Self-Assembly-Induced Ultrafast Photodriven Charge Separation in Perylene-3,4-dicarboximide-Based Hydrogen-Bonded Foldamers

We report the synthesis, self-assembly characteristics, and ultrafast electron transfer dynamics of a perylene-3,4-dicarboximide (PMI) covalently linked to an <i>N,N′</i>-bis­(3,4,5-tridodecyloxyphenyl)­melamine electron donor (D) via a biphenyl spacer (PMI-Ph₂-D). Synchrotron-based small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in methylcyclohexane solution show that PMI-Ph₂-D self-assembles into π–π stacked, hydrogen-bonded foldamers consisting of two or three hexameric rings or helices. Ultrafast transient absorption spectroscopy reveals that photoinduced charge separation within these nanostructures occurs by a unique pathway that is emergent in the assembly, whereas electron transfer does not occur in the PMI-Ph₂-D monomers in tetrahydrofuran

Interrogating the Intramolecular Charge-Transfer State of a Julolidine−Anthracene Donor−Acceptor Molecule with Femtosecond Stimulated Raman Spectroscopy

The nature of the lowest-energy charge-transfer (CT) excited state of the donor−acceptor molecule, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ−An) is investigated using femtosecond stimulated Raman spectroscopy. Transient Raman spectra are presented with subpicosecond time resolution, and peaks are assigned based on the published Raman modes of the reference molecule phenyl anthracene. The results indicate that the CT excited state is dominated by a fully charge separated radical ion pair state with minimal contribution from the local anthracene π−π* state

Vibrational Dynamics of a Perylene–Perylenediimide Donor–Acceptor Dyad Probed with Femtosecond Stimulated Raman Spectroscopy

The ultrafast vibrational dynamics of the photoinduced charge-transfer reaction between perylene (Per) and perylene-3,4:9,10-bis­(dicarboximide) (PDI) were investigated using femtosecond stimulated Raman spectroscopy (FSRS). Specifically probing the structural dynamics of PDI following its selective photoexcitation in a covalently linked dyad reveals vibrational modes uniquely characteristic to the PDI lowest excited singlet state and radical anion between 1000 and 1700 cm^–1. A comparison of these vibrations to those of the ground state reveals the appearance of new ^1*PDI and PDI^–• stretching modes in the dyad at 1593 and 1588 cm^–1, respectively. DFT calculations reveal that these vibrations are parallel to the long axis of PDI and thus then may be integral to the charge separation reaction. The ability to differentiate excited state from radical anion vibrational modes allows the evaluation of the influence of specific modes on the charge transfer dynamics in donor–bridge–acceptor systems based on PDI molecular constructs

Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor–bridge–Acceptor Molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor–bridge–acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ–An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), <b>1</b> and <b>2</b>, respectively, or naphthalene-1,8:4,5-bis­(dicarboximide) (NI) acceptors, <b>3</b> and <b>4</b>, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ^+•–PI^–• or DMJ^+•–NI^–• quantitatively in <b>1</b>–<b>4</b> (τ_CS ≤ 10 ps), and that charge recombination occurs with τ_CR = 268 and 158 ps for <b>1</b> and <b>3</b>, respectively, and with τ_CR = 2.6 and 10 ns for <b>2</b> and <b>4</b>, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (<i>2J</i>) between DMJ^+• and PI^–• or NI^–•, giving 2<i>J</i> > 600 mT for <b>1</b>–<b>3</b> and 2<i>J</i> = 170 mT for <b>4</b>. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of ³*An upon charge recombination occurs by spin–orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2<i>J</i> determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in <b>1</b>–<b>3</b>, whereas both RP-ISC and SOCT-ISC are active for <b>4</b>. The triplet sublevels populated by SOCT-ISC in <b>1</b>–<b>4</b> depend on the donor–acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum

Electron Transfer within Self-Assembling Cyclic Tetramers Using Chlorophyll-Based Donor–Acceptor Building Blocks

The synthesis and photoinduced charge transfer properties of a series of Chl-based donor–acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll <i>a</i> was converted into zinc methyl 3-ethylpyrochlorophyllide <i>a</i> (Chl) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis­(dicarboximide) (NDI) secondary electron acceptors to give Chl–PI–NDI and Chl–PI–NDI₂. The pyridine ligand within each ambident triad enables intermolecular Chl metal–ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene–1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from ^1*Chl to PI to form Chl^+•–PI^–•–NDI and Chl^+•–PI^–•–NDI₂. This initial charge separation is followed by a rapid charge shift from PI^–• to NDI and subsequent charge recombination of Chl^+•–PI–NDI^–• and Chl^+•–PI–(NDI)­NDI^–• on a 5–30 ns time scale. Charge recombination in the Chl–PI–NDI₂ cyclic tetramer (τ_CR = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τ_CR = 10 ± 1 ns in toluene–1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems

