Time Dependent Density Functional Theory (TD-DFT) has been used to assist the design and synthesis of a series thioxanthone triplet sensitizers. Calculated energies of the triplet excited state (ET) informed both the type and position of auxochromes placed on the thioxanthone core, enabling fine-tuning of the UV-Vis absorptions and associated triplet energies. The calculated results were highly consistent with experimental observation in both the order of the λmax and ET values. The synthesised compounds were then evaluated for their efficacies as triplet sensitizers in a variety of UV and visible light preparative photochemical reactions. The results of this study exceeded expectations; in particular [2+2] cycloaddition chemistry that had previously been sensitized in the UV was found to undergo cycloaddition at 455 nm (blue) with a 2 to 9-fold increase in productivity (g/h) relative to input power. This study demonstrates the ability of powerful modern computational methods to aid the design of successful and productive triplet sensitized photochemical reactions. 1 3,3'-MeOTX 354 292 289 298 e 10 ± 3 ps 862 ± 40 ns 0.93 2 3,3'-FTX 362 290 285 289 e 10.5 ± 1.8 ps 456 ± 25 ns 0.92 3 3-MeOTX 367 284 279 283 e 31 ± 4 ps 867 ± 50 ns >0.9 4 3-FTX 370 282 277 282 e 21 ± 3 ps 520 ± 25 ns 0.83 5 TX (R/R'=H) 380 274 268 274 e 70 ps 20 760 ± 30 ns 0.76 17 6 ITX (R'=H, R = 2-i Pr) 385 270 263 266 e 220 ± 8 ps 880 ± 50 ns 0.86 7 2-FTX 388 263 257 261 e 270 ± 10 ps 585 ± 20 ns 0.81 8 4-MeOTX 385 267 260 263 e 1.9 ± 0.4 ns 1.8 ± 0.3 μs 0.70 9 2-MeOTX 399 252 245 242 f 3.3 ± 0.2 ns 1.7 ± 0.6 μs 0.83 10 2-F,2'-MeOTX 408 242 235 235 f 9.1 ± 0.7 ns 1.2 ± 0.2 μs 0.62 11 2,2'-MeOTX 415 235 227 231 f 6.2 ± 0.5 ns 863 ± 60 ns 0.66

Femtosecond transient absorption (fs-TA) and Ultrafast Raman Loss Spectroscopy (URLS) have been applied to reveal the excited state dynamics of bis(phenylethynyl)benzene (BPEB), a model system for one-dimensional molecular wires that have numerous applications in opto-electronics. It is known from the literature that in the ground state BPEB has a low torsional barrier, resulting in a mixed population of rotamers in solution at room temperature. For the excited state this torsional barrier had been calculated to be much higher. Our femtosecond TA measurements show a multi-exponential behaviour, related to the complex structural dynamics in the excited electronic state. Time-resolved, excited state URLS studies in different solvents reveal mode-dependent kinetics and picosecond vibrational relaxation dynamics of high frequency vibrations. After excitation, a gradual increase in intensity is observed for all Raman bands, which reflects the structural reorganization of Franck-Condon excited, non-planar rotamers to a planar conformation. It is argued that this excited state planarization is also responsible for its high fluorescence quantum yield. The time dependent peak positions of high frequency vibrations provide additional information: a rapid, sub-picosecond decrease in peak frequency, followed by a slower increase, indicates the extent of conjugation during different phases of excited state relaxation. The CC triple (–C≡C–) bond responds somewhat faster to structural reorganization than the CC double (>C=C<) bonds. This study deepens our understanding of the excited state of BPEB and analogous linear pi-conjugated systems and may thus contribute to the advancement of polymeric “molecular wires.”

Solvation plays a critical role in various physicochemical and biological processes. Here, the rate of intersystem crossing (ISC) of benzophenone from its S(nπ*) state to its triplet manifold of states is shown to be modified by hydrogen-bonding interactions with protic solvent molecules. We selectively photoexcite benzophenone with its carbonyl group either solvent coordinated or uncoordinated by tuning the excitation wavelength to the band center (λ = 340 nm) or the long-wavelength edge (λ = 380 nm) of its π* ← n absorption band. A combination of ultrafast absorption and Raman spectroscopy shows that the hydrogen-bonding interaction increases the time constant for ISC from <200 fs to 1.7 ± 0.2 ps for benzophenone in CHOH. The spectroscopic evidence suggests that the preferred pathway for ISC is from the S(nπ*) to the T(ππ*) state, with the rate of internal conversion from T(ππ*) to T(nπ*) controlled by solvent quenching of excess vibrational energy.

