**1. Introduction**

Hybrid organic-inorganic materials are recognized as promising systems for many applications in di fferent fields such as optoelectronics, energy, and biomedicine [1–5]. Generally, the final properties are not a mere sum of the individual contributions of each moiety; new and improved features also emerge as a result of synergetic e ffects [1,5,6]. In this way, new materials with interesting features for innovative technologies can be produced.

White light emitters are one of those potential materials that are extremely interesting for lightning systems, necessary in technological devices, and present in everyday life. Hitherto, white light emission has been largely pursued to improve existing lightning technology [7–15]. In this regard, several approaches have been postulated. In particular, fluorescence resonance energy transfer, FRET, is a bioinspired process to develop new systems as light-emitting devices [16–18]. The control of energy transfer processes with the combination of proper dyes allows to tune the color of the resultant emitted light. The most common strategy to achieve white light emission consists of combining fluorophores with emission spectra in the three fundamental regions (blue, green, and red), emitting at well-defined

intensities into a single host system able to impose a proper spatial distribution regarding intermolecular distances and molecular orientations [8,19–23]. Note that although purely organic compounds can allow a low-cost fabrication of white emitter devices, their major drawback is the aging, usually at di fferent rates for each component [23,24]. However, encapsulated organic dyes within nanocavities, preferably inorganic frameworks, are sheltered from chemical-, photo- and thermo-degradation due to the protection provided by the porous host [25,26]. Microporous zeolitic materials are ideal candidates to be used as inorganic stable hosts, in particular microporous aluminophosphates based on AlO4 and PO4 alternate tetrahedral units [26].

Generally, optical applications based on the combination of organic fluorescent dyes with inorganic hosts require optically dense materials. In such cases, molecular aggregation should be completely avoided in order to preserve the optical characteristics of the components. The key to success in the design of these hybrid materials is the matching between dye molecular size and nanochannel dimensions. In this context, the best synthetic approach to reach a tight host-guest fit is via the crystallization inclusion method, in which the dye is embedded while the inorganic framework is being formed [26,27]. Thus, the opening dimension of the pores limits the size of the guest, and since the occlusion process does not involve di ffusion, the gues<sup>t</sup> dyes can be accommodated all along the host crystals, avoiding gues<sup>t</sup> entangling at the pore entrance. However, this is not a trivial approach and several aspects should be considered before the encapsulation of organic dyes within inorganic channelled structures. The first condition when choosing an organic dye for this approach is that its chemical structure should not be very di fferent from that of the organic structure-directing agents (SDA) that typically drive through a template e ffect the crystallization of zeolitic materials, usually amines or quaternary ammonium compounds. In particular, dyes should be able to bear positive charges to maximize interaction with the negatively-charged inorganic framework, enhancing their incorporation. Moreover, the dye must be soluble in an aqueous synthetic gel. For this reason, cationic organic dyes with amine groups are a preferred choice. Besides, a negative net charge should be generated in the zeolitic framework, i.e., by the isomorphic substitution of Al3+ by Mg<sup>2</sup>+ in aluminophosphate networks, to facilitate the entry of those cationic dyes.

In previous works, it has been demonstrated that the one-dimensional Mg-containing aluminophosphate MgAPO-11, with AEL structure, is a perfect host for the incorporation of many commercial dyes due to its adequate pore structure having non-intersecting one-dimensional (1D) channels (6.5 × 4 Å) and their special topology [28,29]. By the encapsulation of dyes through the crystallization inclusion method into this framework, not only is molecular aggregation avoided, but preferential alignment along the pores is also induced [27,30,31]. Moreover, the inorganic 1D-host MgAPO-11 o ffers a very rigid environment limiting molecular motions responsible for the non-radiative deactivation pathways to the ground-state, yielding an astonishing enhancement of fluorescence with respect to the dye in solution, particularly for dyes with flexible molecular structures. As a result, highly fluorescent hybrid materials with an anisotropic response to the linearly polarized light have been obtained using this particular host [31].

In fact, the encapsulation of di fferent commercial fluorescent dyes into the 1D-nanochanneled inorganic host MgAPO-11 (Scheme 1) via a one-pot synthesis has already rendered hybrid materials with diverse optical properties such as (i) an optically switchable system by the simultaneous encapsulation of two chromophores with a complementary response to linearly polarized light [29]; and (ii) a red-emitting hybrid material with Second Harmonic Generation (SHG) properties under IR excitation by the confinement of a nonlinear optics (NLO) dye perfectly aligned within the channels [31].

**Scheme 1.** (**Left**) Molecular structures of dyes: acridine (AC), pyronine Y (PY), and LDS722. (**Right**) Schematic illustrations of AEL-zeolitic type structure, MgAPO-11.

Particularly in this work and as above-mentioned, a critical aspect to attain pure white light after excitation with UV light is to control the e fficiency of energy transfer among the di fferent co-adsorbed fluorescence emitters. Taking into account the requirements for an e ffective Försters Resonance Energy Transfer or FRET [32], acridine (AC), pyronin Y (PY), and hemicyanine LDS722 dyes (Scheme 1) were chosen to achieve a white light hybrid emitter. AC, with bluish-cyan emission, was selected as the primary energy-donor moiety due to its relatively high fluorescent quantum yield and its small dimensions that would provide a high donor-rate into the host. The AC emission spectrum overlaps well with the absorption band of the next energy-acceptor molecule, PY. Indeed, this AC-PY pair was set in a previous work as a suitable donor-acceptor pair for an e fficient FRET process [29]. Finally, a hemicyanine-like dye, 4-[4-[4-(dimethylamino)phenyl]-1,3-butadienyl]-1-ethyl-pyridinium (LDS722), characterized by emission in the red region of the electromagnetic spectrum, is chosen as final acceptor dye in the FRET cascade. Although the fluorescence of this dye is rather poor in solution due to the inherent flexibility of its molecular structure, a highly improved red emission e fficiency is reached by the grea<sup>t</sup> constraint imposed by the AEL nanopores [31].

Here, to achieve a bright emitting system with pure white light, a systematic variation of the relative proportion of the three dyes in the synthesis of the hybrid material is performed. The resultant materials are fully characterized by steady-state and time-resolved spectroscopic techniques. Experimental evidence of FRET processes among the dyes within the pores of the material are also provided via ultrafast spectroscopy experiments where the excited-state dynamics is investigated.

#### **2. Materials and Methods**
