopy, in general, and fluorescence quantum yields (Φ f ), in particular, which were recorded in solvents of different polarities.Notably, the fluorescence spectra of [2]Grid and [3]Grid in toluene and benzonitrile exhibited vibrational fine structure in the 550 to 700 nm range.These are bathochromically shifted by ≈26 nm compared to that of TDPP-Ref.Identical stokes shifts of ≈11 nm and similar fluorescence quantum yields of 68%, 74%, and 70% were derived for [2]Grid, [3]Grid, and TDPP-Ref, respectively, in toluene.The aforementioned findings corroborate that intramolecular exciton couplings between DPPs in the gridized oligomers are exiguous and rule out any other deactivation channels in apolar solvent toluene.Negligible are the changes for the fluorescence quantum yields of [3]Grid in polar benzonitrile with 71%, similar to that of TDPP-Ref.[2]Grid showed, in stark contrast, a significant fluorescence quenching with 45% in benzonitrile.Such a quenching is indicative of non-radiative decay processes.Considering the dependency on solvent polarity charge transfer or charge-separated states are likely to play a role in the excited state dynamics of [2]Grid.

Time-Resolved Absorption and Emission Spectroscopy
Time-resolved single-photon counting (TCSPC) was performed to investigate the excited-state dynamics of TDPP-Ref and gridized oligo-DPPs (Figure S10, Supporting Information, Table 2) following 530 nm photoexcitation.TDPP-Ref fluorescence decayed mono-exponentially with lifetimes of 5.6 and 6.3 ns in toluene and benzonitrile, respectively (Figure S10a, Supporting Information).Likewise, the gridized oligo-DPPs featured mono-exponential fluorescence decays with 5.8 ns for [2]Grid and 4.8 ns for [3]Grid in toluene.In line with the high fluorescence quantum yields in benzonitrile, [3]Grid displayed a monoexponential decay with a lifetime of 4.9 ns as well.However, the fluorescence of [2]Grid in benzonitrile was bi-exponential in nature as shown in Figure S10b (Supporting Information).Next to a short-lived component with 1.3 ns, a longer-lived component with 7.5 ns was deconvoluted.
To elucidate the excited state dynamics in gridized oligo-DPPs, femtosecond and nanosecond transient absorption spectra (fsand ns-TAS) were recorded and analyzed by means of Glotaran with a sequential kinetic model.We first focused on the excited state dynamics in apolar solvent toluene.The fs-and ns-TAS data of TDPP-Ref and gridized oligo-DPPs in toluene are best fitted by a two-state sequential model (Figure S11, Supporting Information).Fs-and ns-TAS of [2]Grid and [3]Grid showed features, which are characteristic of TDPP-ref (Figure 4; Table 2; Figures S12-S13, Supporting Information).At early times, ground state bleaching (GSB) from 487 to 600 nm came along with stimulated emission (SE) at 642 nm.In addition, broad excited state absorptions (ESA) from 680 to 1400 nm that maximize at ≈800 nm were ascribed to the first singlet excited state (S 1 ) characteristics.All of the aforementioned were subject to minor spectral changes within a time window of hundred picoseconds.It involves the relaxation of the molecular structure and solvent reorganization to afford the relaxed first singlet excited state (S 1 ) rel .After 591.2 ps, by which (S 1 ) rel is formed a subsequent decay populates the ground state S 0 with a 5.8 ns lifetime in the case of [2]Grid.Similarly, the (S 1 ) and (S 1 ) rel lifetimes for [3]Grid were 357.0 ps and 4.8 ns, respectively.Notably, the lifetimes of (S 1 ) of the gridized oligo-DPPs, namely 591.2 ps for [2]Grid and 357.0 ps for [3]Grid, are longer than that noted for TDPP-Ref with 110.9 ps in toluene.We conclude slower structural relaxation for the gridized oligo-DPPs (Table 2).More complex geometries and free rotations along the links between DPPs and bifluorenyl spacers after gridization are thought to be responsible for this trend.
