Excited‐State Dynamics of Isolated DNA Bases: A Case Study of Adenine

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the ππ* band. Drawing on our femtosecond time‐resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9‐methyladenine and consider the relative merits of the various proposed mechanisms.     

Excited‐State Dynamics of Pentacene Derivatives with Stable Radical Substituents

The excited‐state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited‐state characteristics of the derivatives.     

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

3′,5′‐Dimethoxybenzoin (DMB) is a bichromophoric system that has widespread application as a highly efficient photoremovable protecting group (PRPG) for the release of diverse functional groups. The photodeprotection of DMB phototriggers is remarkably clean, and is accompanied by the formation of a biologically benign cyclization product, 3′,5′‐dimethoxybenzofuran (DMBF). The underlying mechanism of the DMB deprotection and cyclization has, however, until now remained unclear. Femtosecond transient absorption (fs‐TA) spectroscopy and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopy were employed to detect the transient species directly, and examine the dynamic transformations involved in the primary photoreactions for DMB diethyl phosphate (DMBDP) in acetonitrile (CH₃CN). To assess the electronic character and the role played by the individual sub‐chromophore, that is, the benzoyl, and the di‐meta‐methoxybenzylic moieties, for the DMBDP deprotection, comparative fs‐TA measurements were also carried out for the reference compounds diethyl phosphate acetophenone (DPAP), and 3′,5′‐dimethoxybenzylic diethyl phosphate (DMBnDP) in the same solvent. Comparison of the fs‐TA spectra reveals that the photoexcited DMBDP exhibits distinctly different spectral character and dynamic evolution from those of the reference compounds. This fact, combined with the related steady‐state spectral and density functional theoretical results, strongly suggests the presence in DMBDP of a significant interaction between the two sub‐chromophores, and that this interaction plays a governing role in determining the nature of the photoexcitation and the reaction channel of the subsequent photophysical and photochemical transformations. The ns‐TR³ results and their correlation with the fs‐TA spectra and dynamics provide evidence for a novel concerted deprotection–cyclization mechanism for DMBDP in CH₃CN. By monitoring the direct generation of the transient DMBF product, the cyclization time constant was determined unequivocally to be ≈1 ns. This indicates that there is little relevance for the long‐lived intermediates (>10 ns) in giving the DMBF product, and excludes the stepwise mechanism proposed in the literature as the major pathway for the DMB cyclization reaction. This work provides important new insights into the origin of the 3′,5′‐dimethoxy substitution effect for the DMB photodeprotection. It also helps to clarify the many different views presented in previous mechanistic studies of the DMB PRPGs. In addition to this, our fs‐TA results on the reference compound DMBnDP in CH₃CN provide the first direct observation (to the best of our knowledge) showing the predominance of a prompt (≈2 ps) heterolytic bond cleavage after photoexcitation of meta‐methoxybenzylic compounds. This provides insight into the long‐term controversies about the photoinitiated dissociation mode of related substituted benzylic compounds.     

Excited‐State Proton‐Relay Dynamics of 7‐Hydroxyquinoline Embedded in a Solid Matrix of Poly(2‐hydroxyethyl methacrylate)

Excited 7‐hydroxyquinoline embedded in a solid matrix of poly(2‐hydroxyethyl methacrylate) undergoes a proton‐relay reaction efficiently to form its keto tautomer. However, the reaction mechanism depends on the torsional conformation and the microscopic environment of the molecule at the moment of excitation. Whereas the bridged cis‐enol form undergoes proton relay immediately on absorption of a photon to produce its tautomeric keto species, the unbridged cis form requires 120 ps for bridge formation via solvent reorganization prior to proton relay. Furthermore, the trans form needs 1000 ps for tautomerization because it requires an activated (11 kJ mol^?1) torsional motion to change into its cis form prior to bridge formation and proton relay. Torsional motion rather than solvent reorganization determines the proton relay rate of the trans‐form of the molecule.     

