DABCO-catalyzed one-pot three component synthesis of dihydropyrano[3,2-c]chromene substituted quinazolines and their evaluation towards anticancer activity.

A facile DABCO promoted one-pot three component synthesis of a new series of C-C linked bis-heterocycle containing dihydropyrano[c]chromene as highly fused oxa-heteryl group at C-2 position of quinazoline was developed. Quinazoline-2-carbaldehyde, substituted 4-hydroxycoumarin and ethyl cyanoacetate were used as key components in the Knoevenagel-Michael addition reaction to get the titled compounds. These compounds were screened for anti-cancer activity against the breast cancer cell lines of MDA-MB 231, and MDA-MB 453.

Effects of iptycene scaffolds on the photoluminescence of N,N-dimethylaminobenzonitrile and its analogues.

To understand the effect of iptycene scaffolds on the locally excited (LE) and intramolecular charge transfer (ICT) fluorescence of aminobenzonitriles, a series of triptycene and pentiptycene derivatives were synthesized and their molecular structures and photophysical properties were characterized and compared with the parent phenylene systems, 4-(N-methylamino)benzonitrile (MABN), 4-(N,N-dimethylamino)benzonitrile (DMABN), and 4-(N-phenylamino)benzonitrile (PABN). The iptycene effect does not change the nature of the fluorescing states for each amino donor system, i.e., the MA, PA, and DMA series display LE-only, ICT-only, and LE-ICT dual fluorescences, respectively. However, the iptycene scaffolds impose a significant modification of the absorption and emission spectra, fluorescence quantum efficiency and lifetimes, and the interplay of LE and ICT states. The observed iptycene effect has been discussed with three factors: (1) steric effect on increasing the amino twist angle, (2) steric shielding of solvation to the aminobenzonitrile core, and (3) hyperconjugation interactions of the aminobenzonitrile core with the peripheral phenylene groups of iptycene.

Effects of hydrogen bonding on internal conversion of GFP-like chromophores. II. The meta-amino systems.

To rationalize the efficient quenching of the fluorescence and the Z → E photoisomerization of m-ABDI, the meta-amino analogue of the green fluorescent protein (GFP) chromophore, in protic solvents, the femtosecond time-resolved fluorescence and transient infrared (TRIR) spectra of m-ABDI in CD3CN, CH3OH, and CD3OD are determined. For solutions in CD3CN, the fluorescence decay lifetime is ∼7.9 ns and IR absorption lines near 1513, 1531, 1557, and 1613 cm(-1) of m-ABDI in its electronically excited state were observed with a decay time >5 ns. For solutions in CH3OH, the fluorescence decay is double exponential with time constants of ∼16 and 62 ps. In addition to IR absorption lines of m-ABDI in its electronically excited state with a decay time of ∼16 ps, new features near 1513, 1532, 1554, and 1592 cm(-1) were observed to have a rise time of ∼19 ps and a decay constant of ∼58 ps, indicating formation of an intermediate. The assignments for the IR spectra of the ground and excited states were assisted with DFT and TDDFT calculations, respectively. We conclude that the torsion of the exocyclic C═C bond (the τ torsion) is responsible for the nonradiative decay of electronically excited m-ABDI in CD3CN. However, in CH3OH and CD3OD, the solute-solvent hydrogen bonding (SSHB) interactions diminish significantly the barrier of the τ torsion and induce a new pathway that competes successfully with the τ torsion, consistent with the efficient fluorescence quenching and the diminished yield for Z → E photoisomerization. The new pathway is likely associated with excited-state proton transfer (ESPT) from the solvent to m-ABDI, particularly the carbonyl group, and generates an intermediate (ESPT*) that is weakly fluorescent.

Effects of hydrogen bonding on internal conversion of GFP-like chromophores. I. The para-amino systems.

To understand the effects of solvent-solute hydrogen bonding (SSHB) on the excited-state dynamics of two GFP-like chromophores, p-ABDI and p-CFABDI, we have determined the quantum yields for fluorescence (Φf) and the isomerization Z → E (ΦZE) and the femtosecond fluorescence and transient infrared absorption in selected solvents. The behavior that ΦZE ≅ 0.50 in aprotic solvents, such as CH3CN, indicates that the E-Z photoisomerization adopts a one-bond-flip mechanism through the torsion of the exocyclic C═C bond (the τ torsion) to form a perpendicular species (τ ∼90°) in the singlet excited state followed by internal conversion (IC) to the ground state and partition to form the E and Z isomers with equal probabilities. The observed ΦZE decreased from 0.50 to 0.15-0.28 when CH3CN was replaced with the protic solvents CH3OH and CF3CH2OH. In conjunction with the solvent-independent rapid (<1 ps) kinetics for the fluorescence decay and the solvent-dependent slow (7-20 ps) kinetics for the ground-state recovery, we conclude that the SSHB modifies the potential energy surface for the τ torsion in a way that the IC occurs also for the twisted intermediates with a τ-torsion angle smaller than 90°, which favors the formation of the Z isomers. The possibility of IC induced by torsion of the exocyclic C-C bond (the φ torsion) is also considered but excluded.