Direct Observation of Nanoparticle–Cancer Cell Nucleus Interactions

We report the direct visualization of interactions between drug-loaded nanoparticles and the cancer cell nucleus. Nanoconstructs composed of nucleolin-specific aptamers and gold nanostars were actively transported to the nucleus and induced major changes to the nuclear phenotype <i>via</i> nuclear envelope invaginations near the site of the construct. The number of local deformations could be increased by ultrafast, light-triggered release of the aptamers from the surface of the gold nanostars. Cancer cells with more nuclear envelope folding showed increased caspase 3 and 7 activity (apoptosis) as well as decreased cell viability. This newly revealed correlation between drug-induced changes in nuclear phenotype and increased therapeutic efficacy could provide new insight for nuclear-targeted cancer therapy

Ultrafast Conformational Dynamics of Electron Transfer in ExBox⁴⁺⊂Perylene

Multielectron acceptors are essential components for artificial photosynthetic systems that must deliver multiple electrons to catalysts for solar fuels applications. The recently developed boxlike cyclophane incorporating two extended viologen units joined end-to-end by two <i>p</i>-phenylene linkersnamely, <b>ExBox⁴⁺</b>has a potential to be integrated into light-driven systems on account of its ability to complex with π-electron-rich guests such as perylene, which has been utilized to great extent in many light-harvesting applications. Photodriven electron transfer to <b>ExBox⁴⁺</b> has not previously been investigated, however, and so its properties, following photoreduction, are largely unknown. Here, we investigate the structure and energetics of the various accessible oxidation states of <b>ExBox⁴⁺</b> using a combination of spectroscopy and computation. In particular, we examine photoinitiated electron transfer from perylene bound within <b>ExBox⁴⁺</b> (<b>ExBox⁴⁺</b>⊂perylene) using visible and near-infrared femtosecond transient absorption (fsTA) spectroscopy. The structure and conformational relaxation dynamics of <b>ExBox³⁺</b>⊂perylene⁺ are observed with femtosecond stimulated Raman spectroscopy (FSRS). From the fsTA and FSRS spectra, we observe that the central <i>p</i>-phenylene spacer in one of the extended viologen units on one side of the cyclophane becomes more coplanar with its neighboring pyridinium units over the first ∼5 ps after photoreduction. When the steady-state structure of chemically generated <b>ExBox²⁺</b> is investigated using Raman spectroscopy, it is found to have the central <i>p</i>-phenylene rings in both of its extended viologen units rotated to be more coplanar with their neighboring pyridinium units, further underscoring the importance of this subunit in the stabilization of the reduced states of <b>ExBox⁴⁺</b>

Plasmonic Bowtie Nanolaser Arrays

Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors

Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

Donor–bridge–acceptor (D–B–A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)­julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis­(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN_<i>n</i>) bridges (<i>n</i> = 1–3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ^+•-An^–•, and An^–• acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ^+•-An^–•–FN_<i>n</i>–NI → DMJ^+•-An–FN_<i>n</i>^–•–NI → DMJ^+•-An–FN_<i>n</i>–NI^–•. The charge-shift reaction from An^–• to NI^–• exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å^–1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN_<i>n</i>^–• radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D^+•–B^–•–A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D–B–A molecule

Ultrafast Two-Electron Transfer in a CdS Quantum Dot–Extended-Viologen Cyclophane Complex

Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis­(4,4′-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis­(methylene)) (<b>ExBox</b>^<b>4+</b>) to form both the <b>ExBox</b>^<b>3+•</b> and the doubly reduced <b>ExBox</b>^{<b>2(+•</b>)} states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced <b>ExBox</b>^{<b>2(+•</b>)} state has a lifetime of ∼10 ns, while CdS^+<b>•</b>:<b>ExBox</b>^<b>3+•</b> recombines with multiple time constants, the longest of which is ∼300 μs. The long-lived charge separation and ability to accumulate multiple charges on <b>ExBox</b>^<b>4+</b> demonstrate the potential of the CdS:<b>ExBox</b>^<b>4+</b> complex to serve as a platform for two-electron photocatalysis