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The well-known boron-dipyrromethene (bodipy) dyes have excellent photophysical and photochemical properties, which are significant for practical use in many fields. meso-Phenyl-substituted bodipy (PhBDP) as one of the most classical bodipy derivatives was investigated in the forms of amorphous film, crystalline film, and single crystal by using time-resolved fluorescence and femtosecond transient absorption techniques. The fluorescence of both PhBDP films is strongly quenched because of J-aggregation, which is randomly distributed in the amorphous film but well arranged in the crystalline film. In the crystalline film, the triplet states were populated from the upper excited singlet states (3.54 eV) of the nanoaggregates via singlet fission (SF) within 30 fs. Moreover, SF was also detected after excitation of the lower singlet states (2.58 eV) which are below the SF threshold (3.28 eV). This phenomenon was explained in terms of two-quantum absorption. However, in the amorphous film, the SF process was not observed, indicating the crucial impact of the nanoaggregate morphology on the formation of SF. In the single crystal, SF was also generated owing to the molecular slip-stacked packing which is optimal for the delocalization of excitation. It facilitates exciton fission and energy transfer between the neighboring PhBDP dimer moieties as well. Our findings of PhBDP solids provide a significant theoretical and experimental basis for exploring new SF systems. The prominent quenching, efficient SF, and well-characterized energy transfer of PhBDP solid expands the applications of PhBDP-based organic materials in optoelectronic devices.

The ultrafast excited-state relaxation dynamics of indolo[3,2-b]carbazole (ICZ) molecules were investigated in thin films and single crystals. In the ICZ film, singlet fission (SF) was directly observed both from the upper excited singlet states SNwithin 480–850 fs and from the upper vibrational levels of the singlet state S1within 6–11 ps, which fully suppressed the intramolecular charge transfer (CT) process in S1. In the ICZ crystal, the exciton relaxation through SF and polaron formation compete in the ultrafast timescale (<30 fs), which is accompanied by a thermally induced broadening of the S1–SNabsorption band in the transient absorption spectra. The triplet-state lifetime is shortened from ICZ crystals (>1 ms) to films (2 μs) due to SF with subsequent triplet–triplet annihilation, suggesting the impact of well-structured crystallinity and molecular packing on the exciton transport dynamics. The efficient SF together with well-characterized energy transfer and CT properties of ICZ solids extends the usage of ICZ-core-based organic materials in photovoltaic and electrochemical applications.

Excited-state dynamics in a strong charge-transfer (perylene)3–7,7,8,8-tetracyanoquinodimethane (P3T1) single-crystal system was studied by use of femtosecond transient absorption and upconversion time-resolved fluorescence techniques. Charge separation within 0.6–1.2 ps and subsequent recombination within 6.6–12 ps was demonstrated. Moreover, the excited-state dynamics in the P3T1 crystal was found to depend on excitation energies. Excitation of upper excited electronic states (4.96 eV) leads to efficient direct population of perylene triplet states via singlet fission. At lower excitation photon energies, enhancement of triplet state formation via intersystem crossing as a result of efficient spin–orbit charge transfer was observed. By varying the excitation wavelength, the performance of optoelectronic devices can be tuned, i.e., photon absorption, free carrier transfer, mobility, and recombination.

Ordered porphyrin-based metal organic frameworks (MOFs) may serve as a model for mimicking the natural photosynthesis with highly ordered chlorophylls, i.e., porphyrin-like chromophores. Study of light harvesting and energy transfer as the primary event of photosynthesis is of great importance leading to improvement of photovoltaics overall performance. Detailed characterization of ultrafast dynamics of zinc tetraphenylporphyrin (ZnTPP) surface mounted metal organic framework (SURMOF) is reported by using various steady-state and time-resolved laser spectroscopic techniques, i.e., time-correlated single photon counting, fluorescence up-conversion and transient absorption pump–probe with 20 fs resolution. Obtained results in these nanoporous materials were compared with corresponding results for ZnTPP in ethanol measured under the same conditions. Dramatic quenching of both upper excited singlet state S2and first excited state S1was observed. Subpicosecond and picosecond lifetimes were detected in transient fluorescence and absorption. Analytical formulas are derived for the linear absorption, steady-state fluorescence, and fluorescence up-conversion signals. Theoretical description excellently reproduces experimental time and frequency resolved signals. Strong quenching of the femtosecond transients in SURMOF is explained in terms of highly efficient Förster resonance energy transfer between the neighboring porphyrin moieties which is caused by a strong spectral overlap of absorption and steady-state fluorescence spectra and quantum coherent energy transfer and redistribution.

