Long-wavelength Absorption Band Research Articles

Hydrogenation of di(methoxo)tetraphenylporphyrin phosphorus(V) to isobacteriochlorin, chlorin, and phlorin complex with long-wavelength absorption band

We report the synthesis of nonmetal high-valent phosphorus(V) hydroporphyrin complexes: PV-isobacteriochlorin, PV-chlorin, and two isomers of PV-phlorin, via simple hydrogenation of di(methoxo)tetraphenylporphyrin phosphorus(V) with a hydrogen source and Pd/C catalyst. These PV-hydroporphyrin complexes exhibit long-wavelength absorption bands in the biological optical window. PV-isobacteriochlorin and PV-chlorin complexes are stable against auto-oxidation and exhibit the photoproduction of singlet oxygen under red light irradiation.

Advanced Cd1-xMnxTe:Fe2+ semiconductor crystals for IR applications: The role of the composition and sin-band redshift spectra effect

Doped semiconductor crystals of solid solution Cd1-xMnxTe:Fe2+ were grown by the high-pressure Bridgman method, covering the range of their existence as a zinc blende structure (0 < x < 0.76). The concentration of the Fe2+ impurities was approximately 10−3 wt% in all the studied samples. The structural and optical properties of the crystals were investigated, including the case of high concentrations of x > 0.4. The correlations between the composition of solid solution crystals of Cd1-xMnxTe:Fe2+, the lattice constant, the band gap, and the maxima positions of the Fe2+ active ion absorption and emission spectra were found experimentally and explained physically. A theoretical model based on the principle of compositional additivity for solid solution semiconductor materials was first used for explaining the long-wavelength sin-band linear “redshift” of absorption and emission bands in the spectra of Cd1-xMnxTe:Fe2+ crystals with increasing solid solution concentration. A new effect is discovered for the differentiated redshift of the Jahn-Teller components (bands) of the total absorption spectrum. Longwavelength absorption bands have a stronger redshift than shortwavelength absorption bands. The redshifts in the maxima of the total absorption and emission spectra have shift (slope) coefficients Kab≈1.7 nm/at.% and Kem≈6.1 nm/at.%, respectively. The obtained results can be used to predict and design laser media based on Cd1-xMnxTe:Fe2+ solid solution crystals while controlling the lasing range (for all Mn concentrations).

The effect of substituent position and solvent on thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes: kinetic, thermodynamic, and spectral approaches.

Kinetic and thermodynamic parameters have been investigated for the thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes with alkyl substituents (Me, t-Bu, and Adm) at positions 1 (dyes 2) and 2 (dyes 3) of the tetrazole moiety in two solvents of different polarity, acetonitrile (MeCN) and toluene. The experimental results show crucial dependence of these parameters on a substituent position in the tetrazole moiety and on a solvent. For dyes 2, Eact and ΔH‡ are lower in MeCN than in toluene that results in a high increase in the lifetimes of the Z isomers: from milliseconds in MeCN to minutes in toluene. For dyes 3, the difference in Eact and ΔH‡ in the two solvents is opposite: Eact and ΔH‡ are by more than 20kJmol-1 higher in MeCN, nevertheless, the rate constants for 3 in toluene are comparable with those in MeCN at the ambient temperature and the difference in the behavior is determined by the value of negative entropy of activation. Quantum-chemical calculations of the thermal Z‒E isomerization show the possibility of the process to occur via crossing from the S0 to the thermally induced T1 state. The contribution of this path is highest for 3 in toluene. The analysis of the absorption spectra demonstrates that for the E isomers, the n‒π* and π‒π* transitions are within the long-wavelength absorption band and their positions relative each other are opposite in the solvents: the n‒π* transition is blue-shifted relative to the π‒π* transition in MeCN and is red-shifted in toluene.

Temperature-dependent solvent reorganization entropies, free energies, and transition dipole strengths for the photoexcitation of Reichardt's dye B30.

