Dye Radical Cation Research Articles

Photoprocesses in Bis-Diethylamino Derivatives of 1,4- and 1,3-Distyrylbenzene.

The photoprocesses of diethylamino derivatives of 1,4- and 1,3-distyrylbenzenes in MeCN were studied using absorption, luminescence, 1H NMR, and laser kinetic spectroscopy. Compounds 1 and 2 undergo intersystem crossing to the triplet state and exhibit delayed fluorescence. It was concluded that dye radical anions and radical cations are formed upon dismutation of the triplet state in the presence of an electron donor or acceptor. Quantum mechanical calculations for the structures of diethylamino distyrylbenzenes in the ground and excited states were carried out, making it possible to establish the structures of isomers and the most stable conformers for both compounds.

Intermediates of the Photoinduced 2,4-Bis(4-diethylaminobenzylidene)cyclobutanone Redox Reaction in Methanol

Formation of intermediates (triplet states, radical anions, and radical cations) has been revealed by pulse laser photolysis in the photoinduced redox reaction of 2,4-bis(4-diethylaminpbenzyliden)cyclobutanone in methanol in the presence of an electron donor (ascorbic acid) and electron acceptors (methylviologen and p-nitroacetophenone). The reactions of photoinduced reduction by ascorbic acid and oxidation by methylviologen and p-nitroacetophenone occur via the triplet state with the formation of the dye radical anion and radical cation, respectively. The formation of the radical products of the photoinduced redox reaction is characterized by short-lived changes in the absorption spectra at λmax = 460 nm and a lifetime of 3 × 10−4 s for the radical anion and at λmax = 395 and 620 nm and a lifetime of 5 × 10−5 s for the radical cation.

Visible light photocatalysis of dye-sensitized TiO2: The selective aerobic oxidation of amines to imines

Herein, we report an efficient protocol for the selective oxidation of amines with the sole involvement of conduction band electrons (ecb−) of TiO2 for both the activation of O2 and the ensuing oxidation reaction. Visible light was harvested by dye molecules on the surface of TiO2, which injects electrons into its conduction band, generating dye radical cations. Charge transfers from dye radical cations to amines are relayed by the catalytic cycle of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to TEMPO+, which is an oxidant that can conduct the direct two-electron oxidation of amines to imines. TEMPO was regenerated by the oxidation of TEMPOH with superoxide generated by the interaction of ecb− and O2. This work suggests that one can exploit the strategy of cooperative photocatalysis with each component performing in its optimal role: TiO2 for O2 activation, dye for visible light harvesting, and TEMPO for oxidative transformations.

Femtosecond insights into direct electron injection in dye anchored ZnO QDs following charge transfer excitation.

The role of the charge transfer (CT) state in interfacial electron transfer in dye-sensitized semiconductor nanocrystals is still poorly understood. To address this problem, femtosecond transient absorption (TA) spectroscopy is used as a probe to investigate the electron injection across a newly synthesized coumarin dye (8-hydroxy-2-oxo-4-phenyl-2 benzo[h]chromene-3-carbonitrile, coded BC5) and ZnO quantum dots (QDs). Steady state and time-resolved spectroscopic measurements reveal that BC5 dye interacts strongly with ZnO QDs in the ground state forming a CT complex. The BC5-ZnO QD complex absorbs more towards red compared to only the dye and QDs, and emits fluorescence due to radiative recombination of photogenerated charges. The formation of charges following the excitation of the CT complex has been demonstrated by observing the signature of dye radical cations and electrons in the conduction band (CB) of the QDs in the TA spectra. The TA signals of these charges grow sharply as a result of ultrafast direct electron injection into the QD. We have monitored the complete dynamics of photogenerated charges by measuring the TA signals of the charges up to a couple of nanoseconds. The injected electrons that are free or shallowly trapped recombine with a time constant of 625 fs, whereas deeply trapped electrons disappear slowly (526 ps) via radiative recombination. Furthermore, theoretical studies based on ab initio calculations have been carried out to complement the experimental findings.

Ultrafast Dynamics of the Indoline Dye D149 on Mesoporous ZnO and Al2O3 Thin Films

Abstract Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump–supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370–770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a “non-injecting” mesoporous thin film with large band-gap, consisting of Al2O3 nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250–350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0 → S1 absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+ [N(CN)2] – show that D149 is ideally suited for solar cell applications because of its long S1 lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential.

