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The driving of rapid polymerizations with visible to near-infrared light will enable nascent technologies in the emerging fields of bio- and composite-printing. However, current photopolymerization strategies are limited by long reaction times, high light intensities, and/or large catalyst loadings. The improvement of efficiency remains elusive without a comprehensive, mechanistic evaluation of photocatalysis to better understand how composition relates to polymerization metrics. With this objective in mind, a series of methine- and aza-bridged boron dipyrromethene (BODIPY) derivatives were synthesized and systematically characterized to elucidate key structure–property relationships that facilitate efficient photopolymerization driven by visible to far-red light. For both BODIPY scaffolds, halogenation was shown as a general method to increase polymerization rate, quantitatively characterized using a custom real-time infrared spectroscopy setup. Furthermore, a combination of steady-state emission quenching experiments, electronic structure calculations, and ultrafast transient absorption revealed that efficient intersystem crossing to the lowest excited triplet state upon halogenation was a key mechanistic step to achieving rapid photopolymerization reactions. Unprecedented polymerization rates were achieved with extremely low light intensities (<1 mW/cm2) and catalyst loadings (<50 μM), exemplified by reaction completion within 60 s of irradiation using green, red, and far-red light-emitting diodes. Halogenated BODIPY photoredox catalysts were additionally employed to produce complex 3D structures using high-resolution visible light 3D printing, demonstrating the broad utility of these catalysts in additive manufacturing.

An efficient implementation of the perturb-then-diagonalize nonorthogonal configuration interaction method with second-order Møller–Plesset perturbation theory (NOCI-MP2) is presented. Relative to other low scaling multireference perturbation theories, NOCI-MP2 often requires a much smaller active space because of the use of nonorthogonal reference configurations. Reworking the NOCI-MP2 equations with the resolution of the identity (RI) approximation enables the method to have the same memory requirements and computational scaling as single reference RI-MP2. The working equations are extended to include single substitutions as required when the reference determinants do not satisfy the Hartree–Fock equations. A detailed computational algorithm is presented along with timings to establish the performance of the implementation. NOCI-MP2 is applied to the binding energy and charge resonance energy in dication and monocation π dimers, as well as didiamantane ethane, and hexaphenylethane. A well-defined set of nonorthogonal determinants are obtained using absolutely localized molecular orbitals (ALMOs), as solutions to the self-consistent field for molecular interactions (SCF-MI) equations corresponding to covalent and ionic determinants. Agreement with experimental information where available, and other multireference methods, is satisfactory, with the use of an 0.3 au level shift to guard against large MP2 amplitudes. For didiamantane ethane and hexaphenylethane, large dispersion forces help stabilize the molecules despite the steric repulsion. By contrast, in the case of hexaphenylethane, the energy penalty from the geometric distortion of the fragments significantly weakens the bond.

Singlet exciton fission (SF), the conversion of one spin-singlet exciton (S1) into two spin-triplet excitons (T1), could provide a means to overcome the Shockley–Queisser limit in photovoltaics. SF as measured by the decay of S1has been shown to occur efficiently and independently of temperature, even when the energy of S1is as much as 200 meV less than that of 2T1. Here we study films of triisopropylsilyltetracene using transient optical spectroscopy and show that the triplet pair state (TT), which has been proposed to mediate singlet fission, forms on ultrafast timescales (in 300 fs) and that its formation is mediated by the strong coupling of electronic and vibrational degrees of freedom. This is followed by a slower loss of singlet character as the excitation evolves to become only TT. We observe the TT to be thermally dissociated on 10–100 ns timescales to form free triplets. This provides a model for ‘temperature-independent’ efficient TT formation and thermally activated TT separation.

Exciton dissociation at organic semiconductor interfaces is an important process for the design of future organic photovoltaic (OPV) devices, but at present, it is poorly understood. On the one hand, exciton breakup is very efficient in many OPVs. On the other, electron–hole pairs generated by an exciton should be bound by Coulombic attraction, and therefore difficult to separate in materials of such low dielectric. In this paper, we highlight several electrostatic effects that appear commonly at organic/organic interfaces. Using QM/MM simulations, we demonstrate that the electric fields generated in this fashion are large enough to overcome typical electron–hole binding energies and thus explain the high efficiencies of existing OPV devices without appealing to the existence of nonthermal (“hot”) carrier distributions. Our results suggest that the classical picture of flat bands at organic/organic interfaces is only qualitatively correct. A more accurate picture takes into account the subtle effects of electrostatics on interfacial band alignment.

Current bilayer organic photovoltaics cannot be made thick enough to absorb all incident solar radiation because of the short diffusion lengths (≈10 nm) of singlet excitons. Thus, the diffusion length sets an upper bound on the efficiency of these devices. By contrast, triplet excitons can have very long diffusion lengths (as large as 10 μm) in organic solids, leading some to speculate that triplet excitonic solar cells could be more efficient than their singlet counterparts. In this paper, we examine the nature of singlet and triplet exciton diffusion. We demonstrate that although there are fundamental physical upper bounds on the distance singlet excitons can travel by hopping, there are no corresponding limits on triplet diffusion lengths. This conclusion strongly supports the idea that triplet diffusion should be more controllable than singlet diffusion in organic photovoltaics. To validate our predictions, we model triplet diffusion by purely ab inito means in various crystals, achieving good agreement with experimental values. We further show that in at least one example (tetracene), triplet diffusion is fairly robust to disorder in thin films, as a result of the formation of semicrystalline domains and the high internal reorganization energy for triplet hopping. These results support the potential usefulness of triplet excitons in achieving maximum organic photovoltaic device efficiency.

We present an investigation of the band levels and charge transfer (CT) states at the interface between two organic semiconductors, metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), using a combined quantum mechanics/molecular mechanics (QM/MM) technique. Near the organic–organic interface, significant changes from the bulk, as large as 0.2 eV, are found in the excited state energies, ionization potentials, and electron affinities, due to differences in molecular packing and polarizabilities of the two molecules. The changes in the ionization potential and electron affinity cause the CT states at the interface to be on average higher in energy than fully separated charges in the bulk materials despite having a typical local binding energy of 0.15 eV. Furthermore, we find that thermal fluctuations can induce variations of up to 0.1 eV in the CT binding energy. These results suggest that it is possible for bound interfacial CT states to dissociate in a barrierless fashion without involving “hot” CT states. This observation has direct relevance to the design of more efficient organic photovoltaics.