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. 2026 Aug 26. Online ahead of print. doi: 10.1039/d6sc05125a

Charge-transfer dynamics in bowl-shaped imide-functionalized corannulenes at ensemble and single-molecule levels

Tamizharasan Thavamani a, Philip Daniel Maret a, Amalnadh T a, Alexander Bausback b, Kazutaka Shoyama b, Frank Würthner b,✉, Mahesh Hariharan a,✉
PMCID: PMC13544142  PMID: 42699703

Abstract

Curved aromatic hydrocarbons provide unique platforms for tailoring charge-transfer (CT) excited states for functional optoelectronic and photonic materials. However, tuning CT behavior in bowl-shaped chromophores through molecular design and understanding their excited-state dynamics at the single-molecule level remain challenging. Herein, we investigate CT state engineering in bowl-shaped corannulene chromophores equipped with one (C-NMI) or two (C-NBI) annulated naphthalimide units. Pronounced solvatochromic shifts in fluorescence emission and a substantial change in dipole moment, upon photoexcitation, corroborate the enhanced CT character in C-NBI relative to C-NMI at the ensemble level. To probe the influence of local polarity at the single-molecule level, the chromophores were embedded in nonpolar polystyrene (PS) and polar poly(methyl methacrylate) (PMMA) matrices. The resulting fluorescence intensity trajectories (FITs) distinctly resolved locally excited (LE), hybrid LE-CT, and CT emissive states in C-NMI and C-NBI at the single-molecule level. Subsequently, femtosecond transient absorption spectroscopy captured the evolution of LE and CT states together with triplet-state population dynamics in C-NMI and C-NBI, while theoretical investigations further supported the observed triplet-state population behavior. Overall, this work demonstrates the role of molecular architecture and local dielectric environments in dictating excited-state pathways in bowl-shaped corannulene chromophores, opening new avenues for environment-sensitive probes and nanoscale photonic technologies.

Curved aromatic hydrocarbons provide unique platforms for tailoring charge-transfer (CT) excited states for functional optoelectronic and photonic materials.graphic file with name d6sc05125a-ga.webp

Introduction

Around a hundred years ago, it was recognized that very versatile dyes and pigments can be obtained by appending imide groups to polycyclic aromatic hydrocarbon (PAH) scaffolds.1,2 Accordingly, by imidization the coloristic properties improve, i.e. absorption is shifted bathochromically and the absorption coefficient increases, and the photostability is significantly improved compared to parent PAHs.3,4 Often, fluorescence properties also improve as expected based on the high oscillator strength for the lowest energy transition that typically involves fully conjugated electron-withdrawing imide substituents.3,5 Nevertheless, despite the large number of available imide-functionalized PAHs,4,6–10 only for a few of them in-depth photophysical studies are available that go beyond the characterization of absorption and steady-state emission properties.4,11,12 This motivated the current study in which a PAH bisimide bearing a bowl-shaped corannulene core is investigated by advanced optical spectroscopy techniques, i.e. ultrafast transient absorption spectroscopy and single-molecule fluorescence spectroscopy in various solvents as well as polymeric systems. Transitioning from conventional planar PAHs to curved corannulene-based aromatic systems introduces unique electronic and supramolecular properties arising from their convex–concave topology and curvature-induced out-of-plane dipole moments.13 Accordingly, corannulene has gained attention not only as a fullerene substructure but also as a versatile component in optoelectronic materials.14–17 Moreover, corannulene can be chemically modified in a controlled manner to produce derivatives with varying bowl depth, curvature, dipole moment, polarizability, charge transport or optical band gaps.13,18–20 In this context, several research groups have developed various corannulene architectures possessing diverse excited state landscapes,19,21–24 including annulated corannulene chromophores reported by some of us.25,26 Consequently, corannulene-derived chromophores were investigated as potential materials in solar cells,27 organic light-emitting diodes (OLEDs),28 organic field-effect transistors (OFETs),29,30 and biomedical applications.31 Despite these advances, the excited-state properties of corannulene-based chromophores have received limited attention to date. Among the diverse excited-state dynamics exhibited by corannulene-derived chromophores,26,28,32–34 the charge-transfer (CT) mechanism is especially noteworthy because of its critical involvement in the primary charge-generation events of organic photovoltaic (OPV) devices.35 Previously, various strategies were attempted to tune the CT state in corannulene-based chromophores.36,37 Tang et al. established a molecular engineering strategy to precisely modulate the CT state energy levels in corannulene-derived donor–acceptor dyes through in situ oxidation of the sulfur atom.38 Fukuzumi and co-workers successfully demonstrated the formation of a CT complex between corannulene and lithium ion-encapsulated [60]fullerene in the ground state by the concave-convex π–π CT interaction.39 Our longstanding interest in understanding excited-state dynamics in curved aromatic chromophores26,40–42 at the single-molecule level motivated us to investigate the CT character in bowl-shaped corannulene derivatives. Probing excited-state dynamics at the single-molecule level enables direct visualization of molecular heterogeneity hidden in ensemble photophysical measurements. Though single-molecule excited-state dynamics were widely explored in planar π-conjugated chromophores,43–50 curved aromatic chromophores remain elusive. However, CT emission at the single-molecule level is often challenging to detect owing to its intrinsically weak emissive nature arising from spatially separated frontier molecular orbitals.51,52 To address this challenge, we employed rational molecular design and modulation of the local dielectric environment to induce CT emission in corannulene derivatives, namely C-NMI and C-NBI. Contrasting blinking dynamics observed at the single-molecule level, together with ensemble-level femtosecond (fsTA) and nanosecond transient absorption (nsTA) measurements, consistently revealed the distinct excited-state CT dynamics in C-NMI and C-NBI, suggesting potential applications in nanoscale electronics and imaging.