Folding‐Induced Modulation of Excited‐State Dynamics in an Oligophenylene–Ethynylene‐Tethered Spiral Perylene Bisimide Aggregate

The excited‐state photophysical behavior of a spiral perylene bisimide (PBI) folda‐octamer ( F8 ) tethered to an oligophenylene–ethynylene scaffold is comprehensively investigated. Solvent‐dependent UV/Vis and fluorescence studies reveal that the degree of folding in this foldamer is extremely sensitive to the solvent, thus giving rise to an extended conformation in CHCl₃ and a folded helical aggregate in methylcyclohexane (MCH). The exciton‐deactivation dynamics are largely governed by the supramolecular structure of F8 . Femtosecond transient absorption (TA) in the near‐infrared region indicates a photoinduced electron‐transfer process from the backbone to the PBI core in the extended conformation, whereas excitation power‐ and polarization‐dependent TA measurements combined with computational modeling showed that excitation energy transfer between the unit PBI chromophores is the major deactivation pathway in the folded counterpart.     

Ultrafast Dynamics of o‐Bromofluorobenzene Studied by Time‐Resolved Photoelectron Imaging

Photodissociation dynamics and rotational wave packet coherences of o‐bromofluorobenzene are studied by femtosecond time‐resolved photoelectron imaging (see figure). The decay of different photoelectron rings shows the population decay of states from which the lifetimes of different states are determined. The variation of photoelectron angular distributions reflects the evolution of rotational coherences.

     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.     

Sub‐Picosecond Singlet Exciton Fission in Cyano‐Substituted Diaryltetracenes

Thin films of 5,11‐dicyano‐6,12‐diphenyltetracene ( TcCN ) have been studied for their ability to undergo singlet exciton fission (SF). Functionalization of tetracene with cyano substituents yields a more stable chromophore with favorable energetics for exoergic SF (2E(T₁)?E(S₁)=?0.17 eV), where S₁ and T₁ are singlet and triplet excitons, respectively. As a result of tuning the triplet‐state energy, SF is faster in TcCN relative to the corresponding endoergic process in tetracene. SF proceeds with two time constants in the film samples (τ=0.8±0.2 ps and τ=23±3 ps), which is attributed to structural disorder within the film giving rise to one population with a favorable interchromophore geometry, which undergoes rapid SF, and a second population in which the initially formed singlet exciton must diffuse to a site at which this favorable geometry exists. A triplet yield analysis using transient absorption spectra indicates the formation of 1.6±0.3 triplets per initial excited state.     

Light Harvesting and Efficient Energy Transfer in Dendritic Systems: New Strategy for Functionalized Near‐Infrared BF2‐Azadipyrromethenes

Bright funnels : A series of dendritic systems, which are capable of funneling energy from the periphery to the core, have been synthesized. The photophysical properties of dendrimers have been determined. Selective excitation of the donor leads to an efficient energy transfer (>90 %) to the acceptor. The approach provides a facile synthesis for the modification of near‐infrared BF₂‐Azadipyrromethenes.

     

Studies on the Solvation Dynamics of Coumarin 153 in 1‐Ethyl‐3‐Methylimidazolium Alkylsulfate Ionic Liquids: Dependence on Alkyl Chain Length

Steady‐state and time‐resolved fluorescence behavior of coumarin 153 (C153) is investigated in a series of 1‐ethyl‐3‐methylimidazolium alkylsulfate ([C₂mim][C_nOSO₃]) ionic liquids differing only in the length of the linear alkyl chain (n=4, 6, and 8) in the anion. The aim of the present study is to understand the role of alkyl chain length in solute rotation and solvation dynamics of C153 in these ionic liquids. The blueshift observed in the steady‐state absorption and emission maxima of C153 on going from the C₄OSO₃ to the C₈OSO₃ system indicates increasing nonpolar character of the microenvironment of the solute with increasing length of the alkyl side chain of the anion of the ionic liquids. The average solvation time is also found to increase on changing the substituent from butyl to octyl, and this is attributed to the increase in the bulk viscosity of the ILs. A steady blueshift of the time‐zero maximum of the fluorescence spectrum with increasing alkyl chain length also indicates that the probe molecule experiences a less polar environment in the early part of the dynamics. Rotational dynamics of C153 are also analyzed by using the Stokes–Einstein–Debye (SED), Gierer–Wirtz (GW), and Dote–Kivelson–Schwartz (DKS) theories. Analyses of the results seem to suggest decoupling of the rotational motion of the probe from solvent viscosity.     