Absorption spectra of the solvatochromic dye 2,6-diphenyl-4-2,4,6-triphenyl-1-pyridinophenolate (B30) were measured in seven solvents of varying polarity over temperature ranging from each solvent's freezing point to 300K. The excitation energies and their variances allowed calculations of the solvent reorganization energies, reorganization free energies and reorganization entropies as functions of temperature. The entropies of solvent packing around the chromophore are found to make major contributions to the reorganization free energies. The variances of the excitation energies depend only weakly on temperature, in disagreement with an expression that is often used for solvent reorganization free energies. Polar solvents reduce the transition dipole strength of B30's long-wavelength absorption band, probably because interactions with the solvent enhance the charge-transfer character of the transition. The dipole strength drops further at low temperatures.

Cyclohexene-Embedded Dicyanomethylene Merocyanines - Consecutive Three-Component Coupling-Addition Synthesis and Chromophore Characteristics.

A concise and efficient consecutive three-component alkynylation-addition synthesis of cyclohexene-embedded dicyanomethylene merocyanines furnishes a small library of dyes in moderate to excellent yield. The dyes possess strong absorption coefficients of the longest wavelength absorption bands. According to the crystal structure, the small bond length alternations account for a highly delocalized electronic ground state. The electronic structure of the absorption bands is qualitatively rationalized by TDDFT calculations, which explain that intense HOMO-LUMO transitions along the merocyanine axis lead to cyanine similar Stokes shifts.

Bacteriochlorin syntheses - Status, problems, and exploration

Bacteriochlorins – Nature’s near-infrared (NIR) chromophores – are distinguished by an intense ([Formula: see text] ∼;105 M[Formula: see text]cm[Formula: see text] long-wavelength absorption band in the ∼;700–1000 nm. The development of routes to prepare synthetic, tailorable bacteriochlorins holds promise for multiple disciplines where NIR-light-promoted photoactivity is of interest. A de novo route to bacteriochlorins equipped with a stabilizing gem-dimethyl group in each pyrroline ring was discovered in 2003. Continued development in this arena over 20 years has led to additional routes as well as methods to install substituents at selected positions about the perimeter of the macrocycle. The present paper reports studies that highlight substantial limitations of existing synthetic routes, including stymied access to multi-bacteriochlorin arrays and the inability to install (in a rational way) distinct groups at opposite sides of the macrocycle. The origins of the limitations are traced to particular stages of the chemistry ranging from derivatizing pyrroles, creating pyrrolines, constructing and elaborating dihydrodipyrrins, coupling dihydrodipyrrins, and forming macrocycles. Through exploration of a dozen aspects of bacteriochlorin syntheses, 60 new compounds (and nine known compounds via improved syntheses) have been prepared and characterized; the data include 20 single-crystal X-ray diffraction analyses. The research taken together points to areas of focus to fulfill the promise of this fascinating class of compounds.

Binding of Mammea A/AA (MA) to calf thymus DNA revealed by the ratiometric absorbance of MA in the UV-visible range molecular dynamic simulations and TD-DFT calculations

