S0 Energy Gap Research Articles

Coordination-driven molecular switch on the base of an ESIPT-capable pyrimidine ligand: Synthesis, fine-tuning of emission by the halide anion and theoretical studies

ESIPT-based materials (ESIPT = Excited State Intramolecular Proton Transfer) find diverse applications in optoelectronics and biomedicine owing to the peculiarities of their luminescence properties. Here, an ESIPT-capable compound 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL4,2,Me) featuring a short O–H⋅⋅⋅N intramolecular hydrogen bond and two N,N-sites for metal binding has been synthesized. HL4,2,Me is the first reported molecule which can act as an ESIPT molecular switch triggered by metal ion coordination without its deprotonation. Drastic changes in the HL4,2,Me conformation in the [Zn(HL4,2,Me)X2] (X = Cl, Br,I) complexes significantly alter the photoluminescence response compared to the free ligand. In the solid state, HL4,2,Me exhibits barrierless ESIPT and large Stokes-shifted yellow-orange emission due to the interplay of anti-Kasha S2 → S0 fluorescence and Kasha-like T1 → S0 phosphorescence radiative channels. The violation of Kasha’s rule for HL4,2,Me is justified by an extraordinarily large S2 – S1 energy gap (ca. 0.9 eV), slowing down the rate of S2 → S1 internal conversion. The photoluminescence behavior of the ESIPT–incapable zinc(II) coordination compounds strongly depends on the halide anion: the chlorido complex exhibits only fluorescence, the bromido complex displays a minor phosphorescence channel in addition to a major fluorescence channel, while the iodido complex exhibits predominantly phosphorescence. As a result, the emission color of the [Zn(HL4,2,Me)X2] complexes changes gradually from blue for X = Cl to orange for X = I, providing a platform for the fine-tuning of emission by the halide anion.

Photoluminescence mechanisms of BF2-formazanate dye sensitizers: a theoretical study.

Photodynamic therapy (PDT) is an alternative, minimally invasive treatment for human diseases such as cancer. PDT uses a photosensitizer to transfer photon energy directly to cellular 3O2 to generate 1O2 (Type II), the toxicity of which leads to cancer cell death. In this work, the photoluminescence mechanisms of a BF2-formazanate dye sensitizer (BF2-FORM) and its iodinated derivative (BF2-FORM-D) were studied using complementary theoretical approaches; the photoluminescence pathways in the S1 and T1 states were studied using density functional theory (DFT) and time-dependent (TD)-DFT methods, the kinetic and thermodynamic properties of the pathways using the transition state theory (TST), and the time evolution and dynamics of key processes using non-adiabatic microcanonical molecular dynamics simulations with surface-hopping dynamics (NVE-MDSH). Evaluation of the potential energy surfaces (PESs) in terms of the rotations of the phenyl rings suggested a pathway for the S1 → S0 transition for the perpendicular structure, whereas two pathways were anticipated for the T1 → S0 transition, namely, [T1 → S0]1 occurring immediately after the S1/T1 intersystem crossing (ISC) and [T1 → S0]2 occurring after the S1/T1 ISC and T1 equilibrium structure relaxation, with the T1 → S0 energy gap being comparable to the energy required for 3O2 → 1O2. The PESs also showed that because of the heavy-atom effect, BF2-FORM-D possessed a significantly smaller S1/T1 energy gap than BF2-FORM. The TST results revealed that at room temperature, BF2-FORM-D was thermodynamically more favorable than the parent molecule. Analysis of the NVE-MDSH results suggested that the librational motions of the phenyl rings play an important role in the internal conversion (IC) and ISC, and the S1/T1 ISC and T1 → S0 transitions could be enhanced by varying the irradiation wavelength and controlling the temperature. These findings can be used as guidelines to improve and/or design photosensitizers for PDT.