Results and discussion

Synthesis and structural properties

The bowl-shaped corannulene-based derivatives C-NMI and C-NBI were synthesized and characterized according to the previously reported procedures (Fig. 1).25 The reported crystallographic analysis revealed that both C-NMI and C-NBI exhibit bowl-shaped architectures. From the crystal structures of C-NMI and C-NBI, the bowl depth was determined as the shortest distance between the centroid of the central pentagonal ring and the mean plane defined by the ten carbon atoms constituting the molecular rim.53 The measured bowl depth of C-NMI is 0.921 Å, whereas that of C-NBI is slightly higher i.e. 1.004 Å (Fig. S1a and b).

Fig. 1. Molecular structures for (a) C-NMI and (b) C-NBI.

Fig. 1

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Optical properties

To decipher the effect of different dielectric environments on excited state dynamics, solvent-dependent UV-visible absorption and fluorescence emission measurements were conducted for C-NMI and C-NBI. Steady-state measurements were performed in solvents with distinct dielectric constants, namely nonpolar toluene (TOL, ε = 2.38), moderately polar tetrahydrofuran (THF, ε = 7.58), and highly polar acetonitrile (ACN, ε = 37.50). UV-visible absorption spectra of C-NMI and C-NBI exhibit well-resolved vibronic features with absorption maxima (λAbmax) of ∼504 nm and ∼505 nm respectively in solvents of varying polarity (TOL, THF and ACN) (Fig. 2a, b, Tables S1 and S2). The negligible changes in the spectral profile and λAbmax across the solvents indicate the ground-state electronic structures of C-NMI and C-NBI are largely insensitive to changes in the dielectric environment. Interestingly, despite minimal solvent dependence observed in the UV-visible absorption spectra, the fluorescence emissions of C-NMI and C-NBI show pronounced sensitivity to solvent polarity. The fluorescence emission spectra of C-NMI in TOL (λFlmax = 524 nm) and THF (λFlmax = 524 nm) display distinct vibronic bands, whereas the emission spectrum in ACN exhibits noticeable broadening and a ∼15 nm red-shift relative to C-NMI in nonpolar TOL (Fig. 2a and Table S1). The spectral broadening and enhanced bathochromic shift in polar ACN are indicative of a CT-like excited state in C-NMI.54 Furthermore, the fluorescence emission spectra of C-NBI displayed a much stronger dependence on solvent polarity. Upon moving from nonpolar TOL (λFlmax = 560 nm) to polar ACN (λFlmax = 610 nm) the emission spectra of C-NBI exhibit a bathochromic shift of ∼50 nm along with significant broadening, corroborating the presence of a CT state (Fig. 2b and Table S2).55 Concentration-dependent UV-visible absorption, fluorescence, and excitation measurements were performed for C-NMI and C-NBI in ACN to assess the origin of the observed spectral broadening (Fig. S2–S4). No significant spectral changes were observed with increasing concentration, and the Beer–Lambert plots exhibit