Dynamics of the Formation of a Charge Transfer State in 1,2-Bis(9-anthryl)acetylene in Polar Solvents: Symmetry Reduction with the Participation of an Intramolecular Torsional Coordinate

Tracking Ultrafast Fluorescence Switch-On and Color-Tuned Dynamics in Acceptor-Donor-Acceptor Chromophore

Understanding how the conformational change of conjugated molecules with acceptor-donor-acceptor (A-D-A) architecture affects their physical and optoelectronic properties is critical for determining their ultimate performance in organic electronic devices. Here, we utilized femtosecond transient absorption, time-resolved upconversion photoluminescence spectroscopy, and tunable femtosecond-stimulated Raman spectroscopy, aided by quantum chemical calculations, to systematically investigate the excited state structural dynamics of the intramolecular charge transfer of the tetramethoxy anthracene-based fluorophore 2,3,6,7-tetramethoxy 9,10-dibenzaldehydeanthracene (AnDA) and its derivative 2,3,6,7-tetramethoxy 9,10-diphenylanthracene (TMDPAn) in chloroform. In the AnDA molecule, the tetramethoxy anthracene and benzaldehyde moieties exhibit a strong ability to donate and withdraw electrons. Upon photoexcitation, AnDA shows intriguing ultrafast fluorescence switch-on and red shift dynamics on charge transfer states, and the temporal evolution of AnDA recorded by ultrafast spectroscopy reveals a dynamic picture of two-step intramolecular charge transfer assisted by ultrafast conformational changes and solvation processes. Removing the aldehyde group from TMDPAn significantly decreases the electron pulling capacity of the phenyl unit and disables charge transfer characteristics.

The role of twisting in driving excited-state symmetry breaking and enhanced two-photon absorption in quadrupolar cationic pyridinium derivatives

Two symmetric quadrupolar cationic push-pull compounds with a central electron-acceptor (N+-methylpyrydinium, A+) and different lateral electron-donors, (N,N-dimethylamino and N,N-diphenylamino, D) in a D-π-A+-π-D arrangement, were investigated together with their dipolar counterparts (D-π-A+) for their excited-state dynamics and NLO properties. As for the quadrupolar compounds, attention was focused on excited-state symmetry breaking (ESSB), which leads to a relaxed dipolar excited state. Both electron charge displacements and structural rearrangements were recognized in the excited-state dynamics of these molecules by resorting to femtosecond-resolved broadband fluorescence up-conversion experiments and advanced data analysis, used as a valuable alternative approach for fluorescent molecules compared to time-resolved IR spectroscopy, only suitable for compounds bearing IR markers. Specifically, intramolecular charge transfer (ICT) was found to be guided by ultrafast inertial solvation, while diffusive solvation can drive the twisting of lateral groups to originate twisted-ICT (TICT) states on a picosecond time scale. Yet still, only the bis-N,N-diphenylamino-substituted compound undergoes ESSB, in both highly and sparingly polar solvents, provided that it can experience large amplitude motions to a fully symmetry-broken TICT state. Besides well-known solvation effects, this structural requirement proved to be a necessary condition for these quadrupolar cations to undergo ESSB. In fact, a more efficient uncoupling between the out-of-plane D and A+ groups in the TICT state allows a greater stabilization gained through solvation, relative to the bis-N,N-dimethylamino-substituted derivative, which instead maintains its symmetry. This different behavior parallels the two-photon absorption (TPA) ability, which is greatly enhanced in the case of the bis-N,N-diphenylamino-substituted compound, paving the way for cutting-edge bio-imaging applications.

Controlled Photoinduced Generation of "Visual" Partially and Fully Charge Separated States in Viologen Analogues

Charge-separated states with a lifetime scale of seconds or longer not only favor studies using various steady-state analysis techniques but are important for light-energy conversion and other applications. Through a steric-hindrance-induced method, unprecedented photoinduced generation of a partially charge separated (PCS) state with a lifetime of days has been detected in the "visual" mode during the decay of excited states to a commonly observed fully charge separated (FCS) state for viologen analogues. One pale yellow 4,4'-bipyridine-based metalloviologen compound, with an interannular dihedral angle of 1.84° in 4,4'-bipyridine, directly decays to the purple FCS state after photoexcitation. The other pale yellow compound, with a similar coordination framework but a larger interannular dihedral angle (33.74°), changes first to a yellow PCS state and then relaxes slowly (in the dark in Ar, ca. 2 days; 70 °C in Ar, ca. 1 h) to the purple FCS state. The two-step coloration phenomenon is unprecedented for viologen compounds and their analogues and also rather rare for other photochromic species. EPR and Raman data reveal that photoinduced charge separation first generates univalent zinc and radicals and then the received electron in Zn(I) slowly distributes further to 4,4'-bipyridine. Reduction of π-conjugation and a direct to indirect change in band gap account for the prolongation of the relaxation process and the capture of the PCS state. These findings help to understand and control decay processes of excited states and provide a potential design strategy for multicolor photochromism, light-energy conversion with high efficiency, or other applications.