Different positions of cyano substitution controlled directionality of ESIPT processes with two asymmetric proton acceptors system: A TD-DFT study

In order to systematically investigate the effect of substitution position on excited state intramolecular proton transfer (ESIPT) process and photophysical properties, three 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxy-benzaldehyde (BTHMB) derivatives (BTHMB-1, BTHMB-2 and BTHMB-3) were designed by introducing the strong electron-withdrawing group (–CN) at different positions. Based on the analyses of configurations, electron densities and infrared vibrational frequencies, the intramolecular hydrogen bond (IHB) of BTHMB derivatives are all enhanced in the excited state, which would promote the ESIPT process. All the BTHMB-1, BTHMB-2 and BTHMB-3 compounds can undergo ESIPT process via two possible pathways. The potential energy curves (PECs) showed that the ESIPT barriers decrease along the order of BTHMB-2-O > BTHMB-3-O > BTHMB-1-O, and BTHMB-3-N > BTHMB-2-N > BTHMB-1-N. Due to the lower energy barrier, the ESIPT process in BTHMB-2 tends to happen to the direction of proton acceptor O atom, while the ESIPT processes in BTHMB-1 and BTHMB-3 tend to happen to the direction of proton acceptor N atom. No matter where the substituted position was, the introduction of –CN group resulted in the obvious red-shift of absorption and fluorescence peaks. For BTHMB-1-O/BTHMB-2-O/BTHMB-3-O and BTHMB-1-N/ BTHMB-2-N/BTHMB-3-N, the red-shift values of absorption and fluorescence peaks in BTHMB-1-O and BTHMB-3-N are the largest, respectively. The more red-shift the absorption and emission peaks show, the smaller the S0 and S1 energy gap between HOMO and LUMO are.

The Energy Gap Law at Work: Emission Yield and Rate Fluctuations of Single NIR Emitters.

Internal conversion (IC) often is the dominating relaxation pathway in NIR emitters, lowering their fluorescence quantum yield. Here, we investigate dibenzoterrylene (DBT) by bulk and single molecule spectroscopy. With increasing solvent polarity, the S1-S0 energy gap decreases leading to a decrease of the fluorescence quantum yield and an increase of the IC rate in full accordance with the energy gap law. Making use of the unexpectedly strong fluorescence solvatochromism of this aromatic hydrocarbon, the validity of the energy gap law could also be demonstrated at the single molecule level. The S1-S0 energy gap not only controls the fluorescence lifetime and quantum yield of single molecules but also dictates how these quantities develop during spectral fluctuations. Our results open new avenues into unexplored single molecule photophysics and appear as a promising tool for nanoscale probing of dynamic heterogeneities.

A 1-Hydroxy-1H-imidazole ESIPT Emitter Demonstrating anti-Kasha Fluorescence and Direct Excitation of a Tautomeric Form.

The ability of 1-hydroxy-1H-imidazoles to exist in the form of two prototropic tautomers, the N-hydroxy and the N-oxide forms, can be utilized in the design of new types of ESIPT-fluorophores (ESIPT=excited state intramolecular proton transfer). Here we report the first example of 1-hydroxy-1H-imidazole-based ESIPT-fluorophores, 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL), featuring a short intramolecular hydrogen bond O-H⋅⋅⋅N (O⋅⋅⋅N 2.56 Å) as a pre-requisite for ESIPT. The emission of HL originates from the anti-Kasha S2 →S0 fluorescence in the N-oxide form as a result of a large S2 -S1 energy gap slowing down the S2 →S1 internal conversion. Due to an energy barrier between the N-hydroxy and N-oxide forms in the ground state, the HL molecules can be trapped and photoexcited in the N-oxide form leading to the Stokes shift of ca. 60 nm which is the smallest among known ESIPT-fluorophores.

The low-lying triplet excited state of N-sulfinylanilines explains their reactivity and inclination to cycloaddition across N–S=O group

Density functional theory calculations for p-Br-N-sulfinylaniline and m-nitro-N-sulfinylaniline in the ground singlet (S0) and triplet (T1) excited states are presented and analyzed in terms of their specific physicochemical properties. As all aromatic N-sulfinylamines, these compounds are rather unstable being sensitive to moist air and we assign this instability to the thermally allowed S0T1 excitation induced by internal magnetic forces. Our calculations indicate that the T1–S0 energy gap in these molecules is unexpectedly low and spin-orbit coupling matrix element between these states is relatively high, being allowed by the orbital symmetry selection rules. We also apply the exchange mechanism of spin-catalysis concept in order to explain the prone of N-sulfinylamines to the Diels-Alder cycloaddition with dienes.

