Absorption and Fluorescence Spectroscopic Properties of 1- and 1,4-Silyl-Substituted Naphthalene Derivatives

Silyl-substituted naphthalene derivatives at the 1- and 1,4-positions were synthesized and their UV absorption, fluorescence spectroscopic properties, and fluorescence lifetimes were determined. Analysis of the results shows that the introduction of silyl groups at these positions of the naphthalene chromophore/fluorophore causes shifts of the absorption maxima to longer wavelengths and increases in fluorescence intensities. Bathochromic shifts of the absorption maxima and increases in fluorescence intensities are also promoted by the introduction of methoxy and cyano groups at the naphthalene 4- and 5-positions. In addition, the fluorescence of 9,10-dicyanoanthracene is efficiently quenched by these naphthalene derivatives with Stern-Volmer plot calculated rate constants that depend on the steric bulk of the silyl groups.


Synthesis of Silyl-Substituted Naphthalene Derivatives
In order to evaluate the effects of hydrogen atom and alkyl group containing silyl substituents, an acetylene linkage between the naphthalene ring and silyl groups, and the presence of electron donating (OMe) and electron accepting (CN) groups on the absorption and fluorescence properties of naphthalenes, the naphthalene derivatives 2-14 were prepared ( Figure 1). The 1-silylnaphthalenes 2-5 were prepared by sequences involving lithiation of 1-bromonaphthalene with tert-BuLi followed by addition of the appropriate chlorosilanes. The dimethyl-n-octylsilyl group-containing derivative 6 was prepared using a hydrosilylation reaction of 1-octene with the dimethylsilyl substituted naphthalene 3. The 1,4-disilylnaphthalenes 7-9 were prepared by using an initial dilithiation reaction of 1,4-dibromonaphthalene followed by bis-silylation of the aryl bis-lithium intermediate with chlorosilanes. 1-Bromo-4-methoxynaphthalene was employed as the starting material for the route utilized along with a lithiation-silylation sequence to synthesize the methoxy-substituted derivative 10. The naphthalene derivatives 11 and 12, containing 4-and 5-cyano groups respectively, were prepared by employing a cyanation reaction of 1-bromo-4-(trimethylsilyl)naphthalene and a photoinduced electron transfer reaction between 2 and potassium cyanide, respectively. The trimethylsilylethynyl derivatives 13 and 14 were synthesized by using Sonogashira coupling reactions of the corresponding bromonaphthalenes with trimethylsilylacetylene. The detailed synthetic procedures employed to prepare 1-14 are described in the Experimental Section.

Effects of Hydrogen Atom and Alkyl Group Containing Substituents on the Absorption and Fluorescence Properties of Substituted Naphthalenes
UV-absorption spectra of aerated cyclohexane (ca. 10 −4 M) solutions of the naphthalene derivatives 1-6 are displayed in Figure 2. Evaluation of the spectra shows that the absorption maxima of the silyl-substituted naphthalene derivatives 2-6 shift by 8-9 nm to longer wavelengths and their molecular absorption coefficients () increase, compared with those of naphthalene (1). Similarly, analysis of the fluorescence spectra of aerated cyclohexane solutions of these compounds ( Figure 3) demonstrates that silyl substitution causes emission maxima to shift to longer wavelengths by 4-5 nm and fluorescence intensities to increase, relative to those of naphthalene. Close examination of these spectra shows that substituents on silicon of the silyl groups (SiMe 3 , SiMe 2 H, SiMe 2 n Bu, SiMe 2 t Bu, and SiMe 2 n Oct) have only a small effect on the absorption and fluorescence properties of the naphthalene chromophore/fluorophore. In contrast, silyl-substituents at C-9 of anthracene bring about dramatic changes in absorption and fluorescence properties [20,34]. A possible reason for this phenomenon is steric repulsion, which exists between the C-9 silyl groups and peri-hydrogens at C-1 and C-8 of the anthracene ring. However, this type of effect does normally not occur in naphthalene derivatives, which only possess one peri-hydrogen.

Absorption and Fluorescence Properties of Mono-and Di-silyl Substituted Naphthalene Derivatives
Absorption and fluorescence spectra of cyclohexane solutions of 1,4-bis(trimethylsilyl)naphthalene (7), 1,4-bis(dimethylsilyl)naphthalene (8), and 1,4-bis(tert-butyldimethylsilyl)naphthalene (9) were recorded in order to compare their photophysical properties with those of 1-(trimethylsilyl)naphthalene (2) and naphthalene (1) (Figures 4 and 5). Analysis of the spectra shows that, in general, the absorption maxima of 7-9 occur at longer wavelengths than those of 2 and that the spectrum of the di-tert-butyldimethylsilyl derivative 9 displays the most dramatic bathochromic shift. In addition, the fluorescence intensities and emission wavelengths of the disilylnaphthalenes 7-9 are observed to increase relative to those of 1 and 2.