The in vitro anti-proliferative activity of MA ( 5,7-dihydroxy-8-(3-methylbut-2-enyl)-6-(3-methyl-1-oxobutyl)-4-phenyl[1]2H-[1]benzopyran-2-one)on a variety of cancer cells was previously demonstrated. This work strives to understand the mechanisms by which MA exerts this biological activity. Thereafter, the binding of MA to calf thymus DNA was studied by monitoring the change in the UV-visible absorbance of MA. It was found that, the response of MA to binding with calf thymus DNA is characterised by an increase in the AS/AL ratio of the absorbance of the longest wavelength absorption band to the shortest one, and the appearance of a new band at about 377 nm assigned to S0→S1 transition, which is red shifted as compared to free MA. From the bands ratio, the binding constant is found to be 4.3x105 M−1, indicating strong binding. The deduced binding free energy, enthalpy and entropy are −7.7 kcal/mol, −10.89 ± 0.28 kcal/mol and −54.46 ± 4 J/K, respectively, indicating that MA binds to DNA by a non-bonding Van der Waals type interactions and hydrogen bonds. Further study with classical molecular dynamics shows that MA binds to DNA by intercalation, where it is positioned between two AT base pairs. Unlike isolated MA, TDDFT calculations on ten images extracted from the MD trajectory show that, the frontier molecular orbitals of the complex are distributed over the DNA and MA. This indicates a strong stacking interaction and then explains the hypochromism and the red shift of the S0→S1 transition. The present work demonstrates the potency of MA as antitumor compound and as absorbance-based molecular probe. Communicated by Ramaswamy H. Sarma

Synthesis and Chiroptical Properties of Binaphthyl‐Hinged [5]Helicenes

AbstractThe synthesis, structure, and circularly polarized luminescence (CPL) of two types of shape‐persistent [5]helicenes, hinged by a tethered (−OCH2CH2O−) or untethered (−OCH3) binaphthyl, are reported. The binaphthyl tethering system facilitated an effective Wittig ring‐closure reaction in the synthesis of a macrocyclic precursor, which could be easily converted to the title compound via successive photocyclization. The tethered groups were replaced with untethered −OCH3 groups by treatment with BBr3 followed by methylation. Both compounds exhibited similar spectra, and their longest wavelength absorption band with a low intensity was attributed to the transition from HOMO to LUMO+1. These compounds exhibited CPL with higher dissymmetry factors glum values than unlocked helicenes.

Synthesis of new efficient fluorophores of thieno[2,3-&lt;i&gt;b&lt;/i&gt;]pyridine series

The reaction of 4-amino-6-aryl-2-halopyridine-3,5-dicarbonitriles with thioglycolic acid ethyl ester was used to synthesize a series of new efficient fluorophores of thieno[2,3- b ]pyridine series. The long-wavelength absorption bands of DMSO solutions of the prepared compounds are at the boundary of the UV and visible regions of the spectrum (λabs 375-388 nm) and show an ambiguous influence of the substituent electronic effect, that was explained by quantum chemical calculations. The emission maxima are in the yellow-green region (λem 490- 510 nm) and they are blue-shifted in the presence of electron donor groups with slightly increasing intensity (Φem 37.8-60.6%). It was found that the nitro group presence causes a complete photoluminescence quenching both in solution and in the solid state. In addition, the position of the emission maximum (λem 473-505 nm) and its intensity (Φem 3.6-72.7%) strongly depend on the polarity and the basic properties of the medium, that was established using the Catalan empirical model.

Synthesis and solvatochromism of dipodal and tripodal dyes derived from Brooker’s merocyanine

Brooker’s merocyanine (BM) is a classic solvatochromic probe, which exhibits a band in the visible region sensitive to changes in the polarity of the medium. BM shows a reversion in its solvatochromism, which has inspired much research on studies involving different families of dyes. We synthesized four novel dipodal and tripodal solvatochromic dyes (1b–4b) inspired by BM molecular structure. Characterization of the compounds in their protonated form used spectrometric and differential scanning calorimetry techniques. Dyes 1b–4b have an electron–donating phenolate moiety connected through a CHCH conjugated bridge to two or three N–methylpyridinium electron–acceptor groups. The probes exhibit negative solvatochromism in the solvents studied, showing very different colors with the change in the polarity of the solvent. Multiparameter strategies of Kamlet–Abboud–Taft (KAT), Catalán, and a modified KAT fitted the experimental data. The HBD acidity of the solvent represented the main contribution to the solvation of the probes. Performing electronic–structure calculations of the compounds at LC–BLYP/ma–def2–SVP/D4 level and employing water and DMSO as implicit solvents (SMD model) using software ORCA5.0 revealed that the longest–wavelength absorption bands in the dyes are due to a π → π* transition, from the phenolate (electron–donating) group to the N–methylpyridinium (electron–acceptor) groups. The calculations did not reproduce the experimentally observed negative solvatochromism of the dyes in the solvents studied. However, comparing the theoretical and experimental data of the present study with data from the literature provided important information regarding the results obtained.