Site-selective hydrogen-bonding-induced fluorescence quenching of highly solvatofluorochromic GFP-like chromophores.

The unconstrained green fluorescence protein (GFP)-like chromophore m-DMABDI displays a high solvatofluorochromicity in aprotic solvents, but the fluorescence is quenched in protic solvents. According to the site-specific intramolecularly hydrogen-bonded analogs 1OH and 2OH, the hydrogen bonding to the carbonyl oxygen is more important than that to the imino nitrogen of the imidazolinone group in the fluorescence quenching.

A redox-gated slow-fast-stop molecular rotor.

A pentiptycene-derived p-phenylenediamine mimics a molecular double-rotor system that displays redox-dependent rotation rates for the amino rotors about the pentiptycene-amine C-N bond. The rotation is accelerated in the radical cation state but stopped in the di(radical cation) state. Electronic interplay of the two rotors is also discussed.

Ortho-branched ladder-type oligophenylenes with two-dimensionally π-conjugated electronic properties.

The synthesis, photochemical and electrochemical properties, and electronic structures of a series of star-shaped ladder-type oligophenylenes Sn (n = 7, 10, 13, 16, 19, and 22), including one multibranched case S19mb, are reported and compared with the linear para-phenylene ladders Rn (n = 2-5 and 8) and the stepladder analogues SFn (n = 10, 16, and 22). The n value refers to the number of π-conjugated phenylene rings. Functionalized isotruxenes are the key synthetic building blocks, and S22 is the largest monodispersed ladder-type oligophenylene known to date. The Sn systems possess the structural rigidity of Rn and the ortho-para phenylene connectivity of SFn. Consequently, Sn represents the first class of branched chromophores with fully two-dimensional conjugation in both ground- and excited-state configurations. Evidences include the excellent linear correlations for the optical 0-0 energies or the first oxidation potentials of Sn and Rn against the reciprocal of their n values, delocalized HOMO and LUMO based on density functional theory calculations, and molecule-like fluorescence anisotropy. The resulting model of effective conjugation plane (ECP) for the two-dimensional π-conjugated systems compliments the concept of effective conjugation length (ECL) for one-dimensional oligomeric systems. Other implications of the observed structure-property relationships are also included.

o-Amino conjugation effect on the photochemistry of trans-aminostilbenes.

The excited-state behavior of a series of trans-2-(N-arylamino)stilbenes (aryl = phenyl (o1H), 4-methylphenyl (o1Me), 4-methoxyphenyl (o1OM), and 4-cyanophenyl (o1CN)) and trans-2-(N,N-diphenylamino)stilbene (o2) in both nonpolar and polar solvents is reported and compared to that of the parent trans-2-aminostilbene and the corresponding meta- and para-isomers (m1R and p1R, where R = H, Me, OM, and CN, and m2 and p2). Two types of torsional motions, the D-A torsion that results in a nonfluorescent twisted intramolecular charge transfer (TICT) state and the C═C torsion that leads to the cis isomers, account for the radiationless decays of o1R and o2. The relative efficiencies of these torsions can be readily evaluated from their quantum yields for fluorescence (Φ(f)) and trans → cis isomerization (Φ(tc)). The propensities of the D-A torsion are similar for the ortho and meta isomers, which is 1OM > 1Me and negligible for 1H, 1CN, and 2. The activation parameters determined from temperature-dependent fluorescence lifetimes suggest that the C═C torsion occurs mainly via the triplet state for the ortho systems, a behavior again similar to that of the meta isomers. Whereas the intersystem crossing in o1R, m1R, and m2 is essentially a nonactivated process, it encounters a barrier of 2.7-3.8 kcal mol(-1) in o2. As a result of the barriers that decelerate the radiationless decays and the slow fluorescence rate for o2 in acetonitrile, the observed long fluorescence lifetime 24.5 ns at room temperature reaches a new record for unconstrained trans-stilbenes.

Enhanced diradical nature in oxyallyl derivatives leads to near infra red absorption: a comparative study of the squaraine and croconate dyes using computational techniques.

