Characterization of Excimer Relaxation via Femtosecond Shortwave- and Mid-Infrared Spectroscopy

Excimer formation plays a significant role in trapping excitons within organic molecular solids. Covalent dimers of perylene-3,4:9,10-bis­(dicarboximide) (PDI) are useful model systems for studying these processes as their intermolecular geometries can be precisely tuned. Using femtosecond visible-pump infrared-probe (fsIR) spectroscopy in the shortwave- and mid-infrared regions, we characterize two PDI dimers with a cofacial and a slip-stacked geometry that are coupled through a triptycene bridge. In the mid-infrared region, fsIR spectra for the strongly coupled dimers are highly blue-shifted compared to spectra for monomeric ^1*PDI. The perylene core stretching modes provide a directly observable probe of excimer relaxation, as they are particularly sensitive to this process, which is associated with a small blue shift of these modes in both dimers. The broad Frenkel-to-CT state electronic transition of the excimer, the edge of which has previously been detected in the NIR region, is now fully resolved to be much broader and to extend well into the shortwave infrared region for both dimers and is likely a generic feature of π-extended aromatic excimers

Optical Initialization of Molecular Qubit Spin States Using Weak Exchange Coupling to Photogenerated Fullerene Triplet States

The ability to initialize an electron spin qubit into a well-defined state is an important criterion for quantum information applications. To achieve this goal, a chromophore photoexcited to its triplet state is used to strongly spin polarize a nearby stable radical in a series of C60 fullerene derivatives containing a covalently linked α,γ-bisdiphenylene-β-phenylallyl (BDPA) radical. Selective photoexcitation of C60 results in up to 20-fold enhancement of the BDPA spin polarization observed by pulse electron paramagnetic resonance spectroscopy at room temperature. The sign of the spin polarization depends on the nature of the molecular spacer between C60 and BDPA. In addition, transient absorption spectroscopy and pulse-EPR measurements reveal that the BDPA spin polarization is derived from spin polarization transfer from the C60 triplet state by weak exchange coupling over a 1 nm distance

Effects of Excitation Energy on the Autodetachment Lifetimes of Small Iodide-Doped ROH Clusters (RH–, CH₃–, CH₃CH₂−)

The effect of excitation energy on the lifetimes of the charge-transfer-to-solvent (CTTS) states of small (4 ≤ <i>n</i> ≤ 10) iodide-doped water and alcohol clusters was explored using femtosecond time-resolved photoelectron imaging. Excitation of the CTTS state at wavelengths ranging from 272 to 238 nm leads to the formation of the I···(ROH)_<i>n</i>^– (RH–, CH₃–, and CH₃CH₂−) species, which can be thought of as a vibrationally excited bare solvent cluster anion perturbed by an iodine atom. Autodetachment lifetimes for alcohol-containing clusters range from 1 to 71 ps, while water clusters survive for hundreds of ps in this size range. Autodetachment lifetimes were observed to decrease significantly with increasing excitation energy for a particular number and type of solvent molecules. The application of Klots’ model for thermionic emission from clusters to I^–(H₂O)₅ and I^–(CH₃OH)₇ qualitatively reproduces experimental trends and reveals a high sensitivity to energy parametrization while remaining relatively insensitive to the number of vibrational modes. Experimental and computational results therefore suggest that the rate of electron emission is primarily determined by the energetics of the cluster system rather than by details of molecular structure

Wirelike Charge Transport Dynamics for DNA–Lipid Complexes in Chloroform

The dynamics of charge separation and charge recombination have been determined for lipid complexes of DNA capped hairpins possessing stilbene electron-acceptor and -donor chromophores separated by base-pair domains that vary in length and base sequence in chloroform solution by means of femtosecond time-resolved transient absorption spectroscopy. The results obtained for the DNA–lipid complexes are compared with those previously obtained in our laboratories for the same hairpins in aqueous buffer. The charge separation and charge recombination times for the lipid complexes are consistently much shorter than those determined in aqueous solution and are only weakly dependent on the number of base pairs separating the acceptor and donor. The enhanced rate constants for forward and return charge transport in DNA–lipid complexes support proposals that solvent gating is responsible, to a significant extent, for the relatively low rates of charge transport for DNA in water. Moreover, they suggest that DNA–lipid complexes may prove useful in the development of DNA-based molecular electronic devices

