Porphyrins and phthalocyanines as materials for optical limiting
Introduction
Since the invention of intense light sources based on laser mechanism in 1960s [1], the need for protection of optical sensors and human eyes from accidental or hostile lasers has stimulated considerable research [2]. More recently, several materials and device configurations have been proposed and developed to meet this challenge [3]. Among new materials, organic and organometallic compounds with nonlinear optical (NLO) properties have emerged as promising candidates [4], [5], [6], [7], [8], [9], [10] for the limiting of the laser radiation intensity because of their large nonlinearities, inherently fast response time, broadband spectral response and ease of processing. These materials include porphyrins [4], phthalocyanines [5], [6], [7], [8], [9], fullerenes [10] and organometallic compounds [11]. Among them, porphyrins (Por’s) and phthalocyanines (Pc’s) are especially attractive because their NLO properties can be modified via suitable structural modifications. Current emphasis is given to the establishment of structure–NLO properties relationships for these molecules so that materials with improved or optimised NLO properties can be rationally designed and synthesised for that particular application. In this review, we highlight the recent achievements on molecular design, chemical synthesis and NLO property optimisation for Por’s and Pc’s.
In order to protect optical systems and human eyes from debilitating laser effects, the intensity of incoming laser light has to be opportunely reduced [12]. Optical limiters, whose filtering action is instantaneously activated by the incoming intense light represents a valid solution for the protection of sensors. In this case, the incoming intense light alters the absorptive and refractive properties of the materials in such a way that the resulting transmitted intensity is greatly reduced. Optical limiters based on reverse saturable absorption (RSA) are very transparent for weak light and get opaque for the intense light. Moreover, if only RSA occurs, the quality of the vision can still be maintained during the process of optical limiting (OL). In the RSA process, the absorbing material has an excited-state absorption cross-section, σex, larger than the ground-state absorption cross-section, σg. As the optical excitation intensity increases, more molecules are promoted to the excited state, thus giving rise to higher absorption at intense light excitation. The mechanism of RSA is often described in terms of a four-level model (Fig. 1) [13].
The OL effect of a molecule can be described as follows: when the ground state (Sg) of a molecule is excited to the first singlet state (S1) by absorption of a photon, an intersystem crossing (ISC) process takes place and increases the population in the triplet state (T1) with a time constant τISC. Successively, a second photon is absorbed by the system in the state T1 to the excited state T2. Usually, an efficient OL material has a high ratio of excited-state (T1→T2) to ground-state (Sg→S1) absorption cross-section (σex/σg⪢1), rapid ISC rate (τISC⪡τ), a long triplet lifetime with respect to the pulse width (τT1⪢τ), a long internal conversion lifetime (τIC⪢τ) and a high intersystem crossing quantum yield (φS1→T1∼1). The OL curve can be plotted as input fluence (or energy) versus output fluence (or energy) or input fluence (or energy) versus transmittance (Fig. 2).
Suitable materials, besides good optical limiting properties, should possess optical stability and good processability. An important term for the evaluation of the OL properties of a material is the limiting threshold. It is normally defined as the input fluence (or energy) at which the transmittance is 50% of the linear transmittance. The lower the optical limiting threshold, the better the OL material. Although OL threshold is generally accepted as the key parameter for comparing the OL performance of different materials, one must be cautious in comparing different materials if these materials are characterised under different conditions [9]. A standard method for OL measurements has not been specified yet and different conclusions on OL properties might be drawn for the same material. It is a good practice if different systems are characterised by taking samples with same linear transmission so that the effectiveness of the OL is meaningfully compared. Usually, commercially available C60 or tetra(t-butyl)-indiumchloride Pc (t-Bu4PcInCl) (which will be discussed later) are taken as reference material at 532 nm, for OL evaluations.
Fig. 3 describes a technique for optical limiting determinations. A laser (usually at 532 nm) is focused and the optical limiting sample is placed at the focal plane of the beam; the light transmitted by the sample is then collected by a second lens and directed to a detector. For RSA measurements, no aperture is used, and the sole effect of nonlinear absorption is measured. If nonlinear refraction has to be investigated, an aperture is placed behind the second lens before the detector. The energy of the transmitted beams varies with the change of the incoming beam from low to high energy.
Section snippets
Porphyrins as NLO materials
Porphyrins are an ubiquitous class of naturally occurring compounds with important biological representatives such as hemes, chlorophyll and Vitamin B12. The basic structure of the Por macrocycle consists of four pyrrolic subunits linked by four methine bridges (Fig. 4).
A great number of structural changes consisting of the variation of the central atom and/or peripheral substituents at the pyrrolic free positions can be carried out in the Por structure without altering its chemical stability.
Phthalocyanines and naphthalocyanines as nonlinear optical materials
Pc’s constitute an important class of materials from the industrial standpoint due to their widespread use as dyes and catalysts [38], [39]. On the other hand, Pc’s and numerous of their derivatives, e.g. naphthalocyanines (Nc’s) (Fig. 6), are also playing a relevant role in the emerging field of optoelectronics as NLO materials with relevant OL properties [40], besides their known applications in materials science [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53],
Conclusions
The porphyrins and phthalocyanines described in this work are the product of recent efforts spent in designing effective molecular structures for optical limiting applications. In many cases comparison of the optical limiting performance with different systems can be complicated by the lack of standard procedures in the methodology of characterisation. The deduction of an unambiguous structure–property relationship, represents a further step which has been only partially accomplished in the
References (101)
- et al.
Prog. Quant. Electron.
(1993) - et al.
Coord. Chem. Rev.
(2001)et al.Science
(1996) - et al.
J. Chem. Soc., Faraday Trans.
(1992) - et al.
Opt. Commun.
(1998) - et al.
Appl. Phys. Lett.
(1991) - et al.
Mater. Res. Soc. Symp. Proc.
(1997) - et al.
Thin Solid Films
(1988) - et al.
Thin Solid Films
(1992) - et al.
Opt. Mater.
(2001) - et al.
SPIE Proc.
(1996)