Photoactive Molecular-Based Devices, Machines and Materials: Recent Advances

Abstract

Molecular and supramolecular-based systems and materials that can perform predetermined functions in response to light stimulation have been extensively studied in the past three decades. Their investigation continues to be a highly stimulating topic of chemical research, not only because of the inherent scientific value related to a bottom-up approach to functional nanostructures, but also for the prospective applications in diverse fields of technology and medicine. Light is an important tool in this context, as it can be conveniently used both for supplying energy to the system and for probing its states and transformations. In this microreview we recall some basic aspects of light-induced processes in (supra)molecular assemblies, and discuss their exploitation to implement novel functionalities with nanostructured devices, machines and materials. To this aim we illustrate a few examples from our own recent work, which are meant to illustrate the trends of current research in the field.

Keywords: Azobenzene; Molecular machines; Photochemistry; Quantum dots; Rotaxanes; Supramolecular chemistry.

Figure 1

Schematic diagram of the electronic energy levels of a molecule and of the transitions between them: full line, absorption; dashed lines, radiative decays; dotted lines, non‐radiative decays. See the text for details.

Figure 2

Bimolecular processes that could take place following an encounter between an excited molecule *X and another chemical species Y.

Figure 3

The E and Z configurational isomers of azobenzene, and their photochemically and thermally induced interconversion.

Figure 4

Potential energy diagram for a distorted octahedral complex [Ru(tpy)(bpy)L]²⁺, showing the photoinduced expulsion of the monodentate ligand L as a result of the thermal population of the dissociative ³MC level from the ³MLCT state. Reproduced by permission from ref.40.

Figure 5

The photochemical and thermal interconversion processes between dihydropropene 1, cyclophanediene 2, and endoperoxide 2‐O₂.

Figure 6

Time‐dependent luminescence intensity changes at 1270 nm observed for 2‐O₂ (0.19 mm), obtained in situ upon exhaustive irradiation of 1 with visible light. The emission spectrum of the same solution (inset) shows the characteristic phosphorescence band of ¹O₂. All these measurements were performed without photoexcitation. Conditions: CD₃CN, 0.19 mm, 60 °C. Reproduced by permission from ref.43

Figure 7

Structure formula of the [2]rotaxane 3 and the two catenated regioisomers (antiparallel, HT, and parallel, HH) that can be formed by the [4π + 4π] photocycloaddition of the anthracene‐based stoppers. The experimental data suggest that only the HT isomer is obtained.

Figure 8

Structure formula of the molecular shuttle 4 loaded with cargo 5 and schematic representation of its operation as a nanoscale transporter. The grey spheres represent solvent molecules or other adventitious ligands.

Figure 9

Schematic representation of the threading/dethreading of a pseudorotaxane (a) and of the relative unidirectional transit of a macrocycle along a nonsymmetric molecular axle (b). By incorporating the system shown in (b) in a rotaxane or a catenane, linear or rotary motors may be obtained, respectively (c). Adapted by permission from ref.52

Figure 10

Representation of the self‐assembly equilibria (horizontal processes) and photoisomerization (vertical processes) involving an azobenzene‐containing secondary ammonium axle and the DB24C8 ring (acetonitrile, 298 K).

Figure 11

(a) Structure formula of the axle 6 and ring 7, the components of a photochemical autonomous supramolecular pump. (b) Operation mechanism of the relative unidirectional transit of the ring along the axle, driven by light. (c) Simplified potential energy profiles as a function of the ring‐axle relative position for each structure shown in (b). Dashed lines denote processes that are too slow to take place. Adapted by permission from ref.20

Figure 12

(a) Structure formula of the shape‐persistent azobenzene tetramers 8–10 in their all‐E configuration. (b) Molecular model of compound EEEE‐9.

Figure 13

Stick representation of the crystal structure of the all‐E isomers of 10 (a) and 8 (b) and space filling view of the inner voids. The side view of the pores (channels for 10, non‐communicating cavities for 8) is displayed in the right part of each panel. Hydrogen atoms are omitted for clarity.

Figure 14

(a) CO₂ (circles) and N₂ (diamonds) adsorption isotherms of EEEE‐10 at 273 K. (b) CO₂ adsorption isotherms of the all‐E (filled circles) and all‐Z (empty circles) isomers of 10 at 195 K.

Figure 15

(a) Schematic representation of the light‐induced intercomponent energy transfer processes in QD‐pyrene nanohybrids. (b) Energy‐level diagram for CdSe QDs decorated with 1‐PCA. Samples 12 and 13 can exhibit reversible electronic energy transfer (REET) involving their exciton level and the energy‐matched triplet excited state of 1‐PCA. The QD exciton levels of hybrids 11 and 14 are too high and too low, respectively, for REET to occur at room temperature. Adapted by permission from ref.79

Figure 16

Luminescence decay of 13 monitored at 600 nm (green trace) in deoxygenated solution, as measured by (a) time‐correlated single‐photon counting (log plot, λ _exc = 405 nm) and (b) gated streak camera (log‐log plot, λ _exc = 465 nm). (c) Luminescence decay of 13 (λ _exc = 405 nm, green trace) in air equilibrated solution. (d) Luminescence decay of 11 monitored at 540 nm (λ _exc = 405 nm, purple trace) in deoxygenated solution. The grey traces in all panels refer to the same experiment performed on the same QD sample lacking the 1‐PCA functionalization. Conditions: heptane, room temperature. Adapted by permission from ref.79