Anion effect on crystal structure of Zn-complexes of spironaphthopyran and their chromogenic properties in solution

Effect of the anion nature on crystal structure of Zn-complexes of spironaphthopyran and their chromogenic properties in solution were studied. Complexes with zinc chloride and zinc picrate exhibited tertahedral and octahedral coordination environment of the metal ion respectively. The structure of the complex with zinc picrate demonstrates a rare example of the co-crystallization of two types of complexes differing in the number of coordinated picrate anions. The composition, stability, and absorption properties of zinc complexes in acetone essentially depend on the counter anion nature. When the coordinating ability of the counter ion increases in the range BF4−, ClO4−, NO3–, Pic−, I−, Cl−, CH3COO–, (CH3)3CCOO– the complex stability is reduced from logK = 8.7 to 2.96. This is accompanied by the chromogenic effect which is related to the decrease in the spectral shift of the long-wavelength absorption band of the complex relative to the merocyanine ligand from 846 up to 168 cm−1. The complex photo-initiated processes include fluorescence and thermally reversible dissociation. Fluorescence is characterized by band maxima in the 622–637 nm region and normal Stokes shifts of 1087–1454 cm−1 with an efficiency being weakly dependent on the anion nature. In contrast, the negative photochromism of the complexes demonstrates a significant dependence of the photo-bleaching efficiency on the anion nature in the range 0.012–0.17. The presence of a chromogenic photochromic effect along with fluorescence makes it possible to consider the complex as fluorescent switch.

Solid-state fluorescent 3,3-diarylallylidene indolinones by pseudo-five-component synthesis

One-pot syntheses of 3,3-diarylallylidene indolinones are achieved by pseudo-four- and pseudo-five-component reactions. The title compounds are obtained as a mixture of E/Z-configured isomers. The photophysical characteristics are assessed by absorption and solid-state emission spectroscopy. Irrespective of their configuration, the 3,3-diarylallylidene indolinones fluoresce in the solid state for electron-withdrawing and weakly electron-releasing remote p-phenyl substituted dyes with concomitantly Boc substitution of the lactam moiety. DFT and TD DFT calculation reproduce and rationalize the electronic structure of the longest wavelength absorption and emission bands.

Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes.

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C2- heterocyclic ligand dba2- (H2dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh3, CNtBu and Me2Imd (N,N'-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh3, CNtBu and Me2Imd derivatives and characterized them by elemental analysis, 1H (and 31P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between -1.89 and -2.10 V and increasing along the series Me2Imd &lt; dmso ≈ PPh3 &lt; CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me2Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me2Imd &lt; CNtBu &lt; PPh3 &lt; dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields ΦL ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high ΦL and long τ, the dmso and PPh3 complexes outperform the CNtBu and Me2Imd derivatives. This is remarkable in view of the higher ligand strength of Me2Imd, compared with all other coligands, as concluded from the electrochemical data.

Photophysical Study on the Rigid Pt(II) Complex [Pt(naphen)(Cl)] (Hnaphen = Naphtho[1,2-b][1,10]Phenanthroline and Derivatives.