linearity up to 25 µM for C-NMI and 50 µM for C-NBI, confirming monomeric behavior. Therefore, the ensemble spectroscopic measurements were conducted at concentrations within the monomeric regime (<10 µM) for both C-NMI and C-NBI. To further support the CT nature in C-NMI and C-NBI, relative fluorescence quantum yield (ΦFl) measurements were carried out in TOL, THF and ACN using N,N′-bis(3-pentyl) perylene bisimide (PBI) in toluene as a reference. Although the fluorescence quantum yield (ΦFl) and fluorescence lifetimes (τFl) of C-NMI and C-NBI were previously reported,25 these properties were re-evaluated in TOL, THF, and ACN to assess the influence of solvent polarity on their excited-state behavior. In C-NMI, the ΦFl marginally decreases in ACN (ΦFl = 58.1%) compared to TOL (ΦFl = 69.3%) and THF (ΦFl = 69.3%) (Table S1). The decrease in ΦFl upon moving to polar ACN in C-NMI indicates the formation of non-radiative decay channels. A more pronounced solvent dependency of ΦFl was noted for C-NBI: 42.7% in TOL and 33.3% in THF and 15.5% in ACN (Table S2). The significant decrease in ΦFl for C-NBI with increasing solvent polarity, relative to C-NMI, is consistent with enhanced stabilization of a weakly emissive CT state. Additionally, time-correlated single photon counting (TCSPC) measurements of C-NMI and C-NBI were performed in TOL, THF and ACN with a nanoLED laser excitation source (λex = 440 nm) (Fig. 2c and d). The fluorescence lifetime measurements of C-NMI revealed a monoexponential decay profile with a gradual increase in the fluorescence lifetime (τFl) upon moving from TOL (τFl = 3.07 ns) to THF (τFl = 3.25 ns) to ACN (τFl = 3.85 ns) (Table S3). The monoexponential decay behavior of C-NMI confirms that the emission originates from a single emissive state, under all three solvent conditions.56 Conversely, C-NBI displayed biexponential fluorescence decay profile with distinct lifetimes from nonpolar TOL (τFl1 = 2.17 ns (α1 = 16.0%) and τFl2 = 4.45 ns (α2 = 83.9%)) to polar ACN (τFl1 = 1.25 ns (α1 = 11.6%) and τFl2 = 6.95 ns (α2 = 88.3%)) (Table S4). The short and long fluorescence lifetime components indicate the existence of two distinct emissive states in C-NBI, corresponding to LE and CT states, respectively. Consistent with this assignment, Lippert–Mataga analysis revealed larger dipole moment changes upon excitation for C-NBI (Δµ = 15.92 D) than for C-NMI (Δµ = 8.27 D) (Tables S5, S6, Fig. S5 and S6), indicating significantly enhanced CT character in C-NBI.57,58 Collectively, ensemble steady-state measurements reveal a hybrid LE-CT state in C-NMI, while C-NBI displays dominant CT emission in polar solvents.

Fig. 2. Normalized UV-visible absorption and fluorescence emission spectra of (a) C-NMI and (b) C-NBI in TOL, THF and ACN. Fluorescence lifetime measurements of (c) C-NMI and (d) C-NBI in TOL, THF and ACN.

Fig. 2

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Single-molecule fluorescence measurements

