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Nonlinear Optical Switching Properties in the Furylfulgide Aberchrome 540-Dihydrobenzofuran Derivative Pair of Photochromic Materials Prasenjit Seal and Swapan Chakrabarti* Department of Chemistry, UniVersity of Calcutta; 92, A. P. C. Ray Road, Kolkata - 700 009, India ReceiVed: September 4, 2009; ReVised Manuscript ReceiVed: October 22, 2009
Time-dependent density functional theoretical investigation has been carried out to justify the switching action of nonlinear optical properties in the furylfulgide Aberchrome 540 (FFA) and dihydrobenzofuran derivative (DHBF) photochromic pair of molecules. The effect of solvents on this switching action has also been addressed. The calculations suggest that DHBF has a higher optical coefficient compared to that of FFA. Notably, a dramatic increase in DHBF is observed in the second harmonic data, particularly in the solvent phase. The ratio of SHG tensor gradually decreases as the polarity of the solvent is decreased using a coulomb-attenuating functional irrespective of the basis sets used. An exploration of the excitation parameters suggests that high excited-state dipole moments are responsible for the observed large β value in the case of the DHBF system relative to that of the FFA system. 1. Introduction The field of nonlinear optics has witnessed an ever-increasing importance due to their versatile applications in information processing, electro-optical switching, and telecommunication.1 The nonlinear optical (NLO) processes are also very useful in photonic devices such as erasable optical memory media.2-5 In this context, the phenomenon of photochromism is significant. This is a phenomenon of reversible transformation of a chemical species between two molecular states by the absorption of electromagnetic radiation. The best way to account for the phenomenon is to observe the change of color on exposure to light.2 Besides color change, several other molecular and bulk properties also change along with the change in molecular states. Among the changes in other properties, if appreciable differences in NLO responses are observed between two molecular states of a photochromic pair then that pair is said to act as an efficient NLO switch.5 The relationship between photochromism and NLO properties can lead to the design and fabrication of new devices for data storage or optical switching.2 Photochromism is observed in numerous organic and inorganic compounds including dyes, better known as photochromic materials. These include fulgides, azobenzenes, stilbenes, spiropyrans, diarylethenes, salicylideneimines, etc., while the photochromic processes involved are cis-trans isomerization, photocyclization, photoinduced electron transfer, and keto-enol tautomerism.4-7 Photochromic materials are currently potential candidates for erasable memory media in recordable compact disks.8 Photochromic materials also show ultrafast isomerization upon irradiation. This ultrafast switching ability, in turn, is very useful in optical communication, signal processing, variable frequency filters, attenuators, and phase shifters.9,10 In 2000, a thematic issue was published in Chemical ReViews highlighting the phenomenon of photochromism in various categories of materials and their role in memory and switches.11 Irie, in his work, has reported diarylethenes with heterocyclic aryl groups as photochromic materials and found some promising features to be used for memory and switches.11 Spiropyrans and spirooxazines are another class of photochromic systems that * Corresponding author. E-mail:
[email protected].
have gained a lot of importance. Fischer and Hirshberg12 discovered the photochromic reactions of spiropyrans. Several applications were examined and suggested for these kinds of photochromic systems, which include self-developing photography, actinometry, lenses of variable optical density, etc. Spiropyrans also play a crucial role in three-dimensional optical memory that is based on two-photon generated spiropyranmerocyanine conversion.13 In photoswitching of protein activity, spiropyrans have a significant role.14 The isomerization in stilbenes have been extensively investigated; however, because of unwanted side reactions, its applications in optical switching have been limited. Very recently, Cordes et al.15 reported spectroscopy and dynamics of a novel hemithioindigo-based photoswitch, which forms an amino acid derivative. Apart from these types of photochromes, there are fulgides, which are perhaps the most widely studied.3,16 The phenomenon of photochromism in fulgides occurs between a colorless open form and colored cyclic form. Stobbe first prepared this interesting class of compounds.17 Heller and co-workers18 provided the impetus to study fulgides by publishing a series of papers related to the chemistry of fulgides and some closely associated compounds. In the early 1980s, Heller and co-workers studied the photochromism of a fulgide with a 2,5-dimethyl-3-furyl moiety.19 Since then, the furylfulgide system has been fundamental in the study of the photochromism of fulgides. This system allowed the first demonstration of thermally irreversible photochromism. This thermal irreversibility in photochromic compounds positions these systems as promising candidates for rewritable optical recording media. Besides Heller’s group, there are other groups who also studied photochromism in fulgides. Paetzold and Ilge20 studied benzylidene fulgides via spectroscopic methods and molecular orbital calculations. Yokoyama et al.21 extensively studied the effects of substituents on indolylfulgides. Switching of NLO properties in photochromic materials has been a subject of paramount interest, and modulation of the second harmonic generation (SHG) signal is mainly used to account for the switching actions. Lehn and co-workers22 reported a photochromic system based on nitrobenzylpyridine derivatives that have the potential to switch the SHG efficiency.