In line with previous reports on DPP dimers, charge transfer states, which are heavily favored in polar solvents, play a vital role in the excited-state dynamics. [26,47]Next, we turned to examine the excited state dynamics of gridized oligo-DPPs in polar benzonitrile.Compared to toluene, TDPP-Ref showed in benzonitrile similar spectral evolutions, which are based on lifetimes of 227.2 ps and 6.3 ns for (S 1 ) and (S 1 ) rel , respectively (Figure S14, Supporting Information).The longer lifetime of (S 1 ) in, for example, benzonitrile relates to the higher viscosity.This slows down structural relaxation and solvent reorganization.As shown in Figure S16a (Supporting Information), [3]Grid in benzonitrile exhibited a new weak ESA at ≈680 nm along with (S 1 ) ESA between 680 and 1400 nm in fs-TAS.Importantly, the 677 nm fingerprint resembles the one-electron oxidized form of  b) The second species is (S 1 ) rel for TDPP-Ref.For [2]Grid and [3]Grid, the second species in toluene and benzonitrile is (S 1 ) rel and (S 1 ) rel CT , respectively.
[3]Grid as seen in spectro-electrochemical (SEC) measurements (Figure S18, Supporting Information).Consequently, the fs-and ns-TAS of [3]Grid in benzonitrile are best fitted by a three-state sequential model (Figures S15, Supporting Information).From this, after 461.1 ps, which relates to the relaxation of (S 1 ), the evolution-associated spectra (EAS) of the second species exhib-ited the signatures of (S 1 ) rel and CT with an ESA at 680 nm.Two different scenarios be considered. [20]On one hand, a CT/SB-CS state is reversibly formed from (S 1 ) rel via an incoherent mechanism. [48]Considering the fact that (S 1 ) rel and CT/SB-CS coexist as two inherent states with different optical features and decayed pathways, implicit would be the distinguishable   [2]Grid in argon-saturated benzonitrile.Left: heat map of fs-TAS raw data obtained at 530 nm photoexcitation with time delays up to 5500 ps.Right: evolution-associated spectra showing the singlet excited state (S 1 ) (black), the relaxed singlet excited state mixing with charge transfer (S 1 ) rel CT (red), and symmetry-broken charge-separated state SB-CS (blue).Right: Respective population kinetics.Note that SB-CS is not completely deconvoluted on this time scale.b) ns-TAS of [2]Grid in argon-saturated benzonitrile.Left: heat map of ns-TAS raw data obtained at 530 nm photoexcitation with time delays up to 350 μs.Middle: evolution-associated spectra showing the relaxed singlet excited state mixing with charge transfer (S 1 ) rel CT (red), the symmetry-broken charge-separated state SB-CS (blue), and the first triplet excited state (T 1 ) (green).The spectrum of (T 1 ) in the UV-vis range is amplified by a factor of 20.Right: Respective population kinetics.deconvolution of (S 1 ) rel and CT/SB-CS in TAS data next to a biexponential, monomer-like time-resolved emission.On the other hand, vibronic coupling induces an electronic state mixing between (S 1 ) rel and CT in the "null-aggregates". [20,49,50]The degree of mixing would change as a function of time and the mixed state acts as an intermediate in populating either an excimer or a SB-CS.Some degree of coherence might be maintained during the ground state recovery.In this case, the intermediate state is a superposition of (S 1 ) rel and CT rather than two separated excited states.As such, deconvolution into pure (S 1 ) rel and CT failed.Excimer or SB-CS might also be populated by solvent and nuclear relaxation or spin evolution.Here, for fs-TAS of [3]Grid in benzonitrile, the features of (S 1 ) rel and CT state always coexisted and deconvolution of (S 1 ) rel and CT failed.Moreover, neither charge transfer emission nor a bi-exponential decay of the emission is observed.Hence, we postulate the second species is a superposition of (S 1 ) rel and CT to afford (S 1 ) rel CT .To follow the fate of (S 1 ) rel CT , we turned to ns-TAS (Figure S16b, Supporting Information).After 4.1 ns, the (S 1 ) rel CT of [3]Grid was replaced in ns-TAS by an even longer-lived species.Its transient absorption spectrum includes 625 nm as well as 677 and 906 nm features, which were consistent with the fingerprints of the one-electron reduced and oxidized forms of [3]Grid, respectively.We ascribed it to a SB-CS.SB-CS decays to the ground state within 16.9 ns.It is interesting to note that the lifetime of (S 1 ) rel CT , which is 4.1 ns, matches the sole fluorescent component in TCSPC.In other words, (S 1 ) rel CT is a bright radiative state and coherence prevails throughout the ground state recovery.Considering the low SB-CS intensity and the 3% quenched fluorescence quantum yields compared to toluene, only an insignificant fraction of [3]Grid underwent symmetry-breaking charge separation.
