•Structure-properties relationship of a series of carbazole derivatives is analysed.•A simple structured blue OLED device exhibits high competitive performance.•An emission mechanism for the OLED device is proposed through DFT calculations.The thermal, optical, electrochemical and charge transport properties of a series of nine straightforward carbazole-based compounds have been analysed and interpreted according to their molecular structure by means of the X-ray analysis of single crystals. A non-doped OLED device with low turn-on voltage and maximum luminance up to 1.4 × 104 cd m−2 was achieved. DFT calculations have been performed to explain the high efficiency of radiative exciton production.A series of aryl substituted carbazoles with hole transport properties have been evaluated as blue emitters for organic light-emitting diodes (OLEDs). A simple structured blue OLED exhibits very low turn-on voltage of 4 V, high luminance up to 1.4 × 104 cd m−2 and maximum current efficiency of 3.3 cd A−1. A high efficiency of radiative exciton production (ϕ) of 45% has been calculated.

A series of push–pull carbazole-based compounds has been experimentally and theoretically characterized in combination with the X-ray analysis of the corresponding single crystals. The introduction of the strong electron-withdrawing tricyanovinyl group in the carbazole core affords electron-transporting ability in addition to the characteristic hole-transporting properties exhibited by donor carbazole derivatives.

Push–pull bithienylpyrrole-based azo dyes exhibit thermal isomerisation rates as fast as 1.4 μs in acetonitrile at 298 K becoming, thus, the fastest neutral azo dyes reported so far. These remarkably low relaxation times can be transferred into liquid-crystalline matrices enabling light-triggered oscillations in the optical density of the final material up to 11 kHz under ambient conditions.

Carbazole-based nematic liquid single crystal elastomers switch their fluorescence mechanically on demand enabling a fast optical mechanotransduction under ambient conditions. The identification of the key factors controlling such process is of utmost importance since it might lead to a significant improvement of the transducing abilities of these smart materials. In particular, variations in the length of the fluorophore flexible spacer translates in a distinct mutual interaction between both mesogenic and fluorogenic platforms, giving rise thereby to functional materials with a significantly different mechanofluorescent behaviour.

Two new compounds based on three carbazole units connected by triple bonds as π-spacers have been developed as deep blue emitters for organic light-emitting diodes (OLEDs). Their optical and electrochemical properties were examined and their charge carrier transport properties were investigated by means of the xerographic time-of-flight (XTOF) technique. The prepared diodes demonstrate the feasibility of the new molecules as effective emitters in the deep blue region yielding devices with low turn-on voltages.

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10−5 to 10−2 cm2 V−1 s−1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics.

In search of strategies to improve the switching speeds and fatigue resistances of photochromic molecular switches, we have analysed the kinetic behaviour of a series of photoactive azoderivatives which combine the strong electron withdrawing character of both thiazolium and benzothiazolium salts with the ability of azophenols to establish keto–enolic equilibrium. The excitation of these molecules with a green laser at 532 nm, a wavelength that is compatible with biological applications, induces trans-to-cis photoisomerisation in less than 5 ns. The photogenerated cis isomer reverts to the thermodynamically stable trans form with lifetimes as short as 55 ns in ethanol at room temperature. Thus, a full switching cycle can be completed on a nanosecond timescale. To the best of our knowledge, these are the fastest molecular photochromic switches found heretofore. Furthermore, all of the switches reported herein are able to tolerate thousands of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, which is convenient for further technological applications. In addition, these molecules can be adsorbed onto microcrystalline cellulose thereby exhibiting a thermal isomerisation rate as fast as that observed in alcoholic solutions. This feature affords novel solid switchable materials which operate under similar conditions with nanosecond switching speeds.

Benzothiazole-pyrrole-based azo dyes greatly enhance their thermal isomerisation rate by up to 160 times when they are under the influence of the nematic mean field yielding the LC-based photochromic oscillators with the highest oscillation frequencies reported so far (2.6 kHz at 298 K).

The novel photoswitchable bis-azo derivative reported herein shows a high temporal resolution of 2 × 108 times between the thermal relaxation rates of its two constituting photochromes. Moreover, the slow and fast azo building blocks of this molecular construct can be triggered by using UV and visible light, respectively.