Excited state ultrafast conformational reorganization is recognized as an important phenomenon that facilitates light-induced functions of many molecular systems. This report describes the femtosecond and picosecond conformational relaxation dynamics of middle-ring and terminal ring twisted conformers of the acetylene π-conjugated system bis(phenylethynyl)benzene, a model system for molecular wires. Through excitation wavelength dependent, femtosecond-transient absorption measurements, we found that the middle-ring and terminal ring twisted conformers relax at femtosecond (400-600 fs) and picosecond (20-24 ps) time scales, respectively. Actinic pumping into the red flank of the absorption spectrum leads to excitation of primarily planar conformers, and results in very different excited state dynamics. In addition, ultrafast Raman loss spectroscopic studies revealed the vibrational mode dependent relaxation dynamics for different excitation wavelengths. To corroborate our experimental findings, DFT and time-dependent DFT calculations were carried out. The Franck-Condon simulation indicated that the vibronic structure observed in the electronic absorption and the fluorescence spectra are due to progressions and combinations of several vibrational modes corresponding to the phenyl ring and the acetylenic groups. Furthermore, the middle ring torsional rotation matches the room-temperature electronic absorption, in stark contrast to the terminal ring torsional rotation. Finally, we show that the middle-ring twisted conformer undergoes femtosecond torsional planarization dynamic, whereas the terminal rings relax on a few tens of picosecond time scale.

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm low-frequency phenyl torsional mode. Two vibrational marker bands, C=C stretching (∼1512 cm) and C=C stretching (∼1584 cm) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the C=C stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the C=C stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central C=C twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.

The optical properties of two Re­(CO)3(bpy)Cl complexes in which the bpy is substituted with two donor (triphenylamine, TPA, ReTPA2) as well as both donor (TPA) and acceptor (benzothiadiazole, BTD, ReTPA-BTD) groups are presented. For ReTPA2 the absorption spectra show intense intraligand charge-transfer (ILCT) bands at 460 nm with small solvatochromic behavior; for ReTPA-BTD the ILCT transitions are weaker. These transitions are assigned as TPA → bpy transitions as supported by resonance Raman data and TDDFT calculations. The excited-state spectroscopy shows the presence of two emissive states for both complexes. The intensity of these emission signals is modulated by solvent. Time-resolved infrared spectroscopy definitively assigns the excited states present in CH2Cl2 to be MLCT in nature, and in MeCN the excited states are ILCT in nature. DFT calculations indicated this switching with solvent is governed by access to states controlled by spin–orbit coupling, which is sufficiently different in the two solvents, allowing to select out each of the charge-transfer states.

No abstract

The ability to control photoinduced charge transfer within molecules represents a major challenge requiring precise control of the relative positioning and orientation of donor and acceptor groups. Here we show that such photoinduced charge transfer processes within homo- and hetero-rotaxanes can be controlled through organisation of the components of the mechanically interlocked molecules, introducing alternative pathways for electron donation. Specifically, studies of two rotaxanes are described: a homo[3]rotaxane, built from a perylenediimide diimidazolium rod that threads two pillar[5]arene macrocycles, and a hetero[4]rotaxane in which an additional bis(1,5-naphtho)-38-crown-10 (BN38C10) macrocycle encircles the central perylenediimide. The two rotaxanes are characterised by a combination of techniques including electron diffraction crystallography in the case of the hetero[4]rotaxane. Cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy are employed to establish the behaviour of the redox states of both rotaxanes and these data are used to inform photophysical studies using time-resolved infra-red (TRIR) and transient absorption (TA) spectroscopies. The latter studies illustrate the formation of a symmetry-breaking charge-separated state in the case of the homo[3]rotaxane in which charge transfer between the pillar[5]arene and perylenediimide is observed involving only one of the two macrocyclic components. In the case of the hetero[4]rotaxane charge separation is observed involving only the BN38C10 macrocycle and the perylenediimide leaving the pillar[5]arene components unperturbed.