Effects of Electron-Donating and Withdrawing Groups on Absorption and Fluorescence Properties
Since the presence of electron-donating and withdrawing groups can dramatically influence the photophysical properties of organic chromophores/fluorophores, we investigated the influence of such inductive effects by various substituent groups on the absorption and fluorescence properties of 1-(trimethylsilyl)naphthalenes. For this purpose, absorption and fluorescence spectra recorded using cyclohexane solutions of cyanonaphthalenes 11 and 12 and methoxynaphthalene 10 were compared with those of naphthalene and its 1-trimethylsilyl (2) and 1,4-bis-trimethylsilyl (7) derivatives (Figures 6 and 7). As can be deduced from the recorded spectra, the presence of the 4-methoxy (10), 4-cyano (11), and 5-cyano (12) groups results in long wavelength shifts of the absorption maxima and slight increases in the molar absorption coefficients (), which is in stark contrast to those of the parent arene. Moreover, the fluorescence intensities of 10-12 are larger than those of 1, 2, and 7.

Effects of Silylethynyl Group(s) on Absorption and Fluorescence Properties
The results of recent studies have shown that the silylethynyl group(s) brings about bathochromic shifts of the maxima in the absorption spectra of aromatic substances and that these groups lead to increases in fluorescence intensities [24,[27][28][29]31]. Consequently, we probed the influence of the silylethynyl group(s) on the absorption and fluorescence properties of naphthalenes 13 and 14, and found that the absorption maxima of the naphthalene chromophore/fluorophore shift to longer wavelengths and  increases as a consequence of the presence of silylethynyl group(s) (Figure 8). Especially noteworthy is the observation that, in the 1,4-bis-(trimethylsilylethynyl) derivative 14, the absorption maximum is strongly bathochromically shifted to 347 nm and  increases to 4.4 × 10 4 mol −1 dm 3 cm −1 in comparison with those of other substances studied. Moreover, the fluorescence intensities of the silylethynyl-substituted naphthalenes 13 and 14 are also larger than the other naphthalene derivatives ( Figure 9).

Fluorescence Quenching of 9,10-Dicyanoanthracene by Silylnaphthalenes
Experiments were carried out to evaluate how the electronic and steric effects of silyl groups govern the ability of naphthalene derivatives to serve as quenchers of the singlet excited state of 9,10-dicyanoanthracene. For this purpose, fluorescence emission intensities of benzene solutions of 9,10-dicyanoanthracene in the presence of various concentrations of 1-methylnaphthalene and the naphthalene derivatives 1-3, 6 and 8 were determined ( Figure 10). The rate constants of 9,10-dicyanoanthracene fluorescence quenching, calculated by using Stern-Volmer analysis and falling in range of 5.68 × 10 9 to 1.06 × 10 9 M −1 s −1 , were observed to decrease in the following order:  Additionally, analysis of the fluorescence profiles shows that weak emission from singlet exciplexes, formed between 9,10-dicyanoanthracene and the naphthalene derivatives, occurs at longer wavelengths. The results indicate that the rate constant for fluorescence quenching is dependent on the steric bulk and not on the electronic effects of the silyl substituents present in the naphthalene quenchers.

HOMO-LUMO Energy Calculations, Fluorescence Lifetimes, and Fluorescence Quantum Yields
The results described above demonstrate that the introduction of silyl group(s) promotes a shift in the maxima and an increase in molar absorption coefficients () in the absorption spectra of naphthalenes (Table 1). Bathochromic shifts and incremental increases of  were also observed when electron-withdrawing, electron-donating, and silylethynyl groups are attached to the naphthalene ring. In order to understand these effects, the HOMO and LUMO energies of 1-14 were calculated using the PM3 method. Inspection of the results shows that the HOMO and LUMO energy gaps in these arenes decrease in the following order: 1 > 2-6 > 7-9 > 10-12 > 13-14, a finding that is in good agreement with the experimental absorption spectroscopic results. Specifically, introduction of silyl groups causes an increase in both the HOMO and LUMO energies of naphthalenes and a net overall decrease in energy gaps between these orbitals. In this regard, silyl substituents act as electron-donating groups as a consequence of (C-Si)-*(naphthalene) molecular orbital interactions. Because introduction of silylethynyl group(s) significantly lowers the energy of the naphthalene LUMO, both *(C-Si)-(naphthalene) and (C-Si)-*(naphthalene) orbital interactions are taking place in these substances.  Since both the fluorescence lifetime and the quantum yield are important physical properties of fluorophores, we determined these parameters for a number of the naphthalene compounds synthesized. Fluorescence lifetimes ( s ) of the naphthalenes were determined in both aerated and carefully degassed cyclohexane solutions. The accumulated data show that the fluorescence lifetimes of these substances decrease in the following order: 1 > 2-6 > 7-9 > 10-12 > 13-14. In addition, the large differences observed between the fluorescence lifetimes of aerated and degassed solutions indicate that molecular oxygen has a significant fluorescence quenching effect.
Finally, the fluorescence quantum yields ( f ) of selected members of the series of naphthalenes explored in this investigation were determined in thoroughly degassed solutions to exclude the influence of dioxygen. The data produced, which included quantum yield values of  f 0.30 (2), 0.33 (7), 0.65 (10), 0.66 (11), and 0.85 (14), demonstrate that these substances fluoresce significantly more efficiently than naphthalene ( f = 0.23 [49]).

