Pd(II) biladienes have been developed over the last five years as non-aromatic oligotetrapyrrole complexes that support a rich triplet photochemistry. In this work, we have undertaken the first detailed photophysical interrogation of three homologous Pd(II) biladienes bearing different combinations of methyl- and phenyl-substituents on the frameworks' sp3-hybridized meso-carbon (i.e., the 10-position of the biladiene framework). These experiments have revealed unexpected excited-state dynamics that are dependent on the wavelength of light used to excite the biladiene. More specifically, transient absorption spectroscopy revealed that higher-energy excitation (λexc ∼ 350-500 nm) led to an additional lifetime (i.e., an extra photophysical process) compared to experiments carried out following excitation into the lowest-energy excited states (λexc = 550 nm). Each Pd(II) biladiene complex displayed an intersystem crossing lifetime on the order of tens of ps and a triplet lifetime of ∼20 μs, regardless of the excitation wavelength. However, when higher-energy light is used to excite the complexes, a new lifetime on the order of hundreds of ps is observed. The origin of the 'extra' lifetime observed upon higher energy excitation was revealed using density functional theory (DFT) and time-dependent DFT (TDDFT). These efforts demonstrated that excitation into higher-energy metal-mixed-charge-transfer excited states with high spin-orbit coupling to higher energy metal-mixed-charge-transfer triplet states leads to the additional excitation deactivation pathway. The results of this work demonstrate that Pd(II) biladienes support a unique triplet photochemistry that may be exploited for development of new photochemical schemes and applications.

In this work we untangle the ultrafast deactivation of high-energy excited states in four naphthalene-based azo dyes. Through systematic photophysical and computational study, we observed a structure-property relationship in which increasing the electron donating strength of the substituent leads to longer lived excited states in these organic dyes and faster thermal reversion from the cis to trans configuration. In particular, azo dyes 1-3 containing less electron donating substituents show three distinct excited-state lifetimes of ∼0.7-1.5 ps, ∼3-4 ps, and 20-40 ps whereas the most electron donating dimethyl amino substituted azo 4 shows excited-state lifetimes of 0.7 ps, 4.8 ps, 17.8 ps and 40 ps. While bulk photoisomerization of all four moieties is rapid, the cis to trans reversion lifetimes vary by a factor of 30 with τreversion decreasing from 276 min to 8 min with increasing electron donating strength of the substituent. In order to rationalize this change in photophysical behavior, we explored the excited-state potential energy surfaces and spin-orbit coupling constants for azo 1-4 through density functional theory. The increase in excited-state lifetime for 4 can be attributed to geometric and electronic degrees of freedom of the lowest energy singlet excited-state potential energy surface.

Chemical intuition and well-known design principles can typically be used to create ligand environments in transition metal complexes to deliberately tune reactivity for desired applications. However, intelligent ligand design does not always result in the expected outcomes. Herein we report the synthesis and characterization of a tricarbonyl rhenium (2,2'-bipyridine) 4-pyridylamidine, Re(4-Pam), complex with unexpected photophysical properties. Photoluminescence kinetics of Re(4-Pam) undergoes non-exponential decay, which can be deconvolved into two emission lifetimes. However, upon protonation of the amidine functionality of the 4-pyridylamidine to form Re(4-PamH), a single exponential decay is observed. To understand and rationalize these experimental observations, density functional theory (DFT) and time-dependent density functional theory (TDDFT) are employed. The symmetry or asymmetry of the protonated or deprotonated 4-pyridylamidine ligand, respectively, is the key factor in switching between one and two photoluminescence lifetimes. Specifically, rotation of the dihedral angle formed between the bipyridine and 4-Pam ligand leads to two rotamers of Re(4-Pam) with degenerate triplet- to ground-state transitions.

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp3-hybridized meso-carbon (the 10-position of the biladiene framework) was prepared and studied. In addition to a previously described Pd(II) biladiene complex bearing geminal dimethyl substituents a the 10-position (Pd[DMBil]), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents (Pd[MPBil1]) and geminal diphenyl groups(Pd[DPBil1]) were prepared and structurally characterized. Detailed electrochemical as well as steady-state and time-resolved spectroscopic experiments were undertaken to evaluate the influence of the substituents on the biladiene's tetrahedral meso-carbon. Although all three biladiene homologues are isostructural, Pd[MPBil1] and Pd[DPBil1] display more intense absorption profiles that shift slightly toward lower energies as geminal methyl groups are replaced by phenyl rings. All three biladiene homologues support a triplet photochemistry, and replacement of the geminal dimethyl substituents of Pd[DMBil1] (ΦΔ = 54%) with phenyl groups improves the ability of Pd[MPBil1] (ΦΔ = 76%) and Pd[DPBil1] (ΦΔ = 66%) to sensitize 1O2. Analysis of the excited-state dynamics of the Pd(II) biladienes by transient absorption spectroscopy shows that each complex supports a long-lived triplet excited-state (i.e., τ > 15 μs for each homologue) but that the ISC quantum yields (ΦT) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (ΦΔ) across the biladiene series considered in this work. The results of this study provide new insights to guide future development of biladiene based agents for PDT and other photochemical applications.

