Donating Strength of Azulene in Various Azulen-1-yl-Substituted Cationic Dyes:  Application in Nonlinear Optics

Abstract

Three new azulene-based substituted pyridinium cations, azulene-1-azo(N-methyl-4‘-pyridinium) (1 +), azulene-1-[(E)-2-(N-methyl-4‘-pyridinium)ethenyl] (2 +), and azulene-1-azo[(N-methyl-5‘-quinolinium)] (3 +) were synthesized, and their crystal structures solved by X-ray diffraction. A set of crystallographic, spectroscopic, and computational investigations reveals that, while the pyridinium is a very strong acceptor unit, the azulene counterpart acts as an extremely efficient electron donor in these chromophores. The static quadratic hyperpolarizabilities (β0), measured by the hyper Rayleigh scattering method, are equal to 67 × 10-30, 108 × 10-30, and 80 × 10-30 cm5 esu-1, for 1 +, 2 +, and 3 +, respectively. Interestingly, these values are slightly reduced versus those recorded on the related stilbazolium derivatives built up from the less efficient (dimethylamino)phenyl donor substituent. This observation suggests that the donor strength of azulene is beyond the point which maximizes β in the “push−pull” (arylethenyl)pyridinium series. An important charge delocalization in the ground state is suggested to account for the reduced β value in these chromophores, in agreement with the calculated charge-transfer properties, Mulliken charges, and experimental solvatochromism. These investigations suggest that azulene could attract an additional interest in the design of chromophores with cubic optical nonlinearities.

Chromophoric materials derived from a natural azulene: syntheses, halochromism and one-photon and two-photon microlithography

“…[3][4][5] Similarly, the physical, electronic, and optical properties of some natural pigments could offer access to key building blocks for developing bio-derived chromophoric materials. [9][10][11][12][13][14][15][16] Chemically, guaiazulene is a nonalternant hydrocarbon that is notable for its low transition energy to the S 1 state and unusually large S 1 -S 2 gap, [17][18][19][20][21][22][23][24][25][26][27] where S 1 and S 2 refer to the first and second electronically excited states, respectively. 3,8 Among known natural pigments, 6-8 guaiazulene has received recent popularity for its interesting physical and electronic properties.…”

Synthesis and properties of fluorescent 4′-azulenyl-functionalized 2,2′:6′,2″-terpyridines

“…In this context, the synthesis of tailored terpyridine derivatives with appropriate electron-donor/acceptor moieties may allow for a further improvement of their spectroscopic and electrochemical properties. Owing to its unique fluorescent and remarkable optical and redox properties, azulene proved to be an excellent building block for developing a large variety of materials ranging from NLO chromophores [13] to molecular switches [14–15] and liquid crystals [16] or high-conductance materials [17]. In contrast to most aromatic compounds which exhibit S 1 →S 0 fluorescence under low excitation intensity, azulene shows fluorescence predominantly from the S 2 excited state and only very weakly from S 1 [18].…”

Stimuli‐Responsive Cyclopenta[ef]heptalenes: Synthesis and Optical Properties

“…[1,2] To address these drawbacks, efforts have been reported to exploit the azulene framework as a modular building block for the synthesis of chromophores having stimuli-responsive behavior. [3] Similar to the various azure-blue derivatives of the bicyclic sesquiterpene azulene, guaiazulene exhibits unique electronic and optical properties that allow for its use in chargetransport, [4,5] nonlinear-optics, [6,7] and sensor applications. [8] Unlike other small aromatic hydrocarbons, guaiazulene exhibits a relatively large permanent dipole moment, 1 smaller optical energy gaps of the cyclopenta[ef]heptalenium cations 4a-c + .…”