The electrochemistry and photophysics of the Pt(II) complexes [Pt(naphen)(X)] (Hnaphen = naphtho[1,2-b][1,10]phenanthroline, X = Cl or C≡CPh) containing the rigid tridentate C^N^N-coordinating pericyclic naphen ligand was studied alongside the complexes of the tetrahydro-derivative [Pt(thnaphen)(X)] (Hthnaphen = 5,6,8,9-tetrahydro-naphtho[1,2-b][1,10]phenanthroline) and the N^C^N-coordinated complex [Pt(bdq)(Cl)] (Hbdq = benzo[1,2-h:5,4-h’]diquinoline. The cyclic voltammetry showed reversible reductions for the C^N^N complexes, with markedly fewer negative potentials (around −1.6 V vs. ferrocene) for the complexes containing the naphen ligand compared with the thnaphen derivatives (around −1.9 V). With irreversible oxidations at around +0.3 V for all of the complexes, the naphen made a difference in the electrochemical gap of about 0.3 eV (1.9 vs. 2.2 eV) compared with thnaphen. The bdq complex was completely different, with an irreversible reduction at around −2 V caused by the N^C^N coordination pattern, which lacked a good electron acceptor such as the phenanthroline unit in the C^N^N ligand naphen. Long-wavelength UV-Vis absorption bands were found around 520 to 530 nm for the C^N^N complexes with the C≡CPh coligand and were red-shifted when compared with the Cl derivatives. The N^C^N-coordinated bdq complex was markedly blue-shifted (493 nm). The steady-state photoluminescence spectra showed poorly structured emission bands peaking at around 630 nm for the two naphen complexes and 570 nm for the thnaphen derivatives. The bdq complex showed a pronounced vibrational structure and an emission maximum at 586 nm. Assuming mixed 3LC/3MLCT excited states, the vibronic progression for the N^C^N bdq complex indicated a higher LC character than assumed for the C^N^N-coordinated naphen and thnaphen complexes. The blue-shift was a result of the different N^C^N vs. C^N^N coordination. The photoluminescence lifetimes and quantum yields ΦL massively increased from solutions at 298 K (0.06 to 0.24) to glassy frozen matrices at 77 K (0.80 to 0.95). The nanosecond time-resolved study on [Pt(naphen)(Cl)] showed a phosphorescence emission signal originating from the mixed 3LC/3MLCT with an emission lifetime of around 3 µs.

Reverse solvatochromism in a family of probes having 2,6–di–tert–butylphenolate as electron–donor and 4–nitrophenyl as electron–acceptor groups

Dyes presenting electron–accepting nitrophenyl and electron–donating phenolate groups have been used in the recent years as solvatochromic probes to investigate the polarity of solvents. Although several substituents in the phenolate have been used in studies involving these dyes, the use of tert-butyl groups has not yet been explored for these compounds. Bulky tert–butyl groups in the structure of these probes could bring interesting results because they make the dyes more lipophilic and hinder the formation of hydrogen bonds with hydrogen bond donating solvents. In this study, eight 2,6–di–tert–butylphenols were synthesized and characterized. They were then deprotonated to generate the corresponding phenolate dyes, which exhibited reverse solvatochromism. TD/DFT calculations were performed, being shown that the longest wavelength absorption bands of the dyes are due to π−π* transitions, with an intramolecular charge transfer from the 2,6–di–tert–butylphenolate to the 4–nitrophenyl group. Data indicated that the overall nature of the excitation is the same in all cases, and the differences registered between experimental and theoretical data suggest that a synergetic effect in the solvation of the probes involving many solvent molecules may take place, which can widen the gap between HOMO and LUMO. The tert–butyl groups play an important role in the nature of dye–solvent interactions. Molecular dynamics simulations demonstrated the effect of the tert–butyl groups in hindering hydrogen bonding between the phenolate group and methanol and water solvent molecules, with a more structured solvation shell being observed on the nitro substituent of the probes. In conclusion, the investigation carried out showed that the presence of tert–butyl groups in the electron–donor phenolate groups tunes the types of interactions between the probe and the solvent. This can occur through changes in the molecular and electronic structure of the probe or be a result of steric hindrance, which makes the interaction of the solvent with the oxygen of the donor phenolate group more difficult.

