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Electron donor–acceptor–donor (D–A–D) π-conjugated molecules based on dipyrido[3,2-a:2′,3′-c]phenazine (dppz) were developed as photosensitizers for singlet oxygen generation. These dppz derivatives showed electronic absorption assigned to an intramolecular charge transfer transition in the red region at a wavelength of around 600 nm. Evaluation of singlet oxygen generation quantum yield (ΦΔ) using the trapping agent 1,3-diphenylisobenzofuran showed relatively efficient singlet oxygen generation (ΦΔ; 0.173–0.255) for the dppz derivatives with selenophene-containing π-conjugated side-arms. On the other hand, 2,7-dibromo substitution and Pt(II)Cl2 complexation of the dppz derivative with thiophene-containing side-arms did not facilitate photosensitized singlet oxygen generation. Density functional theory (DFT) and time-dependent DFT calculations, as well as evaluation of the kinetics of the photophysical processes, revealed that appropriate placement of heavy atoms on the frontier orbitals facilitates intersystem crossing from the lowest singlet excited state to the triplet excited state, thus improving ΦΔ. [ABSTRACT FROM AUTHOR]

Five, D–A–D triad type dibenzophosphole oxide (DBPO) derivatives, 1a–e, containing electron‐donating diarylamine or carbazole moieties at their 2‐ and 8‐positions were prepared to investigate relationships that exist between their molecular structures and photophysical properties. Analysis of the crystal structure of the 2,8‐bis(diphenylamino) derivative 1a, revealed that its DBPO moiety possesses quinoid‐like bond‐alternation. In addition, DBPO 1a emits relatively high intramolecular charge transfer (ICT) type photoluminescence (PL) in nonpolar solvents. In contrast, DBPO 1b, possessing two carbazol‐9‐yl groups, has higher PL quantum yields than does 1a especially in polar solvents and the solid state. Analysis of the PL decays profiles of toluene solutions of the 6‐membered cyclic acridin‐10‐yl and phenoxazin‐10‐yl‐substituted DBPOs 1c and 1d reveal the existence of long‐lived components, which likely corresponds to thermally‐activated delayed fluorescence (TADF) at r.t. At low temperature, TADF occurs in a toluene solution of 1d with a lifetime of 188 ms. The related 9‐ethylcarbazol‐3‐yl‐substituted DBPO 1e has a higher PL quantum yield and shorter luminescent lifetime than does 1b. The findings show that the fluorescence properties of DBPOs can be readily controlled by introducing donor moieties. This serves as useful information for developing strategies to design DBPO emitters. [ABSTRACT FROM AUTHOR]

Dyad 4, comprising a triphenylamine (TPA) electron donor and 1,4-pentadien-3-one (pentadienone) electron acceptor tethered by a nonconjugated linker, displays solvatofluorochromism (SFC) and dual fluorescence associated with intramolecular charge transfer (ICT) in the excited state. While the fluorescence arises from a locally excited state of 4 (LE-4*) in saturated hydrocarbon solvents, the fluorescence from the ICT state of 4 (ICT-4*) occurs in aprotic solvents. ICT-4* has a much greater dipole moment than its corresponding ground state. The results of theoretical calculations suggest that the conversion of LE-4* to ICT-4* involves a unique structural change like a leaning of the pentadienone moiety. Two factors are responsible for the significant SFC displayed by 4, the first being the high electron-donating and -accepting abilities of the respective locally excited TPA and pentadienone moieties in LE-4* and the other being a rigid ethano bridge that links the two moieties in ICT-4*. The former property facilitates photoinduced electron-transfer (PET) and the latter prevents full single electron transfer (SET) by prohibiting direct π-conjugation and the spatial approach of the two dyad components. Consequently, these electronic and geometrical features lead to SFC arising from a large dipole moment change caused by ICT and partial intramolecular SET. [ABSTRACT FROM AUTHOR]

The photophysical properties of o-tolyl-, m-tolyl-, and p-xylyl-substituted asymmetric diaroylmethanatoboron difluorides in a mixture of CH2Cl2 and c-C6H12, and in the crystalline state were determined. In solution, the fluorescence (FL) properties of these substances are controlled by the position and number of methyl groups on the phenyl rings. An especially interesting finding is that FL from the p-xylyl derivative occurs from an excited state which possesses intramolecular charge-transfer character caused by the o- and m-methyl groups cooperatively. The results of X-ray crystallographic analysis reveal that these asymmetric diaroylmethanatoboron difluorides form dyads through orbital overlap of neighboring molecules. This phenomenon governs the unique FL properties of these substances in the solid state. [ABSTRACT FROM AUTHOR]

To establish the reaction mechanism of the high-quantum-yield bioluminescence in Cypridina (Vargula), we investigated the chemiluminescence of 6-aryl-2-methylimidazo[1,2-a]pyrazin-3(7H)-ones (1H) as Cypridina luciferin analogues in DMSO—1,1,3,3-tetramethylguanidine and in diglyme—acetate buffer. We found that the chemiluminescence of 1H with an electron-donating aryl group, such as a 4-(dimethylamino)phenyl, 3-indolyl or 3-(1-methyl)indolyl group, gave a high quantum yield (ΦCL) in diglyme—acetate buffer. This indicates that the reaction mechanism producing this high ΦCL involves the chemiexcitation of a neutral dioxetanone intermediate possessing an electron-donating aryl group to the singlet excited state of neutral acetamidopyrazine (the light emitter). In addition, we investigated the fluorescence of acetamidopyrazines and performed DFT calculations for neutral dioxetanones and the transition states (TS) of the dioxetanone's decomposition. The results made it clear that the electron-donating aryl group gives the TS and the singlet-excited acetamidopyrazine (S1) a strong intramolecular charge transfer (ICT) character, and their similar ICT character leads to the ICT TS→ S1 route in the charge transfer-induced luminescence (CTIL) mechanism for efficient chemiexcitation. The reaction mechanism of the chemiluminescence of 1H can explain the highly efficient chemiexcitation of Cypridina bioluminescence. [ABSTRACT FROM AUTHOR]

Abstract: Fluorescence of the phenolate anion (3(O)−) and the amide anion (5(N)−) of coelenteramide analogues in ion pairs with various counter cations was systematically investigated to elucidate the ionic structure of the light emitter in the bioluminescence of the calcium-activated photoproteins aequorin and obelin. The fluorescent properties of 3(O)− in an ion pair with a conjugate acid of an organic base (BASE–H+) were varied depending on the structural variation of the ion pair and the solvent polarity. In particular, the fluorescence of 3(O)− in the ion pair with the conjugate acid of n-butylamine (NBA–H+) indicates that the singlet-excited state of 3(O)− (1 3(O)−∗) and NBA–H+ make a contact ion pair in which the fluorescence emission maxima of 3(O)− is sensitive to the solvent polarity and the fluorescence quantum yields of 3(O)− increase in a less polar solvent. The results also confirm that 1 3(O)−∗ is a twisted intramolecular charge transfer state. By contrast, the fluorescence of 5(N)− in an ion pair depends little on the BASE–H+ or the solvent polarity. Based on these results, we conclude that the light emitter in aequorin and obelin bioluminescences is the singlet-excited state of coelenteramide phenolate anion 2(O)− (1 2(O)−∗) in a contact ion pair with an imidazolium side chain of a histidine residue, which is located at the less polar active sites of the photoproteins. We also propose a mechanism for the bioluminescence reaction, including the chemiexcitation process to give 1 2(O)−∗. [Copyright &y& Elsevier]