Having characterized the ensemble-level steady-state photophysical properties of the bowl-shaped chromophores C-NMI and C-NBI, we examined how distinct dielectric environments influence the excited state properties at the single-molecule level.59 A single-molecule fluorescence time-resolved confocal microscopy technique was employed to investigate fluorescence dynamics of individual fluorescent molecules of C-NMI and C-NBI. The fluorescence lifetime imaging (FLIM) and FITs were extracted to decipher the excited state behaviour of C-NMI and C-NBI chromophores. SMFS measurements were performed using a MicroTime 200 (MT200) time-resolved fluorescence microscope from PicoQuant. An inverted microscope (Olympus IX83) equipped with a piezoelectric scanning stage (P-733.2CD, PI) was employed for the measurements. The FLIM images and the FITs for C-NMI and C-NBI were acquired using a 512 nm pulsed laser (LDH-D-C-512, PicoQuant) operating at a repetition rate of 20 MHz. Fluorescence photons were collected using a water-immersion objective (UPlanSApo 60×, NA 1.2). Spectral filtering was achieved using a primary dichroic mirror (zt405-442/510 rpc-UF3, Chroma) in combination with a 519 nm long-pass emission filter (FF01-519/LP, Semrock). Spatial filtering was accomplished using a 50 µm pinhole. The filtered emission, after passing through a series of optics, was directed onto a single-photon avalanche photodiode (SPAD; Excelitas). The detected signals were processed using a HydraHarp 400 time-correlated single-photon counting (TCSPC) module (PicoQuant). Data acquisition and analysis were performed using SymPhoTime64 software.60 The SMFS measurements of C-NMI and C-NBI were performed in two polymer matrices with distinct dielectric characteristics: nonpolar polystyrene (PS, ε = 2.6) and polar poly(methyl methacrylate) (PMMA, ε = 3.4).61,62 PMMA provides a more polar microscopic environment owing to its polar ester functionalities.63–65 The samples of C-NMI and C-NBI were prepared at femtomolar concentrations in TOL and further mixed with ∼3 w/w% of PS or PMMA in a 1 : 1 ratio. The resulting polymer solutions were then spin-coated on a coverslip and subsequently dried. FLIM images of the C-NMI and C-NBI samples show pronounced fluorescence lifetime contrast images, with bright spots corresponding to individual fluorescent emitters in PS and PMMA matrices (Fig. 3a and e). FITs display intermittent switching between fluorescence on-state (bright/emissive state) and off-state (dark/non-emissive state) or permanent loss of fluorescence (photobleaching).66 By changing the local dielectric environment, fluctuations in fluorescence intensity can be monitored through FITs. Single-molecule FITs were acquired for C-NMI and C-NBI in a nonpolar PS matrix and a polar PMMA matrix over a duration of 200 s. Initially, single-step FITs were observed for C-NMI with distinct photon counts in the PS and PMMA matrices (Fig. 3b and c). FITs of 85 individual C-NMI molecules embedded in the PS matrix were acquired. Of these, 57 molecules exhibited single-step FITs with high photon counts, suggesting photon emission from the LE state, while the remaining 28 molecules underwent rapid photobleaching.67 Additionally, 74 single-molecule FITs of C-NMI were analyzed in the polar PMMA matrix, of which 44 molecules exhibited single-step traces similar to those observed in the PS matrix in C-NMI, albeit with lower photon counts. The single-step FITs with low photon counts obtained for C-NMI in the PMMA matrix suggest emission from a low-emissive state. This emission is plausibly associated with an LE-CT mixed state and strongly agrees with ensemble photophysical behavior in polar dielectric environments. A subset of single-molecule FITs of C-NMI in the PS matrix (∼27 molecules) and in the PMMA matrix (∼30 molecules) exhibits occasional short off-states attributed to triplet state population (Fig. S8 and S9).67,68 Furthermore, the fluorescence lifetime distribution plot of the C-NMI molecules shows the measured fluorescence lifetime in the range of ∼2–6 ns in PS and ∼3–6 ns in PMMA (Fig. S12a and b). The noticeable increase in the fluorescence lifetime centered at around 4–5 ns in PMMA compared to PS is consistent with enhanced LE-CT mixed-state character arising from stabilization of the CT component in the PMMA matrix.

Fig. 3. Representative fluorescence lifetime images of (a) C-NMI and (e) C-NBI. Representative fluorescence intensity trajectories of C-NMI in (b) PS and (c) PMMA matrices and C-NBI in (f) PS and (g) PMMA. Representative antibunching measurements of (d) C-NMI and (h) C-NBI.