The initial kinetics and mechanisms of photo-induced charge transfer in photovoltaic materials are critical to the operation of fabricated devices. Despite the importance of charge transfer in the picosecond to nanosecond timescales, mechanistic understanding of these events is still limited. To address this challenge, a series of p-i-n junction samples that comprises fluorine-doped tin oxide (FTO)/TiO2/ZnS/Sb2S3/P3HT layers was prepared by atomic layer deposition (ALD). ALD allows for carefully controlled film thickness in samples that enable systematic evaluation of photo-induced charge-transfer kinetics by transient absorption spectroscopy (TAS). Sb2S3 serves as the intrinsic light absorber, P3HT is the hole acceptor, and TiO2 is the electron acceptor. An extremely thin, electron-blocking layer of ZnS was deposited between Sb2S3 and TiO2 varied in thickness by ALD to create a series of 20 samples that included (1) five different ZnS thicknesses (0, 2, 5, 10, and 15 ALD cycles) and (2) four combinations of layers, always including ZnS/Sb2S3, that built up to the completed stack. These mechanistic studies confirm our proposed mechanism for photo-induced electron and hole transfer and recombination in these p-i-n junction samples and provide predictive insights into the charge-transfer processes that may be most determinant in the operation of completed devices.

Antimony chalcogenides represent a family of materials of low toxicity and relative abundance, with a high potential for future sustainable solar energy conversion technology. However, solar cells based on antimony chalcogenides present open-circuit voltage losses that limit their efficiencies. These losses are attributed to several recombination mechanisms, with interfacial recombination being considered as one of the dominant processes. In this work, we exploit atomic layer deposition (ALD) to grow a series of ultrathin ZnS interfacial layers at the TiO2/Sb2S3 interface to mitigate interfacial recombination and to increase the carrier lifetime. ALD allows for very accurate control over the ZnS interlayer thickness on the ångström scale (0-1.5 nm) and to deposit highly pure Sb2S3. Our systematic study of the photovoltaic and optoelectronic properties of these devices by impedance spectroscopy and transient absorption concludes that the optimum ZnS interlayer thickness of 1.0 nm achieves the best balance between the beneficial effect of an increased recombination resistance at the interface and the deleterious barrier behavior of the wide-bandgap semiconductor ZnS. This optimization allows us to reach an overall power conversion efficiency of 5.09% in planar configuration.

Experimental and computational studies reveal mechanistic differences in the photophysics of an open- versus closed-shell π-conjugated redox pair.

The combination of oxide and heavier chalcogenide layers in thin film photovoltaics suffers limitations associated with oxygen incorporation and sulfur deficiency in the chalcogenide layer or with a chemical incompatibility which results in dewetting issues and defect states at the interface. Here, we establish atomic layer deposition (ALD) as a tool to overcome these limitations. ALD allows one to obtain highly pure Sb2S3 light absorber layers, and we exploit this technique to generate an additional interfacial layer consisting of 1.5 nm ZnS. This ultrathin layer simultaneously resolves dewetting and passivates defect states at the interface. We demonstrate via transient absorption spectroscopy that interfacial electron recombination is one order of magnitude slower at the ZnS-engineered interface than hole recombination at the Sb2S3/P3HT interface. The comparison of solar cells with and without oxide incorporation in Sb2S3, with and without the ultrathin ZnS interlayer, and with systematically varied Sb2S3 thickness provides a complete picture of the physical processes at work in the devices.

A series of rubrene derivatives were synthesized and the influence of the side group in enhancing photo-oxidative stability was evaluated. Photo-oxidation half-lives were determined via UV-vis absorption spectroscopy, which revealed thiophene containing derivatives to be the most stable species. The electron affinity of the compounds did not correlate with stability as previously reported in literature. Our work shows that shorter excited-state lifetimes result in increased photo-oxidative stability in these rubrene derivatives. These results confirm that faster relaxation kinetics out-compete the formation of reactive oxygen species that ultimately degrade linear oligoacenes. This report highlights the importance of using molecular design to tune excited-state lifetimes in order to generate more stable oligoacenes.

Three bodipy-based (BDP = 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) donor-acceptor dyads were designed and synthesized, and their ground-state and photophysical properties were systematically characterized. The electronic coupling between the BDP chromophore and an electron-donating carbazole (Carb) moiety was tuned by attachment via the meso and the beta positions on the BDP core, and through the use of various chemical linkers (phenyl and alkynyl) to afford mesoBDP-Carb, mesoBDP-phen-Carb, and betaBDP-alk-Carb. meso-Substituted dyads were found to retain ground-state absorption features of the unsubstituted BDP. However, variation of the linkage between the donor and acceptor moieties modulated the photophysical behavior of excited-state deactivation by controlling the rate of photoinduced internal charge transfer (ICT). The beta-substituted dyad dramatically tuned (red shifted) the absorption spectrum, while retaining desired features of the BDP, specifically stability and high extinction coefficients, however the ICT kinetics were accelerated compared to the meso-substituted dyads. Density functional theory (DFT) and time-dependent DFT (TDDFT) were carried out on the six potential dyads formed between BDP and Carb (attachment using the beta and meso positions for all three connections: direct, phenyl and alkynyl) to support the experimental observations. DFT and TDDFT showed molecular orbital density spread across the HOMO level only when attachment occurred through the beta position of BDP. In the meso-substituted BDP-Carb dyads, the molecular orbitals resembled those of the unsubstituted BDP. This work reveals several possible synthetic paradigms to tune photophysical properties with directed synthetic modifications and provides a mechanistic understanding of the ground- and excited- state behavior in these small-molecule donor-acceptor dyads.