The good solubility of azophenols in low molar mass liquid crystals together with the ability of their related polymers to form homogeneous nematic and glassy thin films make such azoderivatives valuable chromophores to get a great variety of photoactivatable systems with fast switching speeds under ambient conditions. In fact, the final applicability of these systems is mainly determined by the thermal cis-to-trans isomerisation rate of the photoactive azophenol used, in other words, by the intimate mechanism the reaction goes through. The kinetico-mechanistic study reported herein shows that the rate of the thermal back reaction for azophenols is very sensitive to the local environment where the azo chromophore is located, mainly to its capability to establish hydrogen bonding with its surroundings. With a proper design, azophenol-based polymers can exhibit thermal isomerisation rates as fast as those of the monomers in solution even without the presence of any solvent.

The use of benzothiazole as an electron-withdrawing group allows obtaining the fastest thermal isomerisation kinetics reported heretofore for neutral azo dyes (70 μs at 298 K). These green light activatable molecules are valuable candidates as molecular photoswitches since they tolerate thousands of working cycles with no sign of fatigue.

Aminoazopyridines are valuable molecules for stable information transmitting systems as well as for light-controlled optical oscillators. Amino-substituted azopyridinium methyl iodide salts transmit optical information within the time scale of nanoseconds, and moreover, show oscillation frequencies up to 1 MHz at room temperature.

Stable photo-controllable electronic switches based on new light-sensitive azopyridines are reported herein. Such systems produce notable variations in the cathodic current density on working at low reduction potentials when UV light falls on them. The appropriate design of the azopyridine chromophore allows modulating the response time of the final opto-electronic switch.

Nematic liquid single crystal elastomers (LSCEs) that contain azobenzenes as cross-linkers have a clear application in the field of light-controlled actuators. They are able to modify their macroscopic dimensions upon irradiation with UV light and furthermore relax back thermally or by irradiation with visible light. Many efforts have been put forward to improve the photo-mechanical effect. However, there are no previous studies about the role of the length of the photo-active cross-linker spacer on the opto-mechanics of light-controlled actuators. In this way, we have prepared several photo-active side-chain nematic polysiloxane LSCEs that contain 4,4′-dialkoxyazobenzenes as cross-linkers, which bear spacers of different lengths. The opto-mechanical behaviour of the different LSCEs prepared has been analyzed. The use of photo-active cross-linkers with long spacers yields actuators with higher mechanical responses under irradiation with UV light. Neither the irradiation nor the thermal relaxation time of the actuator depends on the spacer length.

Azoderivatives show faster thermal cis-to-transisomerisation kinetics when they are under the influence of the nematic mean field than dissolved in isotropic solution. The local order parameter induced in the azo-dye by the host mesogen determines the rate of its thermal cis-to-transisomerisation process. In this way, AZO6-LC mixtures with the same order parameter exhibit identical isomerisation rates independently of the mesogen chemical nature. The temperature dependence of the local order parameter of the chromophore is related with the thermal activation parameters of the isomerisation process and they are characteristic for each AZO6-LC nematic mixture.

The usefulness of azopyridinium methyl iodide salts for designing new promising light-controlled molecular switches is presented. Large absorbance changes have been produced in the samples by irradiation with light at λ = 355 nm. The thermal recovery of the initial state took place completely within 130−450 ms, which is much faster than that reported previously for other push−pull azobenzene-doped nematic mixtures.

The thermal cis-to-trans isomerisation process has been studied for a series of para-, ortho- and polyhydroxy-substituted azobenzenes in different solvents. The kinetics of the thermal back reaction for the p-hydroxy-substituted azobenzenes depend strongly on the nature of the solvent used, with relaxation times ranging from 200–300 milliseconds in ethanol to half an hour in toluene. Otherwise, the process rate is mainly independent of the solvent nature for the ortho substituted analogues. Polyhydroxy-substituted azobenzenes show very much faster kinetics than the para- and ortho- monohydroxyazoderivatives. With relaxation times of 6–12 milliseconds in ethanol, they are optimal molecules for designing fast optical switching devices. All the hydroxyazoderivatives thermally isomerise from the metastable cis form to the thermodynamically stable trans isomer through a rotational mechanism.

The photo-induced phase transition generated by several 4,4′-disubstituted azobenzenes in host nematic low-molar mass liquid crystals was quantified by means of the efficiency, ΔTN–I, using polarized optical microscopy (POM) at variable temperature. Non-covalent interactions established between mesogen and azobenzene were the main determinants of the photo-induced phase transition behaviour. The correct design of the chemical structure of the system components allows controlling the temperature range in which the photo-induced mesophase transition occurs. Therefore, more efficient systems were obtained when azobenzenes with an extended aromatic core were used.