Near-Infrared Photoluminescence and Electrochemiluminescence from a Remarkably Simple Boron Difluoride Formazanate Dye.

A boron difluoride formazanate dye that exhibits near-infrared photoluminescence and electrochemiluminescence was produced via a straightforward two-step synthesis. Examination of its solid-state structure suggested that the N-aryl substituents have significant quinoidal character, which narrows the S1 -S0 energy gap and leads to the unique optoelectronic properties observed. Cyclic voltammetry studies revealed two oxidation waves and two reduction waves that were electrochemically reversible. Electrochemiluminescence properties were examined in the presence of tri-n-propylamine, leading to maximum intensity at 910 nm, at least 85 nm (1132 cm-1 ) red-shifted compared to all other organic dyes. This work sets the stage for the development of future generations of dyes for emerging applications, including single-cell imaging, that require near-infrared photoluminescence and electrochemiluminescence.

Near‐Infrared Photoluminescence and Electrochemiluminescence from a Remarkably Simple Boron Difluoride Formazanate Dye

AbstractA boron difluoride formazanate dye that exhibits near‐infrared photoluminescence and electrochemiluminescence was produced via a straightforward two‐step synthesis. Examination of its solid‐state structure suggested that the N‐aryl substituents have significant quinoidal character, which narrows the S1–S0 energy gap and leads to the unique optoelectronic properties observed. Cyclic voltammetry studies revealed two oxidation waves and two reduction waves that were electrochemically reversible. Electrochemiluminescence properties were examined in the presence of tri‐n‐propylamine, leading to maximum intensity at 910 nm, at least 85 nm (1132 cm−1) red‐shifted compared to all other organic dyes. This work sets the stage for the development of future generations of dyes for emerging applications, including single‐cell imaging, that require near‐infrared photoluminescence and electrochemiluminescence.

A Photophysical Deactivation Channel of Laser-Excited TATB Based on Semiclassical Dynamics Simulation and TD-DFT Calculation.

A deactivation channel for laser-excited 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) was studied by semiclassical dynamics. Results indicate that the excited state resulting from an electronic transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular mrbital (LUMO) is deactivated via pyramidalization of the activated N atom in a nitro group, with a lifetime of 2.4 ps. An approximately 0.5-electron transfer from the aromatic ring to the activated nitro group led to a significant increase of the C–NO2 bond length, which suggests that C–NO2 bond breaking could be a trigger for an explosive reaction. The time-dependent density functional theory (TD-DFT) method was used to calculate the energies of the ground and S1 excited states for each configuration in the simulated trajectory. The S1←S0 energy gap at the instance of non-adiabatic decay was found to be 0.096 eV, suggesting that the decay geometry is close to the conical intersection.

Rational Improvement of Molar Absorptivity Guided by Oscillator Strength: A Case Study with Furoindolizine-Based Core Skeleton.

The rational improvement of photophysical properties can be highly valuable for the discovery of novel organic fluorophores. Using our new design strategy guided by the oscillator strength, we developed a series of full‐color‐tunable furoindolizine analogs with improved molar absorptivity through the fusion of a furan ring into the indolizine‐based Seoul fluorophore. The excellent correlation between the computable values (oscillator strength and theoretical S0–S1 energy gap) and photophysical properties (molar absorptivity and emission wavelength) confirmed the effectualness of our design strategy.