Perylenebisimide (PBI)–phenothiazine (PTZ) and PBI–diphenylamine (DPA) dyads were prepared, with the PTZ or DPA as the electron donor and the 6-subsituted PBI unit as the electron acceptor. The dyads were designed in such a way that electronic coupling (magnitude is the matrix elements, VDAand VDA*) between the electron donor and acceptor is controlled by conformation restriction. The effects of the electronic coupling on UV–Vis absorption and emission were studied. More significant charge-transfer (CT) absorption and CT fluorescence emission bands were observed for dyads with smaller dihedral angles between the electron donor and acceptor, thus stronger electronic coupling (VDA= 3290 cm–1and VDA* = 4317 cm–1) was observed than those with larger dihedral angle, showing weaker coupling effect (VDA= 1210 cm–1and VDA* = 2770 cm–1). Triplet state production was observed only for PBI–DPAbut not for PBI–PTZdyads. With an intermolecular triplet photosensitizing method, the triplet state of PBI–PTZdyads was observed. The femtosecond transient absorption spectra confirmed the ultrafast charge separation (CS, 0.3 ps ∼ 0.6 ps) and slow charge recombination (CR, 130 ps ∼ 240 ps) process. These results indicate that the CR does not produce any triplet state in the PBI–PTZdyads, for instance by the expected spin–orbital charge transfer intersystem crossing (SOCT-ISC). We propose that the lack of triplet state production in PBI–PTZdyads is due to the large separation of the 1CT and the 3CT/3PBI states, and the orthogonal geometry and the CR are not exclusively sufficient criteria for SOCT-ISC. Our results on controlling the absorption, emission, and ISC by tuning the VDAmagnitudes between the donor and acceptor will be useful for molecular design of compact electron donor/acceptor dyads.

Compact electron donor/acceptor dyads were prepared, with perylenemonoimide (PMI) as the electron acceptor and phenothiazine (PTZ) as the electron donor, to study the relationship between the molecular geometry and the spin–orbit charge transfer (CT) intersystem crossing (SOCT-ISC) efficiency. The photophysical properties of the dyads were studied with steady-state and time-resolved optical and magnetic resonance spectroscopies. We found that PTZ is an ideal chromophore for molecular conformation restriction in the compact dyads, which exerts a significant effect on the electronic coupling between the donor and acceptor and thus on the photophysical properties such as UV–vis absorption, fluorescence, and ISC efficiency. Anomalous intensified fluorescence at elevated temperatures was observed, which can be fully rationalized by the bright twisted charge transfer (CT) state. The singlet oxygen quantum yields (ΦΔ) of the dyads are up to 57%, which are highly solvent-polarity-dependent. Femtosecond transient absorption and fluorescence spectroscopies indicate that the charge separation proceeds within 1 ps and the charge-recombination-induced ISC takes 2.8 ns. With time-resolved electron paramagnetic resonance spectroscopy, we confirmed the SOCT-ISC mechanism of the dyad, for which the electron spin polarization and the population rates of the sublevels of the triplet state (zero-field states TX, TYand TZ) are drastically different from those of the spin–orbit-coupling-induced ISC (for monobrominated PMI). Photodriven intermolecular energy-transfer and electron-transfer processes in the presence of a triplet energy acceptor or a sacrificial electron donor were also studied. The radical anion of PMI was reversibly produced with photoreduction for which near-IR absorption band in the range of 750–850 nm was observed.

The ultrafast transient dynamics of a free-base tetraphenylporphyrin (H2TPP) surface-mounted metal organic framework (SURMOF) is spectroscopically characterized and compared with that of H2TPP in ethanol solution. A dramatic increase of the quantum yield of the Soret band fluorescence (S2→ S0) is observed in the H2TPP–SURMOF, which is accompanied by an increase of the S2fluorescence lifetime and a broadening of the fluorescence spectra. In contrast, the Q band (S1) fluorescence is strongly quenched in the H2TTP SURMOF compared with the fluorescence of H2TTP in ethanol solution. The observed transient absorption kinetics supports the fluorescence data. The enhanced fluorescence in the SURMOF compared with H2TPP in solution is attributed to the combination of two factors: the alignment of the vibrational energy levels of the dominant Franck–Condon active mode of the Soret and Q bands and the weaker coupling of these modes to the environment.

The charge carrier photogeneration yield in hybrid polymer/nanocrystal solar cells strongly depends on the interplay between charge transfer across quantum dot (QD) organic capping layers and quantum confinement effects related to the QD size. Here we combine femtosecond transient spectroscopy and density functional theory (DFT) calculations to improve the understanding of charge transfer dynamics at P3HT/InP QD heterointerfaces as a function of core size (2.5 vs 4.5 nm) and length of the surface ligands (oleylamine vs pyridine). We find that, for large core QDs, the polaron generation yield in P3HT is enhanced by efficient exciton dissociation and charge transfer, and is limited by the length of the ligands. Conversely, for smaller size QDs, electron injection from P3HT to InP cores becomes inefficient due to the unfavorable interfacial energetics, even with short pyridine ligands. Thus, we suggest that both QD surface ligand functionalization and core size should be optimized simultaneously for the design of high-performance hybrid nanocrystal/polymer solar cells.

Singlet fission (SF), the spin-allowed process of converting one singlet exciton into two triplet excitons, holds promise for increasing the efficiency of solar cells. Other processes, namely, excimer formation, are regarded as a competing channel of relaxation for SF. In this work, a film of perylene derivative, 2,5,8,11-tetra-tert-butylperylene (TBPe), was prepared to investigate the influence of four tert-butyl groups on the film molecular packing and SF efficiency. In the TBPe film, excimer formation was found to be fully suppressed. Moreover, SF takes place from the upper vibrational states of S1, reaching the highest triplet quantum yield of 1.85. Quantum chemical calculations reveal that the spatial configuration of perylene aggregation plays a key role in tuning the ratio between excimer formation and SF. When the steric hindrance of the tert-butyl groups leads to a slip-stacking spatial arrangement for dimers, it blocks the competing excimer generation and enhances the SF process.