Synthesis, thermal, optical and photoelectrical properties of N,N-diarylated indolo[3,2-b]carbazole derivatives, bearing six strategically placed alkoxy chains of different length, are reported. Electron photoemission spectra of the synthesized derivatives have been recorded and the ionization potentials have been established. It was found that the best charge transport properties were shown by 5,11-di(1,2,3-trioctadecyloxyphenyl)indolo[3,2-b]carbazole. Room temperature hole-drift mobility in its 50% solid solution in bisphenol Z polycarbonate established by the xerographic time of flight technique was found to exceed 10−6 cm2 V−1 s−1 at an electric field of 106 V cm−1.

The conformational preferences and the self-associational behaviors of two hemin-derived porphyrin compounds, a tetramethyl ester and a liquid crystalline tetrakis(3,5-didodecyloxyphenyl)ester, have been studied by UV/vis and 1H NMR spectroscopy in solution. Results indicate that the 3,5-didodecyloxyphenyl units play an important role in both the conformational and the self-associational behaviors of the mesomorphic tetraester. In the monomeric, nonassociated species, the two propionic 3,5-didodecyloxyphenyl esters establish mutual CH/π interactions that restrict the fluctuative behavior of the chains. In the dimeric, self-associated species, intermolecular CH/π interactions occur in addition to the π−π stacking of the porphyrin cores. The temperature-dependent addition of side CH/π interactions to the π−π stacking of the porphyrin rings accounts for the observed tightening and for the slower dynamics of the dimeric structure. The relationship between the self-associational behavior and the mesomorphism of the hemin-derived porphyrin tetraesters is also discussed.

In search of strategies to improve the switching speeds and fatigue resistances of photochromic molecular switches, we have analysed the kinetic behaviour of a series of photoactive azoderivatives which combine the strong electron withdrawing character of both thiazolium and benzothiazolium salts with the ability of azophenols to establish keto–enolic equilibrium. The excitation of these molecules with a green laser at 532 nm, a wavelength that is compatible with biological applications, induces trans-to-cis photoisomerisation in less than 5 ns. The photogenerated cis isomer reverts to the thermodynamically stable trans form with lifetimes as short as 55 ns in ethanol at room temperature. Thus, a full switching cycle can be completed on a nanosecond timescale. To the best of our knowledge, these are the fastest molecular photochromic switches found heretofore. Furthermore, all of the switches reported herein are able to tolerate thousands of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, which is convenient for further technological applications. In addition, these molecules can be adsorbed onto microcrystalline cellulose thereby exhibiting a thermal isomerisation rate as fast as that observed in alcoholic solutions. This feature affords novel solid switchable materials which operate under similar conditions with nanosecond switching speeds.

Nematic liquid single crystal elastomers (LSCEs) that contain azobenzenes as cross-linkers have a clear application in the field of light-controlled actuators. They are able to modify their macroscopic dimensions upon irradiation with UV light and furthermore relax back thermally or by irradiation with visible light. Many efforts have been put forward to improve the photo-mechanical effect. However, there are no previous studies about the role of the length of the photo-active cross-linker spacer on the opto-mechanics of light-controlled actuators. In this way, we have prepared several photo-active side-chain nematic polysiloxane LSCEs that contain 4,4′-dialkoxyazobenzenes as cross-linkers, which bear spacers of different lengths. The opto-mechanical behaviour of the different LSCEs prepared has been analyzed. The use of photo-active cross-linkers with long spacers yields actuators with higher mechanical responses under irradiation with UV light. Neither the irradiation nor the thermal relaxation time of the actuator depends on the spacer length.

Benzothiazole-pyrrole-based azo dyes greatly enhance their thermal isomerisation rate by up to 160 times when they are under the influence of the nematic mean field yielding the LC-based photochromic oscillators with the highest oscillation frequencies reported so far (2.6 kHz at 298 K).

The good solubility of azophenols in low molar mass liquid crystals together with the ability of their related polymers to form homogeneous nematic and glassy thin films make such azoderivatives valuable chromophores to get a great variety of photoactivatable systems with fast switching speeds under ambient conditions. In fact, the final applicability of these systems is mainly determined by the thermal cis-to-trans isomerisation rate of the photoactive azophenol used, in other words, by the intimate mechanism the reaction goes through. The kinetico-mechanistic study reported herein shows that the rate of the thermal back reaction for azophenols is very sensitive to the local environment where the azo chromophore is located, mainly to its capability to establish hydrogen bonding with its surroundings. With a proper design, azophenol-based polymers can exhibit thermal isomerisation rates as fast as those of the monomers in solution even without the presence of any solvent.