Dynamics of Charge Injection and Charge Recombination in DNA Mini-Hairpins

Steady state spectroscopy, femtosecond transient absorption spectroscopy (fsTA), and femtosecond stimulated Raman spectroscopy (FSRS) of DNA mini-hairpins possessing a diphenylacetylenedicarboxamide (DPA) linker and 1–3 adenine–thymine (A-T) or guanine–cytosine (G-C) base pairs have been investigated. Ultraviolet and circular dichroism (UV and CD) spectra are consistent with ground state conformations that are predominantly base-paired and π-stacked for conjugates possessing two or three base pairs; however, they offer no information concerning the conformation of conjugates possessing a single base pair. fsTA spectra are indicative of π-stacked structures excepted in the case of the conjugate possessing a single G-C base pair. All of the conjugates display transient absorption bands characteristic of the DPA^–. anion radical. Conjugates possessing two or three G-C base pairs display a transient absorption band characteristic of the short-lived G^+• cation radical. The mini-hairpins with 1–3 A-T base pairs do not display the transient absorption band characteristic of the (A_n^+•) polaron. This implies that an A-tract with three base pairs is too short to support polaron formation

Direct Observation of Ultrafast Excimer Formation in Covalent Perylenediimide Dimers Using Near-Infrared Transient Absorption Spectroscopy

Energy transfer in perylene-3,4:9,10-bis­(dicarboximide) (PDI) aggregates is often limited by formation of a low-energy excimer state. Formation dynamics of excimer states are often characterized by line shape changes and peak shift dynamics in femtosecond visible transient absorption spectra. Femtosecond near-infrared transient absorption experiments reveal a unique low-energy transition that can be used to identify and characterize this state without overlapping excited singlet-state absorption. Three covalently bound PDI dimers with differing PDI–PDI distances were studied to probe the influence of interchromophore electronic coupling on the PDI excimer transient spectra and dynamics

Efficient Charge Transport via DNA G‑Quadruplexes

The dynamics and efficiency of photoinduced charge transport has been investigated in DNA capped hairpins possessing a stilbenedicarboxamide (Sa) hole donor and stilbenediether (Sd) hole acceptor separated by DNA G-quadruplex structures possessing 2-to-4 tetrads by means of femtosecond and nanosecond transient absorption spectroscopy with global analysis. The results for the quadruplex structures are compared with those for the corresponding duplex structures having G-C base pairs in place of the G-tetrads. Following photoinduced charge separation to form a contact radical ion pair, hole transport to form the Sa^–•/Sd^+• charge-separated state is slower but more efficient for the quadruplex vs duplex structures. Thus, the G-quadruplex serves as an effective conduit for positive charge rather than as a hole trap when inserted into a duplex, as previously postulated

Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State Using Femtosecond Mid-Infrared Spectroscopy

Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, ¹(S₁S₀), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, ¹(T₁T₁), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis­(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (<b>XanTDI</b>_<b>2</b>). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton ¹(T₁T₁) state has absorptions characteristic of the T₁ state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the CC stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CH₂Cl₂ and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character

Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers

The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1–25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τ_CR = 72–85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore–catalyst systems for solar fuel production that utilize multiple perylene chromophores

Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy

Facile exciton transport within ordered assemblies of π-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (<b>ExV</b>^<b>2+</b>) chromophores is varied systematically using a pair of <i>o</i>-, <i>m</i>-, or <i>p</i>-xylylene (<i><b>o</b></i>-, <i><b>m</b></i>-, or <i><b>p</b></i><b>-Xy</b>) covalent linkers to produce <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> (3.5 Å), <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> (5.6 Å), and <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> and <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), the lowest excited singlet state of <b>ExV</b>^<b>2+</b> exclusively photo-oxidizes the <i><b>p</b></i><b>-Xy</b> spacer to give the <i><b>p</b></i><b>-Xy</b>^<b>+•</b>-<b>ExV</b>^<b>+•</b> ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character