Charge transfer vs. proton transfer in the excited-state dynamics of biomimetic pyranoflavylium cations

Pyranoflavylium cations are synthetic analogues of pyranoanthocyanin, colored pigments, formed from grape anthocyanins during the maturation of red wines. Studies of a series of monosubstituted pyranoflavylium cations, ranging from methoxy (PF+-OMe) to cyano (PF+-CN), have shown that they display fluorescence and form triplet states that sensitize singlet oxygen formation in acidified acetonitrile. In alcohol-water mixtures, they behave as photoacids, undergoing adiabatic excited state proton transfer (ESPT) to water on a picosecond timescale, as confirmed in this report by femtosecond pump-probe spectroscopy. In contrast, the corresponding dimethylamino substituted pyranoflavylium cation (PF+-NMe2) is virtually non-fluorescent under the same conditions and exhibits a long-wavelength absorption band that has been attributed to a charge-transfer (CT) transition. Indeed, pump-probe spectroscopy of PF+-NMe2 in acidified acetonitrile shows ultrafast (<1 ps) formation of a CT state that decays back to the ground state with a 12–13 ps lifetime. In acidified methanol, the initial Franck-Condon CT state (ca. 3 ps lifetime) converts to a 13 ps lifetime CT state analogous to that in acetonitrile. In 50:50 ethanol:water and 30:70 methanol:water mixtures, PF+-NMe2 exhibits a short-lived (3–8 ps) initial CT state, an intermediate lifetime (30 ps) CT state and a much longer lived (130 ps) species attributed to a twisted intramolecular CT state. Thus, in addition to demonstrating that the photophysics of PF+-NMe2 is dominated by CT rather than ESPT, pump-probe spectroscopy provides details of the solvent-dependent dynamics of the CT decay pathways.

Dihydroquinolylazotetrazole dyes in aqueous solutions: Effect of substituents and pH on spectral properties, acid-base equilibria and thermal Z‒E isomerization

Spectral, luminescent and photochemical properties of a series of hetarylazo dyes with dihydroquinoline and tetrazole moieties, which differ by the position of methyl substituent at the tetrazole fragment, were investigated in aqueous solutions at various pH by stationary spectrophotometry, fluorimetry and time-resolved methods. The results were supported by quantum chemical calculations. From the pH-dependence of the absorption spectra, it has been shown that azonium cations and dications additionally protonated at the tetrazole fragment are formed upon protonation of the E isomers. Opposite to the azonium cations studied earlier, the long-wavelength absorption band of the azonium cations is hypsochromically shifted relative to that of the neutral form. Weak fluorescence in aqueous solutions growing in acidic media is the specific feature of the dyes. Time-resolved experiments have shown that proton transfer reactions play important role in the transformations of the dyes after excitation in aqueous solutions.

Consecutive multicomponent syntheses of N-substituted 3-arylallylidene indolones – Solid-state emitters and photoisomerization

A series of variably substituted 3-arylallylidene indolones were synthesized as diastereomeric mixtures applying a novel consecutive three-component Heck-condensation sequence in the sense of a one-pot synthesis. The N-substitution pattern was found to be crucial for the fluorescence in the solid state. Accordingly, the synthetic route was extended to a consecutive four-component reaction to generate a diverse compound library of Boc-substituted 3-arylallylidene indolones. Studying the structure-property relationship by Hammett correlation revealed a discontinuous behavior of the longest wavelength absorption bands of these compounds. 3-Arylallylidene indolones exclusively fluoresce in the solid state. Exceptions are strongly donor-substituted derivatives, which also luminesce in solution and reveal positive emission solvatochromism. With the N-tosyl derivative, an AIE (aggregation induced emission) chromophore was discovered and for the (p-anisyl)2N-substituted derivative aggregation induced emission enhancement (AIEE) was found. The absorption spectra of the 3-arylallylidene indolones can be plausibly rationalized by TD(DFT) calculations. Interestingly, the diastereomers, which can be photoisomerized, are very similar in their photophysical properties.