Stable photo-controllable electronic switches based on new light-sensitive azopyridines are reported herein. Such systems produce notable variations in the cathodic current density on working at low reduction potentials when UV light falls on them. The appropriate design of the azopyridine chromophore allows modulating the response time of the final opto-electronic switch.

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10−5 to 10−2 cm2 V−1 s−1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics.

The photo-induced phase transition generated by several 4,4′-disubstituted azobenzenes in host nematic low-molar mass liquid crystals was quantified by means of the efficiency, ΔTN–I, using polarized optical microscopy (POM) at variable temperature. Non-covalent interactions established between mesogen and azobenzene were the main determinants of the photo-induced phase transition behaviour. The correct design of the chemical structure of the system components allows controlling the temperature range in which the photo-induced mesophase transition occurs. Therefore, more efficient systems were obtained when azobenzenes with an extended aromatic core were used.

A series of push–pull carbazole-based compounds has been experimentally and theoretically characterized in combination with the X-ray analysis of the corresponding single crystals. The introduction of the strong electron-withdrawing tricyanovinyl group in the carbazole core affords electron-transporting ability in addition to the characteristic hole-transporting properties exhibited by donor carbazole derivatives.

Azoderivatives show faster thermal cis-to-transisomerisation kinetics when they are under the influence of the nematic mean field than dissolved in isotropic solution. The local order parameter induced in the azo-dye by the host mesogen determines the rate of its thermal cis-to-transisomerisation process. In this way, AZO6-LC mixtures with the same order parameter exhibit identical isomerisation rates independently of the mesogen chemical nature. The temperature dependence of the local order parameter of the chromophore is related with the thermal activation parameters of the isomerisation process and they are characteristic for each AZO6-LC nematic mixture.

The thermal cis-to-trans isomerisation process has been studied for a series of para-, ortho- and polyhydroxy-substituted azobenzenes in different solvents. The kinetics of the thermal back reaction for the p-hydroxy-substituted azobenzenes depend strongly on the nature of the solvent used, with relaxation times ranging from 200–300 milliseconds in ethanol to half an hour in toluene. Otherwise, the process rate is mainly independent of the solvent nature for the ortho substituted analogues. Polyhydroxy-substituted azobenzenes show very much faster kinetics than the para- and ortho- monohydroxyazoderivatives. With relaxation times of 6–12 milliseconds in ethanol, they are optimal molecules for designing fast optical switching devices. All the hydroxyazoderivatives thermally isomerise from the metastable cis form to the thermodynamically stable trans isomer through a rotational mechanism.

Push–pull bithienylpyrrole-based azo dyes exhibit thermal isomerisation rates as fast as 1.4 μs in acetonitrile at 298 K becoming, thus, the fastest neutral azo dyes reported so far. These remarkably low relaxation times can be transferred into liquid-crystalline matrices enabling light-triggered oscillations in the optical density of the final material up to 11 kHz under ambient conditions.

The novel photoswitchable bis-azo derivative reported herein shows a high temporal resolution of 2 × 108 times between the thermal relaxation rates of its two constituting photochromes. Moreover, the slow and fast azo building blocks of this molecular construct can be triggered by using UV and visible light, respectively.

Carbazole-based nematic liquid single crystal elastomers switch their fluorescence mechanically on demand enabling a fast optical mechanotransduction under ambient conditions. The identification of the key factors controlling such process is of utmost importance since it might lead to a significant improvement of the transducing abilities of these smart materials. In particular, variations in the length of the fluorophore flexible spacer translates in a distinct mutual interaction between both mesogenic and fluorogenic platforms, giving rise thereby to functional materials with a significantly different mechanofluorescent behaviour.

Aminoazopyridines are valuable molecules for stable information transmitting systems as well as for light-controlled optical oscillators. Amino-substituted azopyridinium methyl iodide salts transmit optical information within the time scale of nanoseconds, and moreover, show oscillation frequencies up to 1 MHz at room temperature.

The use of benzothiazole as an electron-withdrawing group allows obtaining the fastest thermal isomerisation kinetics reported heretofore for neutral azo dyes (70 μs at 298 K). These green light activatable molecules are valuable candidates as molecular photoswitches since they tolerate thousands of working cycles with no sign of fatigue.