Fig. 3

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Contrasting FITs were obtained for C-NBI in the nonpolar PS and polar PMMA matrices compared to C-NMI. Two-step traces with varying photon intensities were observed for C-NBI in the PS and PMMA matrices (Fig. 3f and g). In the nonpolar PS matrix, 92 single molecules of C-NBI were analyzed. Approximately 58 molecules exhibited two-step FITs, with an initial high-intensity photon count assigned to LE emission and a subsequent lower-intensity photon count attributed to a weakly emissive CT state (Fig. 3f and S10). In addition, the remaining 34 molecules of C-NBI showed rapid photobleaching. Occasionally, C-NBI molecules in the PS matrix displayed frequent short off-state behavior, likely arising from triplet-state population.69 Notably, a dominant CT state was observed for C-NBI in the PMMA matrix (62 molecules), as evidenced by two-step FITs corresponding to distinct LE and CT emissions (Fig. 3g and S11). The low-lying CT state is stabilized by the polar PMMA matrix through dielectric screening and dipole–dipole interactions at the single-molecule level. The CT stabilization at the single-molecule level was consistent with the polar local environment of C-NBI observed in ensemble-level photophysical measurements. Furthermore, the enhanced stabilization of the CT state in the PMMA matrix is evident from the single-molecule fluorescence lifetime distributions of C-NBI. To gain further insight into the CT dynamics, an intensity-step-resolved fluorescence lifetime analysis was performed. For C-NBI in PS, the fluorescence lifetime histogram exhibits two distinct lifetime populations (Fig. S13a). The first-step lifetime, centered at approximately 4 ns, is attributed to the LE state, whereas the second-step lifetime, spanning approximately 4.6–5.5 ns, is assigned to the CT state. Similarly, two distinct lifetime populations are observed for C-NBI in PMMA (Fig. S13b). The first-step lifetime is centered at approximately 4.5 ns and is attributed to the LE state. Notably, the second-step lifetime distribution shifts to longer fluorescence lifetime values, spanning approximately 5–6.5 ns, indicating enhanced stabilisation of the CT state in the PMMA matrix. Antibunching experiments of both C-NMI and C-NBI in PS and PMMA were conducted to confirm single emitter behavior using the Hanbury Brown–Twiss (HBT) experimental setup.70 The second-order photon correlation analysis was employed to examine the photon coincidences in the molecular emitters.43 A dip at zero lagtime (µs) in the second-order correlation function confirms the emission from a single molecule (Fig. 3d and h).40

Femtosecond transient absorption measurements

To rationalize the CT behavior observed in SMFM measurements, we performed solvent dielectric-dependent femtosecond transient absorption (fsTA) spectroscopy on C-NMI and C-NBI in nonpolar TOL and polar ACN. Upon photoexcitation of C-NMI in TOL at 480 nm using a 100 fs pump pulse, the fsTA spectrum exhibits negative ground-state bleach (GSB) and stimulated emission (SE) bands centered at 508 and 559 nm, respectively, along with a positive excited-state absorption (ESA) band spanning ∼579–765 nm (Fig. 4a). The evolution-associated spectra (EAS) of C-NMI in TOL were obtained by global fitting of the transient absorption data using a three-component sequential kinetic model (A → B → C → GS, where GS represents the ground state). The first component (A) is assigned to a locally excited hot singlet state, which undergoes rapid vibrational relaxation (τVR = 30.9 ps) to form the second component (B). Component (B) is attributed to the relaxed singlet excited state of C-NMI, which subsequently decays within 1.2 ns, populating the third component (C). Consequently, the third component (C), characterized by a positive ESA feature at 540 nm, was found to be a long-lived species attributable to the triplet excited state of C-NMI in TOL. Conversely, in polar ACN, C-NMI exhibits a distinct excited-state response compared to nonpolar TOL. Upon photoexcitation at 480 nm, C-NMI in ACN displays a GSB at 499 nm, accompanied by a broad ESA band spanning from ∼581–763 nm, suggesting the CT nature of the transient species (Fig. 4b). Spectral deconvolution of the fsTA data of C-NMI in ACN, using an A → B → C → GS kinetic model, reveals three principal components. Component (A) is assigned to a hot singlet state with CT character, which decays within 33.9 ps to populate component (B). Component (B) is attributed to a relaxed singlet CT (LE-CT) state, which subsequently evolves into the third component (C) with a decay time of 1.4 ns. The third component (C), characterized by an ESA feature at 540 nm, does not fully decay within the 3.6 ns temporal window accessible by our fsTA instrument and is assigned to the triplet excited state.

Fig. 4. (Top row) FsTA spectra of C-NMI in (a) TOL and (b) ACN and C-NBI in (c) TOL and (d) ACN illustrating the evolution of the excited-state species following photoexcitation. (Middle row) EAS reconstructed from global analysis using an A → B → C → GS model for C-NMI in (a) TOL and (b) ACN and with an A → B → GS model for C-NBI in (c) TOL and (d) ACN. (Bottom row) Relative population profiles of the excited states in C-NMI in (a) TOL and (b) ACN and C-NBI in (c) TOL and (d) ACN.