Rational design of near-infrared absorbing organic dyes: Controlling the HOMO-LUMO gap using quantitative molecular orbital theory

Principles are presented for the design of functional near‐infrared (NIR) organic dye molecules composed of simple donor (D), spacer (π), and acceptor (A) building blocks in a D‐π‐A fashion. Quantitative Kohn–Sham molecular orbital analysis enables accurate fine‐tuning of the electronic properties of the π‐conjugated aromatic cores by effecting their size, including silaaromatics, adding donor and acceptor substituents, and manipulating the D‐π‐A torsional angle. The trends in HOMO–LUMO gaps of the model dyes correlate with the excitation energies computed with time‐dependent density functional theory at CAMY‐B3LYP. Design principles could be developed from these analyses, which led to a proof‐of‐concept linear D‐π‐A with a strong excited‐state intramolecular charge transfer and a NIR absorption at 879 nm. © 2018 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc.

Unravelling the solvent polarity effect on the excited state intramolecular proton transfer mechanism of the 1- and 2-salicylideneanthrylamine. A TD-DFT case study

TD-DFT has been used to investigate the relaxation pathways of 1- and 2-salicylideneanthrylamine in different solvent environments.

Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9'-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.

Determination of the ground- and excited-state dipole moments of bromocresol purple in protic and aprotic solvents

Although it has been widely recognized that hydrogen bonds play a significant role in the photophysics of molecules, this phenomenon has rarely been applied to the solvatochromic method for determination of dipole moments. The difference in the dipole moment between the ground and excited state was determined in protic and aprotic solvents using both the Lippert-Mataga equation and the Bilot-Kawski equation for bromocresol purple, a molecule capable of hydrogen-bond donation and acceptance. The dipole change in protic environments was determined to be 15.2 ± 1.0 D for the Lippert-Mataga method and 9.2 ± 1.0 D for the Bilot-Kawski method, while the change in aprotic environments was 10.4 ± 1.0 D and 6.7 ± 1.0 D, respectively. Both methods highlighted the importance of hydrogen bonding in stabilizing increased charge-separation of the excited state, allowing for larger changes in dipole moments in protic environments. This study further validates a simple, rational modification to the commonly used methods that allows access to dipole-moment data on dyes with hydrogen-bonding capabilities through solvatochromic experiments.

A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D–π–D anthracene derivative

The excited state deactivation of the investigated compound involves two distinct fluorescent states. ¹A* → ¹B* transition is accompanied by structural motions, with the ¹B* state exhibiting a certain ICT character.

Photophysical properties of symmetrically substituted diarylacetylenes and diarylbuta-1,3-diynes

A series of symmetrically substituted diarylacetylenes and diaryl-1,3-butadiynes were prepared and studied with an emphasis on their spectral and photophysical properties.

Unexpected multiple activated steps in the excited state decay of some bis(phenylethynyl)-fluorenes and -anthracenes

The temperature effect on the photophysical parameters of four acetylene-derivatives [bis(phenylethynyl)-anthracenes and -fluorenes with substituents of different electron acceptor efficiencies] has been investigated by absorption and emission spectroscopy, using stationary and pulsed (ns/fs resolution) techniques. The nature of the central nucleus (anthracene or fluorene) and the peripheral electron-withdrawing group (nitro or formyl) strongly affect the deactivation of the excited states of these push-pull molecules. In some cases the study evidenced an interesting role of two activated steps in the deactivation of the excited singlet state, namely an activated inter-system crossing to an upper triplet state of n,π* nature (previously hypothesized on the basis of TD-DFT calculations) and a sort of activated internal conversion, discussed also on the basis of maximum entropy method analysis of the fluorescence decay data. Nicely, an efficient ISC was found for the fluorene-derivatives where small energy gaps between S1 (π,π*) and Tn (n,π*) states had been calculated while no activated ISC was evidenced in the case of anthryl-derivatives where higher S1-Tn energy gaps are expected. A peculiar temperature effect for a fluorene-derivative was pointed out and also explained on the basis of quantum-mechanical calculations at the DFT level taking into account the solvation effects by means of the conductor-like polarizable continuum model CPCM. The presence of dual emission, at first evidenced by a shoulder in the emission spectrum of the fluorene-derivative featuring a peripheral formyl group in dichloromethane at low temperatures, was nicely confirmed by femtosecond up-conversion measurements at room temperature.

Experimental evidence of dual emission in a negatively solvatochromic push–pull pyridinium derivative

Dual emission due to LE and ICT excited states was observed in a cationic push–pull system showing negative solvatochromism.

Concomitant conformational dimorphism in 1,2-bis(9-anthryl)acetylene

The structure of a concomitant conformational polymorph of an opto-electronic material raises questions about polymorphism.