Rational Improvement of Molar Absorptivity Guided by Oscillator Strength: A Case Study with Furoindolizine‐Based Core Skeleton

AbstractThe rational improvement of photophysical properties can be highly valuable for the discovery of novel organic fluorophores. Using our new design strategy guided by the oscillator strength, we developed a series of full‐color‐tunable furoindolizine analogs with improved molar absorptivity through the fusion of a furan ring into the indolizine‐based Seoul fluorophore. The excellent correlation between the computable values (oscillator strength and theoretical S0–S1 energy gap) and photophysical properties (molar absorptivity and emission wavelength) confirmed the effectualness of our design strategy.

Spectroscopic and Photophysical Characterization of Acetylenic Fluorophores: The Role of the Proximity Effect on Increasing Internal Conversion.

The spectroscopic and photophysical behavior of a series of six extended arylacetylenes and one 1,3-diyne derivative was studied in solvents of different polarity to gain insight into the relationships between molecular architecture and optical/photophysical properties. The radiative decay channel was revealed to be the most important one for all the compounds, particularly in nonpolar solvents. A notable fluoro-solvatochromism was observed for the 1,3-diyne derivative in line with the large increase of its dipole moment under excitation. A peculiar behavior was observed for nitro-substituted aryl acetylenes on increasing solvent polarity, which was explained by the presence of an upper forbidden singlet state (S2 ) that is more polar than the allowed S1 state; this was confirmed by quantum mechanical calculations. The large decrease of the S2 -S1 energy gap in a moderately polar solvent causes a strong increase of S1 -S0 internal conversion to the detriment of fluorescence, which is in agreement with Lim's proximity effect model.

Click Reaction Synthesis and Photophysical Studies of Dendritic Metalloporphyrins

AbstractSeveral dendritic zinc(II)‐porphyrins bearing carbazole units at the terminals have been prepared through click reaction of azide‐substituted Zn‐porphyrin precursors and carbazole‐based alkynes under [Cu(NCCH3)4][PF6] catalysis. This family of new dendritic metalloporphyrins shows dual luminescence from both the upper S2 and the lowest S1 singlet states. The observed trends in the spectroscopic data and the photophysical properties of the dendrimers have been rationalized in terms of the type of meso‐spacer between the macrocycle and dendritic shell. The key feature of the meso‐spacer is proposed to be the degree of hindrance towards aryl ring rotation relative to the mean porphyrin plane. Based on the observed differences in the S1→S0 fluorescence quantum yield on going from four‐ to five‐coordinate dendrimers, it was shown that dendrimer architectures with all four meso‐aryl spacers sterically hindered, are most appropriate to monitor processes related to axial ligation of the dendrimer core, since they provide larger luminescence response differences between the four‐coordinate and five‐coordinate forms. The blue S2→S0 fluorescence quantum yield has been measured and the observed trend has been rationalized in terms of the S2→S1 energy gap law. No significant differences were observed between the compounds either with different rotational degree of freedom of the meso‐spacers or with different ligation states of the dendrimer core.

Comprehensive investigation of the excited-state dynamics of push–pull triphenylamine dyes as models for photonic applications

A series of emitting push-pull triarylamine derivatives, models of their widely used homologues in photonics and organic electronics, was investigated by steady-state and time-resolved spectroscopy. Their structural originality stems from the sole change of the electron-withdrawing substituent X (-H: 1, -CN: 2, -NO2: 3, -CHC(CN)2: 4), giving rise to efficient emission tuning from blue to red upon increasing the X electron-withdrawing character. All compounds are highly fluorescent in alkanes. The more polar compounds 2-4 undergo considerable Stokes shift and emission quenching in polar solvents. Femtosecond transient absorption data allowed us to identify the nature of the emissive state which varies as a function of the compound and surrounding polarity. A long-lived ππ* excited state with weak charge transfer character was found for 1. This excited state evolves into a long-lived ICT state with red-shifted emission for 2 in polar solvents. For 3 and 4, the ICT state is directly populated in all solvents. Long-lived and emissive in n-hexane, it relaxes in toluene to a new ICT' conformation with stronger charge transfer character and enhanced Stokes shift. In more polar THF, ethanol, and nitrile solvents, ICT relaxes to a dark excited state ICT'' with viscosity-dependent kinetics (<10 ps). The ICT'' state lifetime drops with increasing solvent polarity (150 ps for 3 in THF, 8.5 ps in butyronitrile, 1.9 ps in acetonitrile), denoting an efficient radiationless deactivation to the ground state (back charge transfer). This result reveals a very small S0-S1 energy gap at the relaxed ICT'' geometry, with a possible close-lying S0-S1 conical intersection, which suggests that the ICT → ICT'' process results from a structural change involving a large-amplitude molecular distortion. This fast structural change can account for the strong fluorescence quenching observed for 3 and 4 in polar solvents. Finally, the magnitude of intersystem crossing between the singlet and triplet excited states largely depends on the electron-deficient X unit and the solvent itself. These observations help one conclude on the prevailing role played by the electron-withdrawing groups and the surrounding polarity in the photophysical performances of triphenylamine derivatives, largely employed in numerous emissive solid-state devices.

