Photophysical Properties of The DCM and DFSBO Styryl DyesConsequence for Their Laser Properties

Abstract

The two styryl dyes, 4-dicyanomethylene-2-methyl-6-P-dimethylaminostyryl-4H-pyran (DCM) and 7- dimethylamino-3-(p-formylstyryl)-l, 4-benzoxazin-2-one (DFSBO) exhibit similar solvent-induced shifts of their absorption and emission spectra related to a large intramolecular charge transfer (ICT) in the first singlet excited state. From the Stokes shift values (vA−vF) and a vectorial analysis of their ground state dipole moment (μg= 6.1 D for DCM and 5.8 D for DFSBO), and using the Lippert-Mataga theory, we have estimated the dipole moments of their fluorescent excited states S1 (μe = 26.3 D for DCM and 27.6 D for DFSBO). Intersystem crossing to the triplet state is totally inefficient in DCM but significant in DFSBO. Moreover the absorption of the DFSBO triplet is quite large in the emission band (600–650 nm), which makes of DFSBO a poor laser dye. Although DCM trans-cis photoisomerization can be quite efficient in non polar solvents (chloroform, tetrahydrofuran), DFSBO does not photoisomerize probably due to steric hindrance and to the S1 character which should be more "benzoxazinone" than ethylenic. DFSBO is also shown to exhibit rotamerism.

Photophysical Properties of 4‐Dicyanomethylene‐2‐methyl‐6‐(p‐dimethylamino‐styryl)‐4H‐pyran Revisited: Fluorescence versus Photoisomerization

“…Both conformers are expected to be populated at room temperature with a Boltzmann distribution of ∼96:4 ( s‐trans ‐( E )‐ 1 / s‐cis ‐( E )‐ 1 ), but cannot be easily distinguished by steady‐state spectroscopy, due to fast conformational interconversion and the likely overlap of their respective spectral contributions. The main absorption and emission spectral features are nevertheless attributed to the s‐trans ‐( E )‐ 1 isomer: the large emission Stokes shifts recorded at higher solvent polarity are consistent with previous studies on DCM, [3b, 9] due to extended π‐conjugation and intramolecular charge‐transfer character between the aniline electron‐donating group and the dicyanomethylene‐pyranyl electron‐withdrawing group, with planar geometry and large dipole moment in the ground state, which is dramatically enhanced in the excited state (Δ μ =14.4 D) [9a] . Time‐resolved spectroscopy is a powerful tool to resolve conformational mixtures, identify the number of species and their respective lifetimes.…”

“…The molar absorption coefficient turned out to be rather insensitive to the solvent and the values are between 53 900 L mol −1 cm −1 in MeCN and 47 700 L mol −1 cm −1 in PhMe (Table 1). As previously reported for DCM‐based derivatives, compound ( E )‐ 1 was found to exhibit a marked fluorescence solvatochromism: [3b, 9] the emission band is strongly blue‐shifted as the polarity decreases, and the maximum wavelength moves from 620 nm in MeCN to 548 nm in PhMe, resulting in a shortening of the Stokes shift from 5400 cm −1 in MeCN to 3400 cm −1 in PhMe (Table 1). The complete set of data as a function of the solvent polarity was used to plot the Lippert–Mataga graph shown in Figure S4, providing an enhancement of the dipolar moment between the ground and excited state Δ μ as high as 14.4 D. In a similar manner, the emission quantum yield Φ em is affected by the solvent polarity, from 0.55 in MeCN down to 0.02 in PhMe (Table 1), which is consistent with previous studies on DCM [9b] …”

“…DCM is aw ell-known fluorophore with strong red emission. [2] Moreover,i ts charge-transfer character from the electron-donatinga mine to the electron-withdrawing dicyanomethylene-pyranyl moiety confers strong solvatochromism to this dye, with the possibility of tuning the absorption and emission by means of solvent polarity.F or all these reasons,D CM has found many applications in laser dyes, [2,3] chemo-or biosensors, [4] in dopantso rh osts for OLEDs, [5] in nonlinearo ptics materials, [6] or in supramolecular photoactive architectures [7] and logic gates. [8] In the last decades, the solvatochromism of DCM has been extensively investigated by steady-state spectroscopy [3b, 9] as well as time-resolvedt echniques, [10] showing the strong photoinduced intramolecular charge-transfer nature of its emissive excited state, which has been confirmed by theoretical studies.…”

“…For all these reasons, DCM has found many applications in laser dyes, [2, 3] chemo‐ or biosensors, [4] in dopants or hosts for OLEDs, [5] in nonlinear optics materials, [6] or in supramolecular photoactive architectures [7] and logic gates [8] . In the last decades, the solvatochromism of DCM has been extensively investigated by steady‐state spectroscopy [3b, 9] as well as time‐resolved techniques, [10] showing the strong photoinduced intramolecular charge‐transfer nature of its emissive excited state, which has been confirmed by theoretical studies [11] . The photophysics of DCM has been mostly detailed in solution, but also in a variety of organized media (microemulsions, vesicles, micelles, and aggregates) [12] .…”

Photoisomerization of a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran analog dye: a combined photophysical and theoretical investigation

“…In addition, 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) analogs are well-known red-emitting dyes, which are used in lasers, [15][16][17] in capping layers of organic light emitting diodes (OLEDs) and in dye-sensitized solar cells (DSSCs). [18][19][20][21][22] Its donor-p-acceptor structure confers it a noticeable intermolecular charge transfer character and consequently, strong solvatochromism.…”