More Like this

1. Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state

   https://doi.org/10.1039/D0SC04715E  

   Xu, Zhen; Climent, Clàudia; Brown, Christopher M.; Hean, Duane; Bardeen, Christopher J.; Casanova, David; Wolf, Michael O. (January 2021, Chemical Science)

   null (Ed.)

   Sulfur oxidation state is used to tune organic room temperature phosphorescence (RTP) of symmetric sulfur-bridged carbazole dimers. The sulfide-bridged compound exhibits a factor of 3 enhancement of the phosphorescence efficiency, compared to the sulfoxide and sulfone-bridged analogs, despite sulfone bridges being commonly used in RTP materials. In order to investigate the origin of this enhancement, temperature dependent spectroscopy measurements and theoretical calculations are used. The RTP lifetimes are similar due to similar crystal packing modes. Computational studies reveal that the lone pairs on the sulfur atom have a profound impact on enhancing intersystem crossing rate through orbital mixing and screening, which we hypothesize is the dominant factor responsible for increasing the phosphorescence efficiency. The ability to tune the electronic state without altering crystal packing modes allows the isolation of these effects. This work provides a new perspective on the design principles of organic phosphorescent materials, going beyond the rules established for conjugated ketone/sulfone-based organic molecules. 

   more » « less
2. Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin

   https://doi.org/10.1021/acs.jpca.4c03139  

   Dutta, Abhijit; Ghosh, Sujit Kumar; Mandal, Satyajit; Srinivasan, Varadharajan; Ramamurthy, Vaidhyanathan; Sen, Pratik (August 2024, The Journal of Physical Chemistry A)

   The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7- diethylaminothiocoumarin (TC1), a close analogue of the wellknown probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1’s potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule’s photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations. 

   more » « less
3. Ultrafast Excited State Dynamics of a Verdazyl Diradical System

   https://doi.org/10.3390/photochem4040025  

   Clark, Caitlyn; Pawłowski, Filip; Brook, David_J R; Grieco, Christopher (December 2024, Photochem)

   While the photophysics of closed-shell organic molecules is well established, much less is known about open-shell systems containing interacting radical pairs. In this work, we investigate the ultrafast excited state dynamics of a singlet verdazyl diradical system in solution using transient absorption (TA) spectroscopy for the first time. Following 510 nm excitation of the excitonic S0 → S1 transition, we detected TA signals in the 530–950 nm region from the S1 population that decayed exponentially within a few picoseconds to form a vibrationally hot S0* population via internal conversion. The dependence of the S1 decay rate on solvent and radical–radical distance revealed that the excited state possesses charge-transfer character and likely accesses the S0 state via torsional motion. The ultrafast internal conversion decay mechanism at play in our open-shell verdazyl diradicals is in stark contrast with other closed-shell, carbonyl-containing organic chromophores, which exhibit ultrafast intersystem crossing to produce long-lived triplet states as the major S1 decay pathway. 

   more » « less
4. Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor–bridge–acceptor compounds

   https://doi.org/10.1039/D0CP01235A  

   Li, Xiao; Valdiviezo, Jesús; Banziger, Susannah D.; Zhang, Peng; Ren, Tong; Beratan, David N.; Rubtsov, Igor V. (May 2020, Physical Chemistry Chemical Physics)

   Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor–acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N -isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the CC bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S 0 → S n ) depend strongly on the dihedral angle ( θ ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S 1 excited state in the planar conformations ( θ ∼ 0) but selects an S 2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S 2 → S 1 charge separation reaction. Unprecedented variation of the S 2 –S 1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20–60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics. 

   more » « less
5. DNA scaffold supports long-lived vibronic coherence in an indodicarbocyanine (Cy5) dimer

   https://doi.org/10.1039/D0SC01127D  

   Sohail, Sara H.; Otto, John P.; Cunningham, Paul D.; Kim, Young C.; Wood, Ryan E.; Allodi, Marco A.; Higgins, Jacob S.; Melinger, Joseph S.; Engel, Gregory S. (August 2020, Chemical Science)

   null (Ed.)

   Vibronic coupling between pigment molecules is believed to prolong coherences in photosynthetic pigment–protein complexes. Reproducing long-lived coherences using vibronically coupled chromophores in synthetic DNA constructs presents a biomimetic route to efficient artificial light harvesting. Here, we present two-dimensional (2D) electronic spectra of one monomeric Cy5 construct and two dimeric Cy5 constructs (0 bp and 1 bp between dyes) on a DNA scaffold and perform beating frequency analysis to interpret observed coherences. Power spectra of quantum beating signals of the dimers reveal high frequency oscillations that correspond to coherences between vibronic exciton states. Beating frequency maps confirm that these oscillations, 1270 cm −1 and 1545 cm −1 for the 0-bp dimer and 1100 cm −1 for the 1-bp dimer, are coherences between vibronic exciton states and that these coherences persist for ∼300 fs. Our observations are well described by a vibronic exciton model, which predicts the excitonic coupling strength in the dimers and the resulting molecular exciton states. The energy spacing between those states closely corresponds to the observed beat frequencies. MD simulations indicate that the dyes in our constructs lie largely internal to the DNA base stacking region, similar to the native design of biological light harvesting complexes. Observed coherences persist on the timescale of photosynthetic energy transfer yielding further parallels to observed biological coherences, establishing DNA as an attractive scaffold for synthetic light harvesting applications. 

   more » « less