Complete Photodynamics of the Efficient YD2-o-C8-Based Solar Cell

The interplay between the different channels involved in the dye-sensitized solar cells (DSSCs) photodynamic cycle (photon absorption, dye aggregation and emission quenching, e– injection, dye regeneration, e––dye recombination) is the determining factor for their efficiency and performance. Here, we report on studies of the photodynamics of complete solar cells sensitized with 5,15-bis(2,6-dioctoxyphenyl)-10-(bis(4-hexylphenyl)amino-20-4-carboxyphenyl ethynyl)porphyrinato]zinc(II) (YD2-o-C8), adsorbed on TiO2 and Al2O3 nanoparticles in the presence of [Co(bpy)3]3+/2+ electrolyte, with and without a disaggregating agent (chenodeoxycholic acid, CDCA). We used steady-state and femtosecond to millisecond time-resolved techniques to interrogate the related dynamics. The steady-state UV–vis results show a change in the YD2-o-C8 molecules distribution and orientation as well as a decrease in the ligand to metal coordination between the porphyrin molecules due to the effect of the [Co(bpy)3]3+/2+ redox couple. The picosecond to nanosecond emission results for cells using Al2O3 electrode (as a reference) indicate the increase in emission lifetime (from 260 to 460 ps) upon addition of CDCA. This decreases the population of the dye aggregates, thus reducing the efficiency of the nonradiative processes. For YD2-o-C8/Al2O3 cell, we observed vibrational cooling in the 3.5–5 ps range and vibrational energy transfer (27–30 ps) to the alumina NPs film. However, for YD2-o-C8/TiO2 these processes occur in ∼1 and 6 ps, respectively, due to a difference in the dye/electrode interactions. For the DSSC using TiO2 nanoparticles, electron injection from YD2-o-C8 to titania occurs in ∼100 fs, while from the relaxed vibrational state it is much slower (6 ps). The back electron transfer from the TiO2 trap states to the porphyrin radical cation happens in ∼9 ps. Electron injection efficiencies for the full cell with and without CDCA are different (85–89%), reflecting the effect of the aggregates on the cell performance. In addition, the back electron transfer from the titania conduction band to the dye radical cation is slower in the presence of CDCA (878 and 1577 μs at 550 and 1400 nm, respectively) than without coadsorbent (143 and 252 μs at 550 and 1400 nm, respectively). Thus, the quantum yield of dye regeneration amounts to ∼93% and ∼99% in the absence and presence of the coadsorbent, respectively. These results indicate the role of the YD2-o-C8 aggregation on TiO2 electrode in the DSSCs performance. The findings relevant to the photocycle of porphyrins-based solar cells can help in better design and optimization of such devices.

On different photodecomposition behaviors of rhodamine B on laponite and montmorillonite clay under visible light irradiation

In this study, laponite and montmorillonite clays were found to be able to decompose rhodamine B upon visible light irradiation (λ > 420 nm). Very interestingly, it was found that rhodamine B on laponite underwent a stepwise N-deethylation and its decomposition was terminated once rhodamine 110, as a decomposition product, was formed, whereas the same phenomenon was not observed for rhodamine B on montmorillonite, whose decomposition involved chromophore destruction. Mechanistic study revealed that the different photodecomposition behaviors of rhodamine B on laponite and montmorillonite were attributed to the oxidation by different reactive oxygen species, with laponite involving HO2/O2- while montmorillonite involving OH. It was also found that the degradation pathway of rhodamine B on laponite switched from N-deethylation to chromophore destruction when solution pH was changed from 7.0 to 3.0, which was attributed to a much higher fraction of HO2 relative to O2- under pH 3.0 than under pH 7.0. Based on the results, a mechanism of rhodamine dye decomposition on clay under visible light was proposed, involving the clay as an electron acceptor, electron relay between the adsorbed dye molecules and oxygen molecules, and subsequent reactions between the generated dye radical cations and different reactive oxygen species. The results of this study shed light on how to best utilize visible light for organic pollutant degradation on clays within engineered treatment systems as well as on many of naturally occurring pollutant degradation processes in soils and air involving clay.