Photoluminescence of Ni(II), Pd(II), and Pt(II) Complexes [M(Me2dpb)Cl] Obtained from C‒H Activation of 1,5-Di(2-pyridyl)-2,4-dimethylbenzene (Me2dpbH).

The three complexes [M(Me2dpb)Cl] (M = Ni, Pd, Pt) containing the tridentate N,C,N-cyclometalating 3,5-dimethyl-1,5-dipyridyl-phenide ligand (Me2dpb−) were synthesised using a base-assisted C‒H activation method. Oxidation potentials from cyclic voltammetry increased along the series Pt < Ni < Pd from 0.15 to 0.74 V. DFT calculations confirmed the essentially ligand-centred π*-type character of the lowest unoccupied molecular orbital (LUMO) for all three complexes in agreement with the invariant reduction processes. For the highest occupied molecular orbitals (HOMO), contributions from metal dyz, phenyl C4, C2, C1, and C6, and Cl pz orbitals were found. As expected, the dz2 (HOMO-1 for Ni) is stabilised for the Pd and Pt derivatives, while the antibonding dx2−y2 orbital is de-stabilised for Pt and Pd compared with Ni. The long-wavelength UV-vis absorption band energies increase along the series Ni < Pt < Pd. The lowest-energy TD-DFT-calculated state for the Ni complex has a pronounced dz2-type contribution to the overall metal-to-ligand charge transfer (MLCT) character. For Pt and Pd, the dz2 orbital is energetically not available and a strongly mixed Cl-to-π*/phenyl-to-π*/M(dyz)-to-π* (XLCT/ILCT/MLCT) character is found. The complex [Pd(Me2dpb)Cl] showed a structured emission band in a frozen glassy matrix at 77 K, peaking at 468 nm with a quantum yield of almost unity as observed for the previously reported Pt derivative. No emission was observed from the Ni complex at 77 or 298 K. The TD-DFT-calculated states using the TPSSh functional were in excellent agreement with the observed absorption energies and also clearly assessed the nature of the so-called “dark”, i.e., d‒d*, excited configurations to lie low for the Ni complex (≥3.18 eV), promoting rapid radiationless relaxation. For the Pd(II) and Pt(II) derivatives, the “dark” states are markedly higher in energy with ≥4.41 eV (Pd) and ≥4.86 eV (Pt), which is in perfect agreement with the similar photophysical behaviour of the two complexes at low temperatures.

Solvatochromism of a D205 indoline dye at the interface of a small TiO2-anatase nanoparticle in acetonitrile: a combined molecular dynamics simulation and DFT calculation study

ABSTRACT A combined molecular dynamics (MD) simulation and density functional theory (DFT) calculations were used to investigate the effect of the acetonitrile (ACN) solution on the absorption and the charge transfer in a D205 indoline dye at the interface of a TiO2 nanoparticle. DFT calculations were carried out to estimate the equilibrium geometry of a small Ti30O62H4-anatase nanoparticle and to derive interaction parameters for bidentate binding of a D205 dye to TiO2. A series of the D205 dye configurations anchored to Ti30O62H4 were generated from the MD simulations and used as input for the time-dependent DFT (TD-DFT) calculations. We found that the immersion of the D205-Ti30O62H4 composite system into the polar ACN environment results in conformational changes of the dye moiety, which are accompanied by the solvatochromic redshift of its long-wavelength absorption band up to 1900 cm-1. Our results show that the HOMO and LUMO energy level alignment of the dye and the nanoparticle suggests a favourable driving force for charge transfer from the dye to TiO2. The MD/TD-DFT-estimated solvatochromic shift for the D205 dye at the TiO2/ACN interface is in good agreement with the experiment, showing that such computational approach enables reliable predictions of optical properties of other dye-TiO2 composites in solution.