Fig. 4

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C-NBI exhibits markedly different excited-state dynamics compared to C-NMI. The fsTA spectra of C-NBI in TOL exhibit a negative band in the ∼488–572 nm region, attributed to GSB and/or SE, along with an ESA band observed over the ∼573–765 nm wavelength range (Fig. 4c). The fsTA data of C-NBI in TOL depict a slightly broadened spectral signature in comparison with the spectral signature of C-NMI in TOL. The broadening of the spectral features observed in C-NBI probably originates from its CT character.37 Deconvolution of the total spectra of C-NBI in TOL into two principal components using a two-component sequential kinetic model (A → B → GS) reveals two principal components. The hot singlet excited state with CT character (first component) decays with a time constant of 93.3 ps, forming a relaxed singlet CT state (second component), which does not fully decay within the 3.6 ns experimental time window of our fsTA setup. However, the fsTA spectrum of C-NBI in ACN shows GSB in the ∼486–553 nm range and a significantly broadened ESA band observed between ∼556 and 766 nm (Fig. 4d). The broadness of the ESA feature suggests a CT character of the excited state of C-NBI in polar ACN. Analysis of the EAS and relative population profiles of C-NBI in ACN indicates the presence of two principal components. The hot singlet excited state with CT character relaxes with a time constant of 302.4 ps, accompanied by the growth of a long-lived relaxed singlet state with CT character that persists beyond the accessible fsTA timescale. The blue-shifted differential absorption maximum of the second component, compared to the initially generated singlet excited state, is consistent with the dielectric-dependent evolution of the relaxed singlet CT state.71,72

Nanosecond transient absorption measurements

To probe triplet-state involvement in C-NMI and C-NBI, nanosecond transient absorption (nsTA) measurements were carried out in solvents of different polarity (TOL and ACN). The nsTA experiments were carried out using a 355 nm excitation laser with an 8–10 ns pulse width. Under nitrogen-purged conditions, the nsTA spectra of C-NMI exhibited an intense ESA band at ∼540 nm in TOL and ∼530 nm in ACN (Fig. S14a and S15a). The ESA band observed for C-NMI is attributed to absorption from a long-lived excited state, likely arising from triplet state absorption (T1 → Tn). The positive ESA bands decayed rapidly in oxygen-purged solutions of C-NMI in TOL and ACN, confirming the involvement of triplet-state manifolds (Fig. S14b and S15b). The fitted triplet lifetimes (τT) of C-NMI were determined to be 3.13 µs in TOL and 14.42 µs in ACN (Fig. S16a and b). Likewise, the nsTA spectra of C-NBI in nitrogen-purged solution show a less intense ESA band in TOL and ACN (Fig. S17a and S18a). In oxygen-purged TOL and ACN solutions of C-NBI, the positive ESA bands decayed rapidly during nsTA measurements, corroborating the involvement of triplet-state manifolds in C-NBI following photoexcitation (Fig. S17b and S18b). Due to the poor decay kinetics of C-NBI in TOL, reliable fitting of the decay was challenging, and thus the triplet lifetime could not be determined (Fig. S19a). However, monoexponential decay fitting of C-NBI in ACN yielded a τT of 4.40 µs (Fig. S19b).

Singlet oxygen quantum yield measurements

The singlet oxygen quantum yield (ΦΔ) is a well-known tool for quantifying the amount of triplet state population of chromophoric systems.73,74 The quantification of ΦΔ for C-NMI and C-NBI was carried out in TOL and ACN as the solvents (Section 1.4). During the evaluation of singlet oxygen production, the triplet state energy of chromophores C-NMI and C-NBI was transferred to ground-state triplet oxygen (3O2), resulting in singlet oxygen (1O2) generation. The singlet oxygen quantum yield was found to be 16.6% in TOL and 34.6% in ACN for C-NMI (Fig. S20) and 6.6% in TOL and 12.1% in ACN for C-NBI (Fig. S21).