Hydrophilicity Control of Visible‐Light Hydrogen Evolution and Dynamics of the Charge‐Separated State in Dye/TiO2/Pt Hybrid Systems

Visible-light-driven H(2) evolution based on Dye/TiO(2)/Pt hybrid photocatalysts was investigated for a series of (E)-3-(5'-{4-[bis(4-R(1)-phenyl)amino]phenyl}-4,4'-(R(2))(2)-2,2'-bithiophen-5-yl)-2-cyanoacrylic acid dyes. Efficiencies of hydrogen evolution from aqueous suspensions in the presence of ethylenediaminetetraacetic acid as electron donor under illumination at λ>420 nm were found to considerably depend on the hydrophilic character of R(1), varying in the order MOD (R(1)=CH(3)OCH(2), R(2)=H)≈MO4D (R(1)=R(2)=CH(3)OCH(2))>HD (R(1)=R(2)=H)>PD (R(1)=C(3)H(7), R(2)=H). In the case of MOD/TiO(2)/Pt, the apparent quantum yield for photocatalyzed H(2) generation at 436 nm was 0.27±0.03. Transient absorption measurements for MOD- or PD-grafted transparent films of TiO(2) nanoparticles dipped into water at pH 3 commonly revealed ultrafast formation (<100 fs) of the dye radical cation (Dye(·+) ) followed by multicomponent decays, which involve minor fast decays (<5 ps) almost independent of R(1) and major slower decays with significant differences between the two samples: 1) the early decay of the major components for MOD is about 2.5 times slower than that for PD and 2) a redshift of the spectrum occurred for MOD with a time constant of 17 ps, but not for PD. The substituent effects on H(2) generation as well as on transient behavior have been discussed in terms substituent-dependent charge recombination (CR) of Dye(·+) with electrons in bulk, inner-trap, and/or interstitial-trap states, arising from different solvent reorganization.

Femtosecond to millisecond studies of electron transfer processes in a donor–(π-spacer)–acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.

Mechanism Investigation of Visible Light-Induced Degradation in a Heterogeneous TiO2/Eosin Y/Rhodamine B System

Visible light-induced degradation of rhodamine B (RhB) and eosin Y (EO) in a heterogeneous TiO(2) P-25/EO/RhB system was investigated in the present work. The results showed that the photodegradation of RhB is enhanced significantly when EO is introduced into the P-25/RhB system. Under optimal conditions (50 mg P-25, 20 mg L(-1) EO), RhB (4 mg L(-1)) almost decomposed completely after 35 min of visible light irradiation, though EO was photodegraded simultaneously. The possible photodegradation mechanism was studied by the examination of active species HO*, O(2)(*-) anions, or dye radical cations through adding their scavengers such as methanol, t-butanol, benzoquinone, EDTA, and the I(-) anion. In addition, the electron paramagnetic resonance (EPR) spin trapping technique was also used to monitor the active oxygen species formed in the photocatalytic process. Combined with the contrastive experiments under different atmospheres (N(2)-purged or air) and in different systems, it can be deduced that dissolved O(2) plays a crucial role in dye photodegradation and the O(2)(*-) anion is possibly the major active oxygen species. The low degradation rate with the introduction of EDTA or I(-) indicated that dye radical cations also play a part in photodegradation. Furthermore, except for the dye-sensitized photodegradation on the P-25 surface, reaction in bulk solution also occurs in this system, leading to effective photodegradation of RhB.

Fabrication of Silver Nanoparticle-Embedded Polymer Promoted by Combined Photochemical Properties of a 2,7-Diaminofluorene Derivative Dye

A one step method for the fabrication of silver nanoparticle-embedded polymer is detailed from the mechanistic aspects to the structural analysis of the material. A 2,7-diaminofluorene derivative is employed both as a photoreductant agent of silver precursors and as a free radical photoinitiator to produce silver nanoparticles in a poly(ethylene glycol) matrix at the same time. Each photophysical mechanism is separately investigated and quantified by steady state and time-resolved spectroscopy. The analysis confirms the key role the dye radical cation in the photopolymerization. Moreover, the addition of an amine co-initiator induces a clear acceleration of the photopolymerization rate and leads to a new distinctive mechanism. As a consequence, the morphology of the resultant silver nanocomposite exhibits different size distributions characterized by transmission electron microscopy. This structural observation was also correlated by an unexpected metal-enhancement fluorescence process arising from residu...

Study of Anisotropic Interfacial Electron Transfer Across a Semiconductor/Solution Interface by Time‐Resolved EPR Spectroscopy

Xanthene dyes absorbed onto colloidal TiO2 exhibit spin-polarized EPR spectra when excited with pulses of visible light at low temperatures (see schematic representation). Analysis of the spectra enables the range of distances between the electron and the dye radical cation as well as their relative orientations to be determined. Furthermore, an orientational selectivity in electron transfer across the semiconductor nanoparticle is observed.