Electronic structure of tris(2-phenylpyridine)iridium: electronically excited and ionized states

A computational study of tris(2-phenylpyridine)iridium, Ir(ppy)3, is presented. The perspective is that of using organo-transition-metal complexes as phosphorescent species in light-emitting diodes (OLED's). Quantum yields approaching 100% are possible through a triplet harvesting mechanism. Complexes such as Ir(ppy)3 are amenable to exacting experimental and theoretical studies: small enough to accommodate rigor, yet large enough to support bulk phenomena in a range of host materials. The facial and meridional isomers differ by ∼220 meV, with fac-Ir(ppy)3 having the lower energy. Because fac-Ir(ppy)3 dominates in most environments, focus is on this species. Time-dependent density functional theory using long-range-corrected functionals (BNL and ωB97X) is used to calculate excited states of Ir(ppy)3 and a few low energy states of . The calculated T1 – S0 energy gap (2.30 eV) is in reasonable agreement with the experimental value of 2.44 eV. Only a few percent of singlet character in T1 is needed to explain so short a phosphorescence lifetime as 200 ns, because of the large and absorption cross-sections. Equilibrium geometries are calculated for S0, T1, and the lowest cation state (D0), and several ionization energies are obtained: adiabatic (5.86 eV); vertical from the S0 equilibrium geometry (5.88 eV); and vertical ionization of T1 at its equilibrium geometry (5.87 eV). These agree with a calculation by Hay (5.94 eV), and with the conservative experimental upper bound of 6.4 eV. Molecular orbitals provide qualitative explanations. A calculated UV absorption spectrum, in which transitions are vertical from the S0 equilibrium geometry, agrees with the room temperature experimental spectrum. This is consistent with Franck–Condon factors dominated by , as expected given the delocalized nature of the orbitals. Ir(ppy)3 vibrational frequencies were calculated and used to estimate the probability density for 500 K, i.e. the temperature at which the experiments were carried out. In combination with the vibrational energy imparted through photoexcitation, it is seen that a large amount of vibrational energy appears in without causing its fragmentation. Specifically, for = 15,000 cm−1, the probability density for total vibrational energy peaks at ∼31,000 cm−1 with a 7800 cm−1 width.

The Influence of Temperature on C153 Steady-State Absorption and Fluorescence Kinetics in Hydrogen Bonding Solvents

In a recent paper (J Fluoresc (2011) 21:1547–1557) a temperature induced modulation of Coumarin 153 (C153) fluorescence lifetime and quantum yield for the probe dissolved in the polar, nonspecifically interacting 1-chloropropane was reported. This modulation was also observed in temperature dependencies of the radiative and nonradiative rates. Here, we show that the modulation is also observed in another 1-chloroalkane—1-chlorohexane, as well as in hydrogen bonding propionitrile, ethanol and trifluoroethanol. Change in the equilibrium distance between S 0 an S 1 potential energies surfaces was identified as the source of this modulation. This change is driven by temperature changes. It leads to a modulation of the fluorescence transition dipole moment and it is the primary source of the experimental effects observed. Additionally, we have found that proticity of the solvent induces a rise in the fluorescence transition dipole moment, which leads to a shortening of the fluorescence lifetime. Hydrogen bonds are formed by C153 also with hydrogen accepting solvents like propionitrile. We show that while such bonds do not affect the transition probability, they do change the S0 an S1 energy gap which in turn implies a change in non-radiative transition rate in a similar way as in protic solvents, as well as in the fluorescence spectrum position. Finally, the influence of temperature on the energies of hydrogen bonds formed by C153 when acting as hydrogen donor or acceptor is reported.