10.1021/jp908553f 2010 American Chemical Society Published on Web 11/17/2009
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Seal and Chakrabarti
Lehn et al.23 have also shown switching actions in diarylethene compounds between high and low levels of NLO response. Kim and co-workers,24 in an elegant attempt, have highlighted photoswitching in 1,2-bis(3-thienyl)ethene derivatives. They also explored the photophysics and molecular criteria that are responsible for such behavior in those derivatives. Delaire and co-workers illustrated that photoisomerization of disperse red one dyes dissolved in a poly(methyl methacrylate) (PMMA) matrix can lead to a decrease in SHG.25 Later on, in another work, the Delaire group illustrated the mechanism of photoassisted poling of two important classes of photochromic materials, viz. spiropyran-photomerocyanine and a particular type of furylfulgide-dihydrobenzofuran (FF-DHBF) system.26 Their results indicated 7.5 and 30 times increase in the SHG value for FF-DHBF system in favor of the closed form, i.e., DHBF. Recently, Khedhiri et al.27 studied the conformational changes of photochromic 3-furylfulgide systems in relation to the crystal structure. In the present work, we focus our attention on the switching actions of nonlinear optical properties in the furylfulgide Aberchrome 540 (the name comes from Aberchromics Ltd.) and dihydrobenzofuran derivative (FFA-DHBF) photochromic pair. In order to do this, static and frequency-dependent (second harmonic generation (SHG)) first-order hyperpolarizability tensors are calculated using hybrid and coulomb-attenuating hybrid functionals. The most significant part of the present investigation is the explicit role of solvents in influencing the NLO tensors in these systems. Albeit, there are several works related to the photochromism and NLO properties in furylfulgide systems; however, proper theoretical justification to the switching actions is still missing. The present investigation is an attempt to fill this gap. 2. Computational Details The gas-phase geometries for the photochromic pair furylfulgide Aberchrome 540 (FFA)-dihydrobezofuran derivative (DHBF) have been optimized at B3LYP/6-311++G(d,p) level of theory implemented in the Gaussian 0328 suite of programs, and all solvent-phase geometry optimizations have been performed using the ADF 2008.0129 program package. For solventphase geometry optimization, we have adopted the polarizable continuum model (PCM) at BLYP/TZP level of theory. In the present investigation, four solvents of different polarities are used, viz., dimethyl sulfoxide (DMSO), methanol (CH3OH), chloroform (CHCl3), and benzene (C6H6). The optimized Cartesian coordinates for the FFA and DHBF systems in each phase are given in the Supporting Information. The response calculations are performed via the DALTON 2.030 computational package. For the evaluation of first-order NLO tensor, B3LYP31 and CAM-B3LYP32 hybrid functionals along with Dunning’s correlation consistent double-ζ basis sets (cc-pVDZ and aug-cc-pVDZ) are used. The response properties are calculated from the time-dependent quadratic response theory.33 For the solvent-phase calculations, recently developed parallel implementation of the PCM is employed.34 The molecular cavities that are used for the calculations consist of interlocking spheres. The molecular radii are taken from the work of Ferrighi et al.35 The static dielectric constants for DMSO, CH3OH, CHCl3, and C6H6 are 46.7, 32.63, 4.9, and 2.247, respectively, whereas the corresponding optical counterparts are 2.179, 1.758, 2.085, and 2.244, respectively. 3. Results and Discussion Schematic representations of the photochromic pair of molecules, i.e., FFA and DHBF, are presented in Figure 1a and
Figure 1. Structures of the photochrome pair of molecules: (a) furylfulgide Aberchrome 540 (FFA) and (b) dihydrobenzofuran derivative (DHBF). The molecular axes are also given. Color scheme: carbon (black); oxygen atoms (red); hydrogen atoms (green).