[2]Grid in benzonitrile displayed singlet excited state features similar to that seen in toluene in the early times (Figure 5a).In particular, an ESA is seen at ≈680 to 1400 nm.As time progressed, significant differences are, however, discernable.On one hand, new ESAs appeared at 625, 677, and 906 nm, and, on the other hand, the stimulated emission at 642 nm disappeared.Notably, SEC of [2]Grid disclosed fingerprint absorptions of the one-electron oxidized form at 677 and 895 nm as well as of the one-electron reduced form in the 600 to 640 nm range (Figure S18, Supporting Information).The fs-and ns-TAS data of [2]Grid in benzonitrile is best fitted by a four-state sequential model (Figures 5 and S19, Supporting Information).The first species is clearly (S 1 ) and decays within 322.3 ps.Interesting is the fact that the markers of singlet and one-electron oxidized form seen for the EAS of the second species reflects the superposition of the (S 1 ) rel and CT characteristics in the form of (S 1 ) rel CT .In polar benzonitrile, a more dominant contribution from the charge transfer in (S 1 ) rel CT is observed for [2]Grid and leads to a faster decay of (S 1 ) compared to [3]Grid.EAS of the third species is essentially a combination of the one-electron oxidized and oneelectron reduced features at 677 / 906 and 625 nm, respectively.It is a SB-CS state.Compared to [3]Grid, the SB-CS population in [2]Grid within 1.2 ns is much faster and displayed much higher intensities of the GSBs.Our results demonstrate that SB-CS has been straightforwardly achieved through a coherent intermediate state (S 1 ) rel CT in null-aggregated [2]Grid and [3]Grid in a polar solvent.The closer packing between the DPPs in [2]Grid renders stronger contributions from CT in the intermediate state and lowers its energy to render SB-CS energetically more feasible.The fate of SB-CS was uncovered in ns-TAS.After 8.1 ns, in the EAS of the fourth species, the SB-CS characteristics are replaced by a range of ESAs between 400 and 580 nm, namely maxima at 420, 506, and 544 nm next to broad ESAs in the 593-800 nm range (Figure 5b).Notably, this species lasted 19.5 μs and the ground state is quantitatively recovered.To clarify the nature of this new state, we performed triplet-triplet sensitization experiments using N-methylfulleropyrrolidine (N-MFP) as a triplet sensitizer and at 387 nm photoexcitation (Figure S20, Supporting Information).In the absence of any [2]Grid, only the long-lived first triplet excited state (T 1 ) of N-MFP was recorded with characteristic maxima at 680 nm.In the presence of [2]Grid, (T 1 ) of N-MFP was replaced on longer time scales by (T 1 ) of [2]Grid with ESAs at 400-580 and 600-800 nm and GSB at 550-600 nm.Interestingly, the spectroscopic features of the fourth species are consistent with the fingerprints seen in photosensitized (T 1 ) of [2]Grid (Figure S20, Supporting Information).What stands out is a mono-exponential decay.Thus, we speculate that it must be the triplet excited state (T 1 ), which is generated by charge recombination rather than the correlated triplet pair 1 (T 1 T 1 ) via SF.In the optimized structure of [2]Grid, the two DPPs are in a quasi face-to-face packing mode.Nevertheless, DPP rotation is not completely frozen as the DPPs and the bifluorenyl spacers are linked by single bonds.Especially, in polar solvents, in which SB-CS is likely to occur, breaking the face-to-face packing results in a torsion angle between the two DPPs in a pseudo compact spiro form. [51]In summary, the major channel for (T 1 ) formation is the charge recombination based on a spin-orbit charge-transfer intersystem crossing (SOCT-ISC) mechanism. [52,53]A strong spectral overlap of GSB and fingerprints of (T 1 ) hampers a triplet quantum yield determination via target analyses of the transient absorption spectra. [4]Instead, the triplet quantum yield was approximated by means of singlet oxygen quantum yields (Φ Δ ) using C 60 as a reference.(Full details in Supporting Information Figure 8. Species-associated spectra of (S 1 ) rel CT of [2]Grid (green) and [3]Grid (blue) in benzonitrile, and differential absorption spectra of (T 1 ) of [2]Grid (black).and Figure S21, Supporting Information).Φ Δ in the case of [2]Grid in benzonitrile is ≈32%.This indicates a yield of (T 1 ) formation of [2]Grid in benzonitrile in the range of 32%.