Excited-state dynamics of phenol–pyridinium biaryl

The excited-state dynamics of a donor-acceptor phenol-pyridinium biaryl cation was investigated in various solvents by femtosecond transient absorption spectroscopy and temperature dependent steady-state emission measurements. After excitation to a near-planar Franck-Condon delocalized excited S(1)(DE) state with mesomeric character, three fast relaxation processes are well resolved: solvation, intramolecular rearrangement leading to a twisted charge-shift (CSh) S(1) state with localized character, and excited-state proton transfer (ESPT) to the solvent leading to the phenoxide-pyridinium zwitterion. The proton transfer kinetics depends on the proton accepting character of the solvent whereas the interring torsional kinetics depends on the solvent polarity and viscosity. In nitriles, ESPT does not occur and interring twisting arises with no significant intrinsic barrier, but still slower than solvation. The CSh state is notably fluorescent. In alcohols and water, ESPT is faster than the solvation and DE → CSh relaxation processes and yields the zwitterion hot ground state, which strongly quenches the fluorescence. In THF, solvation and interring twisting occur first, leading to the fully relaxed, weakly fluorescent CSh state, followed by slow ESPT towards the zwitterion. At low temperature (77 K), the large viscous barrier of the solvent inhibits the torsional relaxation but ESPT still arises to some extent. Strong emission from the DE geometry and planar zwitterion is thus observed. Finally, quantum chemical calculations were performed on the ground and excited state of model phenol-pyridinium and phenoxide-pyridinium compounds. Strong S(1) state energy stabilization is predicted upon twisting in both cases, consistent with a fast relaxation towards the perpendicular geometry. A substantial S(0)-S(1) energy gap is still present for the twisted cationic species, which can explain the long-lived emission of the CSh state in nitriles. A quite different situation arises with the zwitterion for which the S(0)-S(1) energy gap predicted at the twisted geometry is very small. This suggests a close-lying conical intersection and can account for the strong fluorescence quenching observed in solvents where the zwitterion is produced by ESPT.