General
Purifications of solvents were carried out in the following manner: acetonitrile was distilled from P 2 O 5 and then from CaH 2 . Benzene was distilled from CaH 2 and then from Na. Isopropanol was distilled from CaH 2 . Piperidine was distilled from KOH. THF was distilled from CaH 2 , and then from Na and Ph 2 C=O. Other chemicals were used as purchased.
Melting points were determined on a Yanagimoto Yanaco MP-500 Micro Melting Point apparatus, and are uncorrected. 1 H-and 13 C-NMR spectra were recorded using a Varian MERCURY-300 (300 MHz and 75 MHz, respectively) spectrometer with Me 4 Si as an internal standard. Mass spectra (CI) were recorded utilizing a JEOL JMS-DX303 spectrometer. High-performance liquid chromatographic (HPLC) separations were performed using a recycling preparative HPLC, equipped with a Jasco PU-2086 Plus pump, RI-2031 Plus differential refractometer, Megapak GEL 201F columns (GPC) with CHCl 3 as the eluent. HPLC separations (packed silica gel) were performed by a recycling preparative HPLC equipped with Jasco PU-987 pump, UV-970 uv-vis detector, CHEMCOSORB I-5Si column (Chemco Scientific Co., Ltd.) with hexane and EtOAc as eluents. Column chromatography was conducted by using MERCK silica gel 60 (0.063-0.200 mm). Thin layer chromatography (TLC) was performed using MERCK silica gel 60 F 254 .
UV-vis and fluorescence spectra were recorded using Jasco UV-160 and FP-770 spectrophotometers, respectively, employing a 1 cm pathlength cell at 298 K. Solutions of the naphthalenes in spectral grade cyclohexane (ca. 10 −4 M (1-12) or 10 −5 M (13, 14)) were prepared under aerated conditions. Excitation wavelengths were the longest wavelength absorption maxima unless otherwise noted and absorbances at the excitation wavelengths were 0.5. Fluorescence lifetimes were measured by using a HORIBA NAES-550 nano-second fluorometer equipped with a SSU-111A photomultiplier, SCU-121A optical chamber, SGM-121A monochromator, and LPS-111 lamp power supply. All decay curves were fitted by utilizing a single exponential decay with chi square values less than 2. Fluorescence quantum yields ( f ) were determined using carefully degassed (freeze-pump-thaw method) cyclohexane solutions. Standards for fluorescence quantum yields are listed in the footnotes of Table 1

Conclusions
In the study described above, the absorption and fluorescence properties of 1-silyl-, 1,4-disilyl-, 1-silylethynyl-and 1,4-disilylethynyl-naphthalenes were evaluated. The findings show that the absorption maxima of the 1-silyl-and 1,4-disilylnaphthalenes occur at longer wavelengths with larger  values than those of naphthalene (1). Bathochromic effects and incremental increases in  were also observed for electron-donating, electron-withdrawing and silylethynyl group substituted naphthalenes. The results of PM3 calculations of HOMO-LUMO energy gaps are in accord with the experimental observations. Furthermore, fluorescence quantum efficiencies were found to increase and fluorescence lifetimes decrease when the silyl substituents are present on the naphthalene ring system. Also, the respective fluorescence quantum yield and lifetime of 1,4-bis(trimethylsilylethynyl)naphthalene (14) were found to be 0.85 and 2 ns. Finally, the fluorescence of 9,10-dicyanoanthracene is efficiently quenched by silylnaphthalenes, with quenching rate constants that depend on the steric bulk and not the electronic properties of these substituents.