Photophysics of novel donor–{saturated rigid hydrocarbon bridge}–acceptor systems exhibiting efficient excitation energy transfer

Photoswitchable Molecular Units with Tunable Nonlinear Optical Activity: A Theoretical Investigation

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the "open-open" and the "closed-closed" isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open-closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers.

Nonradiative inter- and intramolecular energy transfer from the aromatic donor anisole to a synthesized photoswitchable acceptor system

We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N'-1,2-phenylene di-p-tosylamide (B) and the multichromophore (M) where A and B are connected by a spacer containing both rigid triple (acetylenic) and flexible methylene bonds. Both steady state and time correlated single photon counting measurements demonstrate that though intermolecular energy transfer, of Forster type, between the donor and acceptor moieties occurs with rate 10(8)s(-1) but when these two reacting components are linked by a spacer (multichromophore, M) the observed transfer rate ( approximately 10(11)s(-1)) enhances. This seemingly indicates that the imposition of the spacer by inserting a triple bond may facilitate in the propagation of electronic excitation energy through bond. The time resolved fluorescence measurements along with the theoretical predictions using Configuration interaction singles (CIS) method by using 6-31G (d,p) basis set, implemented in the Gaussian package indicate the formations of the two excited conformers of B. The experimental findings made from the steady state and time resolved fluorescence measurements demonstrate that, though two different isomeric species of the acceptor B are formed in the excited singlet states, the prevailing singlet-singlet nonradiative energy transfer route was found from the donor A to the relatively longer-lived isomeric species of B.

Photophysical processes involved within the bichromophoric system 9-benzotriazole-1-ylmethyl-9H-carbazole and its role as an artificial photosynthetic device

From both steady state and fluorescence lifetime measurements it reveals that due to photoexcitation of benzotriazole (BZ) part of the bichromophore, 9(1-H-benzotriazole-lylmethyl)-9H-carbazole (BHC), singlet-singlet energy transfer takes place to populate the lowest excited singlet of carbazole (CZ). CZ, thus being excited indirectly via energy transfer process, undergoes strong charge transfer (CT) reaction with the surrounding polar medium acetonitrile (ACN). On the other hand, very weak CT band was apparent when CZ part, within BHC, was directly excited. In less polar tetrahydrofuran (THF) and polar benzonitrile (BN) environment, lack of formation of CT band strongly suggests in favor of the electron-accepting behavior of ACN. Moreover, by measuring the emission spectra of BHC in microcrystals and of 30 bilayers mixed LB film at high mole fraction of BHC molecules, the possibility of excimer formation or aggregation has been ruled out. Thus, BHC, when dissolved in ACN, acts as a triad system of BZ-CZ-ACN where BZ acts as an antenna molecule and CZ as a reaction center. The possible role of the bichromophoric system BHC as an artificial photosynthetic or solar energy conversion device has been hinted.

Synthesis and luminescence properties of U-shaped polycyclic molecules containing syn-facial functionalized quinoxaline rings

[reaction: see text] Rigid U-shaped molecules containing syn-facially situated quinoxaline rings have been synthesized in three steps, by a combination of the Diels-Alder reaction, the ruthenium-catalyzed oxidation, and the Zn(OAc)(2)-catalyzed condensation of the resulting bis-alpha-diketones with benzene-1,2-diamines. The unsymmetric bis-quinoxalines, with electronically different substituents, and the quinoxaline ring-attached alpha-diketones were also prepared. Their luminescence properties were examined and described.