Remarkably, the lifetimes of (S 1 ) rel CT and SB-CS for [2]Grid in benzonitrile match the shorter and longer-lived components seen in the TCSPC experiments, respectively (vide supra).To shed light onto the nature of SB-CS in [2]Grid, time-resolved emission spectra (TRES) were recorded.As shown in Figure 6, the TRES data is the best fit by global analysis with a sequential model based on two species.Notably, these two species with lifetimes of 1.3 and 7.5 ns, respectively, shared absolutely identical emission spectra without any notable shifts.It illustrates that for [2]Grid the sole emissive state is (S 1 ) rel CT and that the long-lived emission results from up-converting SB-CS.At this point, we postulate that SB-CS exists in equilibrium with (S 1 ) rel CT .Based on the analysis of time-resolved absorption and emission, the excited dynamic of [2]Grid and [3]Grid in toluene and benzonitrile are illustrated in Figure 7.
In this context, the sequential fitting models in Figures S15,S19 (Supporting Information) fail to analyze the TAS of [2]Grid and [3]Grid in benzonitrile, respectively.We further evaluated the TAS based on excited dynamics in Figure 7. Here, we focused on the optimization of the fitting of ns-TAS in the visible range as (S 1 ) rel CT , SB-CS state, and (T 1 ) are completely deconvolutable on this time scale.The optimized kinetic models for [2]Grid and [3]Grid are illustrated in Figures S22,S23 (Supporting Information), respectively.The corresponding rate constants were derived using sequential fitting models, fluorescence quantum yields, and triplet quantum yields (Table S3, Supporting Information).A reversible fitting model for [2]Grid is unsuitable to fit [3]Grid in benzonitrile.This points to the irreversible SB-CS in [3]Grid.Moreover, the (S 1 ) rel CT to SB-CS transition, denoted as k((S 1 ) rel CT → SB-CS), is an order of magnitude faster when comparing [2]Grid with [3]Grid in benzonitrile with 3.8 × 10 8 and 3.5 × 10 7 s −1 , respectively.SB-CS is sensitive to the grid size.Species-associated spectra (SAS) and the population of each state in benzonitrile are displayed in Figures S24,S25 (Supporting Information) for [2]Grid and [3]Grid, respectively.For [2]Grid, the SAS of SB-CS exhibited the distinguishable features of the one-electron oxidized form at 677 nm and the one-electron re-duced form at 625 nm.In addition, the populations of the respective states indicate the repopulation of (S 1 ) rel CT from SB-CS.For [3]Grid, SAS and EAS are identical albeit with a higher intensity of the SAS of SB-CS (Figure S25, Supporting Information).