Dye-sensitized solar cells using a chlorophyll a derivative as the sensitizer and carotenoids having different conjugation lengths as redox spacers

Titania-based Grätzel-type solar cells were fabricated by the use of a chlorophyll a derivative (methyl 3-carboxy-3-devinyl-pyropheophorobide a) as the dye sensitizer. A 10% each of carotenoids, including neurosporene, spheroidene, lycopene, anhydrorhodovibrin and spirilloxanthin with numbers of conjugated double bonds, n = 9–13, was added as a conjugated spacer in order to neutralize the dye radical cation and to block the reverse electron transfer. The short-circuit current density ( J sc) and the solar energy-to-electricity conversion efficiency ( η) systematically increased, with increasing n, from the values of 10.1 mA cm −2 and 3.1% (with no carotenoid) up to 11.5 mA cm −2 and 4.0% (with spirilloxanthin, n = 13), i.e., an enhancement of ≈30%.

Photoinduced Electron Transfer for Pyrromethene Dyes

The photophysics and electrochemistry of pyrromethene dyes (PM), an important class of laser dyes, have been investigated. Selected dyes were found to be moderately good electron donors. Diffusion-limited fluorescence quenching (ca. 6 × 109 M-1 s-1) is observed for the dyes having half-wave oxidation potentials <1.07 V (vs SCE) in the presence of an electron acceptor, such as methyl viologen or pyromellitic dianhydride. For pyrromethene 567, the dye radical cation and triplet state, along with the pyromellitic dianhydride radical anion and the methyl viologen radical cation, were identified using transient absorption spectroscopy. The dye radical cation, which displays a sharp absorption band at 410 nm, is found to be insensitive toward oxygen. Secondary electron transfer involving the reduction of oxygen and producing superoxide radical anion is suggested. Oxygen plays a critical role in PM dye photochemistry as it induces intersystem crossing to generate the dye triplet state and gives rise to products ...

Dynamics of Semiconductor-to-Dye Electron Transfer for Anthracene Dyes Bound to Different Sized TiO2 Particles

The photosensitization of different sized TiO2 nanoparticles by anthracenecarboxylic acid dye molecules has been studied using transient absorption spectroscopy. These experiments primarily yield information about the semiconductor-to-dye electron transfer reaction. Our measurements show that for TiO2 particles in the 4 to 40 nm size range, the rate of this reaction does not depend on the particles' size. The results were analyzed using a model where the electrons are assumed to be evenly distributed over the surface of the particle. To reproduce our experimental results with this analysis, the scaling parameter that describes the distance dependence of the semiconductor-to-dye electron transfer reaction must be > 180 nm. We do not consider this to be physically reasonable. An alternative explanation is that the injected electrons are in localized trap sites that are spatially close to the dye radical cation, i.e., they are not randomly distributed over the particle surface. The observation of single expo...

Free radical induced oxidation of the azo dye Acid Yellow 9

The kinetics and mechanism of the free radical oxidation of the azo dye, Acid Yellow 9, by sulfate radical anions and hydroxyl radicals have been studied using pulse radiolysis and product analysis. Sulfate radicals react via one electron oxidation, generating the dye radical cation, which has an absorption maximum centered on 470 nm. In basic solutions, the radical cation mainly undergoes deprotonation to yield the aminyl radical which has a strong absorption with a maximum centered on 370 nm, and the pKa for the radical cation is 5.5. Formation of a sulfated product, 1, is indicative of a coupling reaction between the radical cation and sulfate radical anions. Studies also indicate that the hydroxyl radicals react with the dye by both electron transfer as well as by adduct formation.

Ultrafast Photophysical Investigation of Cresyl Violet Aggregates Adsorbed onto Nanometer-Sized Particles of SnO2 and SiO2

The relaxation dynamics of cresyl violet H-aggregate dimers adsorbed onto SnO2 or SiO2 colloidal particles has been examined with ca. 200 fs time resolution. These experiments were performed by monitoring both the ground state recovery of the excited dye molecules and the transient absorption signal in the region of the dye radical cation. For cresyl violet−SiO2, the ground state recovery is a single exponential with a 2.9 ± 0.2 ps time constant. Transient absorption measurements that monitored the excited electronic state of the dye show a similar 2.5 ± 0.4 ps decay. The observed dynamics for cresyl violet−SiO2 is assigned to internal conversion followed by vibrational relaxation of the adsorbed cresyl violet dimers. The similarity of the transient absorption and bleach recovery time constants shows that vibrational relaxation is extremely rapid, i.e., internal conversion is the rate-limiting step. For cresyl violet−SnO2, the ground state recovery is biexponential with time constants of 2.4 ± 0.4 ps (∼80% of the amplitude) and 11.3 ± 0.5 ps (∼20%). Transient absorption measurements that monitor both the electronically excited dye aggregates and the dye radical cation also show a biexponential decay with time constants of 2.3 ± 0.3 and 12.3 ± 0.5 ps. The 2.4 ps process is attributed to internal conversion/vibrational relaxation of the excited dye aggregates, analogous to the results for the cresyl violet-SiO2 system. The 12 ps process is assigned to the decay of the cresyl violet dimer radical cation that is produced by electron transfer to the SnO2 semiconductor particles. The radical cation only contributes to the signal for the cresyl violet−SnO2 system because electron transfer to SiO2 is not energetically allowed. The decay mechanism for the radical cation is back electron transfer from SnO2.