We apply many criteria to estimate the diradical character of the ground state singlets of several oxyallyl derivatives. This is carried out as the oxyallyl derivatives like squaraine and croconate dyes can be represented by both mesoionic and diradical formulas, the domination of which would characterize its lowest energy transition. One criterion applied is the singlet-triplet gap, which is known to be inversely proportional to the diradical character. Another criterion is the occupation number; this is determined for the symmetry broken state of the molecules in the unrestricted formalism, and the difference of occupation in the HOMO and LUMO is related to the diradical character. The diradical character of all of the croconates and few squaraines is estimated to be large. All of these have absorption above 750 nm and can be classified as near infrared (NIR) dyes, leading to the inference that NIR absorptions in these molecules are largely due to the dominance of the diradical character. To understand the reliability of the DFT methods for the absorption property predictions of these molecules, TD-DFT studies to calculate the vertical excitation energies have been carried out, using the B3LYP/ BLYP exchange correlation functionals and the LB94 asymptotic functional with and without the inclusion of solvent. The deviations, in both the squaraine series (average lower diradical character), are found to be systematic, and with the inclusion of the solvent in the calculation, the deviations decrease. The best least-squares fit with the experimentally observed values using B3LYP /6-311G(d, p) for the symmetric squaraines yields an R value of 0.92 and, for the unsymmetric squaraines, an R value of 0.936. With inclusion of the solvent, the R value is 0.96 for the symmetric squaraines and 0.961 for the unsymmetric squaraines, indicating that these DFT functionals with linear scaling may be used to study these systems. The croconate dyes, however, have larger deviation from the experimentally observed values in all of the functionals studied even after inclusion of the solvent effects. The deviations are also not systematic. The deviation with respect to the experiment in this case is attributed to the average larger diradical character in this series.

Structure, bonding, and lowest energy transitions in unsymmetrical squaraines: a computational study.

Natural resonance theory (NRT) and natural bond orbital (NBO) analysis have been carried out on a simple symmetrical and an unsymmetrical substituted squaraine with a view of understanding the structure of the latter type of squaraines. It is found that there are some fundamental differences in the structure and bonding between these two types of squaraines particularly in the resonance weights and delocalization energies. These differences are expected to reflect in the low energy transitions and charge transfer in these squaraines. To investigate this, the nature of the lowest energy transitions occurring on excitation in unsymmetrical squaraines has been studied using high-level symmetry adapted cluster-configuration interaction method (SAC/SAC-CI) and compared with reported experimental observations. In general the agreement with the experimental data is very good. The transition dipole moment always lies on the pi-backbone and is quite large in magnitude. The ground state dipole moment in some cases does not change in the excited state upon excitation while in some other cases there is a large reduction/enhancement in the magnitude indicative of some charge rearrangement in this direction. Inclusion of the solvent using the IEFPCM model, a slightly better agreement with the experiment is found in some cases. Studies are carried out with a different basis set and it is found that the change in basis set has very little effect on the transition energies. In the case of weak side donor groups attached to the central ring the larger charge transfer to the central acceptor ring in general takes place from the O- atoms of the squarylium moiety while in the case of strong donors the charge transfer from the O- atoms to the central rings drop down. We have not observed any correlation between the charge transfer in the excited state to the central ring from the side donor groups and the lowest energy excitation in the molecules. Reduction of the HOMO-LUMO gap (an indication of increase of the diradicaloid character) always leads to a bathochromic shift.

Role of the oxyallyl substructure in the near infrared (NIR) absorption in symmetrical dye derivatives: A computational study.

It is well-known from experimental studies that the oxyallyl-substructure-based squarylium and croconium dyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (J. Am. Chem. Soc. 2003, 125, 348) which contains the same basic chromophore, but the absorption is red-shifted by at least 300 nm compared to the former dyes and is observed near 1100 nm. To analyze the reasons behind the large red shift, in this work we have carried out symmetry-adapted cluster-configuration interaction (SAC-CI) studies on some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlier reports, it is seen that the donor groups do not seem to play a major role in the red-shift of the absorption. On the other hand, on the basis of the results of the high-level calculations carried out here and using qualitative molecular orbital theory, it is observed that the orbital interactions play a key role in the red shift. Finally, design principles for the oxyallyl-substructure-based NIR dyes are suggested.

Near-infrared absorption in symmetric squarylium and croconate dyes: a comparative study using symmetry-adapted cluster-configuration interaction methods.

Symmetric croconate (CR) and squarylium dyes (SQ) are well-known near-infrared (NIR) dyes and, in general, are considered to be donor-acceptor-donor type molecules. It is established in the literature that CR dyes absorb in a longer wavelength region than the corresponding SQ dyes. This has been attributed to the CR ring being a better acceptor than the SQ ring. Thus increasing the donor capacity should lead to a bathochromic shift in both SQ and CR. On the other hand, some experiments reported in the literature have revealed that increasing the conjugation in the donor part of the SQ molecule leads first to red shift, which upon a further increase of the conjugation changes to a blue shift. Hence, to understand the role of the central ring and the substitutions in the absorption of these dyes, we carried out high-level symmetry-adapted cluster-configuration interaction (SAC-CI) calculations of some substituted SQ and CR dyes and compare the absorption energy with the existing experimental data. We found that there is very good agreement. We also carried out SAC-CI calculations of some smaller model molecules, which contain the main oxyallyl substructure. We varied the geometry (angle) of the oxyallyl subgroup and the substitution in these model molecules to establish a correlation with the bathochromic shift. We found that the charge transfer is very small and does not play the key role in the red shift, but on the other hand, the perturbation of the HOMO-LUMO gap (HLG) from both the geometry and substitution seems to be responsible for this shift. We suggest as a design principle that increasing the donor capacity of the groups may not help in the red shift, but introducing groups which perturb the HLG and decrease it without changing the MO character should lead to a larger bathochromic shift.