Effect of Solvent-Induced Coil to Helix Conformational Change on the Two-Photon Absorption Spectrum of Poly(3,6-phenanthrene)

Abstract

This paper investigates the effect of solvent-induced conformational changes of poly(3,6-phenanthrene) on their two-photon absorption (2PA). Such effect was studied employing the wavelength-tunable femtosecond Z-scan technique and modeled using the sum-over-essential states approach. We observed a strong reduction of the 2PA cross-section when the sample was prepared in hexane (poor solvent) in comparison to chloroform (good solvent), which is related to the conformation adopted by the polymer in each case. In chloroform it adopts a random coil conformation, as opposed to the one-handed helix conformation in hexane. Our results pointed out that the coil to helix conformation change decreases the degree of molecular planarity of the polymer π-conjugated backbone, which is primarily responsible for their optical nonlinearity, contributing to diminishing the effective transition dipole moments and, consequently, the 2PA cross-section. Moreover, by studying the nonlinear response with different light polarization, we showed that, although the solvent-induced conformational change does not alter the molecular symmetry of the polymer, it modifies considerably the direction of the transition dipole moments between the excited states.

Characterization of two- and three-photon absorption of polyfluorene derivatives

“…In that study, the authors compared the 2PA cross‐section obtained for thiophene/phenylene‐based polymer containing about 15 consecutive units with a thiophene/phenylene‐based oligomer, where they observed only an increment of about 2.5 times on the 2PA cross‐section for the polymer when compared with the oligomer. In another work, Vivas et al observed a reduction of about 50% on the 2PA cross‐section of poly(3,6‐phenanthrene) when the polymer changed its conformation from coil to helix. The authors explained that the difference was directly related to the decrease in the molar absorptivity, corroborating the nonlinear optical results obtained by Vanormelingen et al, and also the results presented here.…”

Two‐Photon Solvatochromism II: Experimental and Theoretical Study of Solvent Effects on the Two‐Photon Absorption Spectrum of Reichardt’s Dye

“…[1,[3][4][5][6][7][8][9][10][11][12][13][14] In 1878, Kundt proposed a rule that related the shift of the UV/Vis absorption-band maximum towards longer wavelengths to the increasing value of the index of refraction. [1,[15][16][17][18] Furthermore, this definition becomes even more broad, covering inter alia solvent effects on fluorescence, [19] Raman and hyper-Raman, [20] as well as twophoton absorption (2PA) [21][22][23][24][25][26][27][28][29][30][31][32][33][34] spectra. [1,[15][16][17][18] Furthermore, this definition becomes even more broad, covering inter alia solvent effects on fluorescence, [19] Raman and hyper-Raman, [20] as well as twophoton absorption (2PA) [21][22][23][24][25][26]…”

“…[17,52,53] More recently, RD has gained interest as a material used for model studies of controlled drug delivery [54,55] and as a promising agent for comparing the electrophilicity of various neutral Lewis acidic species. The available data are based on two measurement techniques, that is, Z-scan and two-photon-induced fluorescence (TPIF), [21][22][23][24][25][26][27][28][29][30][31][32][33][34] supported by the results of theoretical studies. [56][57][58][59][60] Since it was first synthesized in 1920, RD and its derivatives have been extensively examined both experimentally, [61] to better understand its unusual sensitivity for changes in the environment, [36-38, 40, 41, 44, 46, 50-60, 62-83] and theoretically.…”

“…[1,3,4] Nowadays, the term solvatochromism describes a modification of the position and the intensity of UV/Vis absorption bands induced by the change in the polarity of the environment. [1,[15][16][17][18] Furthermore, this definition becomes even more broad, covering inter alia solvent effects on fluorescence, [19] Raman and hyper-Raman, [20] as well as twophoton absorption (2PA) [21][22][23][24][25][26][27][28][29][30][31][32][33][34] spectra. Two main types of solvatochromism are distinguished: a positive solvatochromism, which is observed if the band shifts towards longer wavelengths with increasing medium polarity; and a negative one, which is accompanied by a blueshift (hypsochromic shift).…”

“…Unfortunately, systematic studies of solvent effects on 2PA are rare. The available data are based on two measurement techniques, that is, Z-scan and two-photon-induced fluorescence (TPIF), [21][22][23][24][25][26][27][28][29][30][31][32][33][34] supported by the results of theoretical studies. [35,87,[101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117][118][119][120] In some of the theoretical works, [35,87,[101][102][103]121] the authors have presented a hypothesis that the 2PA cross-section (s 2PA ) and the related 2PA probability (d 2PA ) of the donor-acceptor p-conjugated species can be properly described by using the two-state model.…”