Singlet Energy Transfer in Porphyrin-Based Donor−Bridge−Acceptor Systems: Interaction between Bridge Length and Bridge Energy

Singlet excitation energy transfer is governed by two donor-acceptor interactions, the Coulombic and exchange interactions giving rise to the Förster and Dexter mechanisms, respectively, for singlet energy transfer. In transfer between colliding molecules or between a donor (D) and acceptor (A) connected in donor-bridge-acceptor (D-B-A) system by an inert spacer (B), the distinction between these two mechanisms is quite clear. However, in D-B-A systems connected by a pi-conjugated bridge, the exchange interaction between the donor and acceptor is mediated by the virtual low-lying excited states (unoccupied orbitals) of that bridge and, as a consequence, becomes much more long-range in character. Thus, the clear distinction to the Coulombic mechanism is lost. This so-called superexchange mechanism for singlet energy transfer has been shown to make a significant contribution to the energy transfer rates in several D-B-A systems, and its D-A distance as well as D-B energy gap dependencies have been studied. We here demonstrate that in a series of oligo-p-phenyleneethynylene (OPE) bridged porphyrin-based D-B-A systems with varying D-A distances the Förster and through-bond (superexchange) mechanisms both make considerable contributions to the observed singlet energy transfer rates. The donor is either a zinc porphyrin or a zinc porphyrin with a pyridine ligand, and the acceptor is a free base porphyrin. By comparison to a homologous series where only the D-B energy gaps varies, a separation between the two energy transfer mechanisms was possible and, moreover, an interplay between distance and energy gap dependencies was noted. The distance dependence was shown to be approximately exponential with an attenuation factor beta=0.20 A-1. If the effect of the varying D-B energy gaps in the OPE series was taken into account, a slightly higher beta-value was obtained. Ground-state absorption, steady-state, and time-resolved emission spectroscopy were used. The experimental study is accompanied by time-dependent density functional theory (TD-DFT) calculations of the electronic coupling, and the experimental and theoretical results are in excellent qualitative agreement (same distance dependence).

Bicyclo[2.2.2]octene-Based “Crab-like” Molecules: Synthesis, Complexation, Luminescence Properties, and Solid-State Structures

[structure, reaction: see text] The U-shaped, multifunctionalized tetraetheno-bridged dicyclopenta[b,i]anthracenediol 10 was synthesized to serve as a platform molecule. The molecule was prepared from the Diels-Alder adduct 5a of tricycloundecatriene 3 and bicyclo[2.2.2]octene-fused p-benzoquinone 4. Functionalization of 10 to construct crab-like molecules was achieved via the base-promoted bis-O-alkylation of two endo-oriented hydroxyl groups at termini in 10 with the following alkyl halides: allyl, propagyl, and benzyl bromides; 1-bromo- and 1-iodo-4-(bromomethyl)benzene; 9-(bromomethyl)anthracene; 1-(bromomethyl)pyrene; and isomeric bromomethylpyridines. Single-crystal X-ray structures were obtained for bis-phenyl (21) and bis-pyrenyl (25) crabs, and for the silver(I) complex (32 and 33) crabs. The silver(I) complex 32 from bis-o-pyridyl crab 30 is a [2+2] dimeric dimetallocyclophane, and 33 from bis-m-pyridyl crab 29 is a [1+1] metallo-bridged cyclophane. The self-assembled intramolecular pi-stacking of pyrenyl rings in 25 with an interplanar distance of 3.40 A and the consequent pi-pi interactions were revealed by the X-ray crystal structure and its luminescence property.

Intramolecular sensitization within steroids: Excited-state interaction of C-17 α and β carbon–bromine bonds with a C-6 carbonyl group

The synthesis, photochemistry, and photophysics of 17α-bromo-3α-(triphenylsilyloxy)-5α-androstan-6-one (1) and 17β-bromo-3β-(triphenylsilyloxy)-5α-androstan-6-one (2) have been studied in aqueous tetrahydrofuran. The 17α-bromo isomer gives evidence (reduced ϕf, τ1, and ϕisc for the ketone) for interaction between the ketone and C–Br moieties in the excited singlet state. Some photodehalogenation is also observed upon excitation of the ketone chromophore. This interaction seems to be absent or minimal for the 17β-bromo isomer.Key words: photodehalogenation, bromosteroid, ketosteroid, intramolecular singlet–singlet energy transfer (ISSET).