Photochemistry of Squaraine Dyes: Excited Triplet State and Redox Properties of Crown Ether Squaraines

The excited state and redox properties of three crown-ether derivatives of squaraines have been studied. Direct laser excitation of these dyes leads to the formation of excited triplets, dye radical cations and anions, via a biphotonic photoionization mechanism. All these transient species absorb strongly in the visible region. The triplet states of these dyes generated via triplet−triplet energy transfer from 3(1-pyrenecarboxaldehyde)* and 3C60* photosensitizers also produce dye radical cations and anions via a self-quenching process. The bimolecular rate constants for the self-quenching process are in the range of (0.5−1.9) × 109 M-1 s-1. The redox properties of these dyes have been investigated by electrochemical, spectroelectrochemical, and radiolytic methods. They exhibit two reversible oxidations in the potential range of 0.7−1.2 V vs SCE. The spectral features of electrochemically generated one electron oxidation product (λmax = 680 nm) match those of dye radical cations produced by photochemical and radiolytic methods.

Photoprocesses of eosine and rose bengal ion pairs with cationic surfactant in non-polar solvent: application in photosensitization studies

The anionic dyes (D) eosine (Eo) and rose bengal (RB) form hydrophobic ion pairs (DS2) with cationic surfactants (S) which are soluble in non-polar solvents (Bilski et al., J. Phys. Chem., 95 (1991) 5784). The solubility of DS2 in non-polar solvents increases with increasing size of the surfactant cation. We have studied the photoproperties of ion pairs formed between RB or EO and the dimethyldioctadecylammonium cation (RBS2 and EoS2) in isooctane, CCI4, toluene and Ch2CI2. No significant concentration-dependent aggregation occurred in the dye solutions at room temperature. However, some features of initially formed “loose” aggregates developed in isooctane, CCl4 and toluene, which in CH2CI2 all DS2 spectra were characteristic of the monomeric dyes. DS2 showed a red shift in absorption/fluorescence and low-temperature phosphorescence which is characteristic of xanthene chromophores placed in an aprotic enviroment. The fluorescence intensity was also high in Ch2CI2 owing to its aproticity and the lack of aggregation. By contrast, aggregation lowered the fluorescence of the dyes in isooctane. Transient absorption measurements in degassed toluene indicated of a triplet state (λmax=630 nm for RB and 600 nm for Eo) which was self-quenched giving rise to transient absorbances at 380 and 460 nm, probably due to the formation of the dye radical anion and cation respectively. We found no evidence for photoredox reactions between the ion pair constituents, S+and D2-. In aerated solutions, strong 1O2 phosphorescence at 1268 nm was detected; we have compared the efficiency of 1O2 phosphorescence emission by the DS2 ion pair with that of tetraphenylporphine zinc complex. The quantum yield of photodepletion of 1,3- diphenylisobenzofuran (DPBF) sensitized by RBS2 (7 μM) in toluene was 0.89. The quantum yield of oxygen consumption during this reaction was 0.75; Eo was about three times less efficient as a sensitizer. The photostability of hydrophobic EoS2 and RBS2 ion pairs, as well as the photosensitization in non-polar solvents, are similar to those in H2O for both dyes. DS2 may be the photosensitizer of choice in some non-polar solvents.

Photosensitizing properties of squaraine dyes

The present report summarises our results on the photophysical and photochemical investigations of a series of squaraine dyes which exhibit intense and sharp absorption bands in the visible and near infrared regions. The intramolecular charge-transfer transitions arising from the “donor-acceptor-donor” arrangements of these dyes have an interesting effect on their excited state properties. The major nonradiative decay process of squaraines is by rotation about the C-C bonds between the central cyclobutane unit and its neighbouring phenyl groups. Microencaging of one of the dyes by Β-cyclodextrin or poly(4-vinylpyridine) was found to restrict this motion, bringing about up to 90-fold enhancement in its fluorescence yield. These aspects as well as the dynamics of charge transfer from the excited singlet state of some of the squaraine dyes to TiO2 and the recombination of the injected charge with the dye radical cation are discussed.