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern,Photophysical Studies,and Enantioselective Catalysis by a Chiral Sensitizer

The intramolecular [2+2] photocycloaddition of four 4‐(but‐3‐enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH₂, Z‐CHEt, E‐CHEt, CMe₂) and two 3‐(but‐3‐enyl)oxyquinolones (substitution pattern: CH₂, CMe₂) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83–95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z‐ or E‐substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio‐ and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which–in contrast to simple alkoxyquinolones such as 3‐butyloxyquinolone and 4‐methoxyquinolone–decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4‐diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3‐(but‐3‐enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at ?25 °C, presumably because a significant racemic background reaction occurred. The 4‐substituted quinolones showed higher enantioselectivities (92–96 % ee at ?25 °C) and, for the terminally Z‐ and E‐substituted substrates, an improved regio‐ and diastereoselectivity.     

Watson–Crick Base Pairing Controls Excited‐State Decay in Natural DNA

Excited‐state dynamics are essential to understanding the formation of DNA lesions induced by UV light. By using femtosecond IR spectroscopy, it was possible to determine the lifetimes of the excited states of all four bases in the double‐stranded environment of natural DNA. After UV excitation of the DNA duplex, we detected a concerted decay of base pairs connected by Watson–Crick hydrogen bonds. A comparison of single‐ and double‐stranded DNA showed that the reactive charge‐transfer states formed in the single strands are suppressed by base pairing in the duplex. The strong influence of the Watson–Crick hydrogen bonds indicates that proton transfer opens an efficient decay path in the duplex that prohibits the formation or reduces the lifetime of reactive charge‐transfer states.     

Ultrafast Spectroscopy of Hydroxy‐Substituted Azobenzenes in Water

Ultrafast UV/Vis pump/probe experiments on ortho‐, meta‐ and para‐hydroxy‐substituted azobenzenes (HO‐ABs), as well as for sulfasalazine, an AB‐based drug, were performed in aqueous solution. For meta‐HO‐AB, AB‐like isomerisation behaviour can be observed, whereas, for ortho‐HO‐AB, fast proton transfer occurs, resulting in an excited keto species. For para‐HO‐AB, considerable keto/enol tautomerism proceeds in the ground state, so after excitation the trans‐keto species isomerises into the cis form. Aided by TD‐DFT calculations, insight is provided into different deactivation pathways for HO‐AB, and reveals the role of hydroxy groups in the photochemistry of ABs, as well as their acetylation regarding sulfasalazine. Hydroxy groups are position‐specific substituents for AB, which allow tuning of the timescale of thermal relaxation, as well as the amount and contribution of the keto species to photochemical processes.     

Wavepacket Splitting and Two‐Pathway Deactivation in the Photoexcited Visual Pigment Isorhodopsin

Isorhodopsin is the visual pigment analogue of rhodopsin. It shares the same opsin environment but it embeds 9‐cis retinal instead of 11‐cis. Its photoisomerization is three times slower and less effective. The mechanistic rationale behind this observation is revealed by combining high‐level quantum‐mechanical/molecular‐mechanical simulations with ultrafast optical spectroscopy with sub‐20 fs time resolution and spectral coverage extended to the near‐infrared. Whereas in rhodopsin the photoexcited wavepacket has ballistic motion through a single conical intersection seam region between the ground and excited states, in isorhodopsin it branches into two competitive deactivation pathways involving distinct conical intersection funnels. One is rapidly accessed but unreactive. The other is slower, as it features extended steric interactions with the environment, but it is productive as it follows forward bicycle pedal motion.     

Ultrafast Twisting Dynamics of Thioflavin‐T: Spectroscopy of the Twisted Intramolecular Charge‐Transfer State

Understanding the excited‐state properties of thioflavin‐T (ThT) has been of immense importance, because of its efficient amyloid‐sensing ability related to neurodegenerative disorders. The excited‐state dynamics of ThT is studied by using sub‐pico‐ and nanosecond time‐resolved transient absorption techniques as well as density functional theory (DFT)/time‐dependent DFT calculations. Barrierless twisting around the central C?C bond between two aromatic moieties is the dominant process that contributes to the ultrafast dynamics of the S₁ state. The spectroscopic properties of the intramolecular charge‐transfer state are characterized for the first time. The energetics of the S₀ and S₁ states has also been correlated with the experimentally observed spectroscopic parameters and structural dynamics. A longer‐lived transient state populated with a very low yield has been characterized as the triplet state.