1b, respectively, along with the molecular axis. From the figure, it is quite clear that the fulgide system is the open form (colorless in nature) whereas DHBF is the ring form (colored in nature). As mentioned earlier, emphasis has been given to the coefficient of first-order hyperpolarizability tensors, 〈β(0;0,0)〉 and 〈β(-2ω;ω,ω)〉 [SHG]. The average value for the coefficient of first-order hyperpolarizability tensors can be determined using the following expression.
〈β〉 )
∑ βiβ*i and i
βi )
1 3
∑ (βijj + βjij + βjji) j
(1)
In the above equation, the sums are over the coordinates x, y, z (i, j ) x, y, z) and βi* refers to the conjugate of the vector βi. In general, the importance of a particular substance in the field of NLO and its applications requires description of frequency dependence. Hence, we have also explored the frequency-dependent NLO tensor of this FFA-DHBF photochromic pair mainly in order to see whether there is any switching action of the NLO properties in these systems. The static part will be discussed at first followed by a detailed discussion on the SHG calculations. Tables 1 and 2 depict the
Nonlinear Optical Switching Properties
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TABLE 1: B3LYP and CAMB3LYP Data for 〈β(0;0,0)〉 (in a.u.) for the Furylfulgide Aberchrome 540 (FFA)-Dihydrobenzofuran Derivative (DHBF) Photochromic Pair in Gaseous and Solvent Phasesa phase systems
gaseous
FFA (open form)
gaseous DMSO methanol chloroform benzene
DHBF (ring form)
gaseous DMSO methanol chloroform benzene
FFA (open form)
gaseous DMSO methanol chloroform benzene
DHBF (ring form)
〈β(0;0,0)〉 (a.u.)
contributing β tensors
(βDHBF)/(βFFA)b
B3LYP 410 895 826 819 767 1726 2822 2572 2809 2826
βxzz, βzxx βxzz, βzxx βxzz, βzxx βxzz, βzxx βxzz, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx
4.20 (3.56,3.49) 3.15 (3.04,2.95) 3.11 (2.99,2.92) 3.42 (3.23,3.11) 3.68 (3.39,3.26)
CAM-B3LYP 329 787 732 711 656 1671 3543 3224 3297 3145
βxzz, βzxx βxzz, βzxx βxzz, βzxx βxzz, βzxx βxzz, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx βxxx, βzxx
5.07 (3.92,3.92) 4.50 (4.21,3.95) 4.40 (4.11,3.87) 4.63 (4.20,4.01) 4.79 (4.20,3.97)
solvent
gaseous DMSO methanol chloroform benzene
a The contributing β tensors are also given. The basis set used is cc-pVDZ. b The ratios given in the parentheses are obtained by dividing the corresponding β tensors of FFA and DHBF systems, i.e., the 1st ratio is βDHBFmost contributing tensor/βFFAmost contributing tensor (in the same phase) and whereas the 2nd ratio is βDHBF2nd most contributing tensor/βFFA2nd most contributing tensor (in the same phase).
TABLE 2: B3LYP and CAMB3LYP Data for 〈β(0;0,0)〉 (in a.u.) for Furylfulgide Aberchrome 540 (FFA)-Dihydrobenzofuran Derivative (DHBF) Photochromic Pair in Gaseous and Solvent Phasesa phase systems
gaseous
FFA (open form)
gaseous
solvent
DMSO methanol chloroform benzene DHBF (ring form)
gaseous DMSO methanol chloroform benzene
FFA (open form)
gaseous DMSO methanol chloroform benzene
DHBF (ring form)
gaseous DMSO methanol chloroform benzene