Computational details

To comprehend the triplet state population observed in C-NMI and C-NBI, quantum chemical calculations were performed. The vertical excitation energies (VEEs) were computed using time-dependent density functional theory (TDDFT) at the CAM-B3LYP/def2-SVP level of theory in a vacuum. In addition, spin–orbit coupling (SOC) constants between singlet and triplet states were computed by employing the PySOC package at the CAM-B3LYP/def2-SVP level of theory to assess the intersystem crossing (ISC) efficiency in the chromophoric systems.75 The computed energies indicate that C-NMI exhibits a smaller energy gap (ΔEST) (ES1–T4 = 0.12 eV), whereas C-NBI shows a comparatively larger ΔEST (ES1–T4 = 0.23 eV) for the energetically best matching triplet states to the S1 energy level (Fig. S22 and Tables S7 and S8). Notably, the ΔEST values of both C-NMI and C-NBI are smaller than those of their respective congeners, PMI and PBI, suggesting more favorable energetic conditions for ISC.50,76 Consistently, C-NMI also shows a quantitatively higher SOC matrix element 〈S1|Hsoc|T4〉 = 0.18 cm−1 than C-NBI (〈S1|Hsoc|T4〉 = 0.14 cm−1) (Tables S9 and S10). Together, these results demonstrate improved ISC efficiency, leading to a more pronounced triplet-state population in C-NMI compared to C-NBI. Overall, based on a combination of steady-state, single-molecule, time-resolved, and computational investigations, we mapped the excited-state relaxation pathways of C-NMI and C-NBI in both nonpolar and polar environments (Fig. 5).

Fig. 5. Proposed Jablonski diagram summarizing the excited-state dynamics of C-NMI in (a) nonpolar and (b) polar media and of C-NBI in (c) nonpolar and (d) polar media.

Fig. 5

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Conclusions

In conclusion, the excited state dynamics of the bowl-shaped corannulene chromophores C-NMI and C-NBI were explored at both the ensemble and the single-molecule levels in distinct dielectric environments. A pronounced red shift (∼50 nm from TOL to ACN) together with marked solvatochromic behavior in the emission spectrum of C-NBI suggests a significantly enhanced CT character relative to C-NMI (∼15 nm from TOL to ACN). Furthermore, solvent-dependent ensemble fluorescence measurements, including emission spectroscopy, ΦFl, and fluorescence lifetime measurements, reveal a hybrid LE-CT state in C-NMI, whereas C-NBI predominantly exhibits a CT state under polar conditions. Single-molecule fluorescence measurements of C-NMI and C-NBI in PS and PMMA matrices established the dielectric dependence of the charge-transfer behavior. In C-NMI, single-step FITs observed in both nonpolar PS and polar PMMA matrices reveal LE and hybrid LE-CT states, respectively. In contrast, C-NBI exhibited two-step FITs in both matrices, indicating weak CT character in PS and dominant CT character in PMMA, as evidenced by photon counts and fluorescence lifetime distribution profiles. The evolution of photoexcited states together with triplet-state population in C-NMI and C-NBI was investigated using fsTA spectroscopy across solvents of varying dielectric constants. Complementary nsTA measurements and quantum chemical calculations further substantiated the participation of triplet states in C-NMI and C-NBI. These findings provide molecular-level insight into CT processes in curved aromatic chromophores with implications for future photonic and optoelectronic materials.

Author contributions

All authors have contributed to the final version of the manuscript.

Conflicts of interest

The authors declare no conflicts of interest.

Supplementary Material

SC-OLF-D6SC05125A-s001
SC-OLF-D6SC05125A-s002

Acknowledgments

M. H. acknowledges the Nano Mission project (DST/NM/TUE/EE01/2019) of the Department of Science and Technology (DST), Government of India, for financial support. F. W. and M. H. acknowledge the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Indian Department of Science and Technology (DST) for financial support through IRTG 2991 ‘Photoluminescence in Supramolecular Matrices’ – Project numbers 517122340 (DFG) and INT/FRG/IRTG/01/2024 (DST). T. T., P. D. M. and A. T. acknowledge IISER TVM for financial assistance. A. B. acknowledges the GIANT/SPARC (DAAD Projekt-ID: 57807953) program for supporting his two-month research stay at IISER Thiruvananthapuram. M. H. acknowledges SPARC (PG250063) for financial assistance. We thank the Padmanabha HPC cluster at IISER TVM for the computing facility.

Data availability

All data supporting this finding are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc05125a.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

SC-OLF-D6SC05125A-s001
SC-OLF-D6SC05125A-s001.pdf (20.4MB, pdf)
SC-OLF-D6SC05125A-s002
SC-OLF-D6SC05125A-s002.cif (107.6KB, cif)

Data Availability Statement

All data supporting this finding are available within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6sc05125a.

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