Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

Reaction dynamics of the photochromic systems such as spiropyran, flugide and diarylethene derivatives in femto- to nano-second time region was reviewed. The mechanisms of rapid reaction channels in various phases and future prospects for photochromic systems were discussed.

Ultrafast Dynamics of Photochromic Systems.

Ultrafast vibrational dynamics of hydrogen bonds in the condensed phase.

Property tuning in selected examples of D–π–A molecules has been discussed and summarized in this review article. The tuning and structure–property relationships have been demonstrated on the particular A, π and D parts of the push–pull molecule. Special emphasis has been put on the tuning of the FMO levels and optical properties. Further prospective applications of the given chromophore have also been considered.

https://pubs.rsc.org/en/content/articlepdf/2014/RA/C4RA11264D

Fundamental aspects of property tuning in push-pull molecules†

Azobenzene (azo) chromophores have been incorporated into a wide variety of materials and molecular architectures, including polymers, dendrimers, and molecular glasses. Azobenzene exhibits a uniquely clean and efficient photochemistry, with facile geometric isomerization about the azo bond, converting the molecule from trans to cis . This review discusses the extensive number of investigations of azobenzene photo-switching and photo-modulation. In particular, azos can be used to alter material behaviour with light, switching both molecular and macroscopic properties. A large number of photobiological studies have shown that interfacing the azo chromophore with enzymes and biopolymers is feasible and useful. The all-optical surface patterning unique to azobenzenes is also reviewed. Lastly, azobenzene photomechanical effects are discussed.

Novel photo-switching using azobenzene functional materials

Femtosecond time-resolved UV−visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370−450 nm, the decay of which can be fitted by a sum of two exponential components. The shorter-lived component has a lifetime of 0.9 ± 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ± 0.2 ps in acetonitrile; this is attributed to the S2(ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ± 1 ps in hexane, cyclohexane, and hexadecane and 16 ± 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S†, into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S2 or S1 potential energy surface. The vibrational relaxation of hot t-AB molec...

Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of trans-Azobenzene in Solution

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Internal, hydrogen bonded OH groups of ethanol oligomers (solvent CCl4) are vibrationally excited by intense picosecond pulses at 3320 cm−1. The transient band shape observed in the OH stretching region (3000 to 3700 cm−1) is monitored by an independently tunable picosecond infrared pulse. The bands in this region are direct probes of hydrogen bridges. The time dependent growth and decay of these bands provides strong evidence for rapid bond breaking with a vibrational predissociation time of ≊5 ps, and for partial reassociation with a time constant of ≊20 ps.

Ultrafast dynamics of hydrogen bonds directly observed by time‐resolved infrared spectroscopy

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl4, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm−1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V–V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.

Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Ultrafast time-resolved electronic absorption spectroscopy has been used to study the photochemistry of trans-azobenzene and trans-1, a derivative in which azobenzene is capped by an azacrown ether, on UV excitation to the S2(ππ*) state. Excitation of trans-1 results in transient absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which recovers on a similar time scale. In contrast, excitation of trans-azobenzene results in transient absorption which decays with a dominant component with a shorter lifetime of ca. 1 ps, and in bleaching which recovers on a much longer time scale of ca. 18 ps. The recovery of the ground-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoisomerization of 1 is approximately twice that of azobenzene. These observations demonstrate that the restricted rotational freedom of the phenyl groups in trans-1 has a significant ef...

T. Q. Ye

Papers

Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of trans-Azobenzene in Solution

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Ultrafast dynamics of hydrogen bonds directly observed by time‐resolved infrared spectroscopy

Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Photoisomerization of a Capped Azobenzene in Solution Probed by Ultrafast Time-Resolved Electronic Absorption Spectroscopy