LAUSR

In connection with the widespread use of substituted phthalide derivatives in various areas of technical activity, they remain the subject of intensive research. One of the topical unresolved problems in this field is revealing the nature of photoand electroluminescence of thin polymer films based on phthalide (polyarylene phthalides) with unusual conductivity properties [1]; to solve this problem, it is necessary to have data on phthalide luminescence. However, it was shown [2] that phthalide does not f luorescence, but only phosphoresces at low temperatures. This is due to the rapid transition from the lowest singlet state S1(n,π*) to one of the lower lying triplet states [2, 3]. As shown in [4] using aldehydes as an example, nonfluorescent compounds with the nπ* lower electronic state that have a small splitting between the nπ* and ππ* states begin to f luoresce in polar solvents. The reason for this is a change of the type of the lowest electronically excited state from nπ* to ππ* due to a change in the energies of these states upon the formation of a hydrogen-bonded complex in polar solvents [4, 5]. The energy gap between the lowest excited singlet nπ* state and the subsequent ππ* state of phthalide is small, being 0.11 eV as given by the data of CNDO/S-CI calculation [6] or 0.10 eV according to this work. On the other hand, it is well known that the carbonyl group borne by the phthalide molecule forms a stable hydrogen bond with electron acceptors, such as the hydroxyl group of the methanol molecule [7]. Thus, both these factors can be responsible for the change in the order of sequence of the nπ* and ππ * states in methanol solution and, hence, the appearance of f luorescence. The allowance for the formation of hydrogen bonds between carbonyl groups of solute molecules and methanol molecules in calculating the electronic spectra leads to a fairly good description of the absorption spectrum in a methanol solution [8]. In this study we recorded f luorescence, f luorescence excitation, and absorption spectra of phthalide (Aldrich, 98%) in cyclohexane (Panreac, 99.9%) and methanol (Fluka Hydronal, ≥99.99%) solutions. The absorption spectra were recorded on a Shimadzu UV2401 spectrophotometer. The f luorescence and fluorescence excitation spectra (C = 2 × 10−5 mol/L) were recorded on a Shimadzu RF-5301PC spectrophotometer at a temperature of 296 K; the spectral width of monochromator slits on the side of excitation and luminescence was 5 nm. The geometry of the phthalide molecule in cyclohexane and its hydrogenbonded complex with the methanol molecule in the methanol continuum was optimized using the B3LYP/6-311+G(d,p) method in terms of the polarizable continuum model (PCM). The electronic absorption spectra of phthalide in cyclohexane and the complex in methanol were calculated using the TDDFTB3LYP/6-311+G(d,p) method in terms of PCM as well. The obtained absorption spectra of phthalide in cyclohexane and methanol (Fig. 1) are similar to those obtained earlier by other researchers [2, 3, 6].