Near‐Infrared Dyes and Fluorophores Based on Diketopyrrolopyrroles

Our research activities over the last few decades have concerned correlations between molecular structure and fluorescence. 2] A central theme of the research has been the synthesis and spectroscopic investigations of near-infrared (NIR) dyes to get insights into what degree the obtainable fluorescence quantum yields are restricted by the S0$S1 energy gap. Our results on the condensation of 1,4phthalazinediones with heteroarylacetonitriles in POCl3 prompted us to carry out the reaction with diketopyrrolopyrroles (DPPs) 1. Herein we report the results of the investigations. To date, attempts to activate the carbonyl group of DPPs with POCl3, and to subsequently convert the intermediates with nucleophiles, only led to monosubstitution or to ring opening. 10] The reaction of 1 and 2 in refluxing toluene with an excess of POCl3 according to Scheme 1 afforded disubstituted NIR dyes 3. The progress of the reaction was controlled by recording absorption spectra and stopping the heating as soon as the DPP was used up and/or products absorbing at short wavelengths appeared. Purification was carried out by digesting the product in acetone and subsequent flash chromatography (silica gel/CHCl3 or CH2Cl2). The NIR dyes 3a–3h (the structures and spectroscopic data of 3b–3h can be found in the Supporting Information) were synthesized according to this procedure (Scheme 1) from the reaction of 1 and 2. The course of the reactions was the same in all cases and side-products with strong absorptions below 350 nm were observed. These side-products are most likely compounds formed by the opening of the DPPs pentalene ring system. Prerequisite for the reactions is a certain solubility of the DPPs. This is a feature of all of the DPPs used here with the exception of the 4-methoxy derivative 1b. Remarkably, the 4(N-methyl-N-octyl amino) derivative 1e has a much better solubility than all the other DPPs used. Only in the case of the reaction of 1e with 2a were we able to isolate the 1:1 condensation product (13% yield). The heteroarylacetonitriles 2a and 2b substituted with a tert-butyl group were used to improve the solubility of the condensation products 3. In general, satisfying yields were obtained only if the solubility of both condensation partners was improved by having longer alkyl groups (Table 1). From these observations we draw the following conclusion concerning the reaction pathway: DPP reacts with POCl3 to form a monophosphorylated intermediate, which reacts with the Scheme 1. Reagents and conditions: a) absolute toluene/POCl3, reflux; b) 1,2-dichlorobenzene/BF3·Et2O, reflux, diisopropylethylamine; c) xylene/chlorodiphenylborane, reflux; DPPs 1: R=4-octyloxy (1a), R=4-methoxy (1b), R=4-butyloxy (1c), R=4-(hex-5-enyloxy) (1d), R=4-(N-methyl-N-octylamino) (1e); heteroarylacetonitriles 2 : 2-(6tert-butylquinolin-2-yl)acetonitrile (2a), 2-(6-tert-butylbenzothiazol-2yl)acetonitrile (2b), 2-(quinoxalin-2-yl)acetonitrile (2c), 2-(6-methylpyridin-2-yl)acetonitrile (2d). A: aromatic ring.

On the Formation of Cyclobutane Pyrimidine Dimers in UV-irradiated DNA: Why are Thymines More Reactive?¶

The reaction pathways for thermal and photochemical formation of cyclobutane pyrimidine dimers in DNA are explored using density functional theory techniques. Although it is found that the thermal [2 + 2] cycloadditions of thymine + thymine (T + T → T⋄T), cytosine + cytosine (C + C → C⋄C) and cytosine + thymine (C + T → C⋄T) all are similarly unfavorable in terms of energy barriers and reaction energies, the excited-state energy curves associated with the corresponding photochemical cycloadditions display differences that—in line with experimental findings—unanimously point to the predominance of T⋄T in UV-irradiated DNA. It is shown that the photocycloaddition of thymines is facilitated by the fact that the S1 state of the corresponding reactant complex lies comparatively high in energy. Moreover, at a nuclear configuration coinciding with the ground-state transition structure, the excited-state energy curve displays an absolute minimum only for the T + T system. Finally, the T + T system is also associated with the most favorable excited-state energy barriers and has the smallest S2–S0 energy gap at the ground-state transition structure.

Structure, Spectroscopy, and Spectral Tuning of the Gas-Phase Retinal Chromophore: The β-Ionone “Handle” and Alkyl Group Effect

The low-lying singlet states (i.e. S0, S1, and S2) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers. On the basis of the S0-->S1 vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that measured in solution (2.82 eV in methanol). In addition, the absorption maxima associated with the blue, green, and red cone visual pigments are tentatively rationalized in terms of the spectral changes computed for PSB11 structures featuring differently twisted beta-ionone rings. More specifically, a blue-shifted absorption maximum is explained in terms of a large twisting of the beta-ionone ring (with respect to the main conjugated chain) in the visual S-cone (blue) pigment chromophore. In contrast, the chromophore of the visual L-cone (red) pigment is expected to have a nearly coplanar beta-ionone ring yielding a six double bond fully conjugated framework. Finally, the M-cone (green) chromophore is expected to feature a twisting angle between 10 and 60 degrees. The spectroscopic effects of the alkyl substituents on the PSB11 spectroscopic properties have also been investigated. It is found that they have a not negligible stabilizing effect on the S1-S0 energy gap (and, thus, cause a red shift of the absorption maximum) only when the double bond of the beta-ionone ring conjugates significantly with the rest of the conjugated chain.