Previous investigations on dimers [54] dimers [50,55,56] that one of the several mechanisms is based on coherence.It is mediated by a superposition of the singlet excited state (S 1 S 0 ), the correlated triplet pair 1 (T 1 T 1 ), and CT.For gridized oligo-DPPs, [2]Grid and [3]Grid in benzonitrile, a similar intermediate state observed.of these reveals, however, any SF.To shed light on this, SASs of (S 1 ) rel CT for [2]Grid and [3]Grid in benzonitrile are compared with the differential absorption spectra of [2]Grid (T 1 ) in Figure 8. Particular focus is placed on the signatures of (T 1 ) at 500 nm, CT at 677 nm, and (S 1 ) rel at 795 nm.None of them exhibits any discernible contributions from (T 1 ), which speaks for a non-mixing with 1 (T 1 T 1 ).It is likely that the coupling between (S 1 ) rel and CT is so strong that it effectively confines the population of (S 1 ) rel CT and hinders its interaction with 1 (T 1 T 1 ).Consequently, after decoherence by, for example, solvent and nuclear relaxation or even spin evolution, the formation of either SB-CS or (S 1 ) rel , will govern the decay of the intermediate state.Next, we evaluated the contributions from (S 1 ) rel and CT in the intermediate state (S 1 ) rel CT by calculating the relative CT/(S 1 ) rel ratio by comparing ΔOD(677 nm)/ΔOD(795 nm).0.76 and 0.006 were the values for [2]Grid and [3]Grid, respectively.In [3]Grid, CT contributions are minor, and the radiative decay from (S 1 ) rel dominates.The radiative decay from (S 1 ) rel is largely suppressed in [2]Grid and SB-CS takes over.

Triplet-Triplet Annihilation Upconversion
To study the ability of DPP gridization to foster TTA-UC, Pd(II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine (PdPc(Obu) 8 ) was utilized as a triplet sensitizer.It absorbs, on one hand, near-infrared photons in the range of 650-800 nm and it undergoes, on the other hand, an efficient intersystem crossing to generate a triplet excited state with an energy of ≈1.13 eV, which is comparable to the energy of the first triplet excited of DPPs with the value of ≈1.15 eV. [28,30]Samples for up-conversion measurements were prepared from PdPc(Obu) 8 at a concentration of 4.5 × 10 −5 m and TDPP-Ref, [2]Grid, and [3]Grid at concentrations of 2.5 × 10 −4 m, respectively, in oxygen-free toluene.As shown in Figure 9, TDPP-Ref, [2]Grid, and [3]Grid exhibit TTA-UC and emit high-energy photons in the range of 500-700 nm upon photoexciting at 730 nm.To evaluate the efficiency of TTA-UC, we compared the integrated area of upconverted fluorescence.[2]Grid displays superior TTA-UC performance and the integrated area of TTA-UC fluorescence is ≈1.5 times than seen for TDPP-Ref.In line with previous investigations, the (T 1 ) energies of the DPP derivatives are all very similar. [30]iven a (T 1 ) energy of 1.15 eV, TDPP-Ref and [2]Grid with their (S 1 ) energies of 2.22 and 2.13 eV, [57] respectively, have shown exothermic TTA-UC in thermodynamically driving triplet-triplet annihilation.Nevertheless, the energy difference between two (T 1 )s and one (S 1 ) is higher for [2]Grid with 0.17 eV compared to 0.10 eV for TDPP-Ref results in a stronger TTA driving force for [2]Grid and ultimately higher TTA-UC.The stronger fluorescent [3]Grid with the same (S 1 ) energy than [2]Grid exhibited the weakest upconverted fluorescence.Overall, it is about one-third the TDPP-Ref yield.A reason is the bulky bifluorenyl spacers, which suppress intermolecular triplet-triplet interactions and, thus, TTA-UC performance.In light of this reasoning, [2]Grid, which features the same bulky spacers and outperforms TDPP-Ref and [3]Grid under the same condition, TTA-UC is likely to stem intramolecular contributions. [58,59]The larger size of [3]Grid hinders intramolecular communications between the DPPs.This is independently confirmed by the lower quantum yield of SB-CS in benzonitrile (vide supra).Overall, it underlines the importance of distance between chromophores to TTA-UC.

Conclusion
In this article, different gridized oligo-DPPs, namely [2]Grid and [3]Grid were synthesized via gridization using bifluorenyls as spacers.DPP oligomers with less flexibility mimic stacked chromophores in the solid state.The monomer-like steady-state spectroscopy together with electrochemistry corroborates that in both [2]Grid and [3]Grid null exciton-coupling is present.Moreover, both display high fluorescence quantum yields in toluene with a value of ≈70%, which is comparable to DPP monomer TDPP-Ref.Solvent-dependent time-resolved emission and absorption spectroscopy unravel the polarity-sensitive excited-state dynamics of [2]Grid as well as [3]Grid.In toluene, both [2]Grid and [3]Grid displayed similar excited-state dynamics that are comparable to those of TDPP-Ref.However, for [3]Grid in benzonitrile, the SB-CS is formed slowly and in a negligible yield.Interestingly, the formation of SB-CS in [2]Grid in benzonitrile is reversible, and an equilibrium is established between SB-CS and (S 1 ) rel CT .However, the SB-CS in [2]Grid is long-lived enough to undergo spin-evolution and, in turn, to form free triplet excited state with a 32% triplet quantum yield via SOCT-ISC.The comparison between the different sizes of gridized oligo-DPPs highlights not only the correlation between the relative distances of chromophores and excited-state dynamics but also the role of electronic couplings in singlet fission.Our results provide new strategies toward discrete chromophores assembling into rigid structures via covalent linkages.Our studies benchmark the design of novel singlet fission, up-conversion, and intersystem crossing materials.

Figure 2 .
Figure 2. Side view (top) and top view (bottom) onto the geometry-optimized structure of a) [2]Grid and b) [3]Grid.

Figure 4 .
Figure 4. a) fs-TAS of[2]Grid in argon-saturated toluene.Left: heat map of fs-TAS raw data obtained at 530 nm photoexcitation with time delays up to 5500 ps.Middle: evolution-associated spectra showing the singlet excited state (S 1 ) (black) and the relaxed singlet excited state (S 1 ) rel (red).Right: respective population kinetics.Note that (S 1 ) rel is not completely deconvoluted on this time scale.b) ns-TAS of[2]Grid in argon-saturated toluene.Left: heat map of ns-TAS raw data obtained at 530 nm photoexcitation with time delays up to 350 μs.Middle: evolution-associated spectra showing the relaxed singlet excited state (S 1 ) rel (red).Right: Respective population kinetics.

Figure 5 .
Figure5.a) fs-TAS of[2]Grid in argon-saturated benzonitrile.Left: heat map of fs-TAS raw data obtained at 530 nm photoexcitation with time delays up to 5500 ps.Right: evolution-associated spectra showing the singlet excited state (S 1 ) (black), the relaxed singlet excited state mixing with charge transfer (S 1 ) rel CT (red), and symmetry-broken charge-separated state SB-CS (blue).Right: Respective population kinetics.Note that SB-CS is not completely deconvoluted on this time scale.b) ns-TAS of[2]Grid in argon-saturated benzonitrile.Left: heat map of ns-TAS raw data obtained at 530 nm photoexcitation with time delays up to 350 μs.Middle: evolution-associated spectra showing the relaxed singlet excited state mixing with charge transfer (S 1 ) rel CT (red), the symmetry-broken charge-separated state SB-CS (blue), and the first triplet excited state (T 1 ) (green).The spectrum of (T 1 ) in the UV-vis range is amplified by a factor of 20.Right: Respective population kinetics.

Figure 6 .
Figure 6.Time-resolved emission spectra (TRES) of [2]Grid in argon-saturated benzonitrile at 530 nm photoexcitation with time delays up to 0.1 μs.Left: heat map of TRES raw data.Middle: deconvoluted TRES data.Evolution-associated spectra of the deconvoluted species: the first species is the fluorescent (S 1 ) rel CT (red) and the second species is the fluorescent (S 1 ) rel CT from SB-CS (blue).

Figure 7 .
Figure 7. Schematic illustration of the excited-state dynamics of [2]Grid (green) and [3]Grid (blue) in toluene (left) and benzonitrile (right).The solid arrow and dotted arrow are the kinetic obtained from time-resolved transient absorption and time-resolved emission, respectively.

Table 2 .
Lifetimes obtained from TCSPC and sequential global analysis of transient absorption measurement in toluene and benzonitrile of TDPP-ref,[2]Grid, and[3]Grid.